Chemical Energetics: IGCSE Chemistry 0620

Chemical energetics is short (three subtopics) but it appears every series: 2-3 MCQs, a reaction pathway diagram worth 3-4 marks on Paper 3 or 4, and a bond energy calculation worth 3 marks on Paper 4. Examiner reports name two recurring losses: energy-level diagrams drawn with the arrow pointing the wrong way, and ΔH calculations with break and make swapped. Both are sign errors, and both are fixable in one sitting.

Exothermic and endothermic reactions (Core)

An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings rises. An endothermic reaction takes thermal energy in from the surroundings, so the temperature falls. The word “surroundings” does work in those sentences: the energy is measured by what happens to the thermometer in the solution, not to “the reaction”.

Examples worth one mark each: combustion, respiration and acid-alkali neutralisation are exothermic; thermal decomposition (heating copper carbonate), photosynthesis and the cold packs used on sports injuries are endothermic. Burning fuels, the most-cited exothermic case, links forward to organic chemistry, where complete combustion of hydrocarbons gives carbon dioxide and water.

(S) Extended candidates attach the symbol: the enthalpy change ΔH is negative for exothermic reactions and positive for endothermic ones. The sign is bookkeeping from the system’s point of view: energy leaves, so the products sit lower.

A practical note: Paper 6 measures energy changes with a thermometer in a polystyrene cup. Temperature rise = exothermic. If a question gives a table of temperatures, subtract before and after and say which way the energy moved.

Energy level (reaction pathway) diagrams (Core, with Supplement labelling)

Core candidates interpret the diagrams; (S) Extended candidates draw and label them. Both versions test the same two reads:

• Exothermic: reactants drawn higher than products. The energy difference arrow points downwards from reactants to products and is labelled ΔH (negative).
• Endothermic: products higher than reactants. The arrow points upwards and ΔH is positive.

(S) The activation energy, Ea, is the minimum energy that colliding particles must have in order to react. On the diagram it is the gap from the reactant level to the peak of the curve. Three labels (ΔH, Ea, and the axis “energy” on y against “progress of reaction” on x) convert a sketch into 3 marks.

(S) The diagram is also where catalysts are explained: a catalyst provides an alternative reaction pathway with a lower activation energy, drawn as a lower hump on the same diagram. The reactant and product levels do not move, so ΔH is unchanged. That sentence pair belongs to rates of reaction but is examined on this diagram.

Bond energy calculations (S)

Two facts drive every calculation: bond breaking is endothermic (energy must be put in to pull atoms apart) and bond making is exothermic (energy is released when new bonds form). Whether the overall reaction is exothermic or endothermic depends on which is bigger.

The formula: ΔH = total energy in to break bonds − total energy released making bonds. (Equivalently: bonds broken − bonds formed, using bond energies as positive numbers.)

Worked pattern for H2 + Cl2 → 2HCl with bond energies H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol:

Step	Working
Bonds broken	436 + 242 = 678 kJ in
Bonds formed	2 × 431 = 862 kJ out
ΔH	678 − 862 = −184 kJ/mol

Negative ΔH, so exothermic: more energy is released forming bonds than is taken in breaking them. The factor-of-two on HCl is where the marks die: the equation makes two moles of HCl, so two H–Cl bonds form. Count bonds from the balanced equation, coefficient by coefficient, before touching the calculator. Methane questions need the same care: CH4 contains four C–H bonds, and combustion equations multiply the O=O and O–H counts too.

Worked exam question

Hydrogen bromide is formed from its elements: H2 + Br2 → 2HBr. Bond energies in kJ/mol: H–H 436, Br–Br 193, H–Br 366. (a) Calculate the enthalpy change, ΔH, for this reaction. (b) State whether the reaction is exothermic or endothermic and explain your answer using your value from (a). [4]

Model answer: (a) Energy to break bonds = 436 + 193 = 629 kJ (1). Energy released making bonds = 2 × 366 = 732 kJ (1). ΔH = 629 − 732 = −103 kJ/mol (1). (b) Exothermic, because ΔH is negative: more energy is released forming bonds than is taken in breaking them (1).

Mark-by-mark: mark 1 is the bonds-broken total. Mark 2 needs the ×2 on H–Br; writing 366 caps the answer at 2 marks even with correct method afterwards. Mark 3 is the subtraction in the right order with the negative sign: “+103” is wrong, “103” without a sign is risky. Mark 4 is awarded for the conclusion tied to the sign; “exothermic because it gives out heat” restates the word and earns nothing.

The mistakes that cost marks

1. Arrow direction on the diagram. The ΔH arrow runs from the reactant level to the product level: downwards for exothermic. Diagrams with products drawn above reactants in a combustion question fail at the first mark.
2. Activation energy measured from the products, or from the baseline. Ea starts at the reactant energy level and ends at the peak.
3. Break and make swapped in ΔH calculations, flipping the sign. Anchor it: breaking costs energy (in), making pays energy (out), ΔH = in − out.
4. Missing the coefficients when counting bonds: one H–Cl instead of two, three C–H bonds in methane instead of four.
5. “The reaction gets hot” for exothermic definitions. The mark scheme wants thermal energy transferred to the surroundings; the surroundings warm up, and that is what the thermometer reads.

How to phrase it for full marks

Command words decide the depth here (“state” wants the word exothermic, “explain” wants the energy comparison) and the command words guide maps each one to an answer length.

Student wording	Mark-scheme wording
”The reaction gives off heat"	"Thermal energy is transferred to the surroundings, so the temperature rises"
"It’s exothermic because it feels hot"	"ΔH is negative; more energy is released forming bonds than is taken in breaking bonds"
"The catalyst speeds it up"	"The catalyst provides an alternative pathway with a lower activation energy"
"Energy needed to start the reaction"	"The minimum energy colliding particles must have in order to react"
"Breaking bonds releases energy"	"Bond breaking is endothermic; bond making is exothermic”

One structural habit covers the whole topic: every explanation should contain an energy direction (in or out), a location (surroundings or bonds), and a sign or comparison. Sentences with all three match the marking points automatically.

The Malaysia note

Energetics lands mid-course in most Malaysian international schools, sandwiched between heavier topics, and students treat it as light revision, then drop 3 marks on a bond-energy sign in the May/June Paper 4. Students with SPM-stream siblings will recognise “tindak balas eksotermik” from KSSM Kimia, but 0620 marks the surroundings phrasing and the ΔH sign convention more strictly than school tests do. This topic is a favourite of ours for a free 1-hour trial lesson: it is small enough to fix completely in one session, and the before-and-after on a past-paper diagram question shows exactly what mark-scheme training changes.

Every sub-topic in Chemical Energetics

• Exothermic and Endothermic Reactions
• Energy Level (Reaction Pathway) Diagrams
• Bond Energy Calculations (Supplement)