Energy From Fuels

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レッスンノート

Combustion Reactions

Combustion is a rapid, exothermic reaction involving a fuel, oxygen, and an ignition source, commonly called burning.
It produces heat and light, so it is exothermic.
Complete combustion occurs with sufficient oxygen, while incomplete combustion occurs with limited oxygen.
Metals can combust to form metal oxides; reactive metals like magnesium burn with a bright white flame: 2Mg(s) + O₂(g) → 2MgO(s).
Non-metals such as sulfur burn to form covalent oxides like sulfur dioxide: S(s) + O₂(g) → SO₂(g).
Organic fuels include hydrocarbons (especially alkanes) and alcohols; they release large amounts of energy when combusted.
Complete combustion of organic compounds produces carbon dioxide and water: fuel + oxygen → carbon dioxide + water.
For example, methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l); ethanol: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).

Incomplete Combustion

Incomplete combustion occurs when oxygen is limited; it still produces water but forms carbon monoxide (CO) or carbon (C) instead of carbon dioxide.
In a Bunsen burner, complete combustion gives a blue, non-luminous flame; incomplete combustion gives a yellow, luminous flame.
Carbon monoxide is colourless and odourless, making it hard to detect without a sensor.
CO is toxic because it binds irreversibly to haemoglobin, preventing oxygen transport; this can cause dizziness, loss of consciousness, and death.
With very limited oxygen, carbon (soot) is produced; a yellow, sooty flame indicates incomplete combustion.
Hydrocarbons with higher carbon content are more likely to produce soot; for example, naphthalene (94% C) produces more soot than nitrobenzene (59% C).
Incomplete combustion never produces hydrogen; hydrogen is always preferentially oxidised by any available oxygen.
Example: incomplete combustion of propane to carbon: C₃H₈(l) + 2O₂(g) → 3C(s) + 4H₂O(l).

Fossil Fuels: Advantages and Disadvantages

Fossil fuels include coal, oil, and natural gas; they are made of hydrocarbons formed from buried dead plants and animals.
They are non-renewable (finite) and cannot be replaced quickly once used.
Coal is cheap, widely available, long-lasting, and safe to store; by-products like ash can be used in road construction.
Oil is easy to store and transport, has high energy density, and can be separated by fractional distillation and cracking.
Natural gas is the cheapest fossil fuel, easy to transport in pipelines, and burns with a blue flame producing minimal harmful emissions.
Combustion of fossil fuels releases carbon dioxide (global warming), sulfur dioxide (acid rain), and particulates (global dimming).
Coal mining causes habitat destruction, noise pollution, and health risks; oil spills harm habitats; natural gas is expensive to extract and store under pressure.

Amount of Carbon Dioxide Produced When Fuels Burn

Specific energy is a measure of the energy stored in a substance; different fuels release different amounts of energy when combusted.
As the hydrocarbon chain length increases, carbon content increases, so more carbon dioxide and carbon monoxide/carbon are produced.
Longer chains have stronger London dispersion forces, making the hydrocarbon less volatile.
Longer chains release less energy per unit mass of fuel; methane releases the most energy per unit mass.
As chain length increases, incomplete combustion becomes more likely.
To calculate CO₂ produced, use the balanced equation and mole ratios; for example, 1.00 g butane produces 3.03 g CO₂.

Carbon Dioxide Levels & the Greenhouse Effect

Atmospheric carbon dioxide is rising due to human activities: combustion of fossil fuels (electricity, transport, construction), deforestation, and livestock farming.
The main driver is combustion of fossil fuels, which releases carbon stored for millions of years.
Greenhouse gases include carbon dioxide (CO₂), methane (CH₄), nitrous oxides (NOₓ), and water vapour (H₂O).
The greenhouse effect keeps Earth warm: shortwave radiation from the sun is absorbed and re-emitted as longwave infrared radiation, which is absorbed by greenhouse gases and re-radiated in all directions.
Human activities increase greenhouse gas levels, leading to the enhanced greenhouse effect and global warming.
Mauna Loa Observatory has recorded CO₂ data since 1958 and methane data since 1984; the graph shows annual fluctuations due to seasonal photosynthesis.
Photosynthesis removes CO₂ from the atmosphere, causing seasonal changes in CO₂ levels.

