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© DGMcC Chemical Equilibrium & Rate of Reaction Belfast Lecture.

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1 © DGMcC Chemical Equilibrium & Rate of Reaction Belfast Lecture

2 © DGMcC To understand how reaction conditions affect reactions you should understand: • CHEMICAL EQUILIBRIUM - reversible processes and equilibrium - at equilibrium the rates of the forward and reverse reactions are equal - equilibrium constant - qualitative effect of changes in reactant and product concentration on the position of equilibrium - qualitative effects of changing the temperature and the ……...total pressure to a reaction vessel (including those used ……...on an industrial scale) on the position of equilibrium;

3 © DGMcC Mg + 2HCl  MgCl 2 + H 2 Irreversible reactions Most Chemical reactions are considered irreversible in that products are not readily changed back into reactants. When wood burns it is pretty difficult to un-burn it back into wood again! When magnesium reacts with acid it is not easy to unreact it and get back the magnesium. Wood reacting with oxygen

4 © DGMcC Cotton Wool Soaked in Conc. Ammonia Cotton Wool Soaked in Conc. Hydrochloric Acid NH 3(g) HCl (g) Ring of “White Smoke” NH 3(g) + HCl (g)  NH 4 Cl (s) Reversible Reactions

5 © DGMcC Moist pH Paper Heat Strongly Solid Ammonium chloride NH 4 Cl (s)  NH 3(g) + HCl (g) Reversible Reactions

6 © DGMcC NH 4 Cl (s)  NH 3(g) + HCl (g) NH 3(g) + HCl (g)  NH 4 Cl (s) Reversible Reactions NH 3(g) + HCl (g) NH 4 Cl (s)

7 © DGMcC Reversible Reactions CuSO 4.5H 2 O  CuSO 4 + 5H 2 O CuSO 4.5H 2 O CuSO 4 + 5H 2 O CuSO 4 + 5H 2 O  CuSO 4.5H 2 O

8 © DGMcC Reversible reactions are not uncommon A+B ⇄ C+D On mixing A & B there will be no C & D ∴ the rate of the forward reaction is high and the rate of the reverse reaction will be zero. As the reaction proceeds there is less A & B ∴ the rate of the forward reaction decreases. As C & D are formed in increasing amounts what will happen to the rate of the reverse reaction ? It will increase. Eventually what will happen to the rates? They will become equal.

9 © DGMcC

10 When this happens we say the system is in chemical equilibrium. This is a dynamic equilibrium. The reaction starts then seems to stop. The forward and reverse reactions are both proceeding at the same rate. At equilibrium what happens to the amount of A? For every molecule of A reacting there is one formed ∴ the amount stays constant. This is true of the other reactants and products also. A+B ⇄ C+D

11 © DGMcC It is rather like the situation where a person is running on a treadmill. Neither have stopped but the person could remain in the same place for ever! The symbol  is used to mean dynamic equilibrium. Dynamic equilibrium Equilibrium – Equilibrium – because of the unchanging amounts Dynamic – Dynamic – because reaction is still occurring The person stays in the same place!

12 © DGMcC Which of these is true about a dynamic equilibrium? A. All the product molecules are used up. B. All the reactants molecules are used up. C. The reaction has stopped both in the forward and backward directions. D. The composition of the reaction mixture remains the same.

13 © DGMcC Which of these is a reversible process? A. Reacting acid with alkali. B. Heating hydrated (blue) copper sulphate. C. Burning coal. D. Dissolving magnesium in acid.

14 © DGMcC To understand how reaction conditions affect reactions you should understand: • CHEMICAL EQUILIBRIUM - reversible processes and equilibrium - at equilibrium the rates of the forward and reverse reactions are equal - equilibrium constant - qualitative effect of changes in reactant and product concentration on the position of equilibrium - qualitative effects of changing the temperature and the ……...total pressure to a reaction vessel (including those used ……...on an industrial scale) on the position of equilibrium;

15 © DGMcC To understand how reaction conditions affect reactions you should understand: • CHEMICAL EQUILIBRIUM - reversible processes and equilibrium - at equilibrium the rates of the forward and reverse reactions are equal - equilibrium constant - qualitative effect of changes in reactant and product concentration on the position of equilibrium - qualitative effects of changing the temperature and the ……...total pressure to a reaction vessel (including those used ……...on an industrial scale) on the position of equilibrium;

16 © DGMcC Equilibrium Constants A+B ⇄ C+D The concentration of each substance at equilibrium will therefore remain constant We can write an equilibrium constant for this reaction in terms of the concentrations of the reactants and products K c =[C] x [D] [A] x [B] where [ ] = equilibrium concentration in moles /dm 3 Remember it is always [products] [reactants]

17 © DGMcC A+2B ⇄ 3C+4D K c =[C] 3 x [D] 4 [A] x [B] 2 where [ ] = equilibrium concentration in moles /dm 3 K p =P 3 C x P 4 D P A x P 2 B where P is the equilibrium partial pressure of a gas.

