SL & HL Reactivity · What drives chemical reactions? BETA R1.3

Energy from fuels

IB syllabus topic Reactivity 1.3 — Energy from fuels

Energy from fuels covers combustion reactions of hydrocarbons and other fuels, the difference between complete and incomplete combustion, why burning fossil fuels releases carbon dioxide and drives climate change, biofuels as a renewable alternative, and how fuel cells convert chemical energy directly into electrical energy. This is a largely qualitative topic linked to enthalpy of combustion and calorimetry.

Part of IB Chemistry (2025-2030 syllabus) — Standard and Higher Level.

Key points

Combustion is an exothermic redox reaction of a fuel with oxygen; complete combustion of a hydrocarbon gives only carbon dioxide and water.
Incomplete combustion occurs when oxygen is limited and produces carbon monoxide (CO) and/or carbon (soot, C) as well as water, releasing less energy per mole of fuel.
Carbon monoxide is a toxic, colourless, odourless gas that binds to haemoglobin; soot causes respiratory problems and smog.
Fossil fuels (coal, oil, natural gas) are finite, non-renewable stores of carbon laid down over millions of years; burning them transfers this stored carbon to the atmosphere as $\text{CO}_2$ , a greenhouse gas that enhances global warming.
Biofuels (e.g. bioethanol, biodiesel) are produced from recently living biomass and are considered renewable and broadly carbon-neutral because the $\text{CO}_2$ released was recently absorbed by photosynthesis.
A fuel cell converts the chemical energy of a fuel (e.g. hydrogen) directly into electrical energy by continuous supply of fuel and oxidant; it does not store charge like a battery.
The energy released by a fuel can be measured by calorimetry using $Q = mc\Delta T$ (given in the data booklet) to find the enthalpy of combustion.

Subtopic by subtopic

Combustion reactions

Combustion is an exothermic redox reaction of a fuel with oxygen. The fuel is oxidised (carbon and hydrogen gain oxygen) and oxygen is reduced.

For a hydrocarbon $\text{C}_x\text{H}_y$ in a plentiful supply of oxygen, complete combustion gives only carbon dioxide and water:

$\text{C}_x\text{H}_y + \left(x + \tfrac{y}{4}\right)\text{O}_2 \rightarrow x\,\text{CO}_2 + \tfrac{y}{2}\,\text{H}_2\text{O}$

For example, methane:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

Key points:

Combustion releases a large amount of energy, which is why hydrocarbons are used as fuels.
Complete combustion releases the maximum energy and burns with a clean, often blue, flame.
The standard enthalpy of combustion $\Delta H_c^{\ominus}$ is the energy released when one mole of a substance is completely burned in oxygen under standard conditions.

For alcohols, remember the fuel already contains oxygen, so balance carefully, e.g. ethanol:

$\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

The energy output of a fuel can be measured experimentally by calorimetry (see worked examples).

Incomplete combustion

When the oxygen supply is limited, a hydrocarbon undergoes incomplete combustion. Instead of all the carbon ending up as $\text{CO}_2$ , some forms carbon monoxide (CO) and/or carbon (soot, C).

Example equations for methane:

To carbon monoxide: $\;2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$
To soot: $\;\text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O}$

Consequences:

Less energy is released per mole of fuel, because the carbon is not fully oxidised.
Carbon monoxide is a toxic, colourless, odourless gas. It binds strongly to haemoglobin, preventing the blood from carrying oxygen, and can be fatal.
Soot (particulate carbon) blackens surfaces, causes a smoky yellow/orange flame, and contributes to respiratory problems and smog.

You can recognise incomplete combustion by a yellow, smoky flame rather than a clean blue one. It is a common issue with faulty gas heaters and poorly tuned engines, which is why carbon monoxide detectors are used in homes.

Fossil fuels and carbon dioxide

Fossil fuels - coal, crude oil and natural gas - are formed from the remains of ancient organisms over millions of years. They are non-renewable because they are used far faster than they form, and supplies are finite.

They are stores of carbon that was removed from the atmosphere long ago. Burning them transfers this carbon to the atmosphere as carbon dioxide:

$\text{fuel} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$

$\text{CO}_2$ is a greenhouse gas: it absorbs and re-emits infrared radiation given off by the Earth, trapping heat in the atmosphere (the enhanced greenhouse effect). Rising $\text{CO}_2$ concentrations are linked to global warming and climate change.

Fuels differ in how much $\text{CO}_2$ they emit per unit of energy:

Coal has the highest carbon content, so it releases the most $\text{CO}_2$ per unit of energy.
Natural gas (mainly methane) has the highest hydrogen-to-carbon ratio, so it releases the least $\text{CO}_2$ per unit of energy and is the cleanest-burning fossil fuel.

