Thermochemistry MCQ - Practice Questions & Answers

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Thermochemistry And Enthalpy For Chemical Reaction

Thermochemistry

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

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