Calorimetry (Oxford AQA IGCSE Chemistry)

Worked Example

1.023 g of the fuel propanol was burned in a spirit burner and used to heat 200 g of water in a copper calorimeter.

The temperature of the water rose by 30 ^oC.

Calculate the heat energy change, Q,  for the combustion of propanol using this data.

Answer:

• Q = m x c x ΔT

• Q = 200 g x 4.18 J/g/°C x 30 °C = 25 080 J 

• Combustion is an exothermic reaction so the energy change should be negative

• This calculation gave you a positive answer so you need to add a minus sign in front = - 25 080 J

Worked Example

In a calorimetry experiment, 2.50 g of methane is used to heat up 100 g of water. The temperature increases by 43 ^oC.

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) = 100 g

  •  c (of water) = 4.18 J g^-1 °C^-1

  • ΔT (of water) = 43 ^oC

  • Q = 100 x 4.18 x 43  = 17 974 J

• Step 2:  This is released by 2.50 g of methane

  • Energy released by 1.00 g of methane:

    • 17 974 ÷ 2.50

    • = 7 189.6 J/g

    • = -7.189 kJ/g

Worked Example

The energy from 0.01 mol of propanol was used to heat up 250 g of water.

The temperature of the water rose from 25 °C to 37 °C .

The specific heat capacity of water is 4.18 J/g/°C.

Calculate the enthalpy change in kJ/mol.

Answer:

• Step 1: Q = m x c x ΔT

  • m (of water) = 250 g

  • c (of water) = 4.18 J /g/°C

  • ΔT (of water) = 37 – 25 °C = 12 °C

  • Q = 250 x 4.18 x 12 = 12 540 J

• Step 2:  Calculate the energy released per mole

  •   ΔH = Q ÷ n

  • 12 540 J ÷ 0.01 mol = 1 254 000 J/mol

  •  – 1254 kJ/mol

Exam Tip

When you determine Q your answer will be in joules, but enthalpy change is measured in kJ/mol. Make sure you convert Q to kilojoules by dividing by 1000.

Reactions where energy has been released / the temperature has increased are exothermic- this means your answers for enthalpy changes should be negative.