Chemistry HEAT OF REACTION, FUSION AND VAPOURIZATION

Calorimetry - Measuring Heats of Reactions :

All calorimetric techniques are based on the measurement of heat that may be generated (exothermic process), consumed (endothermic process) or simply dissipated by a sample. There are numerous methods to measure such heat. Any process that results in heat being generated and exchanged with the environment is a candidate for a calorimetric study.

A calorimeter is a device used to measure heat of reaction. In order to measure heats of reactions, we often enclose reactants in a calorimeter, initiate the reaction, and measure the temperature difference before and after the reaction. The temperature difference enables us to evaluate the heat released in the reaction.

Two basic types of calorimetry are discussed :
(a) measurement based on constant volume.

(b) measurements based on constant pressure.

A calorimeter may be operated under constant volume which measures internal energy change `DeltaU` by bomb calorimeter or constant (atmosphere) pressure , which measures enthalpy change `Delta H` by calorimeter Whichever kind to use, heat capacity of the calorimeter is required.

The heat capacity is the amount of heat required to raise the temperature of the entire calorimeter by `1` `K`, and it is usually determined experimentally before or after the actual measurements of heat of reaction. The heat capacity of the calorimeter is determined by transferring a known amount of heat into it and measuring its temperature increase.

Bomb Calorimeter `DeltaU` Measurement :

For chemical reactions, heat absorbed at constant volume, is measured in a bomb Calorimeter. In this Calorimeter, a steel vessel (the bomb) is immersed in a water bath. A combustible substance is burnt in pure oxygen supplied in the bomb. Heat evolved during the
reaction is transferred to the water around the bomb and its temperature is monitored. Since the bomb Calorimeter is sealed, its volume does not change, i.e., the energy changes associated with reactions are measured at constant volume.

Since volume does not change, a bomb calorimeter measures the heat evolved under constant volume,

`q_v,`

`q_v = C Delta T,`

where `Delta T` is the temperature increase. The `q_v` so measured is also called the change in internal energy, `= E`.

`= E = q_v = C xx Delta T`

`text(Note)` : More heat is giving of if the reaction is carried out at constant pressure, since the `P-V` work (`1.5 R T`) due to the compression of `1.5` moles of gases in the reactants would contribute to `dH`. lf `1.0` mole water is decomposed by electrolysis at constant pressure, we must supply an amount of energy equivalent to enthalpy change, `dH`, a little more than internal energy, `dE`. More energy must be supplied to perform the `P-V` work to be done by the products (`H_2` and `O_2`).

`DeltaH` measurement :

Measurement of heat change at constant pressure (generally at atmospheric pressure) can be done in a Calorimeter shown in the figure. In this case, the Calorimeter is left open to atmosphere. As the reaction occurs in the Calorimeter, the temperature change is noticed and then heat of reaction is measured with the knowledge of heat capacity of Calorimeter system.

The heat capacity of the calorimeter can also be determined by burning an exactly known amount of a standard substance, whose enthalpy of combustion has been determined. Benzoic acid, `C_7H_6O_2`, is one such standard. The problem below illustrates the calculations.

Enthalpy Changes during Phase Transformations :

(i) `text(Enthalpy of Fusion)`, `Delta_(fus)H` : It is the enthalpy change that accompanies melting of one mole of a solid substance at constant temperature (melting point of solid) and pressure. For example :

`H_2O(s) -> H_2O(l)`; `Delta_(fus)H = + 6.01 kJ` `mol^(- 1)`

(ii) `text(Enthalpy of Vaporisation)`, `Delta_(vap)H` : It is the enthalpy change required to vapourise one mole of a liquid substance at constant temperature (boiling point of liquid) and pressure for example :

`H_2O(l) -> H_2O(g)`; `Delta_(vap)H = + 40.79 kJ` `mol^(- 1)`

(iii) `text(Enthalpy of Sublimation)`, `Delta_(sub)H` : It is the enthalpy change required to sublime one mole of a solid substance at constant temperature and pressure. For example :

`CO_2(s) -> CO_2(g)`; `Delta_(sub)H = +25.2 kJ mol^(-1)`