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The molar enthalpies of neutralisation of three reactions are given - HSC - SSCE Chemistry - Question 32 - 2021 - Paper 1
Question 32
The molar enthalpies of neutralisation of three reactions are given. Reaction 1: HCl(aq) + KOH(aq) → KCl(aq) + H2O(l) ΔH = -57.6 kJ mol⁻¹ Reaction 2: HNO3(aq) + K... show full transcript
Worked Solution & Example Answer:The molar enthalpies of neutralisation of three reactions are given - HSC - SSCE Chemistry - Question 32 - 2021 - Paper 1
Step 1
Explain why the first two reactions have the same enthalpy value but the third reaction has a different value.
Answer
The net ionic equation for all three reactions is:
H⁺(aq) + OH⁻(aq) → H2O(l).
HCl and HNO3 are strong acids and completely ionised. When they react with aqueous KOH, both reactions have the same exothermic enthalpy value due to the same net ionic equation.
In the case of HCN, it reacts with aqueous KOH, but HCN only partially ionises in an equilibrium reaction with water:
HCN(aq) + H2O(l) ⇌ H3O⁺(aq) + CN⁻(aq).
As the reaction proceeds, the equilibrium shifts to the right, and the HCN will further ionise as the equilibrium shifts to the side of products. This is endothermic, as it removes some heat from the system, resulting in a lower exothermic ΔH⁻ value compared to the first two reactions. Therefore, the reaction involving HCN has a smaller but still exothermic enthalpy value.