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Thermochemistry - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

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Thermochemistry And Enthalpy For Chemical Reaction is considered one of the most asked concept.
19 Questions around this concept.

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Consider an endothermic reaction $X\rightarrow Y$ with the activation energies $E_{b}\: and\: E_{f}$ for the backward and forward reactions, respectively. In general

$E_{b}\: < \: E_{f}$

$E_{b}\: > \: E_{f}$

$E_{b}\: =\: E_{f}$

There is no definite relation between $E_{b}\: and\: E_{f}$

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Calculate the standard Gibbs free energy change $\mathrm{\left(\Delta G^{\circ}\right)}$ for the reaction:

$\mathrm{2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(l)}$
Given the standard Gibbs free energy of formation $\left(\Delta G_{f}^{\circ}\right)$ values: $\Delta G_{f}^{\circ}\left(\mathrm{H}_{2} \mathrm{O}\right)=-237.2 \mathrm{~kJ} / \mathrm{mol}$

$\Delta G_{f}^{\mathrm{o}}\left(\mathrm{H}_{2}\right)=0 \mathrm{~kJ} / \mathrm{mol}$
$\Delta G_{f}^{\mathrm{o}}\left(\mathrm{O}_{2}\right)=0 \mathrm{~kJ} / \mathrm{mol}$

$474.4 \mathrm{~kJ} / \mathrm{mol}$

$11.70 \mathrm{~kJ} / \mathrm{mol}$

$20.40 \mathrm{~kJ} / \mathrm{mol}$

$85.20 \mathrm{~kJ} / \mathrm{mol}$

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Concepts Covered - 1

Thermochemistry And Enthalpy For Chemical Reaction

Thermochemistry

It deals with the heat changes during chemical reactions. It is called chemical energetics and is based on the first law of thermodynamics.

Exothermic Reaction

$\begin{array}{l}{\text { Heat is evolved here. }} \\ {\Delta \text {H is -ve as } \mathrm{H}_{\mathrm{R}}>\mathrm{H}_{\mathrm{P}}} \\ {\mathrm{\Delta H}=\mathrm{H}_{\mathrm{P}}-\mathrm{H}_{\mathrm{R}}=-\text { ve } \quad \text { (at constant pressure) }} \\ {\text { At constant volume, }} \\ {\Delta \mathrm{E}=\left(\mathrm{E}_{\mathrm{P}}-\mathrm{E}_{\mathrm{R}}\right)=-\mathrm{ve}} \\ {\text { that is. } \mathrm{E}_{\mathrm{R}}>\mathrm{E}_{\mathrm{P}}}\end{array}$

Endothermic Reaction

$\begin{array}{l}{\text {Here heat is absorbed. }} \\ { \Delta H \text { or } \Delta E=+\text { ve for endothermic }} \\ {\Delta H=H_{p}-H_{R}=+\text { ve }(\text { at constant pressure as })} \\ {\text { as } H_{p}>H_{R} \text { or } E_{p}>E_{R}} \\ {\text { At constant volume }} \\ {\Delta E=\left(E_{p}-E_{R}\right)=+v e}\end{array}$

Heat or Enthalpy of Reaction

It is the change in enthalpy (amount of heat evolved or absorbed) when the number of gm-moles of the substance indicated by a chemical reaction have undergone complete reaction.

$\begin{array}{l}{\text { It is determined by water and Bomb calorimeters. }} \\ {\text { Mathematically, it is given as }} \\ {=\Sigma \mathrm{H}_{\mathrm{p}}-\Sigma \mathrm{H}_{\mathrm{R}}} \\ {\text { It can be express by }[\mathrm{\Delta H}]_{\mathrm{p}} \text { or }[\Delta \mathrm{E}]_{\mathrm{v}} \text { at constant }} \\ {\text { pressure and volume respectively. }}\end{array}$