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
- 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)
Alternative gas collection set up
- 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
Graph of gas volume evolved 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
- 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
Examiner Tip
If you are asked to write methods for investigating the effect of a factor, e.g temperature, remember to include control variables in your answer.