The glass calorimeter is the instrument used in measuring the heat of reaction. If the heat of reaction is large, we can detect exothermic and endothermic reactions by feeling the temperature of the reaction vessel before and after the reaction. If the reaction is
- Exothermic, there is a rise in temperature so the reaction vessel feels warm;
- Endothermic, there is a fall in temperature so the reaction vessel feels cool.
To determine accurate values of ΔH, we use a calorimeter.
There are several types of calorimeters. The bomb calorimeter is very sensitive and is used in nutrition studies. The glass calorimeter is used to determine the ΔH of most chemical reactions. Reactants in stoichiometric amounts are placed in the calorimeter. As the reaction proceeds, the heat energy evolved or absorbed will either warm or cool the system. The temperatures of the system before and after the reaction are recorded.
Knowing the parameters below such as
- The temperature change,
- The mass of reactions, and
- The specific heat capacity of the reaction mixture, we can calculate the ΔH for the reaction.
Standard Conditions for Measuring ΔH
The heat of reaction depends not only upon the mass of the reactants, but also upon the temperature and pressure of the reacting system. Another important factor is the physical state of the substances involved in the reaction, since heat change accompanies the rearrangement of certain bonds between particles when a substance changes from one physical state to another.
For example, we can represent the formation of water as follows:
- H2(g) + ½ O2(g) H2O(l) ΔH = -286 KJ mol-1
- H2(g) + ½ O2 (g) H2O(g) ΔH = -242 KJ mol-1
In (a) liquid water is formed, while in (b) water vapour if formed. Liquid water absorbs heat from the surroundings to form water vapour. The latent heat of vaporization of water is 44 KJ mol-1.
H2O(l) ⟶ H2O(g) ΔH = +44KJ mol-1
Therefore, the formation of water vapor requires less energy than the formation of liquid water. This is seen in the values of ΔH in (a) and (b) which differ by 44KJ mol-1
Besides the three states of matter, the existence of the reactants and products of a reaction in the aqueous state also affects the heat of reaction. For example, the heat evolved during the neutralization of a given amount of sodium hydrogen solution by hydrochloric acid is greater when hydrogen chloride gas is passed into the solution than when hydrochloric acid solution is used. This is because hydrogen chloride gas dissolves in water with the evolution of heat. The heat evolved when a substance is dissolved in water is known as the heat of solution. This is of considerable importance since a vast majority of reactions occur in aqueous medium.
We now see why it is so important to include the state symbols: solid (s), liquid (l), gas (g) or aqueous (aq) after each substance when writing a chemical equation.
Since the value of ΔH is affected by several factors, it is necessary to define standard conditions for its measurement using a glass calorimeter. These conditions for determining ΔH are as follows:
- The temperature must be 298 K (25oC)
- If gases are involved, the pressure must be at 1.01 x 105Nm-2 (1 atm. At 760 mm Hg)
- If aqueous solutions are involved, their concentrations must be 1 moldm-3
The heat of reaction or enthalpy change obtained under such standard conditions is given the special symbol, ΔHӨ.
It should be noted that in practice, for many reactions, Δ cannot be measured at 298 K. Experimental ΔH values are converted to values at 298 K, or the temperature at which a ΔH value is given is specified within parenthesis.