Verified Solution • 2025–26 Session moderate

QJEEM20Ch98c4: JEE Main 2020 Chemistry - Class 11 JEEM20Ch98c4 Solution: For the following Assertion and Reason, the correct option is : …

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.

Multiple Choice Options

  • 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.

Step-by-Step Verified Solution

H2O ⇌ H+ + OH-

As Δ\Delta H > 0 for this reaction. So Dissociation of H2O is endothermic.

pH = −ln⁡[H+]- \ln \left[ {{H^ + }} \right]

kw = [H+][OH−]\left[ {{H^ + }} \right]\left[ {O{H^ - }} \right]

ln⁡{kT2kT1}=ΔH∘R{1T1−1T2}\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\}

When temperature increases then

{{\Delta H^\circ } \over R}\left\{ {{1 \over {{T_1}}} - {1 \over {{T_2}}}} \right\} > 0 , as reaction is endothermic.

∴\therefore kT2 > kT1

⇒\Rightarrow [H+]T2{\left[ {{H^ + }} \right]_{{T_2}}} > [H+]T1{\left[ {{H^ + }} \right]_{{T_1}}}

∴\therefore [pH]T2 < [pH]T1

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