Rate and Extent of Chemical Change

• Rate of a chemical reaction


  How quickly a product is formed or how quickly a reactant is used up
• Successful collisions


  1. Reactant particles must collide with enough energy

  2. Collisions that produce a chemical reaction
• Mean rate of reaction


  • Quantity of reactant used / time taken
  • Quantity of product formed / time taken
• Measuring the mass of a reaction mixture


  1. Measure the changing mass at regular time intervals

  2. Useful when gases are given off
• Units for measuring rate of reaction


  g/s or g/min
• Measuring the volume of a reaction mixture


  1. Measure the changing volume at regular time intervals

  2. Collect the gas using equipment like gas syringe, measuring cylinder or upside-down burette
• Units for measuring rate of reaction


  cm³/s or cm³/min
• Factors affecting the rate of a chemical reaction

  • Concentration and pressure
  • Catalyst
  • Surface area
  • Temperature
• How increased concentration/pressure affects rate


  More frequent successful collisions between reactant particles
• Catalyst


  • Substance that speeds up a chemical reaction without getting used up itself
  • Offers an alternative pathway at a lower activation energy
• How catalysts affect rate

  Frequency of collisions is unchanged, but more particles have energy greater than activation energy
• How increased temperature affects rate


  Reactant particles gain kinetic energy and move more quickly, increasing successful collisions
• How increased surface area affects rate


  Larger area of solid exposed to other reactant particles, increasing successful collisions
• Calculating gradient


  1. Draw construction lines on straight part of graph

  2. Measure values of x and y
• Haber process


  Industrial process to make ammonia, requires high temperature, high pressure and iron catalyst
• Changing conditions and effect on equilibrium

  1. Increasing temperature shifts equilibrium to endothermic reverse direction

  2. Increasing pressure shifts equilibrium to side with fewer molecules

  3. Increasing reactant concentration shifts equilibrium to products
• Dynamic equilibrium


  In a closed system, forward and reverse reactions occur at same rate, concentrations remain constant
• Calculating rate of reaction from graph


  1. Draw construction lines on straightest part of graph

  2. Rate = change in quantity / change in time
• Reversible reaction


  Reactants form products, products can then reform reactants
• Required Practical 5: Measuring gas production


  1. Measure volume of CO2 produced when HCl reacts with CaCO3

  2. Plot graph of time vs volume of gas

  3. Calculate mean rate of reaction
• Required Practical 5: Investigating colour change


  Measure time for precipitate to form when Na2S2O3 reacts with HCl at different temperatures