C5-Energy Changes
For 9-1 GCSE AQA Combined Science Trilogy
- Created by: Joseph.C.
- Created on: 24-03-19 13:26
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- 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
- Transfers energy to surroundings
- 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.
- Takes in energy from surroundings.
- 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
- Exothermic reactions
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