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. carry out an experiment to measure the enthalpy of combustion of a substance and be able to identify the experimental errors.

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: CH 4 + 2O 2 → CO 2 + 2H 2 O 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.

Exercise Write balanced formulae equations to show the enthalpy of combustion of the following substances: (a)C 2 H 6 (b)C 4 H 8 (c)C 6 H 12 O 6 (d)C 2 H 5 OH

Answers (a)C 2 H ½O 2 → 2CO 2 + 3H 2 O (b)C 4 H O 2 → 4CO 2 + 4H 2 O (c)C 6 H 12 O O 2 → 6CO 2 + 6H 2 O (d)C 2 H 5 OH + 3 O 2 → 2CO 2 + 3H 2 O

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 -1o C -1 m = the mass of water in kg NOTE (1cm 3 of water = 0.001kg) ∆T = the change in temperature in o C

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

Example g of methanol, CH 3 OH, is burned completely and the heat energy produced causes the temperature of 100cm 3 of water to be raised from 20 o C to 27 o C. Calculate the enthalpy of combustion of methanol.

Step 1: Calculate ‘m’ and ‘∆T’ m: Volume of water = 100cm 3 ∴mass of water = 0.1kg ∆T = 27 – 20 = 7 o C

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

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

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

Example 2 0.6g of propane, C 3 H 8, is burned in a spirit burner and the energy produced used to heat up 50cm 3 of water from 21 o C to 29 o C. Calculate the enthalpy of combustion of propane.

Step 1: Calculate ‘m’ and ‘∆T’ m: Volume of water = 50cm 3 ∴mass of water = 0.05kg ∆T = 29 – 21 = 8 o C

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

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

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

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

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