25: Fossil Fuels

Question 1

What are the most common energy sources in Hong Kong?

Answer 1

Electricity (55%), oil and coal products like petroleum and diesel (28%), town gas and liquefied petroleum gas (LPG) (17%)

Question 2

What is the source of coal, petroleum, and natural gas?

Answer 2

Coal was formed from dead plants while petroleum and natural gas were formed from dead sea creatures.

Question 3

Why are fossil fuels called non-renewable resources?

Answer 3

Fossil fuels are still forming year slowly, but humans are using them up much faster than they can form, meaning that they will run out one day.

Question 4

How is coal formed?

Answer 4

Ancient giant plants die and form thick layers over the swamps. Water and dirt began to pile up on top of the dead plant remains. Over millions of years, high pressure, high temperature, and bacteria action changed these remains into coal.

Question 5

How are petroleum and natural gas formed?

Answer 5

Ancient marine plants and animals die and sink to the seabed. Their remains were covered by layers of sand and sediment. Over time, further deposits of sand and sediment buried the remains deeper and deeper. Over millions of years, high pressure, high temperature, and bacteria action turn the remains into petroleum and natural gas.

Question 6

What is the definition of hydrocarbon?

Answer 6

A hydrocarbon is a compound containing only hydrogen atoms and carbon atoms.

Question 7

What is the molecular formula of ethene?

Answer 7

C₂H₄

Question 8

What is the molecular formula of benzene?

Answer 8

C₆H₆

Question 9

What is the molecular formula of cyclopentane?

Answer 9

C₅H₁₀

Question 10

What is the molecular formula of butane?

Answer 10

C₄H₁₀

Question 11

What is the condensed structural formula of butane?

Answer 11

CH₃CH₂CH₂CH₃
or CH₃(CH₂)₂CH₃

Question 12

What are aromatic hydrocarbons?

Answer 12

An aromatic hydrocarbon is a hydrocarbon with at least one benzene ring in it structure.

Question 13

What are aliphatic hydrocarbons?

Answer 13

An aliphatic hydrocarbon is a hydrocarbon with no benzene rings; the chains of carbon atoms may be straight or branched, or with rings that are not benzene rings.

Question 14

List three major classes of hydrocarbons found in petroleum.

Answer 14

Alkanes, cycloalkanes, aromatic hydrocarbons

Question 15

What are saturated hydrocarbons?

Answer 15

A saturated hydrocarbon has the maximum possible number of hydrogen atoms in their molecules since all carbon-carbon bonds in the molecule are single covalent bonds.

Question 16

List two classes of saturated hydrocarbons.

Answer 16

Alkanes and cycloalkanes.

Question 17

What is the general molecular formula of alkanes?

Answer 17

C_nH_2n+2

Question 18

What is the general molecular formula of cycloalkanes?

Answer 18

C_nH_2n

Question 19

What are petrochemicals?

Answer 19

Petrochemicals are chemicals that are derived from petroleum.

Question 20

Name 8 products made from petrochemicals.

Answer 20

Adhesives, cosmetics, pharmaceuticals, shampoo and shower gel, paints, synthetic rubber, textiles, synthetic polymers

Question 21

How and where is petroleum separated into fractions?

Answer 21

Petroleum is separated into different fractions by fractional distillation, carried out in an oil refinery.

Question 22

Why should petroleum be processed before it is used as a fuel?

Answer 22

When petroleum is burned directly, a lot of black smoke (soot) and harmful gases are produced. Therefore, it cannot directly be used as a fuel and it must be refined first.

Question 23

How is industrialised fractional distillation of petroleum carried out?

Answer 23

1. Petroleum is first heated to about 400°C in a furnace near the bottom of the fractionating tower.
2. The mixture of liquid and vapour rises, passing into the tower which is cooler at the top.
3. The vapour of hydrocarbons pass up the tower via a series of trays containing bubble caps. Rising vapour condenses into liquid when it reaches the tray at a temperature just below its boiling point.
4. The larger hydrocarbons have a higher melting point, so they condense lower in the tower; smaller hydrocarbons condense at higher, cooler levels.
5. The substances at each level is then collected separately as fractions of petroleum. Refinery gases leave the top of the fractionating column as gases, while bitumen is collected as a residue at the base of the tower.

