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After completing this lesson you should be able to : • The specific heat capacity, mass and temperature change can be used to calculate the enthalpy change for a reaction using Eh = cm∆T The quantities c, m and ∆T can be calculated given relevant data. • The enthalpy of combustion of a substance is the enthalpy change when one mole of the substance burns completely in oxygen. COPY Enthalpy of combustion • The enthalpy of combustion is the heat energy given out when 1 mole of fuel burns completely in oxygen. • The enthalpy of combustion of methane can be represented by the equation • CH4(g) + O2 (g) CO2(g) + H2 O(l) Enthalpy of combustion COPY The heat energy released when alcohols burn can be measured The enthalpy of combustion of a substance is the amount of energy given out when one mole of a substance burns in excess oxygen. COPY Specific heat capacity Calculating the energy change during a chemical reaction in water. E = c. m. T c m T =specific heat capacity =mass in Kg =temperature change The mass of water can be calculated by using the fact that 1 ml = 1 g. The value for c is usually taken as 4.18 kJ kg –1 oC-1 COPY Measuring the enthalpy of combustion of alcohols Weigh a filled alcohol burner Measure 50 cm3 water into a copper calorimeter Take temperature of the water Light the burner and use it to heat the water to approx 60oC Stir the water and take the highest temperature reached Reweigh the burner and remaining fuel Enthalpy of combustion Procedure 1. Weigh the spirit burner (already containing ethanol) with its cap on and record its mass. (The cap should be kept on to cut down the loss of ethanol through evaporation) 2. Using the measuring cylinder, measure out 100 cm3 of water into the copper can. 3. Set up the apparatus as directed by your teacher/lecturer. 4. Measure and record the temperature of the water. 5.Remove the cap from the spirit burner and immediately light the burner. 6.Slowly and continuously stir the water with the thermometer. When the temperature has risen by about 10 °C, recap the spirit burner and measure and record the maximum temperature of the water. 7. Reweigh the spirit burner and record its mass. Specific heat capacity COPY Calculation (a)The heat energy gained by the water (Eh) can be calculated using the formula: Calculating the energy change during a chemical reaction in water. E = c. m. T c m T =specific heat capacity =mass in Kg =temperature change The mass of water can be calculated by using the fact that 1 ml = 1 g. The value for c is usually taken as 4.18 kJ kg –1 oC-1 CALCULATION Suppose 0.25 g of ethanol had been burned and the temperature of the water had risen by 12.5 °C. The heat energy gained by the water (Eh) is calculated using the formula: Eh = c m T Eh = 4.18 x 0.10 x 12.5 = 5.225 kJ We assume that the heat energy released by the burning ethanol is gained only by the water. The heat energy released on burning 0.25 g of ethanol = 5.225 kJ Ethanol: CH3CH2OH Mass of 1 mole = 2(12) + 6(1) + 16 = 46 g We can now calculate the heat energy released on burning 1 mole of ethanol. 0.25g 46g 5.225kJ 46 0.25 x 5.225 = 961 kJ The enthalpy of combustion of ethanol = - 961 kJ mol-1 (A negative sign is used because combustion is an exothermic reaction) COPY Measuring the enthalpy of combustion of alcohols The heat energy gained by the water (Eh) is calculated using the formula: Eh = c m ∆T Eh = x x = kJ COPY Measuring the enthalpy of combustion of alcohols We assume that the heat energy released by the burning alcohol is gained only by the water. The heat energy released on burning ……….. g of ……………anol So one mole .............. g of ................anol ………….. kJ ....................kJ The enthalpy of combustion of …………anol = -………….. kJ mol-1 (A negative sign is used because combustion is an exothermic reaction) COPY Sources of inaccuracy • Heat loss to surroundings • Ignore heat rise of calorimeter • Incomplete combustion • Possible loss of fuel by evaporation from wick COPY Calorimetry To eliminate these inaccuracies a bomb calorimeter is used The burning fuel (or food) is supplied with oxygen to encourage complete combustion The combustion chamber is entirely surrounded so there is no heat loss to the surroundings Commercial ‘bomb’ calorimeters The calorimeter is heated electrically. Energy required to heat the entire apparatus by 1 0C is calculated. COPY Enthalpy of combustion COPY 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. –704 kJ mol-1 Worked example 2. COPY 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. 33.3oC Calculations for you to try. 1. COPY 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. Calculate the enthalpy of combustion of ethanol 2. 0.01 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. 3. 0.1g of methanol, CH3OH, was burned and the heat given out used to raise the temperature of 100 cm3 of water at 21oC. Use the enthalpy of combustion of methanol in the data booklet to calculate the final temperature of the water. 4. 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. COPY 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. Calculate the enthalpy of combustion of ethanol -1269 kJ mol-1 COPY 2. 0.01 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. -886.2 kJ mol-1 COPY 3. 0.1g of methanol, CH3OH, was burned and the heat given out used to raise the temperature of 100 cm3 of water at 21oC. Use the enthalpy of combustion of methanol in the data booklet to calculate the final temperature of the water. 26.4oC COPY 4. 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. 10.66oC