Enthalpy of Combustion

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Presentation transcript:

Enthalpy of Combustion

HIGHER GRADE CHEMISTRY CALCULATIONS Enthalpy of combustion. The enthalpy of combustion of a substance is the amount of energy given out when one mole of a substance burns in excess oxygen. Worked example 1. 0.19 g of methanol, CH3OH, is burned and the heat energy given out increased the temperature of 100g of water from 22oC to 32oC. Calculate the enthalpy of combustion of methanol. Use DH = -cmDT DH = -4.18 x 0.1 x 10 DH = - 4.18 kJ ( c is specific heat capacity of water, 4.18 kJ kg-1 oC-1) m is mass of water in kg, 0.1 kg DT is change in temperature in oC, 10o) Use proportion to calculate the amount of heat given out when 1 mole, 32g, of methanol burns. 0.19 g  -4.18 kJ So 32 g  32/0.19 x –4.18 = -704 kJ Enthalpy of combustion of methanol is –704 kJ mol-1.

Calculation using enthalpy of combustion Calculations for you to try. 1. 0.25g of ethanol, C2H5OH, was burned and the heat given out raised the temperature of 500 cm3 of water from 20.1oC to 23.4oC. Use DH = -cmDT DH = -4.18 x 0.5 x 3.3 = - 7.92kJ Use proportion to calculate the enthalpy change when 1 mole, 46g, of ethanol burns. 0.25 g  -7.92 kJ So 46g  46/0.25 x -7.92 = -1269 kJ mol-1. 2. 0.1 moles of methane was burned and the energy given out raised the temperature of 200cm3 of water from 18oC to 28.6oC. Calculate the enthalpy of combustion of methane. Use DH = -cmDT DH = -4.18 x 0.2 x 10.6 = - 34.768 kJ Use proportion to calculate the enthalpy change when 1 mole of methane burns. 0.1 mol  -34.768 kJ Higher Grade Chemistry So 1mol  1/0..1 x -34.768 = -347.68 kJ mol-1.

Calculation using enthalpy of combustion Worked example 2. 0.22g of propane was used to heat 200cm3 of water at 20oC. Use the enthalpy of combustion of propane in the data book to calculate the final temperature of the water. From the data booklet burning 1 mole, 44g, of propane DH = -2220 kJ By proportion burning 0.22 g of propane DH = 0.22/44 x –2220 = - 11.1kJ Rearrange DH = -cmDT to give DT = DH -cm DT = = 13.3 oC -11.1 -4.18 x 0.2 Higher Grade Chemistry Final water temperature = 20 + 13.3 = 33.3oC

Calculation using enthalpy of combustion Calculations for you to try. 3. 0.1g of methanol, CH3OH, was burned and the heat given out used to raise the temperature of 500 cm3 of water at 21oC. Use the enthalpy of combustion of methanol in the data booklet to calculate the final temperature of the water. From the data booklet burning 1 mole, 32g, of methanol DH = -727 kJ By proportion burning 0.1 g of methanol DH = 0..1/32 x –727 = - 2.27 kJ Rearrange DH = -cmDT to give DT = DH -cm DT = = 5.4 oC -2.27 -4.18 x 0.1 Final water temperature = 21 + 5.4 = 26.4oC Higher Grade Chemistry

Calculation using enthalpy of combustion Calculations for you to try. 2. 0.2g of methane, CH4, was burned and the heat given out used to raise the temperature of 250 cm3 of water Use the enthalpy of combustion of methane in the data booklet to calculate the temperature rise of the water. From the data booklet burning 1 mole, 16g, of methane DH = -891 kJ By proportion burning 0.2 g of methane. DH = 0..2/16 x –891 = - 11.14 kJ Rearrange DH = -cmDT to give DT = DH -cm DT = = 10.66oC -11.14 -4.18 x 0.25 Higher Grade Chemistry