The Rate & Extent of Chemical Change

• 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/second
  • Centimetres cubed/second
  • Moles/minute
  • Decimetres cubed/second
• Reversible reaction


  A reaction with a double arrow in the middle, indicating it can react in both forward and backward directions
• Reversible reaction


  1. Forward reaction

  2. Backward reaction
• Equilibrium


  The point where the forward and backward reactions are occurring at the same rate, so there is no overall change in concentrations
• Position of equilibrium


  The relative concentrations of reactants and products at equilibrium, which can shift left or right depending on conditions
• Adding heat to the reaction

  Encourages the forward reaction, shifting the position of equilibrium to the right
• Cooling the reaction


  Pushes the position of equilibrium back to the left
• Reversible reactions can only reach equilibrium in a closed system</b>
• Exothermic


  A reaction that releases heat energy
• Endothermic

  A reaction that absorbs heat energy
• Reversible reactions are always exothermic in one direction and endothermic in the other
• Anhydrous

  Containing no water
• Thermal decomposition of hydrated copper sulfate

  1. Heating drives the forward endothermic reaction, producing anhydrous copper sulfate

  2. Adding water drives the backward exothermic reaction, reforming the hydrated copper sulfate
• Le Chatelier's Principle

  Principle about the position of equilibrium during a reversible reaction and how it's affected by temperature, pressure, and concentration
• If you change the conditions of a reversible reaction

  The position of equilibrium will shift to try and counteract that change