18 © DGMcC If K c or K p are small the amount of products will be small and reactants large, ∴ we say the equilibrium position is over to the left hand side (LHS). (The partial pressure of a gas in a mixture is the pressure that that gas would exert if it alone occupied the total volume of the mixture. The sum of the partial pressures of the gases in a mixture is equal to the total pressure of the mixture.) If K c or K p are large the amount of products will be large and reactants small, ∴ we say the equilibrium position is over to the right hand side (RHS). The equilibrium constants K c & K p are numerical constants for a given reaction at a given temperature.

19 © DGMcC N.B. Only dissolved substances appear in K c no gases or solids. Likewise only gases appear in K p no solids or solutions. State symbols are very important. e.g. 1. CaCO 3(s) ⇄ CaO (s) +CO 2(g) K p = e.g. 2. Fe 2+ (aq) + Ag + (aq) ⇄ Fe 3+ (aq) + Ag (s) Kc=Kc= (s), (l), (g), (aq). P CO2 [Fe 3+ (aq) ] [Fe 2+ (aq) ] x [Ag + (aq) ]

20 © DGMcC To understand how reaction conditions affect reactions you should understand: • CHEMICAL EQUILIBRIUM - reversible processes and equilibrium - at equilibrium the rates of the forward and reverse reactions are equal - equilibrium constant - qualitative effect of changes in reactant and product concentration on the position of equilibrium - qualitative effects of changing the temperature and the ……...total pressure to a reaction vessel (including those used ……...on an industrial scale) on the position of equilibrium;

21 © DGMcC To understand how reaction conditions affect reactions you should understand: • CHEMICAL EQUILIBRIUM - reversible processes and equilibrium - at equilibrium the rates of the forward and reverse reactions are equal - equilibrium constant - qualitative effect of changes in reactant and product concentration on the position of equilibrium - qualitative effects of changing the temperature and the ……...total pressure to a reaction vessel (including those used ……...on an industrial scale) on the position of equilibrium;

22 © DGMcC Le Chatelier The French chemist Le Chatelier worked all this lot out!!! In a dynamic equilibrium the position of the equilibrium will shift in order to reverse any changes you introduce.

23 © DGMcC Predict how the above equilibrium position be changed by removing D adding C removing B How would the equilibrium constant be changed by removing D adding C removingB Le Chatelier’s Principle: When a change is forced upon a system in chemical equilibrium the position of the equilibrium moves so that the change is opposed. Move to RHS No change Only Temperature changes K A +B ⇄ C+D Move to LHS

24 © DGMcC Changes in Pressure A (g) +B (g) ⇄ C (g) 2 gas molecules 1 gas molecule increasing P will move equilibrium to RHS A (g) +B (g) ⇄ 2C (g) 2 gas molecules increasing P will have no effect A (g) +B (g) ⇄ 2C (g) + D (g) 2 gas molecules 3 gas molecules increasing P will move equilibrium to LHS

25 © DGMcC Which of these is true about the effect of pressure on the reaction below? A. Increased pressure gives more N 2 O 4. B. Increased pressure does not affect the equilibrium. C. Increased pressure makes N 2 O 4 decompose. D. Increased pressure slows down the reaction. 2NO 2 (g) N 2 O 4 (g)

26 © DGMcC During a chemical reaction heat is either given out by the reaction or heat is required by the reaction. During an exothermic reaction heat is given out During an endothermic reaction heat is taken in. Changes in Temperature

27 © DGMcC heat + A + B ⇄ C + D ExothermicHeat can be considered a product. Changes in Temperature Endothermic Heat can be considered a reactant. If temperature is increased equilibrium moves to LHS If temperature is increased equilibrium moves to RHS A + B ⇄ C + D + heat ∴ K gets smaller ∴ K gets larger

28 © DGMcC Which of these is true about the effect of increased temperature on the reaction? A. gives more N 2 O 4. B. does not affect the equilibrium. C. slows down the reactions. D. Achieves equilibrium more quickly. 2NO 2 (g) N 2 O 4 (g)  H=-58kJ/mol

