About the evidence behind this topic
Most of this topic is new. Under the previous syllabus, fossil fuels, biofuels and fuel cells were not core chemistry at all: they sat in an optional unit that only some students took. When the syllabus changed, that material was brought into the core as Reactivity 1.3.
The practical consequence is that there is very little examination history for it. Earlier examinations assess this content only inside the discontinued option, and option questions are not evidence about your examination — they were a different paper, taken by different students, against a different syllabus. Those questions have been excluded here.
What that leaves is honest and small. Combustion equations and the greenhouse-gas material are well established across sessions, because they existed in the core before. Biofuels have appeared in both current-syllabus sessions. Fuel cells have appeared once. Where this document says an outcome is thinly evidenced, that is a statement about the record, not a hint that you can skip it — a topic with one precedent is one you prepare from the syllabus wording, not from pattern-matching past questions.
1.3.1 · Combustion equations SL core
Reactive metals, non-metals and organic compounds all burn in oxygen. For hydrocarbons and alcohols the products are fixed, so the whole task is balancing.
An alcohol brings its own oxygen. Ethanol, C2H5OH, already carries one oxygen atom, so it needs half a mole less O2 than an alkane of the same carbon and hydrogen count would. A published paper prints the equation as C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g).
Carbon, then hydrogen, then oxygen. Write the CO2 coefficient from the carbon count and the H2O coefficient from half the hydrogen count — both are forced. Only then count the oxygen atoms you need, subtract any the fuel already has, and halve it. A half-coefficient for O2 is normal and correct; double everything only if the question asks for whole numbers.
A multiple-choice question, with Mr(propane) = 44.1 g mol–1 supplied. The printed options were 10.4, 5.20, 2.60 and 1.30.
| Balance first | C3H8 + 5O2 → 3CO2 + 4H2O — 6 oxygen atoms in the CO2 and 4 in the water make 10 atoms, so 5 molecules. |
| Amount of propane | 11.5 / 44.1 = 0.2608 mol |
| Amount of oxygen | 5 × 0.2608 = 1.30 mol |
Look at the four options. They are 1.30 doubled three times: 1.30, 2.60, 5.20, 10.4. Stopping at 10 oxygen atoms instead of 5 oxygen molecules lands you exactly on 2.60. The last division by two is the whole question.
Published examiner feedback on a one-mark combustion equation: 60% of the candidates wrote a balanced equation for the combustion of ethanol. Some candidates did not have the correct products and some candidates struggled with balancing the equation. Two different faults, and the order above prevents both — products are forced before any balancing begins.
One report lists Identifying the hydrocarbon that produces equal amounts of CO2 and H2O upon complete combustion among the things candidates handled well. Which class of hydrocarbon does that, and why can an alkane never do it? Answer at the end.
1.3.2 · Incomplete combustion SL core
Take the oxygen away and the carbon stops half-way. The hydrogen does not.
A multiple-choice question asked which products may form when propane undergoes incomplete combustion. The options were CO2 and H2 only; CO, C and H2; CO2, H2O and H2; and CO2, CO and C. The published key is the last of these.
In contrast, only 22% of students could identify the products of incomplete combustion of a hydrocarbon, with the remaining ~80% believing that hydrogen gas is formed!
Three of the four options contain hydrogen gas, and the key is the only one that does not. Another report lists Thinking that hydrogen is a product of incomplete combustion of hydrocarbons outright as a misconception. The hydrogen in a fuel is oxidised whatever the oxygen supply; it is only the carbon that is left half-burnt.
Carbon dioxide, carbon monoxide, carbon. As the oxygen runs short you move along that sequence, and a real flame produces a mixture from along it — which is why the key names three carbon products together rather than one. What you never move along is the hydrogen: it is water at every point.
The syllabus asks what might be observed when a fuel burns in a limited supply of oxygen. A yellow, smoky flame instead of a clean blue one, and a black deposit of soot. One examination question used exactly that: candidates had to recognise The crucible had black soot on the bottom after heating as evidence of a systematic error, and feedback records that only 30% chose the accumulation of soot. Carbon monoxide, by contrast, is colourless and odourless — you would see nothing at all, which is the health risk.
