Measuring rates of reaction

• You can calculate the rates of reaction from experimental results
• The rate of reaction can be measured/observed either by how quickly the reactants are used up or how quickly the products are formed
• Mean rate of reaction= amount of reactant used or amount of product formed / time
• When the product or reactant is a gas you usually measure the amount in cm cubed, if its a solid you use grams and time is usually measured in seconds
• Units for rate may be in cm cubed/s or g/s
• Rate can also be measured in moles mol/s
• Three different ways of measuring the rate of reaction: Precipitation and colour change, change in mass (usually gas given off), The volume of gas given off
• Precipitation and colour change can be used to record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution
• Precipitation

  You can observe a mark through the solution and measure how long it takes for it to disappear- the faster the mark takes to disappear, the quicker the reaction
• Colour change

  If the reactants are coloured (or vice versa) you can time how long it takes for the solution to lose (or gain) its colour
• CONS of precipitation and colour change?

  The results are very subjective, not everybody will agree over the exact point that the mark disappears or the solution changes colour

  You can't plot a rate of reaction graph from the results
• Change in mass (usually a gas given off)

  • Measuring the speed of a reaction that produces a gas can be carried out using a mass balance
  • As the gas is released, the mass disappearing is measured on the balance
  • The quicker the reading on the balance drops, the faster the reaction
• PROS of change in mass

  • if measurements are taken at regular intervals you can plot a rate of reaction graph
  • Most accurate of the 3 methods as the mass balance is very accurate
• CONS of change in mass

  • Releasing a gas straight into the room
• The volume of mass given off

  • Involves use of gas syringe to measure the volume of gas given off
  • The more gas given off during a given time interval, the faster the reaction
• PROS of volume of gas given off

  • Gas syringes usually gives volumes accurate to the nearest cm cubed, so quite accurate
  • You can take measurements at regular intervals and plot a rate of reaction graph too
• CONS of Volume of gas given off

  • Have to be careful- if reaction is too vigorous, you can easily blow the plunger out of the end of the syringe