Determination of AH in the laboratory

The following section gives an example of a laboratory determination of the enthalpy change for a chemical reaction in which the reactants are mixed in water. After the reaction has taken place the final mixture consists of products, unused reactants and water. The principles of such measurements are as follows. 

The chemical reaction should be rapid, so that the energy change is complete in a short period of time. This is achieved by rapidly mixing the reactants. 

An exothermic chemical reaction increases the amount of heat energy contained in the reaction mixture. This raises the temperature ofthe hnal mixture by A T°C, defined as Tgnai initial- An endothermic reaction decreases the amount ofheat energy contained in the reaction mixture, so lowering the temperature of the final mixture. 

The change in the amount ofheat energy due to chemical reaction, q joules, is calculated using the equation 

At constant pressure, the heat change undergone produced by the chemical reaction is equal to the enthalpy change of the reaction, i.e. q = AH. Note that q and AH have the same sign. 

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