Measuring Rates (OCR GCSE Chemistry A (Gateway))

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, M_r 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