Chemistry walkthrough

• 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
• Gradient or steepness of a rate graph

  Can be used to tell you the rate of the reaction
• The purple line on the rate graph has a faster rate than the blue reaction


  Because the line has a steeper gradient
• For a constant rate graph


  The gradient isn't changing
• For a curved rate graph

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

  Divide the change in the y-axis by the change in the x-axis to get the gradient, which is the rate
• For a curved graph, you can use the start and end points to work out an overall or mean rate
• Drawing tangents to calculate the rate at one particular time


  Make the tangent as long as possible, ideally using a transparent ruler or putting the ruler on top of the curve
• At 4 seconds, the tangent rises from 2 to 9 grams


  The rate is 0.7 grams per second
• At 7 seconds, the graph is flatter


  The rate is 0.2 grams per second
• 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

  • Increasing the pressure
  • Increasing the concentration
  • Increasing the surface area
  • Increasing the temperature
  • Adding a catalyst
• Increasing pressure


  Can be done by increasing the number of particles in a container or making the container smaller
• Increasing pressure increases the rate because the particles will collide more frequently
• Increasing concentration


  Works the same way as increasing pressure, more particles in the same space means more collisions
• Increasing surface area


  Cutting something into smaller pieces exposes more surface area for reactions to occur
• Increasing temperature


  Increases the rate of reaction in two ways: 1) Particles have more kinetic energy and collide more frequently, 2) More particles have enough activation energy to react
• Catalysts


  Chemicals that speed up the rate of reaction without being used up or changed themselves
• Catalysts provide an alternative pathway for the reaction with a lower activation energy
• Enzymes are an example of a biological catalyst
• 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 over water or using a gas syringe


  Take readings at regular time intervals
• You also need to control other variables to make the experiment valid
• Measuring the rate using a turbidity method


  Time how long it takes for a cross on paper to disappear as a solid precipitate forms
• A better method would be to use a light sensor and computer
• Reversible reactions


  Reactions where the products can react to form the original reactants, represented by a double-headed arrow
• Reversible reactions can be exothermic in one direction and endothermic in the other
• Equilibrium


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


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


  Shifts the equilibrium to remove the added reactant
• Increasing pressure


  Shifts the equilibrium towards the side with fewer gas molecules
• Increasing temperature


  Shifts the equilibrium towards the endothermic reaction
• 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