Energy

Class 11 Chemistry, unit 22, book pages 396–407

Practise 43 MCQs Take the unit quiz Review 22 cards

Most of the energy we use comes from fossil fuels, which are refined into fractions and converted into petrochemicals. This unit covers how petroleum is formed, mined and refined, the basic petrochemical processes, and how fuels and batteries are compared by energy density and specific energy. It then explains how nuclear fission and fusion release energy, how solar energy is captured by thermal and PV systems, and how energy use affects the environment through the carbon footprint.

Learning outcomes (12)

Differentiate between petrochemicals and the chemicals derived from them.
Identify different raw materials for the petrochemical industry.
Explain the process of fractional distillation of petroleum, and identify the important fractions.
Describe the basic building block processes in petrochemical technology, and explain the petrochemical process technology.
List some major petrochemicals, and understand the importance of petrochemicals in the modern world.
Distinguish between the energy density and specific energy of different energy sources, and explain the efficiency of energy transfer.
Explain the formation, properties and uses of fossil fuels, and the importance of fossil fuels in the modern world.
Explain the mechanism and importance of nuclear fusion and fission, and explain the importance of nuclear energy in the modern world.
Explain the mechanism and importance of solar energy and its importance as a source of renewable energy in the modern world.
Explain the environmental impact of energy consumption, particularly in relation to global warming, and the importance of reducing carbon footprints and moving towards sustainable energy sources.
Apply knowledge of energy sources and their properties to critically evaluate the advantages and disadvantages of different energy sources, and make informed decisions about energy consumption.
Explain the merits of photocells as sustainable ways of meeting energy demands, with reference to the photovoltaic principle.

22.1Origin of petroleum and natural gas

Where does most of the energy used today come from? Fossil fuels are energy-rich substances formed from the remains of organisms. Coal, oil and natural gas are called fossil fuels because they formed from the remains of organisms that lived underground.

The name petroleum comes from the Latin words petra (stone or rock) and oleum (oil). Petroleum is also called crude oil. It is a thick, dark liquid made mostly of hydrocarbons. Natural gas, which is often found with oil deposits, consists mostly of methane. It also contains significant amounts of ethane, propane and butane.

We get most of our energy needs from fossil fuels. Many compounds derived from them serve as building blocks for many important products. The plastics that are everywhere in our world are mainly made from petrochemicals. They are used to make everything from packaging materials to household items, medical devices and electronics.

How petroleum was formed

Marine plants and animals died, and their remains piled up. This happened hundreds of millions of years ago.
Layers of sand, rock and mud buried the dead organisms. Lack of air meant the remains did not simply rot away.
Over time, the heat and pressure of the sediments, together with the activity of bacteria, transformed the material into a dark brown, viscous liquid called petroleum (crude oil).
Gas products that accumulate on top of the oil are called natural gas.

22.1.1Mining of petroleum

Crude oil is extracted from the earth's crust, where it is found, by drilling holes.

When drilling through rocks, natural gas first comes out at high pressure. After the gas pressure drops, the oil is extracted by pumps.

Extracting crude oil

Drill a hole through the rock to reach the oil deposit.
Natural gas comes out first, at high pressure.
When the gas pressure has dropped, pumps bring the oil to the surface.

22.1.2Refining of petroleumOften examined

The conversion of crude oil into beneficial products is referred to as refining. These beneficial products are called fractions. Each fraction is a collection of hydrocarbons that boil within a certain range of temperatures.

Petroleum is refined by fractional distillation in a tall fractionating tower (Figure 22.1). The crude oil is heated up to $400^{\circ} C$ in a furnace. It is then passed into the fractional distillation column, and its vapour rises through the column.

Fractional distillation of crude oil

Heat the crude oil in a furnace to about $400^{\circ} C$ and pass it into the column.
The vapour rises up the column and cools as it moves up.
Each compound condenses at the level where the temperature falls to its boiling point. Compounds with very high boiling points condense first, near the bottom. Compounds with the lowest boiling points condense last, near the top.
Compounds that do not boil collect at the bottom as residue (bitumen).
The vapour therefore condenses gradually at different levels, and the crude oil is separated into fractions. Each fraction has its own boiling range and composition.

Figure 22.1: Refining of petroleum. Fractions with higher boiling points condense lower in the column.

Table 22.1: Fractions of petroleum and their uses
Fraction	Number of carbon atoms per molecule	Boiling point (°C)	Important uses
Liquefied petroleum gas (LPG)	1–4	Below 20	Cylinder gas for cooking
Petrol	5–10	35–70	Fuel for motor cars and vehicles
Naphtha	8–12	70–120	Chemical feedstock for making drugs, plastics and other chemicals
Kerosene	10–16	170–250	Fuel for jet planes, fuel for heating, lighting and cooking
Diesel	14–20	270–340	Fuel for buses, trucks and trains
Lubricating oil	20–50	350–500	Lubricants for machines and engines, waxes and polishes
Fuel oil	50–70	500–600	Fuel for power stations, factories and ships
Bitumen	More than 70	More than 500	Paving roads and making roofing materials

Worked example: reading the boiling ranges of the fractions

Use Table 22.1. (a) Which fractions have boiled away by

15 0^{\circ} C

? (b) Which fraction boils at about

30 0^{\circ} C

? (c) Does diesel leave the column above or below petrol, and why?

Boiling ranges from Table 22.1. The three fractions whose ranges lie below 150 °C (accent) have boiled by then; kerosene and the heavier fractions have not.

(a) Read the boiling ranges up to $15 0^{\circ} C$ : LPG (below 20), petrol (35 to 70) and naphtha (70 to 120) all lie below it, so they have boiled. Kerosene starts at 170, so it is still liquid.
(b) $30 0^{\circ} C$ lies inside the range 270 to 340 of diesel.
(c) Each compound condenses at the level where the column has cooled to its boiling point. The column is hottest at the bottom, so diesel (higher boiling point) condenses lower in the column than petrol, which leaves near the top.
Common mistake: do not match fractions by the number of carbon atoms alone. The ranges overlap (petrol 5 to 10, naphtha 8 to 12), but the boiling points show the order.