Question 24

How is fractional distillation of synthetic crude oil carried out in a school laboratory?

Answer 24

1. Add mineral wool soaked with synthetic crude oil to a test tube. Place a thermometer to monitor the temperature. Connect it to a delivery tube leading to another test tube in an ice water bath.
2. Clamp the set-up and heat it with a Bunsen flame gently.
3. Collect the fractions in the test tube, and when the temperature increases to the desired range, swap the test tube to collect a new fraction.

Question 25

What are the trends in properties of petroleum fractions?

Answer 25

The higher the boiling point of the fraction: 1. the **more** the carbon atoms in one molecule of hydrocarbon in the fraction 2. the **darker** the colour 3. the **higher the viscosity** 4. the **lower the volatility** 5. the lower the flammability 6. the more **sooty** the flame when burnt

Question 26

What is viscosity?

Answer 26

Viscosity is how **sticky/thick** a liquid is. The higher the viscosity, the thicker the liquid.

Question 27

What is volatility?

Answer 27

Volatility is a measure of how easy a liquid **evaporates**. The higher the volatility, the easier for it to evaporate.

Question 28

As we go down the fractionating tower, what is the change in the length and boiling points of hydrocarbons condensed?

Answer 28

The **lower** the position, the **larger** the hydrocarbon molecules, the **higher** the boiling points.

Question 29

List the 8 major petroleum fractions in the order of increasing molecular size, and their boiling point ranges.

Answer 29

1. Liquefied Petroleum Gas (**LPG / refinery gases**): less than 25°C 2. Petrol (gasoline): 40-100°C 3. **Naphtha**: 90-150°C 4. Kerosene: 150-240°C 5. Diesel: 220-300°C 6. Fuel oil: 250-320ºC 7. Lubricating oil: 300-350ºC 8. **Bitumen**: >350°C

Question 30

List five petroleum fractions used as fuels.

Answer 30

1. LPG (refinery gases): fuels for **taxis** 2. Petrol: fuels for **private cars / vehicles** 3. Kerosene: fuels for **aeroplanes** 4. Diesel: fuels for **heavy vehicles** like lorries and buses 5. Fuel oil: fuels for **ships** and power plants

Question 31

Petroleum fractions having large molecular sizes are cheaper. Why do aeroplanes use kerosene instead of fractions with larger molecular sizes as fuels?

Answer 31

The fractions with larger sizes produce **sootier flame** when burnt; the soot released may **block and damage** the aeroplane engine, therefore aeroplanes use kerosene as fuel.

Question 32

What are the usages of naphtha?

Answer 32

It is used as raw materials for production of **Town Gas, plastics and other chemicals**.

Question 33

What are the usages of kerosene?

Answer 33

It is used as aeroplane fuels, **solvents**, and as **heating oil**.

Question 34

What are the usages of lubricating oil?

Answer 34

It is used as **lubricants** to reduce friction, reduce corrosion, and is used to make **waxes** for cars.

Question 35

What are the usages of bitumen?

Answer 35

It is a material for **covering road surfaces and roofs**. It is waterproof, and prevents **skidding** of vehicles in rainy days.

Question 36

How does complete combustion of coal happen?

Answer 36

Complete combustion of coal happens when coal burns completely in a plentiful supply of oxygen. Its carbon is oxidised to carbon dioxide.

Question 37

What are the products of complete combustion?

Answer 37

Carbon dioxide and water **only**.

Question 38

What are the products of incomplete combustion?

Answer 38

Carbon dioxide, water, **and/or carbon monoxide and carbon soot**.

Question 39

How does combustion change the energy level of the reactants?

Answer 39

Energy is released to the surroundings as heat in combustion; it is an **exothermic reaction**.

Question 40

What is the chemical equation for the complete combustion of hexane?