The greenhouse effect

Biofuels

Renewable resources will not run out in the foreseeable future and are sustainable; non-renewable resources are finite and will run out.
The three main biofuels are bioethanol, biodiesel, and biogas; they are made from organic compounds produced by biological carbon fixation.
Bioethanol is ethanol made from a biological source; plants absorb CO₂ via photosynthesis: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g), then glucose is fermented to ethanol.
Bioethanol can be carbon neutral if the CO₂ absorbed during photosynthesis equals the CO₂ released when burned.
Biodiesel is made from renewable vegetable oils via transesterification with methanol, catalysed by acid or alkali (e.g. NaOH or KOH); excess methanol shifts equilibrium to products, yield up to 98%.
Biogas is made when organic matter is broken down by microorganisms in anaerobic conditions; it is mainly methane and carbon dioxide.
Advantages of biofuels: carbon neutral, renewable, reduce greenhouse gases and landfill waste, generate income and jobs.
Disadvantages of biofuels: high costs, land use conflict with food crops, deforestation, soil nutrient depletion, and lower specific energy than fossil fuels.

Fuel Cells

A fuel cell is an electrochemical cell where a fuel donates electrons at one electrode and oxygen gains electrons at the other, generating a voltage.
In a hydrogen-oxygen fuel cell, the electrolyte is aqueous sodium hydroxide; half-equations: 2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻ and O₂(g) + 2H₂O + 4e⁻ → 4OH⁻(aq); overall: 2H₂(g) + O₂(g) → 2H₂O(l) Eθ = +1.23 V.
The only product is water, so hydrogen fuel cells are environmentally friendly with no harmful emissions; they operate at room temperature and avoid combustion.
Hydrogen fuel cells produce no nitrogen oxides (NOₓ) unlike high-temperature combustion; they have been used on spacecraft where product water can be drunk by astronauts.
Disadvantages of hydrogen fuel cells: hydrogen is highly flammable, requires heavy storage tanks, and is mostly sourced from fossil fuels; low energy density by volume means larger containers.
In a methanol fuel cell, methanol is the source of hydrogen ions; half-equations: CH₃OH(aq) + H₂O(l) → CO₂(g) + 6H⁺(aq) + 6e⁻ and O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l); overall: CH₃OH(aq) + 1.5O₂(g) → CO₂(g) + 2H₂O(l).
Advantages of methanol over hydrogen fuel cells: easier to store and transport, operates at low pressure and temperature, higher energy density, can be made from renewable resources.
Disadvantages of methanol fuel cells: toxic and flammable, commonly made from fossil fuels, lower voltage and power output, low efficiency due to methanol crossover, requires expensive catalysts, and produces CO₂.

A hydrogen–oxygen fuel cell

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練習問題

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1.Which of the following is NOT required for combustion to occur?
Easy
- AA fuel
- BOxygen
- CAn ignition source
- DA catalyst
2.What are the products of the complete combustion of a hydrocarbon?
Easy
- ACarbon monoxide and water
- BCarbon dioxide and water
- CCarbon and water
- DCarbon dioxide and hydrogen
3.Which flame colour indicates complete combustion in a Bunsen burner?
Easy
- AYellow, luminous
- BBlue, non-luminous
- COrange, sooty
- DGreen, non-luminous
4.Which of the following are greenhouse gases? (select all that apply)
Medium
- ACarbon dioxide (CO2)
- BMethane (CH4)
- CNitrogen (N2)
- DWater vapour (H2O)
- ENitrous oxides (NOx)
5.The incomplete combustion of organic compounds can produce hydrogen gas.
Easy
True or false?
6.Which of the following is the main product of the incomplete combustion of a hydrocarbon when oxygen is very limited?
Medium
- ACarbon dioxide
- BCarbon monoxide
- CCarbon (soot)
- DHydrogen
7.Why is carbon monoxide dangerous to inhale?
Medium
- AIt is a greenhouse gas
- BIt binds irreversibly to haemoglobin, preventing oxygen transport
- CIt is highly flammable
- DIt reacts with water to form an acid
8.Which of the following is a biofuel?
Easy
- APetrol
- BDiesel
- CBioethanol
- DNatural gas