(a) LPG, petrol, naphtha; (b) diesel; (c) below petrol

Concept assessment exercise 22.1 (1): define petroleum

Define petroleum.

Petroleum (crude oil) is a thick, dark, viscous liquid made mostly of hydrocarbons. It formed over millions of years from the remains of marine plants and animals buried under sediments, and it is a fossil fuel. Its name comes from the Latin words petra (rock) and oleum (oil).

Petroleum is a dark, viscous liquid mixture of hydrocarbons, formed from buried marine organisms.

Concept assessment exercise 22.1 (2): names of the fractions

List names of fractions obtained by the fractional distillation of petroleum.

From Table 22.1, in order from the lightest (lowest boiling point) to the heaviest, the fractions are:

liquefied petroleum gas (LPG)
petrol
naphtha
kerosene
diesel
lubricating oil
fuel oil
bitumen

LPG, petrol, naphtha, kerosene, diesel, lubricating oil, fuel oil and bitumen.

Concept assessment exercise 22.1 (3): one use of each fraction

List one use of each petroleum fraction.

LPG: cylinder gas for cooking.
Petrol: fuel for motor cars and vehicles.
Naphtha: chemical feedstock for making drugs, plastics and other chemicals.
Kerosene: fuel for jet planes (also for heating, lighting and cooking).
Diesel: fuel for buses, trucks and trains.
Lubricating oil: lubricant for machines and engines (also waxes and polishes).
Fuel oil: fuel for power stations, factories and ships.
Bitumen: paving roads and making roofing materials.

See the list above, one use for each fraction.

Concept assessment exercise 22.1 (4): how petroleum is obtained

How is petroleum obtained?

Petroleum is found in rock deposits in the earth's crust, so a hole is drilled through the rock to reach the oil.
Natural gas comes out first, at high pressure.
When the gas pressure has dropped, pumps bring the crude oil to the surface.
The crude oil is then separated into its fractions by fractional distillation in a refinery.

Petroleum is obtained by drilling through rock to the oil deposit; after the gas pressure drops, pumps lift the oil.

22.2PetrochemicalsOften examined

Organic compounds derived from petroleum are called petrochemicals. The most important petrochemicals are ethylene, propylene, butadiene, methanol, benzene, toluene and xylene. They are mainly hydrocarbons and are obtained from the different fractions of crude oil, gas oil and petroleum gas. Some are also obtained from coal and natural gas, and some from renewable sources such as corn, sugarcane and other biomass.

Petrochemicals are used as raw materials in the petrochemical industry to make many chemical products. These products include plastics, rubbers, synthetic fibres, dyes, adhesives, resins, paints, coatings, pesticides and detergents. Cloths, curtains, carpets, floor tiles and toys are also petrochemical products. Because demand for petroleum products is increasing, more refineries and petrochemical plants are needed (Figure 22.2 in the book is a photograph of a petrochemicals plant).

The skeletal structures of some of these petrochemicals are shown below. Only one isomer of xylene is drawn.

Ethylene

Propylene

Butadiene

Methanol

Benzene

Toluene

o-Xylene

22.2.1Building block processes in petrochemical technologyOften examined

Petroleum fractions are converted into many commercially important products in refineries by various processes. The basic building block processes in petrochemical technology are:

Thermal cracking
Catalytic cracking
Steam reforming

Thermal cracking

Thermal cracking is a process in which the high molecular weight fraction of hydrocarbons is heated at high temperature ( $450$ – $750^{\circ} C$ ) and high pressure ( $70 atm$ ) to produce smaller molecules. It produces light oil (gasoline), medium oil (used in diesel fuel), heavy oil and coke.

Example: thermal cracking of decane

Thermal cracking of decane gives octane and ethene. Check that the book's equation

C X_{10} H X_{22} C X_{8} H X_{18} + CH X_{2} = CH X_{2}

balances.

Count the carbon atoms. Decane has 10. The products have 8 in octane and 2 in ethene.
$10 = 8 + 2$
Count the hydrogen atoms. Decane has 22. The products have 18 in octane and 4 in ethene (two $CH X_{2}$ groups).
$22 = 18 + 4$
Both atoms balance, so the equation is correct.
$C X_{10} H X_{22} C X_{8} H X_{18} + CH X_{2} = CH X_{2}$

C X_{10} H X_{22} C X_{8} H X_{18} + CH X_{2} = CH X_{2}

Catalytic cracking

Catalytic cracking is the process in which a catalyst is used to break down large hydrocarbons into smaller molecules. The high molecular weight petroleum fraction is passed over a layer of zeolite catalyst, which cracks the heavier fraction into lighter hydrocarbons. This method is used to produce high-octane fuels and is ideal for making petrol (gasoline).

Example: catalytic cracking of heptane

Catalytic cracking of heptane gives pentane and ethene. Check that

C X_{7} H X_{16} C X_{5} H X_{12} + CH X_{2} = CH X_{2}

balances.

Count the carbon atoms. Heptane has 7. The products have 5 in pentane and 2 in ethene.
$7 = 5 + 2$
Count the hydrogen atoms. Heptane has 16. The products have 12 in pentane and 4 in ethene.
$16 = 12 + 4$
Both atoms balance, so the equation is correct.
$C X_{7} H X_{16} C X_{5} H X_{12} + CH X_{2} = CH X_{2}$

C X_{7} H X_{16} C X_{5} H X_{12} + CH X_{2} = CH X_{2}

Both cracking processes are important for producing commercial gasoline and light oils. The gaseous products, such as methane, ethane, propane, propene and butane, are important raw materials for petrochemical plants.

Worked example: products of cracking

(a) Dodecane,

C X_{12} H X_{26}

, is cracked to octane,

C X_{8} H X_{18}

, and one alkene. Find the formula of the alkene. (b) Decane,

C X_{10} H X_{22}

, is cracked to propene,

C X_{3} H X_{6}

, and one alkane. Find the formula of the alkane.

