# C5-Energy Changes

For 9-1 GCSE AQA Combined Science Trilogy

• Created by: Joseph.C.
• Created on: 24-03-19 13:26
• C5-Energy Changes
• Exothermic reactions
• Transfers energy to surroundings
• Rise in temperature
• e.g. Combustion and neutralisation
• Used in hand warmers (oxidation of iron) and self heating cans of drinks
• Endothermic reactions
• Takes in energy from surroundings.
• Decrease in temperature.
• e.g. thermal decomposit-ion and the reaction between citric acid and sodium hydrogencar-bonate.
• Used in sports injury packs.
• Energy
• Conserved during reactions.
• The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction.
• If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
• Bond energy calculations
• 1. Calculate energy required to break original bonds.
• 2. Calculate energy released by forming new bonds.
• 3. Find overall energy change
• Energy change = energy required to break bonds - energy released by forming new bonds
• Energy transfer practical
• 1. Put 25cm3 of 0.25 mol/dm3 of hydrochloric acid and sodium hydroxide in separate beakers.
• 2. Place the beakers in a water bath set to 25°C until they're both at the same temperature.
• 3. Add HCl followed by NaOH to a polystyrene cup with a lid.
• 4. Take the temperature of the mixture every 30 secs and record the highest temperature.
• 5.Repeat these steps using 0.5 mol/dm3 then 1 mol/dm3 of HCl.
• Reaction profiles
• The minimum amount of energy that particles must have to react is called the activation energy
• Endothermic reactions
• Products are at a higher energy than the reactants.
• Exothermic reactions
• Products are at a lower energy than the reactants.
• The difference between the energy of the reactants and products is the overall energy change.
• Bond energies
• During a chemical reaction, old bonds are broken and new bonds are formed.
• Energy must be supplied to break bonds - bond breaking is endothermic.
• Energy is released when new bonds are formed - bond formation is exothermic
• Bond energy calculations
• 1. Calculate energy required to break original bonds.
• 2. Calculate energy released by forming new bonds.
• 3. Find overall energy change
• Energy change = energy required to break bonds - energy released by forming new bonds