もっと問題

9.Which of the following is an advantage of hydrogen fuel cells over internal combustion engines?
Medium
- AThey produce only water as a product
- BThey are cheaper to manufacture
- CThey require heavy storage tanks
- DThey produce nitrogen oxides
10.What is the function of the platinum in a methanol fuel cell?
Medium
- AIt acts as a catalyst
- BIt acts as a fuel
- CIt acts as an electrolyte
- DIt acts as a membrane
11.Which of the following is a disadvantage of biofuels compared to fossil fuels?
Medium
- AThey are renewable
- BThey have lower specific energy
- CThey reduce greenhouse gas emissions
- DThey are carbon neutral
12.As the length of the hydrocarbon chain in a fuel increases, what happens to the amount of carbon dioxide produced per gram of fuel burnt?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
13.Which of the following are needed for combustion to take place? (select all that apply)
Medium
- AA fuel
- BOxygen
- CAn ignition source
- DA catalyst
- EWater
14.Which statements about the combustion of metals are correct? (select all that apply)
Medium
- AAll metals can oxidise, but not all metals can combust
- BSome metals only combust if they are finely divided, e.g. as a powder or filings
- CMagnesium burns in air with a bright white flame
- DCopper burns in air with a bright white flame
- EMetal oxides formed by combustion are acidic when dissolved in water
15.Which word equation represents the general combustion of a non-metal?
Easy
- Anon-metal + oxygen → non-metal oxide
- Bnon-metal + water → non-metal hydroxide
- Cnon-metal + oxygen → metal oxide
- Dnon-metal + hydrogen → non-metal hydride
16.Which of the following are products of the incomplete combustion of a hydrocarbon? (select all that apply)
Medium
- ACarbon monoxide
- BCarbon (soot)
- CWater
- DHydrogen gas
- ECarbon dioxide only
17.In a Bunsen burner, a blue non-luminous flame indicates complete combustion.
Easy
True or false?
18.Which observation is the clearest sign that a hydrocarbon fuel is undergoing incomplete combustion?
Medium
- AA yellow, sooty flame
- BA blue, non-luminous flame
- CThe formation of water droplets only
- DA colourless, odourless gas that supports combustion
19.Match each type of combustion of a hydrocarbon with its correct products.
Medium
- Complete combustion with excess oxygen
- Incomplete combustion with limited oxygen
- Incomplete combustion with very limited oxygen
- Carbon dioxide and water
- Carbon monoxide and water
- Carbon (soot) and water
20.Place the steps in the correct order to show how the greenhouse effect warms the Earth.
Medium
- The sun emits shortwave radiation that reaches the Earth's surface
- The Earth absorbs the energy and re-emits it as longwave infrared radiation
- Greenhouse gases in the atmosphere absorb some of the infrared radiation
- The greenhouse gases re-radiate the energy in all directions, trapping heat
21.The graph of atmospheric carbon dioxide concentration at Mauna Loa shows a general upward trend with regular small dips and rises each year. What causes the regular annual fluctuations?
Medium
- ASeasonal changes in the rate of photosynthesis
- BSeasonal changes in volcanic activity
- CThe regular eruption of geysers
- DAnnual variation in the Earth's distance from the sun
22.A student burns a sample of cyclohexane (C6H12, Mr = 84) and a sample of nitrobenzene (C6H5NO2, Mr = 123) in a Bunsen burner using a limited air supply. Which statement best predicts what they observe?
Medium
- ACyclohexane produces a more sooty flame because it has a higher percentage of carbon by mass.
- BNitrobenzene produces a more sooty flame because it has a higher percentage of carbon by mass.
- CBoth produce identical flames because both contain six carbon atoms per molecule.
- DNeither produces soot because soot only forms from fuels containing sulfur.
23.A student burns 5.0 cm³ of biodiesel (density = 0.780 g cm⁻³) in an open flame calorimeter containing 1 dm³ of water. The water temperature rises from 20.0 °C to 55.0 °C. Using the specific heat capacity of water as 4.18 J g⁻¹ K⁻¹, what is the heat absorbed by the water, in kJ?
Medium
- A146 kJ
- B0.146 kJ
- C35.0 kJ
- D2.93 kJ
24.Using the data from the biodiesel calorimetry experiment (5.0 cm³ biodiesel, density 0.780 g cm⁻³, heat absorbed by water = 146 kJ), what is the enthalpy change of combustion, in kJ g⁻¹, for this biodiesel sample?
Hard
- A37.4 kJ g⁻¹
- B0.0267 kJ g⁻¹
