Endothermic reactions
3 marks · EasyThe table shows the enthalpy changes for dissolving three different salts in water.
| Salt | ΔH (kJ/mol) |
|---|---|
| Potassium nitrate | +34 |
| Calcium chloride | –81 |
| Ammonium chloride | +15 |
Which salt dissolving is the most exothermic? Give a reason for your answer.
Which salt dissolving is the most endothermic? State the value of ΔH for this salt.
Explain what the positive sign of ΔH tells you about the dissolving process for ammonium chloride.
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(a) Calcium chloride; it has the most negative ΔH value (–81 kJ/mol).
(b) Potassium nitrate; ΔH = +34 kJ/mol.
(c) A positive ΔH means ammonium chloride dissolving is endothermic; energy is absorbed from the surroundings.
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Marking points
B1 (a) · 1 mark · Identifies calcium chloride as most exothermic; reason: most negative ΔH.
B2 (b) · 1 mark · Identifies potassium nitrate as most endothermic and states ΔH = +34 kJ/mol.
B3 (c) · 1 mark · Explains that a positive ΔH means energy is absorbed/endothermic.
Worked solution
a) Calcium chloride is the most exothermic because its ΔH value is the most negative (–81 kJ/mol).
b) Potassium nitrate is the most endothermic; ΔH = +34 kJ/mol.
c) A positive ΔH tells you that ammonium chloride dissolving is endothermic; heat is taken in.