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Question 67 of 70

Q.At 25°C25\degree C, ionic product constant KwK_w of water is 1.00×10−141.00 \times 10^{-14}. Its value at 40°C40\degree C is ________.

(a) 1.00×10−141.00 \times 10^{-14}
(b) 1.14×10−151.14 \times 10^{-15}
(c) 2.71×10−142.71 \times 10^{-14}
(d) 2.95×10−152.95 \times 10^{-15}
Tamil Nadu DgeTamil Nadu HSC (DGE) Board 2025MCQ· 1mImportance★★★★★
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Water's self-ionisation is an endothermic process, so its ionic product KwK_w must rise as temperature increases above 25°C25°C; checking each option against this physical requirement (and the correct order of magnitude) singles out 2.71×10−142.71\times10^{-14} as the only consistent value.

The ionisation of water, H2O(l)⇌H+(aq)+OH−(aq)H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq) is an endothermic process (bond-breaking dominates). By Le Chatelier's principle, raising the temperature shifts this equilibrium further to the right, increasing the concentrations of H+H^+ and OH−OH^-, and hence increasing Kw=[H+][OH−]K_w = [H^+][OH^-]. At 25°C25°C, Kw=1.00×10−14K_w = 1.00\times10^{-14}; at a higher temperature (40°C40°C), KwK_w must therefore be larger than this value, not smaller and not unchanged.

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