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For the reaction H₂O(l) ⇌ H⁺(aq) + OH⁻(aq), K_w = 10⁻¹⁴ at 25°C 55.9 kJ mol⁻¹ of heat is evolved when one mole of H⁺(aq) reacts with one mole of OH⁻(aq) - VCE - SSCE Chemistry - Question 13 - 2004 - Paper 1
Question 13
For the reaction H₂O(l) ⇌ H⁺(aq) + OH⁻(aq), K_w = 10⁻¹⁴ at 25°C 55.9 kJ mol⁻¹ of heat is evolved when one mole of H⁺(aq) reacts with one mole of OH⁻(aq). At 80°C,... show full transcript
Worked Solution & Example Answer:For the reaction H₂O(l) ⇌ H⁺(aq) + OH⁻(aq), K_w = 10⁻¹⁴ at 25°C 55.9 kJ mol⁻¹ of heat is evolved when one mole of H⁺(aq) reacts with one mole of OH⁻(aq) - VCE - SSCE Chemistry - Question 13 - 2004 - Paper 1
Step 1
K_w and pH at 80°C
Answer
To determine the values of K_w and pH at 80°C, we start by noting that the reaction of water is endothermic, as it evolves heat when evolving H⁺ and OH⁻ ions. This means that as the temperature rises, the equilibrium will shift to favor the products, leading to an increase in the concentration of H⁺ and OH⁻ ions.
Since K_w is the ion product of the concentrations of H⁺ and OH⁻ ions, it will increase at elevated temperatures. This implies that at 80°C, K_w is greater than 10⁻¹⁴.
In pure water, as K_w increases, the pH will decrease because the concentration of H⁺ ions will increase. This results in a pH value that is less than 7.
Thus the correct choices are:
K_w: greater than 10⁻¹⁴ pH: less than 7.