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Higher Chemistry Unit 1(b) Enthalpy of combustion

Higher Chemistry Unit 1(b) Enthalpy of combustion. After today’s lesson you should be able to: write balanced formulae equations, with state symbols, to show the enthalpy of combustion of a substance. calculate the enthalpy of combustion of a substance given relevant data.

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Higher Chemistry Unit 1(b) Enthalpy of combustion

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  1. Higher ChemistryUnit 1(b) Enthalpy of combustion

  2. After today’s lesson you should be able to: • write balanced formulae equations, with state symbols, to show the enthalpy of combustion of a substance. • calculate the enthalpy of combustion of a substance given relevant data. • carry out an experiment to measure the enthalpy of combustion of a substance and be able to identify the experimental errors.

  3. Enthalpy of combustion The enthalpy of combustion is the enthalpy change that occurs when 1 mole of a substance is burned completely in an excess of oxygen. e.g. the equation showing the enthalpy of combustion of methane will be: CH4 + 2O2 → CO2 + 2H2O NOTE Fractions may have to be used instead of whole numbers in the balanced equation to ensure that 1 mole of the substance is represented.

  4. Exercise Write balanced formulae equations to show the enthalpy of combustion of the following substances: • C2H6 • C4H8 • C6H12O6 • C2H5OH

  5. Answers (a)C2H6 + 3½O2 → 2CO2 + 3H2O (b)C4H8 + 6O2 → 4CO2 + 4H2O (c)C6H12O6 + 6O2 → 6CO2 + 6H2O (d)C2H5OH + 3O2 → 2CO2 + 3H2O

  6. Calculating the enthalpy of combustion The enthalpy change for a chemical reaction is calculated, in kJ mol-1, using the equation ∆H = -cm ∆T Where: c = the specific heat capacity of water = 4.18kJkg-1oC-1 m = the mass of water in kg NOTE (1cm3 of water = 0.001kg) ∆T = the change in temperature in oC

  7. All combustion reactions are exothermic so ∆H will have a negative value for all combustion reactions. Remember to include the negative sign in your answer

  8. Example 1 0.16g of methanol, CH3OH, is burned completely and the heat energy produced causes the temperature of 100cm3 of water to be raised from 20oC to 27oC. Calculate the enthalpy of combustion of methanol.

  9. Step 1: Calculate ‘m’ and ‘∆T’ m: Volume of water = 100cm3 ∴ mass of water = 0.1kg ∆T = 27 – 20 = 7oC

  10. Step 2: Calculate ∆H ∆H = -cm∆T = -4.18 x 0.1 x 7 = -2.926kJ

  11. Step 3: Find the number of moles of substance undergoing combustion n = m / gfm = 0.16 / 32 = 0.005mol

  12. Step 4: Calculate the enthalpy of combustion 0.005moles methanol → -2.926kJ ∴ 1 mol methanol → -2.926 / 0.005 = -585.2kJmol-1 NOTE The change of units from kJ to kJmol-1 when ∆H for 1 mol is calculated.

  13. Example 2 0.6g of propane, C3H8, is burned in a spirit burner and the energy produced used to heat up 50cm3 of water from 21oC to 29oC. Calculate the enthalpy of combustion of propane.

  14. Step 1: Calculate ‘m’ and ‘∆T’ m: Volume of water = 50cm3 ∴ mass of water = 0.05kg ∆T = 29 – 21 = 8oC

  15. Step 2: Calculate ∆H ∆H = -cm∆T = -4.18 x 0.05 x 8 = -1.672kJ

  16. Step 3: Find the number of moles of substance undergoing combustion n = m / gfm = 0.6 / 44 = 0.014mol

  17. Step 4: Calculate the enthalpy of combustion 0.014moles propane → -1.672kJ ∴ 1 mol propane → -1.672 / 0.014 = -119.43kJmol-1

  18. Exercise Answer the following questions from ‘Test your Higher Chemistry Calculations’: p51 Q 6.11, 6.13, 6.15. 6.17, 6.19

  19. Example 3 Copy worked example 6.7 from p52 of ‘Test your Higher Chemistry Calculations’ p53 Q6.22, 6.24 & 6.26

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