Chemical Energetics - Calorimetry

Almost all chemical reactions produce heat. In the production of heat, experiments are conducted to investigate the amount of heat the reaction produced per mole of substance reacted. However, there are experiments which are conducted to investigate the amount of heat the reaction produce per mole of product formed.

Enthalpy change of reaction is defined as the amount of heat produced by the reaction per mole of substance reacted. Specific chemical reactions have their own enthalpy change definition such as Enthalpy change of neutralisation and enthalpy change of combustion. These definitions will be highlighted in another entry.

Hence, this post aims to explain how an experiment is done so that we can investigate the amount of heat the reaction produced per mole of substance reacted. These experiments are know as calorimetry. There are three different situations that can occur in a common laboratory.

(1) Using a water-beaker calorimeter.

In this experiment, a water-beaker calorimeter is set up as shown below. The combustion reaction occurs below the calorimeter and heat is transferred from the reaction to the calorimeter. The temperature of the solution in the calorimeter increase and using the formula shown below, we can obtain the enthalpy change of combustion. Do note that c = specific heat capacity of water, m = mass of water in the beaker and the negative sign implies the reaction is exothermic (releases heat).

Another formula is in the above picture. This formula uses C, which heat capacity of the calorimeter. The capital C is independent of the mass of the calorimeter. Hence, in this situation, it is the temperature change of the calorimeter which we measured.

(2) Mixing two solutions together.

The diagram below illustrates the steps taken. Usually, this method is done when the two reactants are in aqueous solutions. When these reactants are mixed together, the chemical reaction will transfer or absorb heat from the solution. When the reaction absorbs heat, such reaction is endothermic and the enthalpy change has a positive sign.

(3) Adding solid into an aqueous reactant.

The illustration below illustrates the steps taken. This method is used when one of the reactant is a solid and the other is an aqueous solution. From the formula below, note that V = volume of the solution, while c = specific heat capacity of the solution.

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Article written by Kwok YL 2009.}

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