JEE Main 2020 · 8 January · evening · Single correct · +4 / −1
For the following Assertion and Reason, the correct option is :
Assertion : The pH of water increases with increase in temperature.
Reason : The dissociation of water into H⁺ and OH – is an exothermic reaction.
- (a) Both assertion and reason are false.
- (b) Both assertion and reason are true, but the reason is not the correct explanation for the assertion.
- (c) Both assertion and reason are true, and the reason is the correct explanation for the assertion.
- (d) Assertion is not true, but reason is true.
Answer and solution
Answer (a) Both assertion and reason are false.
Solution
Chemistry ma’am · 4 steps
Breaking water into ions needs heat, so warming water pushes it to ionise more. That lowers pH.
- Check the reason. Splitting H₂O into H⁺ and OH⁻ means breaking an O–H bond, and that takes in energy. So the dissociation is endothermic, not exothermic, and the reason is false.
H₂O ⇌ H⁺ + OH⁻ Δ H > 0
- Apply Le Chatelier. Heating an endothermic equilibrium shifts it forward, so K_w grows with temperature. It is 1.0 × 10⁻¹⁴ at 298 K and about 5.1 × 10⁻¹³ at 373 K.
- Follow to pH. In pure water [H⁺] = √K_w, so a larger K_w gives more H⁺ and a lower pH. At 373 K the pH is about 6.1. The water is still neutral because [H⁺] = [OH⁻], but its pH falls, so the assertion is false.
- So. The assertion and the reason are both false, which is option (a).
The idea to keep. Water's ionisation is endothermic, so K_w rises with temperature and the pH of pure water drops below 7. Neutral means [H⁺] = [OH⁻], not pH 7.
Answer from the NTA final answer key. Solution by PrepWiser.