Exothermic and endothermic reactions

Does the flask get hot or cold?

Almost every change gives out or takes in heat. The heat given out or taken in by a reaction is its heat of reaction.

Exothermic reactions give heat out to the surrounding, so the container gets hot and ΔH is negative. Hydrogen burning in oxygen to make water gives out 571.6 kJ for 2 moles of water: ΔH is −571.6 kJ. Carbon burning to CO₂ gives out 393.5 kJ per mole.

Endothermic reactions take heat from the surrounding, so the container cools and ΔH is positive. Hydrogen and iodine form HI only at high temperature, absorbing 53.08 kJ. Nitrogen and oxygen form NO only in lightning, absorbing 180.6 kJ.

Run a reaction backwards and the same amount of energy is absorbed instead: water splitting back into hydrogen and oxygen takes in 571.6 kJ. Everyday life runs on exothermic reactions: fuels for cooking and heating, food in our bodies, power stations, petrol and diesel engines, and fireworks.

Same reaction, opposite direction: same size of ΔH, opposite sign.

Key terms

Heat of reaction

Sizing a reaction's energy.

Exothermic

Fuels, food, engines.

Endothermic

Reactions that need constant heating.

Combustion

Heat for homes, power stations and vehicles.

Short questions with model answers

1. Q1. 2H₂(g) + O₂(g) → 2H₂O(l) + 571.6 kJ. Write ΔH, and ΔH for the reverse reaction. (p. 70)

   • Heat is on the product side
   • ΔH = −571.6 kJ
   • 2H₂O → 2H₂ + O₂: ΔH = +571.6 kJ

   ΔH = −571.6 kJ; reverse ΔH = +571.6 kJ

2. Q2. Is melting ice exothermic or endothermic? (Exercise 2, p. 72)

   • System: the ice
   • Ice takes heat from your hand or the air

   Endothermic: heat flows into the ice.

Common mistakes

• ✗ "+ 571.6 kJ" on the product side means ΔH = +571.6 kJ.

  ✓ Heat written as a product leaves the system: ΔH = −571.6 kJ.

• ✗ The reverse reaction has the same ΔH.

  ✓ Same size, opposite sign (p. 70).

• ✗ A reaction that needs high temperature must be exothermic.

  ✓ H₂ + I₂ needs high temperature and absorbs 53.08 kJ: endothermic.

MCQs

1. 1. When NaOH and HCl are mixed the temperature rises. The reaction is:

   1. (a) Endothermic, ΔH positive
   2. (b) Endothermic, ΔH negative
   3. (c) Exothermic, ΔH positive
   4. (d) Exothermic, ΔH negative
   Show answer

   (d) Warmer surroundings: heat out, negative ΔH (MCQ, p. 80).

2. 2. C(s) + O₂(g) → CO₂(g) + 393.5 kJ. ΔH is:

   1. (a) +393.5 kJ mol⁻¹
   2. (b) −393.5 kJ mol⁻¹
   3. (c) 0
   4. (d) +787 kJ mol⁻¹
   Show answer

   (b) p. 71: heat evolved, ΔH = −393.5 kJ mol⁻¹.

3. 3. Formation of NO from N₂ and O₂ (ΔH = +180.6 kJ) happens in:

   1. (a) Lightning
   2. (b) Ordinary air at room temperature
   3. (c) Ice
   4. (d) A self-cooling pack
   Show answer

   (a) p. 71: it is endothermic, so it needs the energy of lightning.

4. 4. If 2H₂ + O₂ → 2H₂O has ΔH = −571.6 kJ, then 2H₂O → 2H₂ + O₂ has ΔH =

   1. (a) −571.6 kJ
   2. (b) +571.6 kJ
   3. (c) 0 kJ
   4. (d) −285.8 kJ
   Show answer

   (b) Reverse: same amount absorbed (p. 70).

5. 5. In an endothermic reaction the container:

   1. (a) Gets hot
   2. (b) Cools down
   3. (c) Glows
   4. (d) Does not change
   Show answer

   (b) p. 70: heat taken from the surrounding lowers the container's temperature.

Quick revision

• Exothermic: heat out, container warms, ΔH negative (H₂ + O₂, C + O₂).
• Endothermic: heat in, container cools, ΔH positive (H₂ + I₂, N₂ + O₂).
• Reversing a reaction keeps the size of ΔH and flips its sign.