Thermochemistry - Practice Questions & MCQ

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

Heat is evolved here.
$\Delta \mathrm{H}$ is -ve as $\mathrm{H}_{\mathrm{R}}>\mathrm{H}_{\mathrm{P}}$
$\Delta \mathrm{H}=\mathrm{H}_{\mathrm{P}}-\mathrm{H}_{\mathrm{R}}=-$ ve (at constant pressure)
At constant volume,

$
\Delta \mathrm{E}=\left(\mathrm{E}_{\mathrm{P}}-\mathrm{E}_{\mathrm{R}}\right)=-\mathrm{ve}
$

that is. $E_R>E_P$

Endothermic Reaction

Here heat is absorbed.
$\Delta H$ or $\Delta E=+$ ve for endothermic
$\Delta H=H_p-H_R=+$ ve (at constant pressure as ) as $H_p>H_R$ or $E_p>E_R$
At constant volume

$
\Delta E=\left(E_p-E_R\right)=+v e
$
 

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.

It is determined by water and Bomb calorimeters. Mathematically, it is given as

$
=\Sigma \mathrm{H}_{\mathrm{p}}-\Sigma \mathrm{H}_{\mathrm{R}}
$
It can be express by $[\Delta \mathrm{H}]_{\mathrm{p}}$ or $[\Delta \mathrm{E}]_{\mathrm{v}}$ at constant pressure and volume respectively.