Answer 40

C₆H₁₄ + 9½O₂ -> 6CO₂ + 7H₂O

Question 41

Hexane undergoes incomplete combustion and produces carbon dioxide, carbon monoxide, and carbon soot in the molar ratio of 3:2:1. What is the chemical equation of the reaction?

Answer 41

C₆H₁₄ + 7½O₂ -> 3CO₂ + 2CO + C + 7H₂O

Question 42

Hexane undergoes incomplete combustion and produces carbon dioxide, carbon monoxide, and carbon soot in the molar ratio of 1:1:1. What is the chemical equation of the reaction?

Answer 42

C₆H₁₄ + 6½O₂ -> 2CO₂ + 2CO + 2C + 7H₂O

Question 43

Two samples of benzene are combusted. The first sample undergoes complete combustion. The second one undergoes incomplete combustion and produces carbon dioxide, carbon monoxide, and carbon soot in the molar ratio of 1:1:1. What are the chemical equations of reaction? What is the difference between the amount of oxygen required for the two reactions?

Answer 43

Sample 1: C₆H₆ + 7½O₂ -> 6CO₂ + 3H₂O Sample 2: C₆H₆ + 4½O₂ -> 2CO₂ + 2CO + 2C + 3H₂O Incomplete combustion requires **less oxygen** per molecule of benzene.

Question 44

Why do hydrocarbons with larger molecular size burn with more orange and sooty flame?

Answer 44

The larger the molecular size, the more oxygen molecules are required for complete combustion. Therefore, there is a **higher chance** of incomplete combustion. Carbon soot is released from incomplete combustion, and the flame colour is due to the burning of carbon.

Question 45

How does an increased concentration of carbon dioxide in the atmosphere cause climate change?

Answer 45

As the concentration of carbon dioxide in the atmosphere increases, the **greenhouse effect** is enhanced, causing **global warming** which leads to climate change.

Question 46

What are greenhouse gases?

Answer 46

Greenhouse gases are gases which **absorb infrared radiation** emitted from the Earth and **re-emit them in all directions**, preventing infrared radiation from being re-radiated into space, thus **trapping heat** from the Sun in the atmosphere.

Question 47

List two sources of methane hydrate.

Answer 47

Under Arctic permafrost and beneath the ocean floor.

Question 48

Why is methane hydrate hard to extract?

Answer 48

It is a **crystalline solid** consisting of **methane** molecules surrounded by **cages of interlocking water molecules**. If it is warmed or pressurised, it will **revert back into water and methane gas**, but the release of methane gas could have a damaging impact on the environment.

Question 49

List 5 major types of air pollutants produced by burning fossil fuels.

Answer 49

1. Suspended particulates (mainly carbon soot) 2. Carbon monoxide 3. Volatile compounds (unburnt hydrocarbons) 4. Sulphur dioxide 5. Nitrogen oxides

Question 50

How are suspended particulates produced in the process of burning fossil fuels?

Answer 50

Small carbon particles are produced during the **incomplete combustion** of fuels that contain carbon. Some diesel-powered vehicles with poor maintenance may release them as **black smoke**.

Question 51

What are the harmful effects of suspended particulates?

Answer 51

They can be breathed into the lungs to cause breathing problems and worsen asthma. They also reduce visibility.

Question 52

What are the solutions to suspended particulate pollution?

Answer 52

Install **electrostatic precipitators** in power plants, install **particulate reduction devices** in motor vehicles, or use a better fuel with lower chance of incomplete combustion.

Question 53

How is carbon monoxide produced in the process of burning fossil fuels?

Answer 53

Carbon monoxide is a product of **incomplete combustion** of fossil fuels.

Question 54

What are the harmful effects of carbon monoxide?

Answer 54

It is a poisonous gas.

Question 55

What are the solutions to carbon monoxide pollution?

Answer 55

Use a **catalytic convertor** or use a better fuel with lower chance of incomplete combustion.

Question 56

How are volatile organic compounds produced in the process of burning fossil fuels?

Answer 56

They are released as **unburnt fuels** from petrol engines.

Question 57

What are the harmful effects of volatile hydrocarbons in air?

Answer 57

Some hydrocarbons are carcinogenic.

