Calculating enthalpy changes
The enthalpy changeThe change in the amount of heat as a result of a chemical reaction, such as combustion and neutralisation. for a reaction can be calculated using the following equation:
\(\Delta H=cm\Delta T\)
\(\Delta H\) is the enthalpy change (in kJ or kJ mol-1)
c is the specific heat capacity of water.
It is a constant, 4.18 kJ kg-1藲颁-1 and is found in the data book.
m is the mass of water (in kg) (Remember that 100 cm3 = 0.1 kg)
\(\Delta T\) is the change in temperature (藲颁)
Example
Question
A solution was made by dissolving a spatula of potassium nitrate into 50 cm3 of water. The temperature changed from 20.4藲颁 to 18.7藲颁. Calculate the enthalpy change for this reaction.
To calculate the enthalpy change (\(\Delta H\)) we must know the values for c, m and \(\Delta T\). The specific heat capacity (c) is a constant, with a value of 4.18. Since 50 cm3 of water have been used, the mass of water (m) is 0.05 kg.
From the question we can see that the temperature has decreased by 1.7 藲颁. This means that the reaction is endothermic (so \(\Delta H\) will be positive).
\(\Delta H=cm\Delta T\)
= 4.18 x 0.05 x 1.7
= 0.3553 kJ