Energetics Calculations (Edexcel IGCSE Chemistry (Modular))

Worked Example

Excess iron powder was added to 100.0 cm³ of 0.200 mol dm^-3  copper(II) sulfate solution in a calorimeter.

The reaction equation was as follows.

Fe (s) + CuSO₄ (aq) →    FeSO₄ (aq) + Cu (s)

The maximum temperature rise was 7.5 ^oC. Determine the heat energy change of the reaction, in kJ.

Answer:

The solution is assumed to have the same density as water, so 100.0 cm³  has a mass of 100 g

• Q = m x c x ΔT

• Q = 100 g x 4.18 J/g/°C x 7.5 ^oC = – 3135 J = -3.13 kJ

• The temperature increased indicating an exothermic reaction so the value must be negative

Worked Example

1.023 g of propan-1-ol (M = 60.11 g mol^-1) 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 for the combustion of propan-1-ol 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 = -25 kJ

• Combustion reactions are always exothermic so your answer must be negative

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