Energetics Calculations (Edexcel IGCSE Chemistry (Modular))
Revision Note
Calculating heat energy change
In order the calculate heat energy changes you need to know the mass of the substance being heated, the temperature change and the specific heat capacity of the substance
The specific heat capacity, c is the energy needed to raise the temperature of 1 g of a substance by 1 °C
The specific heat capacity of water is 4.18 J/g/°C
The heat energy change, Q, can be calculated by:
Where:
Q = the heat energy change, J
m = the mass of the substance being heated, g
c = the specific heat capacity, J/g/°C
ΔT = the temperature change, °C
The temperature change in degrees Celsius is the same as the temperature change in Kelvin
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
Calculating molar enthalpy change
We can compare the amount of energy released per gram and per mole for different fuels
In both cases, the energy released (Q) is calculated first
To calculate the energy released per gram of fuel:
energy released per gram= energy released / mass of fuel burned
To calculate the energy released per mole of fuel:
energy released per mole= energy released / number of moles
The energy released per mole is also known as the molar enthalpy change
The units are kJ / mol
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
Examiner 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
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