Exothermic and endothermic — the surroundings
An exothermic reaction transfers thermal energy to the surroundings, so their temperature rises — e.g. combustion, neutralisation, respiration. An endothermic reaction takes in energy from the surroundings, so their temperature falls — e.g. thermal decomposition, dissolving ammonium nitrate. The mark scheme needs "to/from the surroundings"; "gives off heat" alone is not enough. A temperature rise means exothermic, a fall means endothermic.
Reaction-profile diagrams
A reaction-profile diagram plots energy against progress of reaction. Reactants sit on the left, products on the right. The activation energy is a single-headed arrow from the reactants level up to the peak — it must reach the top. The overall change is a single-headed arrow from reactants to products: down for exothermic (products lower), up for endothermic (products higher). is always positive.
(Extended) Bond energy: ΔH = broken − formed
Breaking bonds absorbs energy (endothermic); forming bonds releases energy (exothermic). Overall, = bonds broken − bonds formed. A negative means more energy is released making bonds than absorbed breaking them, so the reaction is exothermic; a positive value is endothermic. Method: total each set, then take broken minus formed. Bond energies are always positive — the sign comes only from the subtraction order.
Drawn from real examiner reports.
Neutralisation is exothermic, not endothermic
Calling an exothermic reaction endothermic is common — especially neutralisation, where "endothermic" is the most frequent wrong answer even though the temperature rises. Combustion, neutralisation, respiration and metal–acid reactions are all exothermic. Keep endothermic for changes where the surroundings cool, e.g. thermal decomposition.
W22 P12 Q11; W22 P13 Q11; W22 P23 Q11; W22 P31 Q3(c) — most common wrong answer for neutralisation is "endothermic".
Define exo via the surroundings
Cambridge mark schemes require "transfers thermal energy to the surroundings" (or that the surroundings warm up). "Heat is released" or "energy is released" alone scores zero for the definition. The same holds for endothermic: "absorbs heat" is not enough — say "takes in energy from the surroundings", with the surroundings cooling.
S23 P32 Q4(c)(i) — "gives off heat" / "energy is released" named as insufficient; credit needs the surroundings.
Reaction-profile diagram errors
Three diagram errors recur. (1) The activation energy arrow is too short — it must reach the peak of the hump from the reactants level. (2) For an exothermic reaction the products are wrongly drawn above the reactants; they must be below. (3) A double-headed arrow is used for — it must be single-headed (down for exothermic, up for endothermic).
W22 P41 Q5(d)(i); W22 P42 Q5(e)(i)(ii) — all three diagram errors flagged.
Read exothermic off the profile
When a question says "use the diagram", do not just write "heat is given off" — that ignores the diagram and scores zero. The mark-scheme point is that the products' energy level is lower than the reactants' level, so energy is transferred to the surroundings and the reaction is exothermic. Read the energy levels off the profile.
W22 P42 Q5(e)(iii); S22 P31 Q4(d)(ii); S23 P32 Q4(c)(iii) — explanation must reference the products/reactants levels, not just "heat given off".
(Extended) Subtracting bonds the wrong way
The formula is = bonds broken − bonds formed. Candidates often find both totals correctly but then do formed minus broken — right size, wrong sign. E.g. broken , formed : kJ/mol (exothermic); reversing gives and a wrong endothermic verdict. Always take broken minus formed.
W22 P42 Q5(f); S22 P41 Q7(e) — correct values found but subtracted in the wrong direction, giving the wrong sign.
Bond breaking absorbs, forming releases
Students often reverse this. Breaking a bond always absorbs energy — an endothermic step. Forming a bond always releases energy — an exothermic step. Whether the whole reaction is exo- or endothermic depends on which total is larger: more energy released forming bonds than absorbed breaking them means exothermic overall; the reverse is endothermic.
(Extended) Bond-energy calculation method
Total the energies of all bonds broken in the reactants, then all bonds formed in the products. Watch counts — has six N−H bonds, not three. Then = broken − formed, and always write the sign: a bare number loses a mark, negative is exothermic.
Drawing a reaction profile
Decide first: exothermic or endothermic? Draw the hump with its peak above the reactants ( is always positive). Products go lower (exo) or higher (endo) than the reactants. Draw from reactants to the peak, and a single-headed arrow to the products.
Name the surroundings in definitions
In any energy-change definition, give the direction AND name the surroundings: exothermic transfers thermal energy to the surroundings (they warm up); endothermic takes it in from the surroundings (they cool). Naming the temperature change too secures the mark.
Mark-scheme precision trap:
| Wrong phrasing (no credit) | Correct phrasing |
|---|---|
| "Gives off heat" | Transfers thermal energy to the surroundings |
| "Absorbs heat" | Takes in energy from the surroundings |
| "Energy is released" | Transfers thermal energy to the surroundings (surroundings temperature rises) |
Define exothermic reaction.
Sketch a reaction-profile diagram for an exothermic reaction. Label the following on your diagram:
