IGCSEChemistryEdexcelRevision Notes3. Physical ChemistryEnergeticsEnergetics Calculations

Energetics Calculations (Edexcel IGCSE Chemistry): Revision Note

Exam code: 4CH1

Stewart Hird

Written by: Stewart Hird

Reviewed by: Lucy Kirkham

Updated on

Edexcel ChemistryEdexcel Science (Double Award): ChemistryEdexcel Science (Double Award) (Modular): Chemistry Unit 1Edexcel Chemistry (Modular): Unit 1

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

    • The specific heat capacity of the substance

  • 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

  • Heat energy change, Q, can be calculated using the formula:

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 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

  • State the equation:

Q = m x c x ΔT

  • Known values:

    • m = 100 g

    • c = 4.18 J/g/°C

    • ΔT = 7.5 oC

  • Substitute the values into the equation:

Q = 100 g x 4.18 J/g/°C x 7.5 oC

  • Solve the equation:

Q = 3135 J

  • Apply the sign:

    • The temperature increased

    • So, the reaction is exothermic (energy was released to the surroundings)

    • Exothermic reactions have a negative enthalpy change:

Q = -3135 J

  • Convert from J to kJ (divide by 1000):

Q = fraction numerator negative 3135 over denominator 1000 end fraction = -3.14 kJ (to 3 significant figures)

  • 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:

  • State the equation:

Q = m x c x ΔT

  • Known values:

    • m = 200 g

    • c = 4.18 J/g/°C

    • ΔT = 30 oC

  • Substitute the values into the equation:

Q = 200 g x 4.18 J/g/°C x 30 oC

  • Solve the equation:

Q = 25080 J

  • Apply the sign:

    • Combustion reactions are always exothermic:

Q = -25080 J

  • Convert from J to kJ (divide by 1000):

Q = fraction numerator negative 25080 over denominator 1000 end fraction = -25.1 kJ (to 3 significant figures)

  • 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 = fraction numerator energy space released over denominator mass space of space fuel space burned space end fraction

  • To calculate the energy released per mole of fuel:

energy released per mole = fraction numerator energy space released over denominator number space of space moles end fraction

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.

a) Calculate the energy released per gram.

b) Calculate the enthalpy change in kJ/mol.

Answer:

  • State the equation:

Q = m x c x ΔT

  • Known values:

    • m = 250 g

    • c = 4.18 J/g/°C

    • ΔT = 37 – 25 °C = 12 °C

  • Substitute the values into the equation:

Q = 250 g x 4.18 J/g/°C x 12 oC

  • Solve the equation:

Q = 12540 J

  • Apply the sign:

    • The temperature increased, so the reaction is exothermic:

Q = -12540 J

  • Convert from J to kJ (divide by 1000):

Q = fraction numerator negative 12540 over denominator 1000 end fraction = -12.54 kJ

a) To calculate the energy released per gram:

  • Find the mass of propanol burned:

mass = moles × Mr

mass = 0.01 mol × 60.11 g mol-1

mass = 0.601 g

  • Apply the formula:

energy released per gram = fraction numerator energy space released over denominator mass space of space fuel space burned space end fraction

energy released per gram = fraction numerator negative 12.54 space kJ over denominator 0.601 space straight g space end fraction

energy released per gram = −20.9 kJ/g (to 3 s.f.)

b) To calculate the enthalpy change in kJ/mol:

  • Calculate the energy released per mole:

ΔH = straight Q over straight n

ΔH = fraction numerator 12.540 space kJ over denominator 0.01 space mol end fraction

ΔH = – 1254 kJ/mol

Examiner Tips and Tricks

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.

Exothermic reactions release energy to the surroundings, causing the temperature to rise. By convention, enthalpy changes for exothermic reactions are always negative, which reflects the fact that the reaction loses energy. If the temperature increased in your experiment, your final answer must be negative.

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

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 Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Lucy Kirkham

Reviewer: Lucy Kirkham

Expertise: Head of Content Creation

Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.