rates of reaction

• what are the 3 different ways to measure rate of reaction?

  • precipitation and colour change
  • change in mass (usually gas given off)
  • volume of gas given off
• what is the formula for rate of reaction

  rate of reaction = amount of reactant used or amount of product formed ÷ time
• summarise precipitation and colour change

  • you can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution (it becomes opaque)
  • you can observe a mark through the solution and measure how long it takes for it to disappear - the faster the mark disappears, the quicker the reaction
  • if the reactants are coloured and the products are colourless (or vice versa) you can time how long it takes for the solution to lose (or gain) its colour.
  • the results are very subjective - different people might not agree over the exact point when the mark disappears or the solution changes colour. also, if you use this method, you can't plot a rate of reaction graph from the results
• summarise change in mass (usually 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
  • if you take measurements at regular intervals, you can plot a rate of reaction graph and find the rate quite easily
  • this is the most accurate of the 3 methods described because the mass balance is very accurate, but it has the disadvantage of releasing the gas straight into the room.
• the volume of gas given off

  • this involves the use a gas syringe to measure the volume of gas given off
  • the more gas given off during a given time interval, the faster the reaction
  • gas syringes usually give volumes accurate to the nearest cm cubed, so they're quite accurate. you can take measurements at regular intervals and plot a rate of reaction graph using this method too. you have to be quite careful though - if the reaction is too vigorous, you can easily blow the plunger out the end of the syringe.
• how would you perform the volume of gas experiment?

  1. start by adding a set volume of dilute hydrochloric acid to a conical flask
  2. now add some magnesium ribbon to the acid and quickly attach an empty gas syringe to the flask with a bung and delivery tube
  3. start the stopwatch. take readings of the volume of gas, which in this case is hydrogen, in the syringe at regular intervals
  4. plot the results in a table
  5. plot a graph with time on the x axis and volume of gas produced on the y axis
• how can the experiment done to investigate the effect of using different acid concentrations?

  1. repeat the experiment with a number of different concentrations of acid. variables such as the amount of magnesium ribbon and the volume of acid used should be kept the same each time - only change the acid's concentration. this makes the experiment a fair test
  2. plot all the results on the same graph to see how the concentration of acid affects the rate of reaction
  3. the 3 graphs show that a higher concentration of acid gives a faster rate of reaction.
• how can the experiment be done with a precipitate producing reaction?

  1. sodium thiosulfate and hydrochloric acid are both clear solutions. they react together to form a yellow precipitate of sulfur
  2. start by adding a set volume of dilute sodium thiosulfate to a conical flask
  3. place the flask on a piece of paper with a black cross drawn on it
  4. add some dilute hydrochloric acid to the flask and start the stopwatch
  5. watch the black cross disappear through the cloudy sulfur and time how long it takes to go
• how can the effect of the concentration of acid on the rate of reaction be investigated

  1. the reaction can be repeated with solutions of either reactant of different concentrations (only change the concentration of one reactant at a time though). the depth of the liquid must be kept the same each time
  2. the higher the concentration, the the quicker the reaction and therefore the less time it takes for the mark to disappear.
  3. only readings can be taken from this reaction, no graphs,