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.
Solution
H<sub>2</sub>O ⇌ H<sup>+</sup>
+ OH<sup>-</sup>
<br><br>As $\Delta$H $>$ 0 for this reaction. So Dissociation of H<sub>2</sub>O is endothermic.
<br><br>pH = $- \ln \left[ {{H^ + }} \right]$
<br><br>k<sub>w</sub> = $\left[ {{H^ + }} \right]\left[ {O{H^ - }} \right]$
<br><br>$$\ln \left\{ {{{{k_{{T_2}}}} \over {{k_{{T_1}}}}}} \right\} = {{\Delta H^\circ } \over R}\left\{ {{1 \over {{T_1}}} - {1 \over {{T_2}}}} \right\}$$
<br><br>When temperature increases then<br><br> $${{\Delta H^\circ } \over R}\left\{ {{1 \over {{T_1}}} - {1 \over {{T_2}}}} \right\} > 0$$, as reaction is endothermic.
<br><br>$\therefore$ k<sub>T<sub>2</sub></sub> $>$ k<sub>T<sub>1</sub></sub>
<br><br>$\Rightarrow$ ${\left[ {{H^ + }} \right]_{{T_2}}}$ $>$ ${\left[ {{H^ + }} \right]_{{T_1}}}$
<br><br>$\therefore$ [pH]<sub>T<sub>2</sub></sub> $<$ [pH]<sub>T<sub>1</sub></sub>
About this question
Subject: Chemistry · Chapter: Equilibrium · Topic: Chemical Equilibrium and Kc, Kp
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