This is one of the main motivations for switching to renewable fuels and to fuel cells.

Biofuels

Biofuels are made from recently living biomass rather than from ancient fossil deposits. Two common examples:

Bioethanol - made by fermentation of sugars from crops such as sugar cane or maize, then used (often blended with petrol):

$\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

Biodiesel - made from vegetable oils or animal fats.

Biofuels are considered renewable because the crops can be regrown, and broadly carbon neutral because the $\text{CO}_2$ released on burning was recently absorbed from the atmosphere by photosynthesis during the plants' growth.

Advantages:

Renewable and approximately carbon neutral.
Can use existing engines and distribution infrastructure.

Disadvantages:

Growing fuel crops competes with food production and uses farmland and water.
Processing, fertilisers and transport mean they are not perfectly carbon neutral.
Land clearing for crops can cause deforestation.

So while biofuels lower net $\text{CO}_2$ emissions compared with fossil fuels, they carry real ethical and environmental trade-offs.

Fuel cells

A fuel cell converts the chemical energy of a fuel directly into electrical energy through redox reactions, as long as fuel and oxidant are continuously supplied. Unlike a battery, it does not run down because it is not storing the reactants inside.

The most common example is the hydrogen-oxygen fuel cell. Hydrogen is oxidised at the negative electrode (anode) and oxygen is reduced at the positive electrode (cathode). The overall reaction is simply:

$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$

The only product is water, so at the point of use there are no carbon emissions.

Advantages:

High efficiency - chemical energy goes straight to electrical energy without a heat-engine step.
Clean at point of use (water is the only product for a hydrogen cell).
Quiet, with no moving combustion parts.

Limitations:

Hydrogen is expensive to produce and store safely.
The hydrogen may itself be made from fossil fuels, shifting rather than removing the $\text{CO}_2$ .
Electrodes often need costly catalysts.

Fuel cells differ from combustion: there is no flame, and the energy is delivered as electricity rather than heat.

Formulae

Q = mc\Delta T

Given in the data booklet. Used in calorimetry to find the heat transferred when a fuel burns: $m$ is the mass of water (or solution) heated, $c$ the specific heat capacity, and $\Delta T$ the temperature rise. Combine with moles of fuel to get the enthalpy of combustion.

\Delta H_c = -\dfrac{Q}{n_{\text{fuel}}}

Constructed by the student (NOT in the data booklet). Converts the measured heat $Q$ (in J or kJ) to the molar enthalpy of combustion by dividing by the moles of fuel burned; the negative sign shows combustion is exothermic.

\text{C}_x\text{H}_y + \left(x + \tfrac{y}{4}\right)\text{O}_2 \rightarrow x\,\text{CO}_2 + \tfrac{y}{2}\,\text{H}_2\text{O}

General balanced equation for complete combustion of a hydrocarbon $\text{C}_x\text{H}_y$ . Constructed by the student; use it to balance any alkane, alkene or alcohol combustion (add oxygen already in the fuel for alcohols).

Definitions

Combustion: An exothermic redox reaction in which a fuel reacts rapidly with oxygen, releasing energy (usually as heat and light).
Complete combustion: Combustion in a plentiful supply of oxygen; for a hydrocarbon the only products are carbon dioxide and water, and the maximum energy is released.
Incomplete combustion: Combustion in a limited supply of oxygen, giving carbon monoxide and/or carbon (soot) in addition to water, with less energy released per mole of fuel.
Fossil fuel: A non-renewable carbon-based fuel (coal, crude oil or natural gas) formed from the remains of ancient organisms over millions of years.
Biofuel: A renewable fuel obtained from recently living biomass, such as bioethanol from fermented crops or biodiesel from vegetable oils.
Carbon neutral: Describes a process whose net release of carbon dioxide to the atmosphere is zero, because any CO2 emitted was recently removed from the atmosphere (e.g. by the plants the fuel was made from).
Fuel cell: An electrochemical cell that converts the chemical energy of a continuously supplied fuel and oxidant directly into electrical energy.

Worked examples

(SL) Write balanced equations for (a) the complete combustion of propane $\text{C}_3\text{H}_8$ and (b) its incomplete combustion to carbon monoxide and water.

