Measuring Rates

• Measuring Rates:

  • Different reactions take place at different rates
  • Some are extremely slow e.g. rusting and others are extremely fast e.g. explosives
  • Rates of reaction can be measured either by how fast a reactant is used up or by how fast the product is made
  • Rate is concerned with amounts of substances and time and can be calculated using the formula
• A formula triangle for calculating the rate of reaction:

  
• • In order to provide sufficient data to establish a conclusion several measurements need to be made during the reaction
  • The product is usually the one that is measured as it is usually easier to measure a product forming than it is a reactant disappearing
  • The quantity to be measured depends on the reaction and may be in grams for mass or cm3 or dm3 for volume if the product is a gas
• • The units of the rate of reaction would therefore be g s-1 or cm3 / dm3 s-1
  • Time is usually in seconds as many reactions studied in the lab are quite quick
  • There are several practical methods that can be done to measure the amount of product formed 
• Measuring Mass:

  • If one of the products is a gas which is given off, then the reaction can be performed in an open flask on a balance to measure the loss in mass of reactant
  • For example, the reaction of calcium carbonate with hydrochloric acid produces CO2
  • The mass is measured every few seconds and change in mass over time is plotted as the CO2 escape
• Measuring Mass:

  • Cotton wool is usually placed in the mouth of the flask which allows gas out but prevents any materials from being ejected from the flask (if the reaction is vigorous)
  • This method is not suitable for hydrogen and other gases with a small relative formula mass, Mr as the loss in mass may be too small to measure
• Diagram showing the set-up for measuring the rate of reaction by loss in mass:

  
• • The mass loss provides a measure of the amount of reactant, so the graph is the same as a graph of amount of reactant against time. Mass loss of a product against time.

  
• Measuring the Volume of Gas:

  • When a gas is produced in a reaction, it can be trapped and its volume measured over time
  • This can be used to measure the rate of reaction.
  • For example, the reaction of magnesium with hydrochloric acid produces hydrogen
• Collecting gases experimental set up:

  
• • An alternative gas collection set up involves collecting a gas through water using an inverted measuring cylinder ( as long as the gas is not water soluble)

  
• • The volume can be measured every few seconds and plotted to show how the volume of gas varies with time
  • The volume provides a measure of the amount of product, so the graph is a graph of amount of product against time

  
• Measuring the Rate a Precipitate Forms

  • Precipitation reactions form a solid precipitate when two clear solutions are mixed together
  • The precipitate clouds the reaction mixture so if the flask is placed over a piece of paper with a cross on it, the time it takes for the cross to disappear from view (due to the formation of the precipitate) can be measured
• Measuring the Rate a Precipitate Forms:

  • An example of this is the reaction between sodium thiosulfate and hydrochloric acid which slowly produces a yellow precipitate of sulfur that obscures a cross when viewed through the solution:

  Na2S2O3 (aq) + 2HCl (aq)   →  2NaCl aq) + SO2 (g) + H2O (l) + S(s)
• Diagram showing the study of a rate of precipitation:

  
• • This method is susceptible to error though as they are subjective, given that different people may not agree on the exact moment that the cross disappears
  • Another disadvantage is that only one data point is produced per experiment, so a rate of reaction graph cannot be plotted