Fundamentals of Enthalpy Changes

When a chemical reaction occurs, there is an exchange of energy between the reaction and its environment. This exchange of energy is in the form or either taking the heat in or giving the heat out to the surrounding.  This exchange of energy is called the “Enthalpy Change”.

The chemical reaction is called a “system” and its environment is called the “surrounding”.  When the system gives out heat to the surrounding, the reaction is said to be “Exothermic” reaction. When the system takes in heat from the surrounding, the reaction is said to be “Endothermic “reaction.  When the heat is given out the enthalpy change is negative and when the heat is absorbed, the enthalpy change is positive. The symbol used for enthalpy change is ΔH. So to recap, in exothermic reactions, heat is given out therefore ΔH is negative whereas in Endothermic reactions, heat is being absorbed so ΔH is positive.

ΔH is easily worked out for a reaction through a simple experiment.  In order to calculate ΔH for a reaction, three things have to be known or worked out. First important value is a constant which is specific heat capacity of water and this is always 4.2 J/g/°C. Second we must know the mass of our system or reactants. And finally we must work out how much the temperature changes during the chemical reaction.

For example if we are reacting zinc to an acid solution, we must know how much of the solution are we using. We can use volume and convert that into mass. If 50cm3 of solution is used we can take that as 50g. This is our mass. We measure the temperature of the solution and as we add the zinc to it, we measure the temperature change until the temperature changes no longer. This will tell us the temperature change.

 If the temperature goes up that means heat is being absorbed by the surrounding so the reaction is Endothermic. If the temperature is going down, this shows the surrounding is absorbing heat from the system, therefore cooling it down and making the reaction an Exothermic on.

The simple equation to work out the enthalpy change is:

Enthalpy Change (ΔH) = Specific heat capacity × Mass × Temperature change

In this experiment excess zinc is added to 50cm3 of copper sulphate and the temperature rose by 10°C. So to work out ΔH in J, 4.2 × 50 × 10 = -2100 J

The negative sign is because the heat given out in the reaction and it is an Endothermic reaction.

Sometimes the Enthalpy Change value worked out from an experiment may not be exactly same as quoted in many data books. This different is only due to the experimental error.

The apparatus used to work out Enthalpy changes is called a calorimeter. A simple calorimeter involves a polystyrene cup with a lid that has a thermometer pierced through it.  The reaction takes place in the cup and the temperature change is measured by the thermometer. These measurements can be used to work out the enthalpy change for the reaction.