LO- Understand how enthalpy changes of combustion can be measured using calorimetry. Measuring Enthalpy Changes What can you remember from GCSE? The energy.

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LO- Understand how enthalpy changes of combustion can be measured using calorimetry. Measuring Enthalpy Changes What can you remember from GCSE? The energy released in a combustion reaction can be measured by heating water. The energy change can be calculated using- q = mc ∆T Where q is the heat energy change in J, m is the mass of water in grams (1cm 3 =1g), c is the specific heat capacity of water (4.18 J g -1 K -1 ), ∆T is the change in temperature Questions 1.Calculate the amount of heat energy needed to increase the temperature of 150cm 3 of water from 20 0 to 30 0 C. 2.Calculate the temperature change that occurs when 10 Joules of heat energy is released from the combustion of 25g of water.

Example- Calculate the mass of water required to release 1kJ of energy by increasing the temperature of the water from 50 0 to C.

Calculating enthalpy changes of combustion Example 1 In a laboratory experiment, 1.80 g of ethanol, C 2 H 5 OH were burned completely in oxygen. The heat from this combustion was used to raise the temperature of 100 g of water from 300 K to 350 K. Calculate a value, in kJ mol –1, for the enthalpy of combustion of ethanol. If you the number of moles of fuel that has undergone combustion in heating the water, you can calculate the enthalpy of combustion in kJ mol -1.

Example 2: Methanol, CH 3 OH, is a convenient liquid fuel. An experiment was conducted to determine the enthalpy of combustion of liquid methanol. The energy obtained from burning 2.12 g of methanol was used to heat 150 g of water. The temperature of the water rose from 298 K to 362 K. (The specific heat capacity of water is 4.18 J K –1 g –1 ) (i) Calculate the number of moles of methanol in the 2.12 g. (ii) Calculate the heat energy required to raise the temperature of 150 g of water from 298 K to 362 K. (The specific heat capacity of water is 4.18 J K –1 g – 1 ) (iii) Use the data above to calculate a value for the enthalpy of combustion of one mole of liquid methanol.

Example 3 (a) Write an equation for the complete combustion of propanone, C 3 H 6 O, to form carbon dioxide and water (1) (b) In a laboratory experiment, 1.45 g of propanone were burned completely in oxygen. The heat from this combustion was used to raise the temperature of 100 g of water from K to K. (i) Calculate the number of moles of propanone in the 1.45 g. (ii) Calculate the heat energy required to raise the temperature of 100 g of water from K to K. (The specific heat capacity of water is 4.18 J K –1 g –1 ) (iii) Hence, calculate a value, in kJ mol –1, for the enthalpy of combustion of propanone. (5)

Example 4 In an experiment, an excess of solid magnesium oxide was added to 50 cm3 of 3.0 mol dm –3 hydrochloric acid. The initial temperature of the solution was 21 °C. After reaction, the temperature had risen to 53 °C. (The specific heat capacity of water is 4.2 J K –1 g –1 ) Use this information to calculate the enthalpy change for the reaction of one mole of magnesium oxide with hydrochloric acid. For your calculation you should assume that all the heat from the reaction is used to raise the temperature of 50 g of water.