AQA A Level Chemistry

Revision Notes

1.6.4 Calorimetry

Test Yourself

Calorimetry

Measuring enthalpy changes

  • Calorimetry is the measurement enthalpy changes in chemical reactions
  • A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or metal can

 

Chemical Energetics Calorimeter, downloadable AS & A Level Chemistry revision notes

A polystyrene cup can act as a calorimeter to find enthalpy changes in a chemical reaction

  • The energy needed to increase the temperature of 1 g of a substance by 1 oC is called the specific heat capacity (c) of the liquid
  • The specific heat capacity of water is 4.18 J g-1 oC-1
  • The energy transferred as heat can be calculated by:

Chemical Energetics Equation for Calculating Energy Transferred in Calorimeter, downloadable AS & A Level Chemistry revision notes

Equation for calculating energy transferred in a calorimeter

Worked example

Specific heat capacity calculations

In a calorimetry experiment, 2.50 g of methane is burnt in excess oxygen.

30% of the energy released during the combustion is absorbed by 500 g of water, the temperature of which rises from 25 °C to 68 °C

The specific heat capacity of water is 4.18 J g-1 °C−1  

What is the total energy released per gram of methane burnt?

Answer

Step 1: q = m x c x ΔT

             m (of water) = 500 g

             c (of water) = 4.18 J g-1 °C-1

             ΔT (of water) = 68 oC - 25 oC

                                  = 43 oC

Step 2: q = 500 x 4.18 x 43

       = 89 870 J

Step 3:  This is only 30% of the total energy released by methane

Total energy x 0.3 = 89 870 J

Total energy = 299 567 J

Step 4:  This is released by 2.50 g of methane

Energy released by 1.00 g of methane = 299 567 ÷ 2.50

= 119 827 J = 120 000 J

= 120 kJ g-1

Exam Tip

When new bonds are formed the amount of energy released is equal to the amount of energy absorbed when the same bonds are broken.

For example:

O2 (g) → 2O (g)           E (O=O) = +498 kJ mol-1

2O (g) → O2 (g)           E (O=O) = -498 kJ mol-1       

Aqueous solutions of acid, alkalis and salts are assumed to be largely water so you can just use the m and c values of water when calculating the energy transferred.

To calculate any changes in enthalpy per mole of a reactant or product the following relationship can be used:Chemical Energetics Equation for Calculating Enthalpy Change in a Chemical Reaction, downloadable AS & A Level Chemistry revision notesWhen there is a rise in temperature, the value for Δbecomes negative suggesting that the reaction is exothermic and when the temperature falls, the value for ΔH becomes positive suggesting that the reaction is endothermic.

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