1.3.3 · Fossil fuels and carbon dioxide SL core
Coal, crude oil and natural gas. The syllabus asks you to evaluate how much carbon dioxide each adds, to know the link with the greenhouse effect, and to cover the tendency to burn incompletely and the energy released per unit mass.
| Property | What it means, and how the fuels compare |
|---|---|
| Energy released per unit mass | kJ per gram, from the enthalpy of combustion divided by the molar mass. Natural gas is the highest, coal the lowest — a bigger proportion of hydrogen means more energy per gram. |
| Tendency to incomplete combustion | Rises with carbon content and with molecular size. Coal is the worst; natural gas the cleanest. This links straight back to section 1.3.2. |
| Carbon dioxide added | Per unit of energy, coal releases the most and natural gas the least, because the fuels differ in how much of their energy comes from burning hydrogen rather than carbon. |
A note on this table. The syllabus requires these comparisons to be covered, and the chemistry behind them is standard: the more of a fuel’s energy that comes from burning hydrogen rather than carbon, the more energy per gram and the less carbon dioxide. But the specific rankings above are teaching content, not quotations — no published question or report in the evidence behind these notes states them, so they are given as chemistry rather than as something an examiner has said.
This is the single most repeated error in the evidence for this outcome, and it runs in both directions. Published examiner feedback: Some candidates handled the question well talking about the reduction of CO2 concentration which is a greenhouse gas. Many candidates talked about acid rain gaining no marks. Some answers were vague indicating the students did not know how to approach this question. The average mark was 0.9 out of 2 marks.
And the reverse, from another session: Over 60% of the candidates were aware that SO3 causes acid deposition. Greenhouse gas was the most common incorrect answer. A third report records students who considered NO is a greenhouse gas. Carbon dioxide and methane cause the greenhouse effect. Sulfur and nitrogen oxides cause acid deposition. Naming the wrong one scores nothing, in either direction.
Feedback on one item: the main mistake was stating that it was a greenhouse gas or contributed to global warming, often in a list of effects which contradicted the correct answer. A list containing the right answer and a wrong one scores zero. Give one effect, and give the one you are confident in.
1.3.4 · Biofuels SL core
A biofuel is made from carbon that a plant fixed out of the air, recently, by photosynthesis. That one sentence carries both the advantage and the argument against.
Renewable against non-renewable is the distinction underneath. Fossil fuels fixed their carbon over hundreds of millions of years and we are returning it in two centuries; a biofuel fixes and returns it over one growing season. The carbon is the same carbon; the timescale is the whole difference.
A structured question asked for two advantages of using ethanol as a fuel instead of gasoline (petrol). The published scheme accepts any two of:
| 1 | «ethanol is» renewable / sustainable resource |
| 2 | «ethanol has» low/zero carbon footprint / produces less CO2 |
| 3 | less sulfur dioxide «than fossil fuels» or less acid rain «than fossil fuels» |
| 4 | less incomplete combustion «than fossil fuels» or less carbon monoxide/soot «than fossil fuels» |
| 5 | Accept “ethanol is biodegradable /less toxic than gasoline” |
And one refusal, stated outright: Do not accept just “less harmful”. Every accepted answer names what is less — less CO2, less sulfur dioxide, less soot. A comparison without a quantity named is not an answer.
A multiple-choice question: which statement is a disadvantage of using biofuels instead of fossil fuels?
| A | Land is used that could be used to grow food. | a real disadvantage — the published key |
| B | Combustion releases more energy per mol of fuel. | would be an advantage, and is not true of ethanol against petrol |
| C | Emissions of CO2 to the atmosphere are lower overall. | true, and an advantage |
| D | Crop waste can be used as a fuel. | true, and an advantage |
Two of the four distractors are correct statements pointing the wrong way. Reading the question for its direction matters as much as knowing the chemistry — and feedback records that about 75% got it right, so it discriminated on exactly that. The food-against-fuel argument is the disadvantage the examination expects; the others worth knowing are the land, water and fertiliser needed to grow the crop, and the fossil energy used in farming and processing it.
1.3.5 · Fuel cells SL core
A fuel cell converts the chemical energy of a fuel directly to electrical energy — no flame, no turbine, no generator. The syllabus asks for half-equations at the electrodes, and names hydrogen and methanol as the fuels to cover.
