Investigating The Rate of a Reaction (Cambridge (CIE) O Level Chemistry): Revision Note

Investigating The Rate of a Reaction

• To measure the rate of a reaction, we need to be able to measure either how quickly the reactants are used up or how quickly the products are formed

• The method used for measuring depends on the substances involved

• There are a number of ways to measure a reaction rate in the lab; they all depend on some property that changes during the course of the reaction

• That property is taken to be proportional to the concentration of the reactant or product, e.g., colour, mass, volume

  • Fast reactions can be easier to measure once the reaction is over, by averaging a single collected measurement over the whole reaction

  • Slower reactions can be measured as they proceed, which generates more data

• Three commonly used techniques are:

  • measuring mass loss on a balance

  • measuring the volume of a gas produced

  • measuring a reaction where there is a colour change at the end of the reaction

Investigating the effect of surface area on the rate of reaction 

Diagram showing the process of downwards displacement to investigate the effect of the surface area of a solid on the rate of reaction

 Method

• Add dilute hydrochloric acid into a conical flask

• Use a delivery tube to connect this flask to a measuring cylinder upside down in a bucket of water (downwards displacement)

• Add calcium carbonate chips into the conical flask and quickly put the bung back into the flask

• Measure the volume of gas produced in a fixed time using the measuring cylinder

• Repeat with different sizes of calcium carbonate chips (lumps, crushed and powdered)

Result

• Smaller sizes of chips cause an increase in the surface area of the solid, so the rate of reaction will increase

• This is because more surface area of the particles will be exposed to the other reactant so there will be more frequent and successful collisions, increasing the rate of reaction

Effect of concentration of a solution on the rate of reaction

Diagram showing the apparatus needed to investigate the effect of concentration on the rate of reaction

 Method

• Measure 50 cm³ of sodium thiosulfate solution into a flask

• Measure 5 cm³ of dilute hydrochloric acid into a measuring cylinder

• Draw a cross on a piece of paper and put it underneath the flask

• Add the acid into the flask and immediately start the stopwatch

• Look down at the cross from above and stop the stopwatch when the cross can no longer be seen

• Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)

Result

• With an increase in the concentration of a solution, the rate of reaction will increase

• This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction

Effect of temperature on the rate of reaction

Diagram showing the apparatus needed to investigate the effect of temperature on the rate of reaction

Method

• Dilute hydrochloric acid is heated to a set temperature using a water bath

• Add the dilute hydrochloric acid into a conical flask

• Add a strip of magnesium and start the stopwatch

• Stop the time when the magnesium fully reacts and disappears

• Repeat at different temperatures and compare results

Result

• With an increase in the temperature, the rate of reaction will increase

• This is because the particles move faster and a greater proportion of them have energy greater than the activation energy, so collisions are both more frequent and more often successful, increasing the rate of reaction

Effect of a catalyst on the rate of reaction

Diagram showing the apparatus needed to investigate the effect of a catalyst on the rate of reaction

Method

• Add hydrogen peroxide into a conical flask

• Use a delivery tube to connect this flask to a measuring cylinder upside down in a tub of water (downwards displacement)

• Add the catalyst manganese(IV) oxide into the conical flask and quickly place the bung into the flask

• Measure the volume of gas produced in a fixed time using the measuring cylinder

• Repeat experiment without the catalyst of manganese(IV) oxide and compare results

Result

• Using a catalyst will increase the rate of reaction

• The catalyst will provide an alternative pathway requiring lower activation energy so more colliding particles will have the necessary activation energy to react

• This will allow more frequent and successful collisions, increasing the rate of reaction

Monitoring changes in mass

• Many reactions involve the production of a gas which will be released during the reaction

• The gas can be collected and the volume of gas monitored as per some methods above

• Alternatively, the reaction can be performed in an open flask on a balance to measure the loss in mass of reactant

• 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)

Diagram showing the set-up for measuring the rate of reaction by loss in mass

• 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

Evaluating Investigations of Rates of Reactions

• When investigating rates of reaction, there are a number of different methods that can be used to carry out the same investigation

• Evaluating what is the best method to use is part of good experimental planning and design

• This means appreciating some of the advantages and disadvantages of the methods available

Table showing some Examples of Advantages and Disadvantages of Methods of Investigating Rates of Reaction