1
For complete combustion, products are only CO2 and water. Carbon: 3 C give 3 CO2. Hydrogen: 8 H give 4 H2O.
2
Count oxygen needed on the right: 3 CO2 has 6 O, 4 H2O has 4 O, total 10 O = 5 O2.
3
Complete: C3H8 + 5 O2 -> 3 CO2 + 4 H2O.
4
For incomplete combustion to CO, all 3 carbons form CO (3 CO) and 8 H form 4 H2O.
5
Count oxygen on right: 3 CO has 3 O, 4 H2O has 4 O, total 7 O. That needs 7/2 O2.
6
Multiply through by 2 to clear the fraction: 2 C3H8 + 7 O2 -> 6 CO + 8 H2O.

Answer: (a) $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$ . (b) $2\text{C}_3\text{H}_8 + 7\text{O}_2 \rightarrow 6\text{CO} + 8\text{H}_2\text{O}$ . Incomplete combustion uses less oxygen and releases less energy per mole of propane.

(SL) In a simple calorimetry experiment, burning 0.46 g of ethanol ( $\text{C}_2\text{H}_5\text{OH}$ , $M = 46.0\ \text{g mol}^{-1}$ ) raised the temperature of 100.0 g of water by 25.0 K. Calculate the experimental enthalpy of combustion of ethanol. ( $c_w = 4.18\ \text{J g}^{-1}\text{K}^{-1}$ )

1
Heat absorbed by the water: Q = mcΔT = 100.0 × 4.18 × 25.0.
2
Q = 10450 J = 10.45 kJ.
3
Moles of ethanol burned: n = 0.46 / 46.0 = 0.0100 mol.
4
Enthalpy of combustion per mole: ΔH_c = -Q / n = -10.45 kJ / 0.0100 mol.
5
ΔH_c = -1045 kJ mol^-1, which rounds to about -1.05 × 10³ kJ mol^-1.

Answer: $\Delta H_c = -\dfrac{10.45\ \text{kJ}}{0.0100\ \text{mol}} \approx -1.05 \times 10^{3}\ \text{kJ mol}^{-1}$ . The value is negative because combustion is exothermic; the experimental figure is smaller in magnitude than the data-book value because of heat lost to the surroundings and incomplete combustion.

(SL) Explain why natural gas (methane) is described as a cleaner fossil fuel than coal in terms of carbon dioxide emitted per unit of energy released.

1
Write the combustion of methane: CH4 + 2 O2 -> CO2 + 2 H2O. One CO2 is made and a lot of energy comes from forming strong C=O and O-H bonds, including energy from burning the 4 hydrogens.
2
Coal is essentially carbon: C + O2 -> CO2. Energy comes only from oxidising carbon, with no hydrogen to contribute.
3
Methane has a high hydrogen-to-carbon ratio, so much of its energy comes from burning hydrogen, which produces only water, not CO2.
4
Coal has a low hydrogen content, so almost all its energy is tied to producing CO2.
5
Therefore methane releases less CO2 for the same amount of energy.

Answer: Methane releases less $\text{CO}_2$ per unit of energy because its high hydrogen-to-carbon ratio means a large share of its energy comes from burning hydrogen (giving water, not $\text{CO}_2$ ), whereas coal is almost pure carbon and produces $\text{CO}_2$ for nearly all of its energy. So natural gas is the cleaner-burning fossil fuel.

Common mistakes

×Forgetting that combustion of an alcohol uses less external oxygen because the fuel already contains oxygen - balance the equation carefully rather than treating it like a hydrocarbon.
×Confusing carbon monoxide (CO, from incomplete combustion) with carbon dioxide (CO2, from complete combustion), or thinking incomplete combustion releases more energy - it actually releases less.
×Calling biofuels completely carbon neutral. They are only approximately carbon neutral; fertilisers, processing and transport still emit CO2, and growing them competes with food production.
×Describing a fuel cell as a type of battery or as combustion. A fuel cell needs a continuous external supply of fuel and oxidant and produces electricity directly via redox, with no flame.

Exam tips

✓When balancing combustion equations, deal with C first, then H, then balance O last; doubling through to clear a $\tfrac{1}{2}\text{O}_2$ is fine and often expected.
✓In calorimetry questions, use $Q = mc\Delta T$ with the mass of WATER (not the fuel) and remember $c_w = 4.18\ \text{J g}^{-1}\text{K}^{-1}$ ; convert $Q$ to kJ before dividing by moles of fuel, and give $\Delta H_c$ a negative sign.
✓If asked to evaluate biofuels or fuel cells, give a balanced answer with at least one advantage AND one disadvantage - examiners reward the trade-offs, not just benefits.
✓Be ready to explain that experimental enthalpies of combustion are less exothermic than data-book values because of heat loss to surroundings, evaporation, and incomplete combustion.

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