Calorimetry Calculations (College Board AP® Chemistry)
Study Guide
Written by: Philippa Platt
Reviewed by: Stewart Hird
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
Examiner Tips and Tricks
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
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