chem paper 2

• This is a summary video for AQA GCSE Chemistry Paper 2
• It gives an overview of everything that could come up on the paper
• It will allow you to do a last-minute cram on the night before the exam
• If you're taking GCSE Combined Science, watch out for the green headings at the top of the screen which tell you whenever there's triple science content that you'll need to skip
• You can also use the timestamps in the description below
• Rate of a chemical reaction

  Its speed, how fast it's going
• Measuring the rate of a chemical reaction

  1. How quickly the reaction is using up the reactants

  2. How quickly it is making products
• Calculations to work out the rate


  An amount of one of the chemicals divided by the time that it takes to either make it or use it up
• The amount can be a mass in grams or a volume in centimetres cubed
• For higher tier, you should also be able to express rate in moles per second
• If a reaction produced 20 centimetres cubed of gas in two seconds

  The rate would be 10 centimetres cubed per second
• Even if you forget how to calculate the rate, as long as the exam board have given you the units, the calculation is wrapped up inside it
• The word 'per' means divided by
• Interpreting rate graphs


  • The gradient or steepness of the graph can be used to tell you the rate of the reaction
  • A steeper gradient indicates a faster rate
• Constant rate graphs


  • The gradient isn't changing
• Curved rate graphs


  • The rate is changing, it's faster at the start and gradually slows down
• Calculating the rate from a curved graph

  1. Calculate the gradient at the start and end to get an overall or mean rate

  2. Draw tangents to calculate the rate at a particular time
• Collision theory


  Chemical reactions only happen when the reacting particles collide with each other, and they need sufficient energy (activation energy)
• Ways to speed up the rate of reaction

  • Increase pressure
  • Increase concentration
  • Increase surface area
  • Increase temperature
  • Add a catalyst
• Increasing pressure


  More particles in the same space, so they collide more frequently
• Increasing concentration


  More particles in the same space, so they collide more frequently
• Increasing surface area


  More surface area for the same amount of stuff, so more collisions
• Increasing temperature


  • Particles have more energy and move faster, so they collide more frequently
  • More particles have enough energy to reach the activation energy
• Catalysts


  Chemicals that speed up the rate of reaction without being used up or changed themselves, they provide an alternative pathway with lower activation energy
• Enzymes are an example of a biological catalyst
• Heat is not a catalyst, it has to be a chemical
• The first required practical in AQA GCSE Chemistry Paper 2 investigates the effect of concentration on the rate of reaction
• Measuring the rate by collecting gas

  1. Use an upturned measuring cylinder or a gas syringe

  2. Take readings at regular time intervals
• Measuring the rate by turbidity


  Time how long it takes for a cross on paper to disappear as a solid precipitate forms
• Reversible reactions have a double-headed arrow between reactants and products
• Equilibrium


  The point where the forward and backward reactions are happening at the same rate, so the concentrations stop changing
• Le Chatelier's principle


  If a system at equilibrium is disturbed, it will shift to counteract the change
• Adding a reactant


  The system will shift to remove it
• Heating a reaction


  The system will shift to cool it down
• Increasing pressure


  The system will shift towards the side with fewer gas molecules
• Decreasing pressure


  The system will shift towards the side with more gas molecules
• For a reversible reaction, if the forward reaction is endothermic, the backward reaction must be exothermic
• Le Chatelier's principle tells me


  The system will shift to counteract that change and increase the pressure so it's going to move the equilibrium towards the higher pressure side which here is my reactants
• The backward reaction is favored


  Because there are more molecules on the left
• The equilibrium will shift to the left


  Therefore the yield of sulfur trioxide will be lower