JEE MAIN - Chemistry (2020 - 8th January Evening Slot - No. 6)
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.
Assertion : The pH of water increases with increase in temperature.
Reason : The dissociation of water into H+ and OH– is an exothermic reaction.
Both assertion and reason are false.
Both assertion and reason are true, but the
reason is not the correct explanation for the
assertion.
Both assertion and reason are true, and the
reason is the correct explanation for the
assertion.
Assertion is not true, but reason is true.
Explanation
H2O ⇌ H+
+ OH-
As $$\Delta $$H $$>$$ 0 for this reaction. So Dissociation of H2O is endothermic.
pH = $$ - \ln \left[ {{H^ + }} \right]$$
kw = $$\left[ {{H^ + }} \right]\left[ {O{H^ - }} \right]$$
$$\ln \left\{ {{{{k_{{T_2}}}} \over {{k_{{T_1}}}}}} \right\} = {{\Delta H^\circ } \over R}\left\{ {{1 \over {{T_1}}} - {1 \over {{T_2}}}} \right\}$$
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
$$ \Rightarrow $$ $${\left[ {{H^ + }} \right]_{{T_2}}}$$ $$>$$ $${\left[ {{H^ + }} \right]_{{T_1}}}$$
$$ \therefore $$ [pH]T2 $$<$$ [pH]T1
As $$\Delta $$H $$>$$ 0 for this reaction. So Dissociation of H2O is endothermic.
pH = $$ - \ln \left[ {{H^ + }} \right]$$
kw = $$\left[ {{H^ + }} \right]\left[ {O{H^ - }} \right]$$
$$\ln \left\{ {{{{k_{{T_2}}}} \over {{k_{{T_1}}}}}} \right\} = {{\Delta H^\circ } \over R}\left\{ {{1 \over {{T_1}}} - {1 \over {{T_2}}}} \right\}$$
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
$$ \Rightarrow $$ $${\left[ {{H^ + }} \right]_{{T_2}}}$$ $$>$$ $${\left[ {{H^ + }} \right]_{{T_1}}}$$
$$ \therefore $$ [pH]T2 $$<$$ [pH]T1
Comments (0)
