rate + extent of chemical change

• Factors that affect the rate of chemical reactions

  • Temperature
  • Concentration
  • Pressure
  • Surface area
  • Presence or absence of a catalyst
• Collision theory


  In order for particles to react they have to collide with each other with sufficient energy (activation energy)
• Rate of reaction


  Depends on the amount of energy the particles have and the frequency of the collisions
• As temperature increases


  Particles gain more energy, move faster, collide more frequently, and collide with more energy
• As concentration or pressure increases


  There are more particles per unit of volume, making collisions more frequent
• As surface area increases

  There is a higher area over which collisions can take place, increasing the frequency of collisions
• Catalyst


  Substances that speed up a reaction without being used up in the reaction themselves
• Catalyst mechanism


  Lowers the activation energy by providing an alternative reaction pathway, increasing the proportion of successful collisions
• Common catalysts


  • Transition metals like cobalt and nickel
  • Enzymes (catalysts made by living organisms)
• Catalysts are not included in the reaction equation as they are not reactants or products
• Rate of chemical reaction

  The speed with which the reactants get turned into products
• Measuring the rate of reaction


  1. Measure how fast the reactants are being used up

  2. Measure how fast the products are being formed
• Rate of reaction = quantity of reactants used / time taken
• Rate of reaction = quantity of products formed / time taken
• Slow reaction


  • Rusting of iron (years or decades)
• Typical reaction


  • Reaction between magnesium and acid (gentle stream of hydrogen bubbles)
• Fast reaction


  • Explosions like fireworks (fraction of a second)
• The rate of reaction starts off fast when there are lots of reactants, then slows down as the reaction progresses and the reactants get used up
• Plotting graphs to show rate of reaction

  1. Time on x-axis

  2. Mass of reactants remaining or volume of product produced on y-axis
• The graph of mass of reactants remaining starts high and decreases rapidly at first, then slows down as the reaction progresses
• The graph of volume of product produced starts at zero, then increases rapidly at first, then slows down as the reaction progresses
• Units for rate of reaction

  Grams/seconds, centimetres cubed/seconds, moles/minutes, decimetres cubed/seconds
• The units used for the rate of reaction depend on what is being measured (reactants or products) and the time period
• Measuring the mean rate of a reaction and the rate of reaction at a specific time

  1. Calculate the rate of reaction by dividing either the amount of reactants used or the amount of product formed over the time taken for that change to occur

  2. Plot the volume of hydrogen produced during the reaction on a graph with time on the x-axis and volume on the y-axis

  3. To calculate the mean rate of reaction over a period, find the amount of hydrogen produced in that period and divide by the time

  4. To calculate the rate of reaction at a specific time, draw a tangent to the graph at that time and find the gradient of the tangent
• The rate of reaction is fastest at the beginning and slows down as the reaction progresses
• The graph plateaus once all of the magnesium has been used up
• Mean rate of reaction

  The average rate of reaction over a certain period
• Rate of reaction at a specific time


  The gradient of the tangent to the graph at that time
• Examiners allow a small range of values for tangent-based calculations as they are judged by eye
• Measuring rate from a graph of magnesium remaining over time

  1. Find the change in magnesium and change in time at the desired point, divide to get the rate

  2. Draw a tangent to the graph at the desired point and use the gradient to calculate the rate