Comparing the Energy Released by fuels

How much energy comes from burning fuels? At the end of the lesson, you should be able to: Describe how we can measure the energy produced by different fuels Be able to state the unit of energy Be able to calculate the energy given off when a fuel is combusted (in kJ/mole) from given data

Method Put 100 cm3 of water in the calorimeter. Record the temperature of the water. Clamp the calorimeter in position about 10 cm above the protective mat. Find the mass of a spirit burner containing an alcohol. Place the burner under the calorimeter and light the wick. Stir the water with the thermometer. Stop heating when the temperature has risen by 20 °C. Replace the cap on the burner. Reweigh the burner as soon as possible. Keep noting the temperature – record the highest value reached. If you can, repeat the experiment with other alcohols. Try to keep all the factors the same

Mass of fuel burned = 196.5 – 195.4 = 1.1g Calculation Find out the amount of energy transferred to the water: Energy (J) = m(water)xSHC(water)xtemp change Energy (J) = 100 x 4.2 x 20 = 8 400J Energy (kJ) = 8.4 kJ Work out the mass of fuel burnt: Mass of burner (before) – mass of burner (after) For example: if the burner and ethanol fuel had a mass of 196.5g before and 195.4 g after. Mass of fuel burned = 196.5 – 195.4 = 1.1g

Calculation Number of moles of fuel burned: You need to know the mass of the fuel burned and the molar mass. For example: mass of ethanol = 1.1 g Mr of ethanol = Ethanol (CH3CH2OH): (12x2) + 16 + 6 = 46 g/mole Moles of ethanol (CH3CH2OH) = mass / molar mass 1.1 / 46 = 0.024 moles

Calculation Divide the energy transferred to the water by the number of moles of alcohol to find the enthalpy change of combustion (ΔH). For example: Energy transferred to water = 8.4 kJ Moles of ethanol burned = 0.024 moles Enthalpy change of combustion (ΔH) = 8.4 / 0.024 Enthalpy change of combustion (ΔH) = 350 kJ/mole

Mass of fuel burned = 196.5 – 194.5 = 2.0g Calculation 2 Find out the amount of energy transferred to the water: Energy (J) = m(water)xSHC(water)xtemp change Energy (J) = 100 x 4.2 x 20 = 8 400J Energy (kJ) = 8.4 kJ Work out the mass of fuel burnt: Mass of burner (before) – mass of burner (after) For example: if the burner and methanol fuel had a mass of 196.5g before and 195.4 g after. Mass of fuel burned = 196.5 – 194.5 = 2.0g

Calculation 2 Number of moles of fuel burned: You need to know the mass of the fuel burned and the molar mass. For example: mass of methanol = 2 g Mr of methanol = Methanol (CH3OH): (12+ 3+ 16 + 1 = 32 g/mole Moles of methanol = mass / molar mass 2 / 32 = 0.0625 moles

Calculation 2 Divide the energy transferred to the water by the number of moles of alcohol to find the enthalpy change of combustion (ΔH). For example: Energy transferred to water = 8.4 kJ Moles of methanol burned = 0.0625 moles Enthalpy change of combustion (ΔH) = 8.4 / 0.0625 Enthalpy change of combustion (ΔH) = 134.4kJ/mole

What is Really Happening? Write a word equation for the reaction between the alcohol and oxygen (the first has been done for you) methanol + oxygen  carbon dioxide + water CH3OH (l) + Where did the energy come from that was transferred to the water? 1½ O2 (g) CO2 (g) + H2O(l) 2 All of the bonds in the reactant molecules are broken and all of the bonds in the product molecules are formed.

The Energy in our Food… We can find the energy in our food in a very similar way, if we can make it burn: Food Energy per 100g Bread 1,200 kJ Chocolate biscuit 2,240 kJ Butter 3,310 kJ Apple 160 kJ Celery 32 kJ Sausages (fried) 1,450 kJ What nutrient type is present in the three most ‘high energy’ foods? What sort of problems do scientists face, when trying to measure these values accurately?

Example Susie burned some propanol (C3H7OH). She set up her equipment. She added 100g of water to a copper calorimeter (100ml). She weighed her mass of fuel and the burner before the experiment; it had a mass of 193.45g. She allowed the fuel to raise the temperature of the water by 30.C. She weighed her mass of fuel and the burner after the experiment; it had a mass of 192.55g. How can she calculate a ΔH for the fuel (in kJ/mole)? Step 1: Heat change = m x C x ΔT = 100 x 4.2 x 30 = 12600 J = 12.6 kJ Step 2: Mass of fuel burned = 193.45 – 192.55 = 0.90 g

Example Step 3: Moles of fuel burned Jezebel burned some propanol (C3H7OH). She set up her equipment. She added 100g of water to a copper calorimeter (100ml). She weighed her mass of fuel and the burner before the experiment; it had a mass of 193.45g. She allowed the fuel to raise the temperature of the water by 30.C. She weighed her mass of fuel and the burner after the experiment; it had a mass of 192.55g. How can she calculate a ΔH for the fuel (in kJ/mole)? Step 3: Moles of fuel burned Mr (propanol) = 60 g/mole n = m/M = 0.90/60 = 0.015 moles Step 4: Heat change (kJ/mole) = 12.6/0.015 = 840 kJ/mole

YOUR TURN… These are the techniques that you can use to work out the heat changes (in kJ/mole) for chemical reactions. It is a classic Higher Tier CHEM3 skill. Have a go at a few examples yourself now. Prove these answers are correct! Q1: 2565 kJ/mole Q2: 2800 kJ/mole Q3: 458.2 kJ/mole