In cracking a large molecule splits into smaller ones, so the atoms of the reactant must all appear in the products. Subtract the known product from the reactant.
(a) Carbon: $12 - 8 = 4$ . Hydrogen: $26 - 18 = 8$ . The alkene is $C X_{4} H X_{8}$ (butene), which fits $C X_{n} H X_{2 n}$ .
$C X_{12} H X_{26} C X_{8} H X_{18} + C X_{4} H X_{8}$
(b) Carbon: $10 - 3 = 7$ . Hydrogen: $22 - 6 = 16$ . The alkane is $C X_{7} H X_{16}$ (heptane), which fits $C X_{n} H X_{2 n + 2}$ .
$C X_{10} H X_{22} C X_{3} H X_{6} + C X_{7} H X_{16}$
Check each answer by counting atoms on both sides: (a) 8 + 4 = 12 C and 18 + 8 = 26 H; (b) 3 + 7 = 10 C and 6 + 16 = 22 H.

(a) C X_{4} H X_{8}; (b) C X_{7} H X_{16}

Steam reforming

In steam reforming, lower gaseous hydrocarbons are heated with steam to $700$ – $1000^{\circ} C$ in the presence of a catalyst. This produces a mixture of hydrogen and carbon monoxide, with small amounts of $CO X_{2}$ . Natural gas, refinery gas, liquefied petroleum gas (LPG) and low-boiling naphtha are used as the feedstock. The $CO$ and $H X_{2}$ are further converted into methanol, which is an important petrochemical.

Example: steam reforming of methane

The book prints steam reforming as

CH X_{4} + H X_{2} O CO + H X_{2}

. Balance this equation.

Count the hydrogen atoms. $CH X_{4}$ and $H X_{2} O$ give 4 + 2 = 6 on the left, but the printed $H X_{2}$ gives only 2 on the right.
$4 + 2 = 6 \neq = 2$
Carbon and oxygen already balance (one each). Use three molecules of hydrogen on the right to balance the hydrogen: 3 × 2 = 6.
$CH X_{4} + H X_{2} O CO + 3 H X_{2}$
Check: C is 1 = 1, O is 1 = 1, and H is 6 = 6.

CH X_{4} + H X_{2} O CO + 3 H X_{2}

Comparison of the three block processes
Process	Conditions (from the book)	Main products
Thermal cracking	$450$ – $750^{\circ} C$ and $70 atm$	Light oil (gasoline), medium oil, heavy oil, coke and small alkenes such as ethene
Catalytic cracking	Zeolite catalyst (temperature not given in the book)	High-octane fuels such as petrol, and gaseous products
Steam reforming	Steam, catalyst, $700$ – $1000^{\circ} C$	A mixture of $H X_{2}$ and $CO$ , with small amounts of $CO X_{2}$ , then methanol

22.3Energy contents of fuelOften examined

Many materials can store energy, and this energy can be used in many ways. Food, coal, petroleum products and uranium all contain stored energy. These materials are collectively known as fuels. The amounts of energy that fuels provide are listed in Table 22.2.

The energy contents of fuels are expressed by two terms, energy density and specific energy.

Quantity	Meaning (from the book)	Unit
Energy density	The amount of energy stored in a material or fuel per unit volume	$J m^{- 3}$ (the battery table uses $MJ L^{- 1}$ )
Specific energy	The amount of energy a fuel can release when it is burned, per unit mass	$J k g^{- 1}$ or $Wh k g^{- 1}$

For example, the specific energy of coal is $24 MJ k g^{- 1}$ and that of natural gas is $55 MJ k g^{- 1}$ . Table 22.2 shows the specific energy of various fuels.

Table 22.2: The specific energy density (MJ/kg) of a variety of different fuels
Fuel type	Reaction type	Specific energy density (MJ/kg)	Typical uses
Biodiesel	Chemical	38	Automotive engine
Coal	Chemical	24	Power plants, electricity generation
Crude oil	Chemical	44	Refinery, petroleum products
Diesel	Chemical	45	Diesel engines
Ethanol	Chemical	26.8	Gasoline mixture, alcohol, chemical products
Gasoline	Chemical	46	Gasoline engines
Natural gas	Chemical	55	Household heating, electricity generation
Uranium-235	Nuclear	3 900 000	Nuclear reactor electricity generation
Wood	Chemical	16	Space heating, cooking

Worked example: energy released by a mass of fuel

Using Table 22.2, find (a) the energy released by burning

2.5 kg

of coal, in MJ and in J, (b) the mass of ethanol and (c) the mass of natural gas that would release the same energy.

Given

$m = 2.5 kg$ mass of coal
$e_{coal} = 24 MJ k g^{- 1}$ specific energy of coal
$e_{ethanol} = 26.8 MJ k g^{- 1}$ specific energy of ethanol
$e_{gas} = 55 MJ k g^{- 1}$ specific energy of natural gas

To find

E
m_{\mathrm{ethanol}}
m_{\mathrm{gas}}

Specific energy is energy per unit mass, so the energy released is mass times specific energy.
$E = m e = 2.5 \times 24 = 60 MJ = 6.0 \times 1 0^{7} J$
(b) For the same energy, the mass needed is the energy divided by the specific energy of ethanol.
$m = \frac{60}{26.8} = 2.24 kg$
(c) In the same way for natural gas.
$m = \frac{60}{55} = 1.09 kg$
The larger the specific energy, the smaller the mass needed. Natural gas needs the least. Common mistake: specific energy is per kilogram, not per cubic metre (that is energy density).

E = 60 MJ; m_{ethanol} = 2.24 kg; m_{gas} = 1.09 kg

The energy density of a battery

Table 22.3 gives the specific energy and energy density of three types of battery.

Table 22.3: Energy density of a battery
Storage type	Specific energy (MJ/kg)	Energy density (MJ/L)
Lithium-ion battery	0.46–0.72	0.83–3.6
Lithium-sulphur battery	1.80	1.26
Silver-oxide battery	0.47	1.8

The efficiency of a battery is the amount of energy a battery can provide compared with the amount of energy required to charge it. No battery has 100% efficiency, because some energy is lost during storage, charging and discharging. Lithium-ion batteries have the highest efficiency, 99%. The efficiency of a lead-acid battery is about 85%.

