Standard Enthalpy Of Formation, Combustion And Bond Dissociation - Practice Questions & MCQ

Heat of Combustion

1. It is, changes in enthalpy when one mole of a substance is completely oxidized or combusted or burnt.

2. $\Delta \mathrm{H}$ is - ve here as heat is always evolved here that is, exothermic process.

3. Heat of combustion is useful in calculating the calorific value of food and fuels.

4. It is also useful in confirming the structure of organic molecules having C, H, O, N, etc.

5. Enthalpy change by combustion of 1 gm solid or 1 gm liquid or 1 cc gas is called calorific value.

calorific value $=\frac{\text { Heat of combustion }}{\text { Molecular wt. }}$

$\Delta \mathrm{H}($ heat of reaction $)=-\Sigma \Delta \mathrm{H}_{\mathrm{P}}^{\circ}-\Sigma \mathrm{H}_{\mathrm{R}}^{\circ}$

Enthalpy of Dissociation or Ionization

It is defined as, "The quantity of heat absorbed when one mole of a substance is completely dissociated into its ions". Example,

$\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}^{+}+\mathrm{OH}^{-} \quad \Delta \mathrm{H}=13.7 \mathrm{Kcal}$

Heat of Atomization

It is the enthalpy change (heat required) when bonds of one mole of a substance are broken down completely to obtain atoms in the gaseous phase (isolated) or it is the enthalpy change when one mole of atoms in the gas phase is formed from the corresponding element in its standard state. In case of diatomic molecules, it is also called bond dissociation enthalpy.

It is denoted by $\Delta \mathrm{H}_{\mathrm{a}}$ or $\Delta \mathrm{H}^{\circ}$.
Example,

$
\begin{aligned}
& \mathrm{H}_2(\mathrm{~g}) \rightarrow 2 \mathrm{H}(\mathrm{~g})-435 \mathrm{~kJ} \\
& \Delta \mathrm{H}=+435 \mathrm{~kJ} / \mathrm{mol} \\
& \mathrm{CH}_4(\mathrm{~g}) \rightarrow \mathrm{C}(\mathrm{~g})+4 \mathrm{H}(\mathrm{~g})+1665 \mathrm{~kJ} \\
& \Delta \mathrm{H}=+1665 \mathrm{~kJ} / \mathrm{mol}
\end{aligned}
$

Phase Transition and Transition Energy

• The change of matter from one state (solid, liquid, or gas) to another state is called Phase Transition.
• Such changes occur at definite temperatures such as melting point (solid to liquid), boiling point (liquid to vapours) etc., and are accompanied by absorption or evolution of heat. The enthalpy change during such phase transitions is called heat of transition or transition energy.

$\begin{aligned} & \text { Example, } \\ & \qquad \mathrm{C} \text { (diamond) } \rightarrow \mathrm{C} \text { (amorphous) } \\ & \Delta \mathrm{H}=3.3 \mathrm{Kcal}\end{aligned}$