Chemistry

Key points

• When fuels and energy react with oxygen, energy is released in an exothermic reaction
• Calorimeter's can be used for the comparison of energy released by different fuels/foods in a school laboratory

Study tip

Remember the specific heat capacity of water, and the value used to convert joules to calories is given to you. Remember that to calculate the energy released by burning a fuel use the equation: Q=mc delta T. You should be able to calculate the energy released from burning a known massof fuel in kJ/g and also when given the formula of fuel or is relative formula mass in kJ/mol

Worked example; 0.50g of a fuel was burned and used to heat 200g of water in a calorimeter. The temperature of the water increased by 14 degrees celcius, the specific heat capacity of water is- c=4.2J/g degrees celcius. Energy released=200 X 4.2 X 14=11760J=11.76kJ, energy released per g of fuel=11.76/0.5=23.52kJ/g

Key points

• You can calculate energy change for reactions in a solution by measuring the temperature change using the equation Q=mc delta T
• Neutralisation and displacement reactions are examples of reactions where you may use this technique

Study tip

Use only tthe volume of the solution when calculating energy change in aqueous solutions, and assume that the specific heat capacity of ann aqueous solution is the same as water

Worked example; a student added 25cm*3 of dilute nitric acid to 25cm*3 of potassium hydroxide solution ina polystyrene cup. The temperature rise was recorded as 12 degrees celcius. To calculate the energy change; Q=mc delta T, the volume of the solution=25+25=50cm*3, the energy change=50 X 4.2 X 12=2520J=2.52kJ

Key points

• Relative difference of the energy of reactants and products can be shown on energy level diagrams
• Catalysts aid the reaction to speed up, and provide a pathway for lower activation energy(energy needed to start a reaction) and so the rate of reaction can increase
• Bond breaking=endothermic, (energy is taken in, reactants lower than products)
• Bond making=exothermic, (energy is released, reactants higher than products)

Study tip

Energy increases up the diagrams, endothemic changes (energy taken in), go up and exothermic changes (energy is released) go down

Key points

• Exothermic-energy released when new bonds formed is greater than energy absorbed when bonds are broken
• Endothermic-energy released when new bonds are formed is less than energy absorbed when bonds are broken
• You can calculate the energy change using bond energies (the energy needed to break the bond between two atoms); bond energies are measured in kJ/mol, you need the balanced equation for the reaction, then need to calculate the total amount of energy needed to break all bonds, the total amount of energy released in making all of the bonds in the products and he difference between two totals

Study tip

Bond energies are generally found in tables, you will be given the values for any bond energies