Worked example: energy stored in a battery and the energy to charge it

A lithium-sulphur battery has a mass of 0.50\,\mathrm{kg}. Using Table 22.3, find (a) the energy it stores, (b) the volume it needs for that energy, and (c) the energy needed to charge a lithium-ion battery that delivers 9.9\,\mathrm{MJ} (efficiency 99\%).

Given

m = 0.50\,\mathrm{kg} mass of the battery
e = 1.80\,\mathrm{MJ\,kg^{-1}} specific energy of Li-S
\rho_E = 1.26\,\mathrm{MJ\,L^{-1}} energy density of Li-S
\eta = 99\% efficiency of Li-ion

To find

E
V
E_{\mathrm{charge}}

(a) Energy stored is mass times specific energy.
E=0.50\times1.80=0.90\,\mathrm{MJ}
(b) Energy density is energy per unit volume, so the volume is the energy divided by the energy density.
V=\dfrac{0.90}{1.26}=0.71\,\mathrm{L}
Check the unit: 1.26\,\mathrm{MJ\,L^{-1}}=1.26\times10^{9}\,\mathrm{J\,m^{-3}}, and 0.71\,\mathrm{L}=7.1\times10^{-4}\,\mathrm{m^3}, which gives the same energy of 0.90\,\mathrm{MJ}.
(c) Efficiency is the energy the battery provides compared with the energy needed to charge it.
E_{\mathrm{charge}}=\dfrac{9.9}{0.99}=10\,\mathrm{MJ}
The extra 0.1\,\mathrm{MJ} is lost during storage, charging and discharging, because no battery has 100% efficiency.

E=0.90\,\mathrm{MJ};\quad V=0.71\,\mathrm{L};\quad E_{\mathrm{charge}}=10\,\mathrm{MJ}

22.3.1Nuclear energy

Uranium can produce millions of times more energy than any fuel can produce (see Table 22.2). This energy is stored in the nucleus of uranium atoms. Uranium can produce energy through two processes, fission and fusion. A fusion reaction releases more energy than a fission reaction. The fission process also produces long-lived radioactive waste as by-products. During fusion, harmful substances such as \ce{CO2} and other greenhouse gases are not released into the atmosphere. Its main by-product is non-toxic helium, and it does not produce long-lived nuclear waste.

Nuclear fission

Nuclear fission occurs when a heavy nucleus splits into two or more smaller nuclei, with the release of neutrons and a tremendous amount of energy. The process can be spontaneous, but more often it is started by the absorption of a neutron. A heavy nucleus, such as uranium-235 or plutonium-239, absorbs a neutron.

Mechanism of fission

The absorption of the neutron makes the nucleus excited and unstable.
This instability causes the nucleus to split into two lighter nuclei.
Three neutrons are emitted during the splitting.
The mass of the reactants is greater than the mass of the products. The lost mass (the mass defect) is converted into energy according to Einstein's equation.
E=mc^2

Example: fission of uranium-235

Check the book's fission equation by mass number and atomic number: \ce{^{235}_{92}U + ^{1}_{0}n -> ^{144}_{56}Ba + 3 ^{1}_{0}n + ^{89}_{36}Kr + energy}.

Mass numbers. Left: 235 + 1 = 236. Right: 144 + 3 × 1 + 89 = 236.
236 = 236
Atomic numbers. Left: 92 + 0 = 92. Right: 56 + 3 × 0 + 36 = 92.
92 = 92
Both balance, so the equation is correct as printed. The products are barium, krypton, three free neutrons and a large amount of energy.

\ce{^{235}_{92}U + ^{1}_{0}n -> ^{144}_{56}Ba + 3 ^{1}_{0}n + ^{89}_{36}Kr + energy}

Nuclear fusion

Nuclear fusion is the process in which two light atomic nuclei combine to form a heavier nucleus, releasing energy in the process.

Mechanism of fusion

Two light nuclei, such as isotopes of hydrogen (deuterium), are brought close together. This usually needs extremely high temperatures to overcome the electrostatic repulsion between the positively charged nuclei.
Under the intense conditions of high temperature and pressure, the nuclei fuse to form a heavier nucleus. Deuterium and tritium fuse to form helium-4, a neutron and an enormous amount of energy.
The mass of the resulting nucleus is less than the sum of the masses of the original nuclei. The difference in mass is released as energy according to Einstein's equation.
E=mc^2

Example: fusion of hydrogen isotopes

The book gives two fusion equations. Check each one by mass number and atomic number. The second equation has a printed misprint, so correct it as well.

Equation (a). Mass numbers: 2 + 2 = 4 = 3 + 1. Atomic numbers: 1 + 1 = 2 = 2 + 0. Balanced.
\ce{^{2}_{1}H + ^{2}_{1}H -> ^{3}_{2}He + ^{1}_{0}n}
Equation (b) as printed uses tritium written as ^{3}_{2}\mathrm{H} and a hydrogen atom as product. The atomic number of tritium is 1, so the symbol should be ^{3}_{1}\mathrm{H}.
\ce{^{3}_{1}H + ^{2}_{1}H -> ^{4}_{2}He + ^{1}_{0}n + energy}
With ^{3}_{1}\mathrm{H}, the mass numbers are 3 + 2 = 5 = 4 + 1 and the atomic numbers are 1 + 1 = 2 = 2 + 0. The product must therefore be a neutron, as the text says.

\ce{^{3}_{1}H + ^{2}_{1}H -> ^{4}_{2}He + ^{1}_{0}n + energy}

22.3.2Importance of nuclear energy

Nuclear energy is an essential source of electricity generation. It can provide large amounts of electricity without producing \ce{CO2} and other air pollutants, so it does not affect air quality. Nuclear power plants produce less atmospheric pollution than thermal power plants. A small amount of nuclear fuel can produce electricity over a more extended period of time.

22.3.3Solar energy

We derive most of our energy from fossil fuels such as coal, oil and natural gas. Burning fossil fuels releases large amounts of greenhouse gases, such as carbon dioxide, oxides of nitrogen and oxides of sulphur. These gases are responsible for global warming. Its impacts include climate change, sea level rise, extreme weather conditions and ecosystem disruption.