Fuel cells have appeared once in the current-syllabus papers behind this document. Everything earlier is from the discontinued option and is excluded. So prepare this one from the syllabus wording rather than from question patterns: deduce half-equations for the electrode reactions in a fuel cell, with hydrogen and methanol as the fuels, and the use of proton exchange membranes will not be assessed. That last line is the guide telling you where the boundary is.
| Electrode | What happens | Where the electrons go |
|---|---|---|
| Anode | Oxidation | electrons are released, so they appear on the right |
| Cathode | Reduction | electrons are consumed, so they appear on the left |
An ox — oxidation at the anode. Then balance atoms, then balance charge with electrons. Both must balance, and the charge balance is what most answers miss.
A structured question showed a molten carbonate fuel cell — carbonate ions carrying the charge through the electrolyte, CO2 and H2 fed to one side, CO2 and O2 to the other — and gave the skeletons to complete:
The published answers:
Check the charge, not just the atoms. At the anode the left side carries 2– and the right carries 2– in the two electrons. At the cathode the left carries 2– from the electrons and the right carries 2– on the carbonate. Add the two and the carbonate, the electrons and the carbon dioxide all cancel, leaving H2 + ½O2 → H2O — the cell is burning hydrogen, without a flame.
The next part asked candidates to suggest why the cell is environmentally friendly. The published answer is zero net carbon «dioxide» emissions or does not contribute to global warming, with the note Ignore references to water. That note is the interesting part. The obvious answer — “the only product is water” — earns nothing here, because the question is about the carbon dioxide, which this cell consumes at one electrode exactly as fast as it produces it at the other.
Putting it together
| Outcome | What is assessed | The mark that is separately lost |
|---|---|---|
| 1.3.1 | Balancing combustion equations, and the amounts that follow | Oxygen atoms counted where molecules were wanted |
| 1.3.2 | Products of incomplete combustion | Offering hydrogen gas — about four fifths of one cohort did |
| 1.3.3 | Fossil fuels, CO2 and the greenhouse effect | Confusing the greenhouse effect with acid deposition, in either direction |
| 1.3.4 | Biofuels: photosynthesis, renewable against non-renewable | A vague comparison — “less harmful” is refused outright |
| 1.3.5 | Half-equations for fuel-cell electrodes | Charge left unbalanced; electrons on the wrong side |
Say what is less, and say than what. Three of the five outcomes are answered by comparisons — less carbon dioxide than a fossil fuel, less incomplete combustion than coal, zero net emissions rather than none at all. Every published scheme in this topic rewards a comparison that names its quantity and its reference point, and refuses one that does not.
- Balance the complete combustion of butane, C4H10.
- Which class of hydrocarbon gives equal amounts of CO2 and H2O, and why can an alkane never do it?
- Why does ethanol need less oxygen than an alkane with the same number of carbons and hydrogens?
- Name the three carbon-containing products that may form as the oxygen supply falls, in order.
- What is never a product of incomplete combustion, and why?
- Give the equation for photosynthesis.
- Write the anode half-equation for a hydrogen fuel cell in alkaline conditions, given the hydroxide ion carries the charge.
- A student writes “biofuels are less harmful”. Why does that score nothing?
1. C4H10 + 6½O2 → 4CO2 + 5H2O, or doubled: 2C4H10 + 13O2 → 8CO2 + 10H2O. 2. Alkenes, CnH2n: n carbons give n CO2 and 2n hydrogens give n H2O. An alkane is CnH2n+2, so it always makes one more water than carbon dioxide — never equal. 3. Its own oxygen atom counts towards the total, saving half a mole of O2. 4. Carbon dioxide, then carbon monoxide, then carbon (soot). 5. Hydrogen gas. The hydrogen is oxidised to water whatever the oxygen supply; only the carbon is left part-oxidised. 6. 6CO2 + 6H2O → C6H12O6 + 6O2. 7. H2 + 2OH– → 2H2O + 2e– — oxidation, so the electrons are on the right, and the charge is 2– on each side. 8. It names no quantity and no comparison. A published scheme refuses it in those words.