Topic #23: Chemical Kinetics

1. Topic #23: Chemical Kinetics • EQ: How do the factors that affect the rate of reaction slow down or speed up a chemical reaction?

2. Chemical Kinetics • Studies the rate at which a chemical process occurs. • Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (how the reaction occurs) 2

3. Rate of Reaction Some reactions occur rapidly Inflation of an airbag Explosion of nitrogen triiodide Some reactions occur slowly Reaction of pigments in paintings with light & pollutants Tarnishing of silver/Iron rusting 3

4. Rate of Reaction An increase in concentration of one of the products per unit time or as the decrease in concentration of one of the reactants per unit time. OR How fast reactants are being converted to products during a chemical reaction – either the rate of formation of a product of the rate of consumption of a reactant may be used (they are related to each other by dividing the corresponding coeffficient in the stoichiometric equation) Units of rate of reaction: moles per dm3 per second We will use mol dm-3 s-1 4

5. Rate of Reaction Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time. [A] vst or [B] vst 5

6. Kinetic Theory • Tiny particles in all forms of matter are in constant motion. • The energy an object has because of motion is called kinetic energy. • The particles in any collection of atoms or molecules at a given temperature have a wide range of kinetic energies, from very low to very high. Overall, there is an average kinetic energy. 6

7. The Maxwell-Boltzmann Distribution • In a sample gas or liquid, the molecules are flying about and bumping into each other and the walls all the time. • When they do this, no energy is lost from the system - they’re elastic collisions. • So in a sample of gas or liquid at any given time, the energy that molecules have is going to be spread out, with some particles having more or less energy than others. • If you were to draw a graph of how the energy was spread over different molecules, you’d get the Maxwell-Boltzmann distribution!

9. This graph is important if you want to get those easy marks. You have to remember that: • the area under the curve = the total molecules in the sample, and doesn’t change • there are no molecules with no energy • very few molecules have high energies, but • there is no maximum energy for a molecule, and • the most probable energy value is where the curve is at its highest, as shown.

10. If you increase the temperature of the sample, the distribution changes as shown below. It becomes stretched out and has a lower peak, but the area under the curve remains the same. The reverse happens when you cool down a sample.

11. Activation Energy (Ea) • Activation Energy: The minimum amount of energy required for reaction: to happen is called the activation energy, Ea. • Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.

12. Activation energy is the minimum amount of kinetic energy that must be given to the reactants before they will react. (a) Activation energy (Ea) of forward reaction (b) Activation energy (Ea) of reverse reaction (c) Heat of reaction (ΔH) What is the heat of reaction for each of the graphs? Which is exothermic and which is endothermic?

13. In a chemical reaction, bonds are broken and new bonds are formed. • Molecules can only react if they collide with each other with enough energy. (Ea) 13

14. Collision theory: Furthermore, molecules must collide with the correct orientation and with enough energy to cause bond to break and reform again. 14

15. COLLISION THEORY: Know this!!! • In order for a reaction to occur, reactants (atoms, ions, radicals or molecules) must collide • Molecules must collide in the right orientation so that the reactive parts of each of the two particles come into contact with each other • Reactants must collide with sufficient kinetic energy to bring about the reaction (at least the activation energy). • Not all collisions result in a reaction. • The likelihood of these happening determines the rate.

16. Changing the rate of reactions Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors affect the rate of reactions? • increased temperature • increased concentration of dissolved reactants, and increased pressure of gaseous reactants • increased surface area of solid reactants.(decrease particle size) • use of a suitable catalyst.

17. Effect of temperature on rate Increasing the temperature will make the particles move faster, so there will be more collisions. At a higher temperature, many more particles will have the necessary activation energy. The ratio of successful collision to unsuccessful collisions will increase. Which one will contribute more towards increasing the rate of reaction?

18. Temperature: greater T = more collisions with the required Ea • Increasing the temperature increases the frequency of collisions but, more importantly, the proportion of molecules with E ≥ Ea increases

19. Effect of concentration on rate of reaction lower concentration higher concentration The higher the concentration of a dissolved reactant, the faster the rate of a reaction. Why does increased concentration increase the rate of reaction? At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react. The ratio of successful collisions to unsuccessful collisions will stay the same, but there will be more successful collisions.

20. Effect of pressure on rate of reaction lower pressure higher pressure Why does increasing the pressure of gaseous reactants increase the rate of reaction? As the pressure increases, the space in which the gas particles are moving becomes smaller. The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.

21. Concentration: higher conc = more particles in given volume = more collisions per unit of time

22. Effect of surface area on rate of reaction low surface area high surface area Any reaction involving a solid can only take place at the surface of the solid. If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction? This means that there is an increased area for the reactant particles to collide with. The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.

23. Particle size: greater surface area = greater # of collisions per unit of time

24. What are catalysts? Ea withoutcatalyst energy (kJ) Ea withcatalyst reaction (time) Catalysts are substances that change the rate of a reaction without being used up in the reaction. Catalysts never produce more product – they just produce the same amount more quickly. Different catalysts work in different ways, but most provide an alternative path with lower activation energy (Ea).

25. Catalysts:provide alternative mechanism w/ lower Ea = more collisions with sufficient Ea **You must not say “the number of collisions” only... You must say “number of collisions per unit time” or “frequency”

26. In order to increase reaction rates…

27. Decreasing the particle size of solid reactants increases the reaction rate. • Increasing the temperature increases the reaction rate. • Only the surface particles of a solid react. Breaking the solid into smaller pieces increases the surface area so more particles available to react. • At higher temperatures the particles move faster and have more energy. Faster movement means greater frequency of collisions, more energy means more collisions have the activation energy required for reacting. 27

28. Increasing the concentration of the reactants increase the reaction rate • Increasing the pressure of a gaseous reaction increases the reaction rate. • More particles present means greater frequency of collisions. • Increasing pressure translates to increased concentration by either reducing the volume or increasing the number of particles. Higher concentration means greater frequency of collisions. 28