- C146 kJ g⁻¹
- D5.85 kJ g⁻¹
25.Which of the following statements about the combustion of metals are correct? (select all that apply)
Medium
- AAll metals can oxidise, but not all metals can combust.
- BCopper burns with a bright flame to form copper oxide.
- CMagnesium burns with a bright white flame to form magnesium oxide.
- DSodium does not react with oxygen.
- EMetal oxides formed from combustion of s-block metals are usually acidic.
26.When a hydrocarbon undergoes incomplete combustion, hydrogen gas is always produced alongside carbon monoxide or carbon.
Easy
True or false?
27.Match each fuel or fuel-related term with its correct description.
Medium
- Bioethanol
- Biodiesel
- Biogas
- Fossil fuel
- Ethanol produced by fermentation of glucose from plants
- Methyl esters made by transesterification of vegetable oils
- Mostly methane produced by anaerobic breakdown of organic waste
- Non-renewable hydrocarbon formed from buried remains of dead organisms
28.A fuel cell uses methanol as the fuel and operates under acidic conditions. Which pair of half-equations correctly represents the reactions at the negative and positive electrodes?
Hard
- ANegative: CH3OH + H2O → CO2 + 6H⁺ + 6e⁻; Positive: O2 + 4H⁺ + 4e⁻ → 2H2O
- BNegative: O2 + 4H⁺ + 4e⁻ → 2H2O; Positive: CH3OH + H2O → CO2 + 6H⁺ + 6e⁻
- CNegative: 2H2 + 4OH⁻ → 4H2O + 4e⁻; Positive: O2 + 2H2O + 4e⁻ → 4OH⁻
- DNegative: CH3OH → CO2 + 4H⁺ + 4e⁻; Positive: O2 + 2H⁺ + 2e⁻ → H2O
29.Which of the following is the correct unit for specific energy?
Medium
- AJ mol⁻¹
- BkJ g⁻¹
- CJ K⁻¹
- DkJ mol⁻¹
30.The incomplete combustion of organic compounds never produces hydrogen gas.
Easy
True or false?
31.What type of radiation do all greenhouse gases absorb?
Easy
- AUltraviolet radiation
- BVisible light
- CInfrared radiation
- DGamma radiation
32.As the length of the hydrocarbon chain increases, what happens to the amount of carbon dioxide produced per gram of fuel burnt?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt first increases then decreases
33.A student sets up an open-flame calorimeter: a known mass of fuel burns beneath a copper can containing 200 g of water, and the temperature rise is measured. Which change would most improve the accuracy of the calculated enthalpy of combustion?
Medium
- AUsing a draught shield and a lid to reduce heat loss to the surroundings
- BUsing a larger mass of fuel so that the flame burns for longer
- CReading the thermometer to the nearest 0.5 °C instead of 0.1 °C
- DStirring the water only after the flame has gone out
34.In a school experiment, an alcohol burner is weighed before and after burning to heat a beaker of water. The mass of fuel burnt is found to be 0.85 g and the temperature rise of 150 g of water is 25.0 °C. Using q = mcΔT with c = 4.18 J g⁻¹ °C⁻¹, what is the heat absorbed by the water?
Medium
- A15.7 kJ
- B3.14 kJ
- C157 kJ
- D0.0157 kJ
35.A student investigates the combustion of ethanol using a simple calorimeter. Which of the following would cause the experimental value of the enthalpy of combustion to be lower than the true value? (select all that apply)
Medium
- AHeat escaping from the flame to the surroundings
- BIncomplete combustion of the ethanol producing soot
- CEvaporation of the ethanol before it burns
- DUsing a thermometer with a 1 °C graduation
- EThe water being stirred continuously during heating
36.When comparing fuels in a simple calorimetry experiment, the same mass of water and the same starting temperature should be used to make the results fair.
Easy
True or false?
37.In a hydrogen–oxygen fuel cell, the electrolyte is aqueous sodium hydroxide. Which half-equation represents the reaction at the negative electrode (anode)?
Medium
- A2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻
- BO₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
- C2H₂(g) + O₂(g) → 2H₂O(l)
- DO₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)
38.A proton exchange membrane (PEM) is used in a methanol fuel cell. Which statements about the membrane are correct? (select all that apply)
Hard
- AIt allows H⁺ ions to pass through while blocking electrons.
- BIt prevents methanol from mixing directly with oxygen.
- CIt conducts electrons from one electrode to the other.
- DIt must be kept moist to function.
- EIt is consumed as the cell operates.
39.Two hydrocarbons are burned in a limited supply of air. One produces a much sootier flame than the other. Which conclusion is best supported by this observation?
