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Lesson: Chapter 8 — Heat changes in chemical reactions

Putting the chapter together 2 of 3

The exercise

Worked answers to the chapter's exercise.

Question 1

Exothermic or endothermic?

  1. What do you mean by an exothermic reaction and an endothermic reaction?
  2. Is each of these exothermic or endothermic? Burning a match; dropping a piece of sodium into water; dissolving urea fertiliser in water; adding glucose to water; adding water to quicklime.

The answer

  1. An exothermic reaction gives out heat; an endothermic reaction absorbs heat.
  2. Burning: exothermic. Sodium in water: exothermic. Urea dissolving: endothermic — the water cools. Glucose in water: endothermic. Water on quicklime: exothermic.

Question 1 (iii) — an energy level diagram

822 kJ mol-1 is released in this reaction. Show it on an energy level diagram.

2Na (s) + Cl2 (g) → 2NaCl (s)

The answer

Sodium and chlorine above, sodium chloride 822 kJ mol-1 below, with the arrow pointing down.

Question 2

40 cm3 of vinegar (dilute acetic acid) is mixed with 60 cm3 of very dilute lime water (calcium hydroxide). The temperature of the mixture rises by 10 °C.

Vinegar and lime water

  1. Calculate the heat change. Take the density of water as 1000 kg m-3 and its specific heat capacity as 4200 J kg-1 °C-1.
  2. What did you assume? Is the reaction exothermic or endothermic?

The answer

  1. Volume = 40 + 60 = 100 cm3 = 1 × 10-4 m3, so m = 1000 × 10-4 = 0.1 kg. Q = 0.1 × 4200 × 10 = 4200 J.
  2. That the mixture has the density and specific heat capacity of water, and that all the heat went into the mixture with none lost. The temperature rose, so the reaction is exothermic — it is a neutralisation.