Reducing greenhouse gas emissions is therefore necessary to control global warming. The carbon footprint indicates the total amount of greenhouse gases, especially carbon dioxide, emitted directly or indirectly by human activity.

Moving to sustainable energy sources is an important strategy for reducing carbon dioxide emissions. Renewable sources such as solar, wind, hydropower and geothermal produce electricity with significantly less or no emissions than fossil fuels. Switching to these cleaner options will not only help combat climate change, but will also increase energy independence, reduce air pollution and promote long-term environmental sustainability.

22.3.4Mechanism of solar energy conversion into heat and electricityOften examined

Solar panels are used to capture sunlight and convert it into electricity and heat. There are two main solar energy technologies:

Solar thermal power (CSP) systems
Photovoltaic (PV) cells

Thermal systems

Thermal systems use sunlight to absorb energy and convert it into heat, which can then be used for heating purposes. Solar energy is first absorbed at the surface by a receiver. The receiver is connected to a liquid or oil, and it heats up the liquid until it becomes hot.

Figure 22.3: Solar thermal system (schematic of the book's photograph). The receiver absorbs sunlight and heats the liquid or oil.

Photovoltaic cells (PV cells)

Photovoltaic cells convert sunlight into electricity. The basic principle of a PV cell is that when the sun shines on the solar panel, the solar energy is absorbed by the solar cells.

How a PV cell produces electricity

Sunlight (photons) falls on the semimetallic silicon used in the cells.
The photons excite the outermost electrons of the silicon, creating an internal electric field.
This causes an electric current to flow. The current is direct current (DC).
The solar panel wiring captures the direct current.
An inverter converts the direct current into alternating current (AC).
The alternating current operates electrical equipment.

Solar thermal and PV systems compared
System	Sunlight is converted into	Use shown in the book	How it works
Solar thermal (CSP)	Heat	Heating purposes	A receiver absorbs the sunlight and heats a liquid or oil.
Photovoltaic (PV) cells	Electricity (DC, then AC after an inverter)	Operating electrical equipment	Photons excite the outermost electrons of semimetallic silicon, creating an internal electric field and a current.

Figure 22.4: Solar panel. Sunlight excites electrons in the cells, and they flow round the circuit through the load.

Advantages of PV cells as an energy source

The photovoltaic principle involves the generation of an electric current when certain materials are exposed to sunlight, as in PV cells. The merits of PV cells as sustainable ways of meeting energy demands are as follows:

Renewable energy source: solar energy is abundant and inexhaustible. Unlike fossil fuels, solar power does not deplete, and it can be harnessed as long as the sun shines.
Low environmental impact: solar energy production has lower environmental impacts than fossil fuels. It produces no air or water pollution during operation, so it contributes to cleaner air and water.
Reduced greenhouse gas emissions: solar power does not emit greenhouse gases during electricity generation. Using it reduces dependence on fossil fuels and helps reduce carbon footprints.
Reduced dependency on fossil fuels: PV cells provide an alternative to fossil energy sources, so they reduce our dependence on fossil fuels.
Long lifespan and low maintenance: PV cells often last 25 years or more with minimal maintenance. This durability contributes to the sustainability of solar power systems.

22.4.5Advantages and disadvantages of different energy sources

Table 22.4: Advantages and disadvantages of different energy sources
Energy resource	Advantages	Disadvantages
Fossil fuels	Reliable	Create pollution and will run out
Nuclear power	Reliable	Will run out and produce dangerous waste that is hard to dispose of
Wind turbines	Clean and cheap to run	Expensive to set up, and wind does not always blow
Hydroelectric power stations	Clean and cheap to run	Expensive to set up, and output could be affected by drought
Solar cells	Clean and cheap to run	Not always sunny, and output does not always outweigh the initial cost to set up

Considering the advantages and disadvantages, we should consider combining energy sources to obtain balanced and sustainable energy. We must constantly monitor the activity of the chosen energy source. In addition, the chosen energy source must meet future energy needs and environmental goals.

22.4.6What is a carbon footprint?

A carbon footprint is the grand sum of all the greenhouse gases emitted by human activity. Greenhouse gases trap heat in the atmosphere and so contribute to global warming. The carbon footprint is measured to lessen the impact of human activities on the environment and to tackle climate change head-on.

To gauge the impact of \ce{CO2} and other greenhouse gases, the carbon footprint is usually expressed as a measure of weight, in tonnes of \ce{CO2} or \ce{CO2} equivalent per year. This allows us to measure the potential warming effect of various greenhouse gases on a global scale.

To minimise your carbon footprint, there are several strategies:

Optimising energy usage.
Adopting renewable energy sources.
Maximising transportation efficiency.
Making sustainable choices in both consumption and production.

The environmental impact and finite nature of fossil fuels have led to increased interest in renewable energy sources such as solar, wind and hydropower. They are alternatives that reduce dependence on fossil fuels.

Worked example: carbon dioxide per megajoule from natural gas and petrol

Take natural gas as methane and petrol as octane, \ce{C8H18}. For 1.0\,\mathrm{kg} of each, find the mass of \ce{CO2} formed on complete burning, and compare the mass of \ce{CO2} per MJ of energy. Use e=55 and 46\,\mathrm{MJ\,kg^{-1}} for natural gas and petrol.

