Energetics Calculations (Edexcel IGCSE Chemistry)

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

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:

Calculate Q

Q = m x c x ΔT

Q = 200 g x 4.18 J/g/°C x 30 °C = – 25 080 J = -25 kJ

The energy from 0.01 mol of propan-1-ol 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 molar enthalpy of combustion.

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

• Step 2: Q = 250 x 4.18 x 12

= 12 540 J

• Step 3:  This is the energy released by 0.01 mol of propan-1-ol

Total energy    ΔH = Q ÷ n = 12 540 J ÷ 0.01 mol = 1 254 000 J/mol

Total energy = – 1254 kJ/mol