Question 58

What are the solutions to volatile organic compound pollution?

Answer 58

Use a **catalytic convertor** or use a better fuel.

Question 59

How is sulphur dioxide produced in the process of burning fossil fuels?

Answer 59

They are released when fuels containing sulphur are burnt.

Question 60

What are the harmful effects of sulphur dioxide pollution?

Answer 60

It irritates the eyes and throat, causes breathing problems, and forms acid rain.

Question 61

What are the solutions to sulphur dioxide pollution?

Answer 61

Use **flue gas desulphurisation plants** or **scrubbers** in power plants, and use fuels with ultra low sulphur content.

Question 62

How are nitrogen oxides produced in the process of burning fossil fuels?

Answer 62

They are produced from the **reaction between nitrogen and oxygen** in the combustion engine in power plants and vehicles, which is very hot.

Question 63

What are the harmful effects of nitrogen oxides pollution?

Answer 63

They can cause **breathing problems** and worsen asthma. They cause **acid rain** and give **photochemical smog** with unburnt hydrocarbons.

Question 64

What are the solutions to nitrogen oxides pollution?

Answer 64

Install low nitrogen oxide burners in power plants, and install **catalytic convertors** in exhaust systems of motor vehicles.

Question 65

List 4 measures the Hong Kong Government has implemented to reduce air pollutant emissions from vehicles.

Answer 65

1. An **incentive programme** is set up to replace diesel taxis / light buses with **liquefied petroleum gas vehicles**. 2. A higher fuel and vehicle **emission standard** was adopted. 3. An **incentive programme** is set up to retrofit old diesel vehicles with **particulate reduction devices**. 4. The **first registration tax** for environmentally friendly vehicles are reduced.

Question 66

What is the working principle of flue gas desulphurisation plants and scrubbers?

Answer 66

They use **calcium oxide** or **calcium hydroxide** to absorb sulphur dioxide, producing insoluble **calcium sulphite**.

Question 67

What is the working principle of electrostatic precipitators?

Answer 67

They are installed in the chimneys, **inducing charges** on suspended particulates and making them **stick onto the negative plate**.

Question 68

Which pollutants can catalytic converters reduce?

Answer 68

Catalytic converters reduce the amounts of **carbon monoxide, unburnt hydrocarbons, and nitrogen monoxide** by converting them into less harmful substances.

Question 69

List 3 catalysts in the catalytic convertor.

Answer 69

Rhodium, palladium, platinum

Question 70

What is the internal structure of the catalytic convertor and the advantage?

Answer 70

The metals are spread thinly over a **ceramic honeycomb-shaped support** to provide a large surface area for reactions. This increases the **rate of conversion** while minimising the amount of expensive catalytic metals used.

Question 71

What is the chemical equation for the conversion of carbon monoxide in a catalytic convertor?

Answer 71

2CO + O₂ -> 2CO₂

Question 72

What is the chemical equation for the conversion of unburnt heptane in a catalytic convertor?

Answer 72

C₇H₁₆ + 11O₂ -> 7CO₂ + 8H₂O

Question 73

What is the chemical equation for the conversion of nitrogen monoxide and carbon monoxide in a catalytic convertor?

Answer 73

2NO + 2CO -> N₂ + 2CO₂

Question 74

List 4 benefits of using fossil fuels.

Answer 74

1. They are **readily available** at a relatively **cheap** price. 2. They are **efficient** and can generate a lot of energy. 3. They are **easy and safe to transport**. 4. Fossil-fuel related industries generate **job opportunities**.

Question 75

List 6 risks of using fossil fuels.

Answer 75

1. Burning fossil fuels releases carbon dioxide, which increases the greenhouse effect and contributes to **global warming**. 2. Burning fossil fuels produce sulphur dioxide and nitrogen oxides, which contribute to **acid rain**. 3. Exploration and mining of fossil fuels harm the environment. 4. Coal mining is very **dangerous**. 5. The extraction and transportation of fossil fuels require energy, increasing fossil fuel consumption. 6. Petroleum leaks cause **pollution** and harm local wildlife.