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Energetics Calculations (Edexcel IGCSE Chemistry: Double Science)
Revision Note
Calculating heat energy change
- In order to 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
-
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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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