S5 Chemistry: Enthalpy Change And Energy (Paper 1: Basics)

S5 Chemistry: Enthalpy Change and Energy (Paper 1: Basics)

2026-11-12

S5 Chemistry: Enthalpy Change and Energy (Paper 1: Basics)

Ten items building the basics of enthalpy: signs for exothermic and endothermic changes, heat from q = mcDT, molar enthalpy, bond energies and energy diagrams, with answers and one-line notes.

Direct answers　Answers in order: negative, exothermic, 2100 J, 2.73 kJ, -54.6 kJ mol-1, -56.3 kJ mol-1, -183 kJ mol-1, -250 kJ mol-1 (exothermic), 272 kJ, no (it is positive).

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S5 Chemistry: Enthalpy Change and Energy
Level	S5

02　Questions (10 items)

FoundationIs the enthalpy change of an exothermic reaction positive or negative?
FoundationMagnesium burns in air with bright light and much heat. Is the reaction exothermic or endothermic?
Foundation50 g of water warms by 10 °C. Taking the specific heat capacity of water as 4.2 J g-1 K-1, how much heat does the water absorb?
Intermediate100 cm3 of water (density 1 g cm-3) rises by 6.5 °C during a reaction. How much heat does the water absorb in kJ?
IntermediateBurning 0.050 mol of a fuel releases heat of which 2.73 kJ is absorbed by the water. Find the molar enthalpy change of combustion.
Intermediate25.0 cm3 of 1.0 M hydrochloric acid is mixed with 25.0 cm3 of 1.0 M sodium hydroxide solution and the temperature rises by 6.7 °C. Find the molar enthalpy change of neutralisation (assume the solution density is 1 g cm-3).
ChallengeGiven bond energies H-H 436, Cl-Cl 243 and H-Cl 431 kJ mol-1, find ΔH for H2 + Cl2 -> 2HCl.
ChallengeAn energy level diagram shows the products 250 kJ mol-1 lower in enthalpy than the reactants. What is ΔH?
ChallengeThe enthalpy change of combustion of ethanol is 1360 kJ mol-1 (exothermic). How much heat is released when 0.20 mol burns completely?
Error redoA student thinks the enthalpy change of an endothermic reaction is negative. Is that right?

03　Answers and one-line notes

#	Answer	One-line note
1	Negative (ΔH < 0)	An exothermic reaction releases heat to the surroundings, so the products have lower enthalpy than the reactants and ΔH is negative.
2	Exothermic	Burning converts chemical energy into heat and light, so the surroundings warm up and the reaction is exothermic.
3	2100 J	Use q = mcDT: 50 times 4.2 times 10 gives 2100 J.
4	2.73 kJ	The mass is 100 g, and q = 100 times 4.2 times 6.5 gives 2730 J, which is 2.73 kJ.
5	-54.6 kJ mol-1	Per mole the heat is 2.73 divided by 0.050, giving 54.6 kJ, and since heat is released the value is -54.6 kJ mol-1.
6	-56.3 kJ mol-1	The solution is 50 g, so q = 50 times 4.2 times 6.7 gives 1407 J; 0.025 mol of water is formed, so ΔH is minus 1.407 over 0.025, which is -56.3 kJ mol-1.
7	-183 kJ mol-1	Breaking bonds needs 436 plus 243, which is 679, and forming bonds releases 2 times 431, which is 862; 679 minus 862 gives -183 kJ mol-1.
8	-250 kJ mol-1, so the reaction is exothermic	The products sit lower, so 250 kJ is released per mole and ΔH is -250 kJ mol-1.
9	272 kJ	Each mole releases 1360 kJ, so 0.20 times 1360 gives 272 kJ.
10	No, it is positive for an endothermic reaction	An endothermic change takes heat from the surroundings, so the products have higher enthalpy and ΔH is positive.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Work out the heat with q = mcDT first, then state the sign and the number of moles clearly.Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.