Methane: write the equation and find the moles in 1000\,\mathrm{g}. M(\ce{CH4})=16 and M(\ce{CO2})=44\,\mathrm{g\,mol^{-1}}.
\ce{CH4 + 2O2 -> CO2 + 2H2O}\qquad n=\dfrac{1000}{16}=62.5\,\mathrm{mol}
One mole of methane gives one mole of \ce{CO2}.
m(\ce{CO2})=62.5\times44=2750\,\mathrm{g}=2.75\,\mathrm{kg}
Octane: M(\ce{C8H18})=8(12)+18=114\,\mathrm{g\,mol^{-1}}. Two moles of octane give sixteen moles of \ce{CO2}, so one mole gives eight.
\ce{2C8H18 + 25O2 -> 16CO2 + 18H2O}\qquad n=\dfrac{1000}{114}=8.77\,\mathrm{mol}
Moles and mass of \ce{CO2}.
n(\ce{CO2})=8\times8.77=70.2\,\mathrm{mol},\quad m=70.2\times44=3088\,\mathrm{g}=3.09\,\mathrm{kg}
Divide by the energy released per kilogram to compare fuels on equal energy.
\text{methane: }\dfrac{2.75}{55}=0.050\,\mathrm{kg\,MJ^{-1}},\qquad \text{octane: }\dfrac{3.09}{46}=0.067\,\mathrm{kg\,MJ^{-1}}
Natural gas gives about 25\% less \ce{CO2} for the same energy, so burning it gives a smaller carbon footprint than burning petrol. Both still add greenhouse gas.

2.75\ \text{and}\ 3.09\,\mathrm{kg\ of\ }\ce{CO2};\quad 0.050\ \text{and}\ 0.067\,\mathrm{kg\,MJ^{-1}}

Formula sheet

Mass-energy equivalence (nuclear energy)

E=mc^2

E is the energy released, m is the mass lost in the reaction (the mass defect), and c is the speed of light in vacuum.

Units: E in \mathrm{J}, m in \mathrm{kg}, c in \mathrm{m\,s^{-1}}

Key terms

Petroleum (crude oil): A thick, dark liquid made mostly of hydrocarbons, formed from the remains of organisms buried under sand, rock and mud.
Natural gas: The gas products that accumulate on top of oil deposits. It consists mostly of methane, with some ethane, propane and butane.
Fossil fuels: Energy-rich substances, such as coal, oil and natural gas, formed from the remains of organisms.
Refining: The conversion of crude oil into beneficial products called fractions.
Fractional distillation: Separation of crude oil into fractions in a tall tower, according to the boiling points of the compounds.
Fraction: A collection of hydrocarbons that boil within a certain range of temperatures and have a specific composition.
Petrochemicals: Organic compounds derived from petroleum, mainly hydrocarbons, used as raw materials in the petrochemical industry.
Thermal cracking: Heating high molecular weight hydrocarbons at high temperature and pressure to produce smaller molecules.
Catalytic cracking: The process in which a catalyst is used to break down large hydrocarbons into smaller molecules.
Steam reforming: Heating lower gaseous hydrocarbons with steam in the presence of a catalyst to produce a mixture of hydrogen and carbon monoxide.
Energy density: The amount of energy stored in a material or fuel per unit volume.
Specific energy: The amount of energy a fuel can release when it is burned, per unit mass.
Efficiency of a battery: The amount of energy a battery can provide compared with the amount of energy required to charge it.
Nuclear fission: The splitting of a heavy nucleus into two or more smaller nuclei, with the release of neutrons and a large amount of energy.
Nuclear fusion: The combining of two light atomic nuclei to form a heavier nucleus, with the release of energy.
Mass defect: The mass lost in a nuclear reaction, which is converted into energy.
Greenhouse gases: Gases such as carbon dioxide, oxides of nitrogen and oxides of sulphur that trap heat in the atmosphere and contribute to global warming.
Carbon footprint: The total amount of greenhouse gases emitted by human activity, usually expressed as tonnes of \ce{CO2} or \ce{CO2} equivalent per year.
Photovoltaic cells: Cells that convert sunlight directly into electricity.
Photovoltaic principle: The generation of an electric current when certain materials are exposed to sunlight.
Inverter: A device that converts direct current (DC) into alternating current (AC).

Multiple choice questionsp. 406

Which is a petrochemical product?

AXylene
BToluene
CRubberAnswer
DCoal

Why

Rubber is named by the book as one of the important petrochemical products, together with plastics, synthetic fibres, dyes, adhesives, resins, paints, pesticides and detergents. These are products made from petrochemical raw materials.

Xylene and toluene are petrochemicals (the compounds themselves, listed in Section 22.2), so they are the wrong choice for a question about a product. Coal is a fossil fuel and a raw material, not a product made from petrochemicals.

Note. The book prints no key. Xylene and toluene are also named as petrochemicals in Section 22.2, so the question could be read as having two keys. C is chosen because the question asks for a product and the book lists rubbers among petrochemical products.

Which of the following has the highest specific energy density?

ACoal
BGasoline
CUraniumAnswer
DWood

Why

Table 22.2 gives uranium-235 as 3\,900\,000\,\mathrm{MJ\,kg^{-1}}, against 24 for coal, 46 for gasoline and 16 for wood. Uranium is therefore about 8.5\times10^{4} times higher than gasoline.

The reason is that the energy of uranium is stored in the nucleus of its atoms and is released in a nuclear reaction, not by burning, so it is far greater than any chemical fuel can give.

iii

\mathrm{J/L} is the unit of

AEnergy densityAnswer
BSpecific energy density
CEnergy
DDensity

Why

Energy density is the energy stored per unit volume, so its unit is joules per unit volume: \mathrm{J\,m^{-3}}, or \mathrm{J\,L^{-1}}. The book gives battery energy density in \mathrm{MJ\,L^{-1}} in Table 22.3.

Specific energy density is per unit mass (\mathrm{J\,kg^{-1}}), joules alone is the unit of energy, and density is measured in \mathrm{kg\,m^{-3}}.

Petrol fraction consists of ______ carbon atoms.

A1 to 4
B5 to 10Answer
C8 to 12
D10 to 16

Why

Table 22.1 gives the petrol fraction as 5 to 10 carbon atoms per molecule, with a boiling range of 35 to 70 °C.

Option A (1 to 4) is liquefied petroleum gas, option C (8 to 12) is naphtha and option D (10 to 16) is kerosene.

Which of the following is not a petrochemical?

APropene
BEthene
CPlasticAnswer
DMethanol

Why

Propene (propylene), ethene (ethylene) and methanol are all named in Section 22.2 as petrochemicals. Plastic is a product made from petrochemicals, not one of the petrochemical compounds the book lists.

So plastic is the odd one out, and the answer is C.

Note. The book prints no key. The answer relies on the book's distinction between petrochemical compounds (propene, ethene, methanol) and the products made from them (plastics, rubbers, fibres).

Short questionsp. 407

2(i)3 marks

Differentiate between energy density and specific energy density.

Solution

Energy density is the amount of energy stored in a material or fuel per unit volume. Its unit is joules per cubic metre (\mathrm{J\,m^{-3}}); the book also quotes it in \mathrm{MJ\,L^{-1}} for batteries.

Specific energy (called specific energy density in Table 22.2) is the amount of energy a fuel can release when it is burned, per unit mass. Its units are \mathrm{J\,kg^{-1}}, \mathrm{MJ\,kg^{-1}} or \mathrm{Wh\,kg^{-1}}.

	Energy density	Specific energy
Measured per	Unit volume	Unit mass
Unit	\mathrm{J\,m^{-3}} (or \mathrm{MJ\,L^{-1}})	\mathrm{J\,kg^{-1}} (or \mathrm{MJ\,kg^{-1}})
Example	Lithium-ion battery: 0.83 to 3.6\,\mathrm{MJ\,L^{-1}}	Coal: 24\,\mathrm{MJ\,kg^{-1}}; natural gas: 55\,\mathrm{MJ\,kg^{-1}}

Note. The book uses 'specific energy density' in Table 22.2 and in the question, and 'specific energy' in its text. Both mean the same quantity: energy per unit mass.

2(ii)3 marks

Define and give examples of petrochemicals.

Solution

Petrochemicals are organic compounds derived from petroleum. They are mainly hydrocarbons obtained from the different fractions of crude oil, gas oil and petroleum gas. Some are also obtained from coal and natural gas, and from renewable sources such as corn, sugarcane and other biomass.

Petrochemical compounds: ethylene, propylene, butadiene, methanol, benzene, toluene and xylene.
Products made from petrochemicals: plastics, rubbers, synthetic fibres, dyes, adhesives, resins, paints, coatings, pesticides and detergents.

2(iii)3 marks

List differences between fission and fusion reactions.

Solution

Both processes release energy from the nucleus, and in both the lost mass is converted into energy according to E=mc^2. The differences are:

Feature	Nuclear fission	Nuclear fusion
Nuclei involved	A heavy nucleus, such as uranium-235, splits into two or more smaller nuclei	Two light nuclei, such as isotopes of hydrogen, combine to form a heavier nucleus
Trigger	Usually started when the nucleus absorbs a neutron	Needs extremely high temperature and pressure to overcome the repulsion between the nuclei
Energy released	Large, but less than fusion (the book's comparison)	More than fission (the book's comparison)
By-products	Long-lived radioactive waste	Non-toxic helium; no long-lived nuclear waste
Effect on the atmosphere	Not compared in the book	No \ce{CO2} or other greenhouse gases released

Fusion is the cleaner process, because its main by-product is non-toxic helium, while fission leaves long-lived radioactive waste.

Note. Misprints in Section 22.3.1: the fission text says barium-141 but the equation gives \ce{^{144}_{56}Ba} (the mass balance 235 + 1 = 144 + 89 + 3 confirms 144). The fusion equation ends in \ce{^{1}_{1}H}, a proton, while the text says the product is a neutron; the deuterium-tritium reaction is \ce{^{2}_{1}H + ^{3}_{1}H -> ^{4}_{2}He + ^{1}_{0}n}.

2(iv)3 marks

List advantages of solar energy.

Solution

The book gives these advantages of solar (photovoltaic) energy:

Renewable: the sun's energy is abundant and inexhaustible, and it can be harnessed as long as the sun shines.
Low environmental impact: solar energy production produces no air or water pollution during operation.
Reduced greenhouse gas emissions: no greenhouse gases are emitted during electricity generation, so the carbon footprint is reduced.
Reduced dependence on fossil fuels: PV cells provide an alternative to fossil energy sources.
Long lifespan and low maintenance: PV cells often last 25 years or more with minimal maintenance.

Table 22.4 also describes solar cells as clean and cheap to run.

2(v)3 marks

What is catalytic cracking of petroleum?

Solution

Catalytic cracking is the process in which a catalyst is used to break down large hydrocarbons into smaller molecules. The high molecular weight petroleum fraction is passed over a layer of zeolite catalyst, which cracks the heavy fraction into lighter hydrocarbons.

The process is used to produce high-octane fuels and is ideal for making petrol (gasoline). For example, heptane gives pentane and ethene:

\ce{C7H16 -> C5H12 + CH2=CH2}

The gaseous products, such as ethene and propene, are important raw materials for petrochemical plants.

33 marks

Why is solar energy so important?

Solution

Most of our energy comes from fossil fuels. Burning them releases greenhouse gases such as carbon dioxide, oxides of nitrogen and oxides of sulphur, which cause global warming, with effects such as climate change, sea level rise, extreme weather and ecosystem disruption. Reducing greenhouse gas emissions is therefore necessary, and the transition to sustainable energy is an important strategy. Solar energy matters because:

it is renewable and inexhaustible, so it does not run out like fossil fuels;
it produces electricity with little or no emission of greenhouse gases, which reduces the carbon footprint;
it reduces dependence on fossil fuels and so increases energy independence;
it reduces air pollution and promotes long-term environmental sustainability.

43 marks

How is solar energy converted into electricity?

Solution

Solar energy is converted into electricity by photovoltaic (PV) cells, using the photovoltaic principle: the generation of an electric current when certain materials are exposed to sunlight.

Sunlight is absorbed by the solar cells of the panel.
Photons excite the outermost electrons of the semimetallic silicon used in the cells, which creates an internal electric field.
The electrons flow, producing an electric current in the form of direct current (DC), which is captured by the panel wiring.
An inverter converts the DC into alternating current (AC), which is used to operate electrical equipment.

55 marks

Explain the mechanism of solar energy conversion into heat and electricity.

Solution

There are two main solar energy technologies: thermal systems, which convert sunlight into heat, and photovoltaic (PV) cells, which convert sunlight into electricity.

