Energetics Calculations (Edexcel IGCSE Chemistry: Double Science)

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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:

bold Q bold space bold equals bold space bold m bold space bold cross times bold space bold c bold space bold cross times bold space bold increment bold T

  • 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 cmof 0.200 mol dm-3  copper(II) sulfate solution in a calorimeter.

The reaction equation was as follows.

Fe (s) + CuSO(aq)     FeSO(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 cm 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

space straight capital delta H equals straight Q over straight n

  • 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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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.