Medium
- AThe sootier fuel has a higher percentage of carbon by mass.
- BThe sootier fuel has a lower percentage of carbon by mass.
- CThe sootier fuel has a higher oxygen content.
- DThe sootier fuel has a lower boiling point.
40.A student compares the combustion of two fuels, A and B, using the same apparatus. Fuel A gives a temperature rise of 30 °C; fuel B gives a rise of 20 °C. Both fuels have the same mass. Which statements are valid conclusions? (select all that apply)
Hard
- AFuel A released more heat per gram than fuel B.
- BFuel A definitely has a higher carbon content than fuel B.
- CThe experiment should be repeated to check reliability.
- DFuel B must have undergone incomplete combustion.
- EThe difference could be due to heat losses in the apparatus.
41.Name the type of radiation emitted by the Earth that is absorbed by greenhouse gases.
Easy
42.Name the process by which glucose is converted into ethanol in the production of bioethanol.
Easy
43.Name the process used to convert vegetable oils into biodiesel.
Medium
44.Calculate the mass of carbon dioxide produced when 1.00 g of butane (C4H10) undergoes complete combustion. (Mr: C=12.01, H=1.01, O=16.00)
Medium
45.Complete the word equation for the complete combustion of a fuel.
Easy
fuel + oxygen → ____ + ____
46.Complete the sentence about incomplete combustion.
Medium
Incomplete combustion occurs when oxygen is ____ and produces carbon monoxide or carbon, and ____.
47.Methanol has a density of 0.792 g cm⁻³ at room temperature and pressure. Use volume = mass ÷ density to calculate the volume, in cm³, of one mole of methanol (Mᵣ = 32.0).
Medium
48.Decane (C10H22) undergoes complete combustion. Calculate the mass of carbon dioxide produced when 1.00 g of decane burns completely. (Mr: C = 12.01, H = 1.01, O = 16.00)
Medium
49.Complete the sentence about the combustion of non-metals.
Medium
Non-metals such as sulfur burn in oxygen to form ____ oxides, which dissolve in water to give solutions with a pH ____ 7.
50.Name the process by which vegetable oils are converted into biodiesel using methanol.
Medium
51.Calculate the mass of carbon dioxide produced when 1.00 g of butane (C4H10) undergoes complete combustion. (Mr: C = 12.01, H = 1.01, O = 16.00). Give your answer in grams to 3 significant figures.
Medium
52.At room temperature and pressure, methanol has a density of 0.792 g cm⁻³. Use volume = mass ÷ density to calculate the volume, in cm³, of one mole of methanol (Mr = 32.0). Give your answer to 3 significant figures.
Easy
53.State the difference between complete and incomplete combustion of a hydrocarbon fuel, such as octane, including the products formed.
Hard
54.Explain why carbon monoxide is dangerous if inhaled.
Medium
55.Explain why fossil fuels contribute more to climate change than biofuels.
Hard
56.Explain why methanol is easier to store and transport than hydrogen for use in a fuel cell.
Hard
57.A student observes that a sample of tetradecane (C14H30) burns with a sooty yellow flame, while a sample of methane burns with a blue flame. Explain these observations in terms of the carbon content of the fuels and the type of combustion occurring.
Hard
58.Calculate the mass of carbon dioxide produced when 1.00 g of butane (C4H10) undergoes complete combustion. (Mr: C=12.01, H=1.01, O=16.00)
Medium
- A4.03 g
- B5.03 g
- C2.03 g
- D3.03 g
59.Methanol has a density of 0.792 g cm⁻³ at room temperature and pressure. Use volume = mass ÷ density to calculate the volume, in cm³, of one mole of methanol (Mᵣ = 32.0).
Medium
- A44.4 cm³
- B36.4 cm³
- C40.4 cm³
- D48.4 cm³
60.Decane (C10H22) undergoes complete combustion. Calculate the mass of carbon dioxide produced when 1.00 g of decane burns completely. (Mr: C = 12.01, H = 1.01, O = 16.00)
Medium
- A4.09 g
- B5.09 g
- C3.09 g
- D2.09 g
61.Calculate the mass of carbon dioxide produced when 1.00 g of butane (C4H10) undergoes complete combustion. (Mr: C = 12.01, H = 1.01, O = 16.00). Give your answer in grams to 3 significant figures.
Medium
- A5.03 g
- B2.03 g
- C4.03 g
- D3.03 g
62.At room temperature and pressure, methanol has a density of 0.792 g cm⁻³. Use volume = mass ÷ density to calculate the volume, in cm³, of one mole of methanol (Mr = 32.0). Give your answer to 3 significant figures.
Easy
- A48.4 cm³
- B44.4 cm³
- C40.4 cm³
- D36.4 cm³

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