UNIT General principlesand of chemistry I UNIT 44 Rates Equilibria Further Organic Chemistry Running head Rates equilibria and further organic chemistry 1 1 How fast Rates of chemical change Techniques to measure rate of reaction 4 3b The rate of chemical reactions and the factors affecting the rate of chemical change have been studied closely for many years Why do chemists study reaction rates kinetics The fact that reactions can occur at very different rates is intriguing and chemists want to understand what is going on Chemists need to understand how to change the rate of a reaction In industry it is essential to understand rates of reaction to help work out the economics of a manufacturing process Reaction rates can provide evidence of the mechanism of a chemical reaction the individual steps by which a reaction takes place The ability to manipulate the rate of a chemical reaction is as vital in the school lab as it is in the chemical industry As you saw in your AS level Chemistry book pages 196 7 concentration pressure temperature the surface area of the reacting substances and catalysts all affect the rate at which chemical reactions happen Using the chemistry of fireworks as an example it is important that a firework fuse burns for long enough to allow the person who lit the firework to get out of the way However it mustn t burn too long or people are tempted to go back and make sure it hasn t gone out The Health and Safety Executive has fixed the minimum burn time at 5 seconds and the maximum at 15 seconds The blue touch paper which acts as a fuse is paper impregnated with potassium nitrate it will burn even in wet windy conditions The speed at which it burns is controlled by the potassium nitrate content of the paper the higher the concentration the faster it burns To understand and control the rate of chemical reactions chemists need to be able to measure how fast reactions occur You can calculate the rate of a reaction by measuring the change in concentration of one of the reactants or products and the time taken rate of reaction c or rate t fig 1 1 1 Controlling the rate at which the fuse on a firework burns is crucial for everyone s safety change in concentration change in time A number of different methods can be used to collect the data we need to measure the rate of a reaction They all investigate changes in concentration either directly or indirectly As you saw in your AS course the techniques largely depend on measuring either how quickly the reactants are used up or how quickly the products are formed You will be considering and using many of these methods 1 10 M02 CHEM SB A2 6058 C1 1 indd 10 14 5 09 09 22 02 UNIT 4 General principles of chemistry I Rates equilibria and further organic chemistry a measuring cylinder gas produced Measuring the volume of a gas produced In a reaction in which a gas is given off the progress of the reaction can be measured by monitoring the rate at which the gaseous product is given off The technique you use will depend on the level of accuracy demanded the simplest is simply to count the number of bubbles produced in a certain time reactants b gas syringe gas produced reactants fig 1 1 2 Using a gas syringe is a method of measuring the volume of gas produced during a reaction quite accurately For more accurate results you can measure the volume of gas given off at regular time intervals collecting the gas in a measuring cylinder by water displacement particularly useful for larger volumes of gas or in a gas syringe fig 1 1 2 You plot a graph of the total volume of gas produced against time to show how the rate of reaction changes By changing the reaction conditions for example the concentrations of the reactants or the temperature you can investigate how these factors affect reaction rates Measuring the change in mass of a reaction mixture This technique also depends on having gaseous substances as products of the reaction You measure the decrease in mass of the total reacting mixture as the reaction proceeds fig 1 1 3 It is relatively easy to carry out in the laboratory needing only a sensitive balance but there are many opportunities for experimental error to creep in unless great care is taken to avoid splashes etc Monitoring a colour change colorimetry fig 1 1 3 Measuring changes in mass is a very useful technique for investigating the effect of surface area on reaction rate fig 1 1 4 In reactions with a colour change colorimetry is an excellent way of measuring the rate of the reaction The techniques involved in using colour changes to measure reaction rate range from simple observation to the use of a colorimeter fig 1 1 4 The appearance of a coloured product or the loss of a coloured reactant is used By changing the reaction conditions and measuring the effect on the appearance or loss of colour the impact of the change on the rate of the reaction can be measured The iodine clock an experimental technique you will look at later is an example where you use your eyes iodine is formed in a reaction and this reacts with starch indicator to form a noticeable blue black colour In a similar investigation the reaction of potassium iodide with potassium peroxodisulfate is carried out in a colorimeter As the oxidation takes place and iodine is produced the brown colour of the iodine is monitored extremely precisely in the colorimeter The change detected by the colorimeter is far more subtle than the human eye could observe 11 M02 CHEM SB A2 6058 C1 1 indd 11 14 5 09 09 22 36 UNIT 4 General principles of chemistry I Rates equilibria and further organic chemistry Titrimetric analysis This involves removing small portions aliquots of the reacting mixture at regular intervals These aliquots are usually added to another reagent which immediately stops or quenches the reaction Alternatively the reaction is slowed by immersing the portion in an ice bath so that there are no further changes to the concentrations in the reacting mixture until further analysis can be carried out The quenched aliquots are then titrated to find the concentrations of known compounds in them fig 1 1 5 This technique gives a detailed picture of changes in rate during a reaction also enabling detailed comparisons when investigating the effect of different factors on the rate of the reaction In a reaction in which acid is being used up titration could be carried out with a standard solution of sodium hydroxide A similar technique of titrimetric analysis is useful