The Rate and Extent of Chemical Change

• Rate of reaction


  Amount (e.g. grams, cm3) of reactant used or product formed / time
• Rate of reaction (mol/s)


  Moles of reactant used or product formed / time
• Units for rate of reaction

  • g/s
  • cm3/s
  • mol/s
• Common ways of measuring rate of reaction

  • Loss in mass of reactants
  • Volume of gas produced
  • Time for a solution to become opaque
• Measuring rate by monitoring mass loss

  Place the reaction flask on a balance. In these reactions (e.g. metal carbonate + acid) a gas is given off, so record the decrease in mass in time intervals (note hydrogen is too light). Plot a graph of mass vs time.
• Measuring rate by monitoring volume of gas

  Connect a gas syringe to a reaction flask and measure the volume of a gas formed in time intervals. Plot a graph of volume vs time.
• Measuring rate by monitoring disappearance of a cross


  Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross.
• Finding rate of reaction at time t from graph of amount of reactant vs time


  Pick a point corresponding to the time t, and find the tangent to the curve at this point. The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate. Gradient of tangent can be expressed in change in y values over change in x values.
• Factors affecting rate of reaction

  • Concentration of reactants
  • Pressure of gases (volume)
  • Surface area
  • Temperature
  • Catalysts
• Collision theory


  Chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy)
• Increasing temperature


  Faster reaction
• Increasing concentration


  Faster reaction
• Increasing pressure of gases

  Faster reaction
• Increasing surface area


  Faster reaction
• Catalyst


  Changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
• Enzyme


  A molecule that acts as a catalyst in a biological system.
• Reversible reaction


  A reaction where the products can react backwards to produce the original reactants.
• Dynamic equilibrium


  In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.
• Le Chatelier's Principle


  If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.