Calorimetry Calculations
Measuring enthalpy changes
- Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
- A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can
Diagram to show how to set up a simple calorimeter
A polystyrene cup can act as a calorimeter to find enthalpy changes in a chemical reaction
- The energy needed to raise the temperature of 1 g of a substance by 1 K is called the specific heat capacity (c) of the liquid
- The specific heat capacity of water is 4.18 J g-1 K-1
- The energy transferred as heat can be calculated by:
Equation for calculating energy transferred in a calorimeter
Worked example
The energy from 0.01 mol of propan-1-ol was used to heat 250 g of water. The temperature of the water rose from 298K to 310K (the specific heat capacity of water is 4.18 J g-1 K-1).
Calculate the enthalpy of combustion.
Answer:
- Step 1: q = m x c x ΔT
- m (of water) = 250 g
- c (of water) = 4.18 J g-1 K-1
- ΔT (of water) = 310 - 298 K = 12 K
- Step 2: q = 250 x 4.18 x 12
- q = 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-1
- Total energy = - 1254 kJ mol-1
Exam Tip
There's no need to convert the temperature units in calorimetry as the change in temperature in oC is equal to the change in temperature in K
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 enthalpy of reaction, in kJ.
Answer:
- Step 1: Calculate q:
- q = m x c x ΔT
- q = 100 g x 4.18 J g-1 K-1 x 7.5 K = - 3135 J
- Step 2: Calculate the amount of CuSO4 (aq)
- moles = volume in dm3 x concentration = 0.1 x 0.2 = 0.02 mol
- Step 3: Calculate ΔH
- ΔH = q ÷ n = -3135 J ÷ 0.02 mol = - 156 750 J = -156.75 kJ
- ΔH = -160 kJ (2 sig figs)
Enthalpy of combustion experiments
- The principle here is to use the heat released by a combustion reaction to increase the heat content of water
- A typical simple calorimeter is used to measure the temperature changes in the water
Diagram to show the set-up of a typical calorimeter
Not all the heat produced by the combustion reaction is transferred to the water. Some heat is lost to the surroundings and absorbed by the calorimeter
- To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter
- Shielding can be used to reduce draughts
- In this experiment the main sources of error are
- Heat losses
- Incomplete combustion
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 enthalpy of combustion of propan-1-ol using this data.
Answer:
- Step 1: Calculate q:
- q = m x c x ΔT
- q = 200 g x 4.18 J g-1 K-1 x 30 K = - 25 080 J
- Step 2: Calculate the amount of propan-1-ol burned:
- moles = mass ÷ molar mass = 1.023 g ÷ 60.11 g mol-1 = 0.01702 mol
- Step 3: Calculate ΔH:
- ΔH = q ÷ n = -25 080 J ÷ 0.01702 mol = - 1 473 560 J = -1 474 kJ
- ΔH = -1.5 x 103 kJ
