Energetics: IGCSE Chemistry Exam Guide

Energetics questions on 0620 test whether you can classify reactions as exothermic or endothermic, draw and interpret energy level diagrams, and (for Extended candidates) calculate enthalpy changes from bond energy data. The topic carries fewer marks than stoichiometry or organic chemistry, but the questions are predictable and the marks are available to any student who learns the three question types below. The full content is under chemical energetics.

Exothermic and endothermic: classification

The first task in any energetics question is to classify the reaction. Two clues are given:

1. Temperature change. If the temperature of the surroundings rises, the reaction is exothermic (energy is released to the surroundings). If it drops, the reaction is endothermic (energy is absorbed from the surroundings).
2. Type of reaction. The syllabus expects you to know these classifications:

Exothermic	Endothermic
Combustion	Thermal decomposition
Neutralisation	Photosynthesis
Oxidation reactions	Dissolving some salts (e.g., NH4NO3)
Respiration	Reaction of citric acid with sodium hydrogencarbonate

The exam error to avoid: saying “the reaction produces heat” instead of “the reaction transfers energy to the surroundings.” The precise phrasing scores; the vague version may not.

Energy level diagrams

Energy level diagrams appear on Paper 4 as drawing questions and on Paper 2 as interpretation questions. You need to draw two types.

Exothermic reaction diagram

• Reactants line drawn at a higher energy level.
• Products line drawn at a lower energy level.
• The vertical distance between them is the enthalpy change (labelled with a downward arrow).
• If activation energy is required, draw a curved hump above the reactants level. The activation energy is the vertical distance from the reactants to the top of the hump.

Endothermic reaction diagram

• Reactants line drawn at a lower energy level.
• Products line drawn at a higher energy level.
• The vertical distance between them is the enthalpy change (labelled with an upward arrow).
• Activation energy is drawn the same way: the distance from reactants to the top of the hump.

What the mark scheme checks

1. Correct relative positions of reactants and products (1 mark).
2. Labels on both horizontal lines (reactants, products) (1 mark).
3. Correct labelling of the enthalpy change or activation energy (1 mark).
4. If a catalyst is shown, a second hump drawn lower than the first, same reactant and product levels (1 mark).

The most common drawing error is placing the products above the reactants for an exothermic reaction or omitting the activation energy hump when the question asks for it. More drawing guidance is in drawing diagrams in chemistry exams.

Bond energy calculations (Extended)

Bond energy calculations follow a fixed method. The exam gives you a table of bond energies and a balanced equation.

Step 1: List every bond broken in the reactants and add up the energies. This is the total energy input (bond breaking is endothermic, it requires energy).

Step 2: List every bond formed in the products and add up the energies. This is the total energy output (bond forming is exothermic, it releases energy).

Step 3: Enthalpy change = energy to break bonds - energy released forming bonds.

• If the result is negative, the reaction is exothermic (more energy released than absorbed).
• If the result is positive, the reaction is endothermic (more energy absorbed than released).

Worked example

Q: Calculate the enthalpy change for: CH4 + 2O2 -> CO2 + 2H2O

Bond energies: C-H = 412, O=O = 496, C=O = 743, O-H = 463 (all in kJ/mol).

Bonds broken (reactants):

• 4 x C-H = 4 x 412 = 1648
• 2 x O=O = 2 x 496 = 992
• Total = 2640 kJ

Bonds formed (products):

• 2 x C=O = 2 x 743 = 1486
• 4 x O-H = 4 x 463 = 1852
• Total = 3338 kJ

Enthalpy change = 2640 - 3338 = -698 kJ/mol

The negative value confirms the reaction is exothermic, which makes sense because combustion of methane releases energy.

Common errors in bond energy calculations

1. Counting the wrong number of bonds. CH4 has four C-H bonds, not one. 2H2O has four O-H bonds, not two. Use the displayed formula if in doubt.
2. Subtracting in the wrong order. It is bonds broken minus bonds formed. Reversing this gives the wrong sign.
3. Forgetting to multiply by the coefficient. If the equation shows 2O2, you are breaking two O=O bonds, not one.
4. Using bond energies for the wrong type of bond. C-O is a single bond; C=O is a double bond. Check the table carefully.

Show every step clearly. The method for showing working is covered in how to show working in calculations.

Activation energy and catalysts

Activation energy is the minimum energy that colliding particles must have for a reaction to occur. It is represented on the energy diagram as the height of the hump above the reactants level.

A catalyst lowers the activation energy by providing an alternative reaction pathway. On the energy diagram:

• The reactants and products remain at the same levels (the overall enthalpy change is unchanged).
• A second, lower hump is drawn, labelled “with catalyst.”

The exam mark is specifically for stating that the catalyst lowers the activation energy while the enthalpy change stays the same.

Worked exam question

Q (Paper 4): Neutralisation of hydrochloric acid with sodium hydroxide causes the temperature of the mixture to rise. (a) State whether this reaction is exothermic or endothermic. (1) (b) Draw an energy level diagram for this reaction. Label the reactants, products and the enthalpy change. (3) (c) Explain, in terms of energy, why the temperature rises. (2)

Model answer: (a) Exothermic (1). (b) Diagram with reactants line higher, products line lower, downward arrow labelled as enthalpy change (3: 1 for correct levels, 1 for labels, 1 for enthalpy change arrow). (c) The reaction releases energy to the surroundings (1). This energy increases the kinetic energy of the particles in the solution, so the temperature rises (1).

The second mark in (c) requires linking energy released to kinetic energy and temperature. Simply saying “energy is given out” earns 1 mark.

Energetics is a compact topic with a high mark-per-hour-of-revision ratio. If energy diagrams or bond calculations are causing losses, a trial lesson can fix the specific drawing or calculation step that is going wrong.