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Ionic Equilibrium question

2020 · 8 Jan · Shift 2 · Q17
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Ionic Equilibrium question

2020 · 8 Jan · Shift 2 · Q17

JEE MainChemistryIonic EquilibriumMCQ+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+H^+H+ and OH–OH^–OH– is an exothermic reaction.
  1. A
    Both assertion and reason are false.
  2. B
    Both assertion and reason are true, but the reason is not the correct explanation for the assertion.
  3. C
    Both assertion and reason are true, and the reason is the correct explanation for the assertion.
  4. D
    Assertion is not true, but reason is true.
View written solutionFree

Correct answer: A

  1. Examine the assertion

    Assertion: The pH of water increases with increase in temperature.

    For pure water, Kw=[H+][OH−]K_w = [H^+][OH^-]Kw​=[H+][OH−] and at neutrality, [H+]=[OH−]=Kw[H^+] = [OH^-] = \sqrt{K_w}[H+]=[OH−]=Kw​​

    Hence, pH=−log⁡[H+]=−log⁡(Kw)\text{pH} = -\log[H^+] = -\log(\sqrt{K_w})pH=−log[H+]=−log(Kw​​)

    It is known that the ionization of water increases with temperature, so KwK_wKw​ increases as temperature increases.

    Therefore [H+][H^+][H+] increases, and hence pH decreases with increase in temperature.

    So the assertion is false.

  2. Examine the reason

    Reason: The dissociation of water into H+H^+H+ and OH−OH^-OH− is an exothermic reaction.

    The ionization of water is actually endothermic, not exothermic. That is why increasing temperature increases KwK_wKw​.

    By Le Chatelier’s principle, if temperature increases and equilibrium shifts toward more ionization, the forward reaction must absorb heat.

    So the reason is false.

  3. Conclusion

    • Assertion: False
    • Reason: False

    Therefore, the correct option is: A\boxed{\text{A}}A​

  4. Comparison with stored answer

    Stored correct answer: A

    My derived answer is also A, so they agree.

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