Kinetics

1. Kinetics

2. Rates of reaction • Some reactions proceed very fast, whereas others can proceed so slowly that it can take millions of years before any appreciable amount of products are formed. • An example of a very fast reaction that we have come to rely on is the rapid inflation of airbags used in automobiles. • Typical reactants are a mixture of sodium azide (NaN3), potassium nitrate (KNO3) and silicon dioxide. • The sodium azide and the potassium nitrate react to produce inert nitrogen gas which inflates the bag: • Write a balanced symbol equation to show this reaction.

3. An airbag inflating 10NaN3(s) + 2KNO3(s) → K2O(s) + 5Na2O(s) + 16N2(g) • The silicon dioxide is present to convert the oxides of potassium and sodium that are produced into harmless silicates. • When a crash occurs, a sensor in the car’s airbag will trigger the reaction if the driver (and passengers) continues to move forward with a momentum greater than the equivalent of hitting a brick way at 16-24 km h-1 • The nitrogen has to be produced to inflate the bag more quickly than the time taken for the driver to hit the steering wheel. • The whole reaction from triggering to complete inflation of the bag is complete in less than one 25th of a second.

4. Other reactions • Other reactions are much slower. • It typically takes several days for equilibrium to be reached for the reaction between ethanoic acid and ethanol in the presence of a catalyst, H+(aq), to form ethyl ethanoate and water: CH3COOH(l) + C2H5OH(l) → CH3COOC2H5(l) + H2O(l) • Many famous paintings have needed cleaning and restoring after several hundred years, because of the slow reaction of the pigments and varnish both with light and with pollutants in the air.

5. What is rate of reaction? • It is necessary to understand what is meant by rate of reaction. • In any reaction the concentration of reactants decreases and the concentration of products increases over time until the reaction reaches equilibrium or is complete. • The rate of a reaction can be defined either as the decrease in concentration of one of the reactants per unit time or the increase in concentration of one of the products per unit time. • The units of rate of reaction are normally moles per cubic decimetre per second (mol dm-3 s-1)

6. Measuring rate of reaction • The change in concentration can be measured by using any property that changes during the conversion of reactants to products. These may include; • Mass or volume changes for gaseous reactions • Change in pH for reactions involving acids and bases • Change in conductivity measurements for reactions involving electrolytes • Use of a spectrometer or colorimeter for reactions involving colour changes.

7. Graphing results • To obtain the rate of reaction at any time during the reaction, a graph of concentration (or some property directly related to concentration) against time can be plotted. • The rate of reaction is given by the gradient of the graph at the specified time. • Usually the rate decreases with time as the concentration of the reactants decreases.

8. Collision Theory • For a reaction to take place between two reactant particles, three conditions are necessary; • The two particles (atoms, ions, radicals or molecules) must come into physical contact (collide) with each other. • They must collide in the correct orientation, so that the reactive parts of the two particles come into contact with each other. • The reactant molecules must collide with sufficient kinetic energy to bring about the reaction. • These three conditions are collectively described as the collision theory.

9. Activation energy • The minimum amount of energy required to bring about the reaction is known as the activation energy. • This is the minimum amount of kinetic energy that must be given to the reactants before they will actually react. • If two reactants molecules collide without this minimum amount of kinetic energy, they will simply bounce off each other without reacting. • In simple terms, what the activation energy provides is the necessary energy to break bonds in the reactant particles before energy can be released as new bonds are formed in the products. • The enthalpy change for the reaction, ΔH, depends only on the initial and final states (as does the Gibbs free energy, ΔG,) and so it cannot give any indication about the rate of the reaction as it does not involve the activation energy.

10. Factors affecting the rate of reaction • Any factor that can alter any of the three conditions of collision theory will have an effect on the rate of reaction. • To understand collision theory we need to look at the movement of particles in the different states of matter. • Solid • Liquid • gas

11. Solids • In a solid the particles are held in fixed positions. • They vibrate about these fixed positions, but have no translational velocity, so that it is hard for two solids to react unless they are in powder form.

12. Liquids • In the liquid or aqueous state the particles are free to move, although there are still attractive forces between the particles. • The particles in a liquid are not travelling at the same velocity. • Some will be moving fast; others will be moving more slowly. • The absolute temperature of the liquid in kelvin is directly proportional to the average kinetic energy of all the particles. • As heat is supplied to the liquid, the average velocity of the particles will increase, and hence the temperature will increase

13. Gases • Once the liquid has turned into a gas there are virtually no attractive forces between the particles, and their average velocity is much greater than in the liquid state. • A graph showing the distribution of kinetic energy amongst the particles for a fixed amount of gas has a characteristic shape and is known as a Maxwell-Boltzmann distribution.

14. Maxwell-Boltzmann distribution • This graph has several key features. • The area under the graph is directly related to the number of particles present, and so for a fixed mass of gas this area must be constant. • At a particular temperature most of the particles possess a kinetic energy close to the average value. However, a few are moving much faster and therefore possess a much higher kinetic energy. At the average value the area under the graph is equal on both sides, because the same number of particles will have a lower kinetic energy as have a higher kinetic energy.

15. Factors that will increase the rate of a chemical reaction • There are four main factors that will affect the rate of a reaction • Changing the concentration of the reactants or the pressure of a gaseous reaction • Changing the temperature of the reaction • Changing the particle size of solid reactants • Adding a suitable catalyst. • Look up each of these factors on pages 114 – 115 in your chemistry companion and write a short paragraph explain using the collision theory how each of these factors affects the rate.

16. Increasing concentration of reactants or pressure of a gaseous reaction • As the quantity of particles in a given volume increases, the number of collisions between the reactant particles will increase. • The ratio of successful collisions to unsuccessful collisions will remain the same, but as there are more collisions the number of successful collisions (i.e. Those with the necessary activation energy) increases and therefore the rate increases.

17. Increasing the temperature of the reaction • At a higher temperature the reactant particles are moving faster, so there will be more collisions, which will make a relatively small contribution towards increasing the rate. • However, much more important is the fact that many more reactant particles will now possess the necessary activation energy; therefore the ratio of successful collisions to unsuccessful collisions will increase, so that many more of the collisions will be successful

18. Decreasing the particle size of solid reactants • When a solid reacts with a liquid, only the surface particles of the solid can come into contact with the liquid reactant particles. • Breaking the solid into smaller pieces greatly increases the surface area, and hence more solid particles are available to react. • If the solid is in powder form the surface area is considerably increased, and the rate of reaction will be much faster.