Energy changes in reactions — GCSE Chemistry practice questions

Question 1

A self-heating coffee can has a sealed inner compartment of calcium oxide. Pressing a button lets water into the compartment. The calcium oxide reacts with the water and the coffee around the compartment becomes hot.

Which row describes the reaction in the compartment?

Accepted Answer

D: exothermic · from the reacting chemicals to the coffee. The coffee gets hotter, so thermal energy is transferred from the reacting chemicals to the coffee (the surroundings). A reaction that transfers thermal energy to its surroundings is exothermic.

Question 2

A catalytic converter in a car exhaust speeds up this reaction:

2CO + 2NO → 2CO₂ + N₂

Fig. 1 shows the reaction pathway with and without the catalyst.

Which row is correct?

Accepted Answer

A: X · Y. Activation energy is the energy from the reactants level up to the top of the hump. X reaches the top of the solid hump (without the catalyst) and Y the top of the lower, dashed hump (with the catalyst). Z is ΔH, which the catalyst does not change.

Question 3

Ethanol is the fuel in spirit burners. It burns completely:

C₂H₅OH(g) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

| bond | bond energy in kJ/mol |
|---|---|
| C–C | 347 |
| C–H | 413 |
| C–O | 358 |
| O–H | 464 |
| O=O | 498 |
| C=O | 805 |

What is the enthalpy change, ΔH, for this reaction?

Accepted Answer

B: −1276 kJ/mol. Bonds broken: 347 + 5 × 413 + 358 + 464 + 3 × 498 = 4728 kJ. Bonds made: 4 × 805 + 6 × 464 = 6004 kJ. ΔH = 4728 − 6004 = −1276 kJ/mol; negative because more energy is released making bonds than is taken in breaking them.

	type of reaction	thermal energy is transferred
A	endothermic	from the coffee to the reacting chemicals
B	endothermic	from the reacting chemicals to the coffee
C	exothermic	from the coffee to the reacting chemicals
D	exothermic	from the reacting chemicals to the coffee