Quick Notes Calorimetry

  • Calorimetry is a technique used to find the heat energy change that occurs during a reaction.
    • Heat energy change is found by measuring the temperature change of a substance that surrounds the reaction.
  • Specific heat capacity (c) refers to the energy required to raise the temperature of 1g (or 1kg) of a substance by 1oC.
  • The temperature change of a substance can be used with its specific heat capacity to find the energy it has absorbed or released.
    • Change in energy = mass x specific heat capacity x change in temperature
      • Q = mcΔT
  • Energy is often lost from the substance, whose temperature is being measured, to the surroundings.
    • To increase the accuracy of calorimetry, heat loss must be minimised (extra insulation can help).

Full Notes Calorimetry

To find the enthalpy change that occurs in a reaction, we need to know how much heat energy has either been released to a reaction’s surroundings or how much energy has been taken in from the surroundings. The process of finding heat energy changes from experimental data is called calorimetry.

In calorimetry, we measure the temperature of a substance that surrounds a reaction before and then after the reaction has occured. From this temperature change, we can calculate the energy change of the reaction.

To then find the enthalpy change for the reaction, we would need to divide the energy change by the number of moles reacting.

 

calorimetry experiment adding magnesium to water temperature change enthalpy exothermic a-level chemistry

 

We use the equation:

Q = mcΔT

Where:
Q = heat energy change
m = mass of surroundings
c = specific heat capacity of surroundings
ΔT = temperature change of surroundings

Different substances require different amounts of energy to heat up by a given temperature. For example, warming 10g of water by 10oC requires a different amount of energy to warming 10g of solid copper by 10oC. To account for this, we use specific heat capacity: the amount of energy it takes to heat 1g of a particular substance by 1oC.

The mass is simply how much of the substance is being heated (or cooled) - it is not the mass of reactants! Remember, for water we can use ‘formula mass = density x volume’ to find the mass of the water being used (density of water = 1g per cm3).

Note - when dealing with solutions, water isn’t the only substance being heated, meaning the specific heat capacity of the solution won’t be exactly the same as for water. However, the difference between the value for water and for the solution is often negligable, so just the specific heat capacity for water is used. Be aware though, it’s a potential source of inaccuracy.

 

using specific heat capacity to find energy change

 

Note, the surroundings of a reaction are, technically, the entire universe!

For calorimetry, we assume the surroundings of the reaction to be a particular substance (for example, a certain volume of water). Obviously, as the temperature of this substance raises, it will also lose energy to its own surroundings – for example, the air or walls of its container. This is why calorimetry isn’t completely accurate and, especially in classrooms(!), leads to lower energy change values than the actual energy change that occurs. This is due to heat loss from the substance, whose temperature is being measured, to the wider surroundings. Insulation can help minimise this heat loss.