Thermal systems (heat)

Sunlight is absorbed at the surface of a receiver.
The receiver is connected to a liquid or oil, and it heats the liquid.
The hot liquid is then used for heating purposes.

Photovoltaic cells (electricity)

Sunlight shines on the solar panel and the solar cells absorb the energy.
Photons excite the outermost electrons of the semimetallic silicon, creating an internal electric field and an electric current (DC).
An inverter converts the DC into AC for the electrical equipment.

Solar energy is captured either as heat by a receiver (top) or as electricity by PV cells (bottom); the inverter changes DC to AC.

Note. The book says the receiver 'contains an oris', which is not a recognised term and is most likely a misprint. The answer describes the receiver without naming it.

65 marks

What are the block building processes in petrochemical technology?

Solution

The basic building block processes in petrochemical technology are thermal cracking, catalytic cracking and steam reforming. They convert petroleum fractions into many commercially important products.

Thermal cracking: the high molecular weight fraction is heated at 450 to 750\,^\circ\mathrm{C} and a pressure of about 70\,\mathrm{atm}. It gives smaller molecules: light oil (gasoline), medium oil (used in diesel fuel), heavy oil and coke. Example: \ce{C10H22 -> C8H18 + C2H4} (decane gives octane and ethene).
Catalytic cracking: the high molecular weight fraction is passed over a zeolite catalyst, which breaks it into lighter hydrocarbons. It is used to produce high-octane petrol. Example: \ce{C7H16 -> C5H12 + C2H4} (heptane gives pentane and ethene).
Steam reforming: lower gaseous hydrocarbons (natural gas, refinery gas, LPG or low boiling naphtha) are heated with steam at 700 to 1000\,^\circ\mathrm{C} in the presence of a catalyst, to give a mixture of \ce{CO} and \ce{H2}. Example: \ce{CH4 + H2O -> CO + 3H2}. A small amount of \ce{CO2} is also produced.

The \ce{CO} and \ce{H2} are further converted into methanol, which is an important petrochemical.

Note. The book writes the steam reforming equation as \ce{CH4 + H2O -> CO + H2}, which is not balanced. The answer uses the balanced form \ce{CH4 + H2O -> CO + 3H2}.

73 marks

How is petroleum refined?

Solution

Refining converts crude oil into useful fractions by fractional distillation in a tall fractionating tower (Figure 22.1 in the book).

The crude oil is heated to about 400\,^\circ\mathrm{C} in a furnace, so that it becomes vapour.
The hot vapour is passed into the fractional distillation column and rises through it.
As the vapour rises it cools, and each compound condenses at the level that matches its boiling point: compounds with high boiling points condense near the bottom and those with low boiling points near the top.
Each fraction is drawn off at its own level, and has its own boiling range and composition (Table 22.1). The compounds that do not boil collect at the bottom as residue.

From the top, the main fractions are liquefied petroleum gas (LPG), petrol, naphtha, kerosene, diesel, lubricating oils, fuel oil and bitumen (the residue).

Petroleum is separated in the column by boiling point: the fraction with the lowest boiling point is drawn off at the top.

85 marks

Debate the role of solar energy in addressing global warming and climate change.

Solution

For: global warming is driven by greenhouse gases released when fossil fuels are burned, and it leads to climate change, sea level rise and extreme weather. Solar energy addresses the cause directly, because a solar panel emits no greenhouse gases during electricity generation. It reduces dependence on fossil fuels and so lowers the carbon footprint.

Renewable sources such as solar produce electricity with significantly less or no emissions than fossil fuels.
The transition to sustainable energy is an important strategy to reduce carbon dioxide emissions.
Solar energy is abundant and inexhaustible, so its use can continue as long as the sun shines.

Against, or limits: solar cells are not always sunny, so their output is not constant, and the initial cost of setting up a system does not always outweigh its output. The book therefore recommends combining energy sources to obtain balanced and sustainable energy, and monitoring the chosen source.

Conclusion: solar energy plays a significant part in reducing greenhouse gas emissions, but its benefit depends on sunlight and cost, so it works best as part of a combination of sources.

95 marks

Evaluate the environmental impact of large-scale solar energy production.

Solution

Solar energy production has a lower environmental impact than fossil fuels. According to the book, its main environmental advantages are:

No air or water pollution during operation, so it contributes to cleaner air and water.
No greenhouse gases during electricity generation, so it reduces the carbon footprint and helps to tackle climate change.
No depletion: unlike fossil fuels, solar power does not run down, and it can be harnessed as long as the sun shines.

The limits are practical rather than environmental: solar cells are not always sunny, and the output does not always outweigh the initial cost of setting up. A large system is therefore most useful where the sun is reliable and its cost is justified, and the book recommends combining it with other sources.

Overall: large-scale solar production affects the air, water and climate far less than burning fossil fuels, which is its main environmental merit.

Note. The book does not discuss manufacture, land use or disposal of solar panels, so the evaluation is limited to the impacts the book describes.

Exam focus

How is petroleum refined? Fractional distillation and the important fractions (petrol with its carbon range, kerosene, diesel). (long)
Not in the 2025 or 2026 paper; book short question 10 and an SLO.
Go to answer
Energy density and specific energy: differences, units (J/L, J/kg) and comparison of fuels and batteries. (short)
Not in the 2025 or 2026 paper; book short question 1 and MCQs 2 and 3.
Go to answer
Define petrochemicals with examples, and the building block processes of petrochemical technology. (short)
Not in the 2025 or 2026 paper; book short questions 2 and 9 and MCQs 1 and 5.
Go to answer
Fission and fusion: differences and the importance of nuclear energy. (short)
Not in the 2025 or 2026 paper; book short question 3 and an SLO.
Go to answer
Solar energy: advantages, importance and conversion into heat and electricity (PV cell). (short)
Not in the 2025 or 2026 paper; book short questions 4 to 8.
Go to answer
Catalytic cracking of petroleum. (short)
Not in the 2025 or 2026 paper; book short question 5.
Go to answer
Environmental impact of energy use, carbon footprint and global warming. (short)
Not in the 2025 or 2026 paper; book short questions 11 and 12 and an SLO.
Go to answer