29 © DGMcC To understand how reaction conditions affect reactions you should understand: • CHEMICAL EQUILIBRIUM - reversible processes and equilibrium - at equilibrium the rates of the forward and reverse reactions are equal - equilibrium constant - qualitative effect of changes in reactant and product concentration on the position of equilibrium - qualitative effects of changing the temperature and the ……...total pressure to a reaction vessel (including those used ……...on an industrial scale) on the position of equilibrium;

30 © DGMcC To understand how reaction conditions affect reactions you should understand: • CHEMICAL EQUILIBRIUM - reversible processes and equilibrium - at equilibrium the rates of the forward and reverse reactions are equal - equilibrium constant - qualitative effect of changes in reactant and product concentration on the position of equilibrium - qualitative effects of changing the temperature and the ……...total pressure to a reaction vessel (including those used ……...on an industrial scale) on the position of equilibrium;

31 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect - reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

32 © DGMcC What is the “rate" of a reaction? The rate of a reaction is the speed of the reaction. It is not “how much” of a product is made, but instead “how quickly” a reaction takes place.

33 © DGMcC 1. Concentration (solutions only) Reaction rates vary widely, from instantaneous to very slow, from a bomb exploding to concrete setting. The factors that may influence a rate are; 2. Temperature (all) 3. Catalyst (all) 4. Particle size (solids only) 5. Pressure (gases only) 6. Light intensity (photochemical reactions only) Reaction Rates

34 © DGMcC THE EFFECT OF CONCENTRATION ON REACTION RATES  For many reactions involving liquids or gases, increasing the concentration of the reactants increases the rate of reaction.  In a few cases, increasing the concentration of one of the reactants may have little noticeable effect of the rate.  Don't assume that if you double the concentration of one of the reactants that you will double the rate of the reaction. It may happen like that, but the relationship may well be more complicated. The mathematical relationship between concentration and rate of reaction is related with the orders of reaction.

35 © DGMcC Changing Concentration By increasing [ ] the number of particles in the given volume will increase ∴ the total number of collisions/sec will increase ∴ the number of effective collisions/sec will increase ∴ the reaction rate will increase.

36 © DGMcC THE EFFECT OF SURFACE AREA ON REACTION RATES  The more finely divided the solid is, the faster the reaction happens.  A powdered solid will normally produce a faster reaction than if the same mass is present as a single lump.  The powdered solid has a greater surface area than the single lump.

37 © DGMcC The effect of particle size Solids with a smaller particle size (e.g. powders or small chips) react more quickly than solids with a larger particle size (e.g large chips).

38 © DGMcC THE EFFECT OF TEMPERATURE ON REACTION RATES As you increase the temperature the rate of reaction increases. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10°C rise in temperature.

39 © DGMcC Changing Temperature By increasing the temperature the total kinetic energy of the mixture is increased ∴ The individual particles, on average, will have a higher kinetic energy ∴ More collisions/sec will be effective, as more will exceed the activation energy ∴ The rate increases. Less importantly, there are more collisions as the particles are moving faster.

40 © DGMcC THE EFFECT OF PRESSURE ON REACTION RATES  Changing the concentration of a gas is achieved by changing its pressure.  Increasing the pressure on a reaction involving reacting gases increases the rate of reaction. Changing the pressure on a reaction which involves only solids or liquids has no effect on the rate.

41 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect - reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

42 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect - reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

43 © DGMcC COLLISION THEORY Collision theory states that... particles must COLLIDE before a reaction can take place not all collisions lead to a reaction reactants must possess at least a minimum amount of energy - ACTIVATION ENERGY plus particles must approach each other in a certain relative way - the STERIC EFFECT

44 © DGMcC COLLISION THEORY According to the collision theory, to increase the rate of reaction you need... more frequent collisionsincrease particle speedor have more particles present more successful collisionsgive particles more energyor lower the activation energy

45 © DGMcC Energy Reaction Coordinate EAEA ΔH -ive Energy Level Diagrams Reactants Products This is an EXOTHERMIC reaction

46 © DGMcC Energy Reaction Coordinate EAEA ΔH +ive Energy Level Diagrams Reactants Products This is an ENDOTHERMIC reaction

47 © DGMcC Activation Energy You can think if the particles collide with less energy than the activation energy, nothing important happens. They bounce apart. Only those collisions which have energies equal to or greater than the activation energy result in a reaction.

48 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect - reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

49 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect -reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

50 © DGMcC Catalysts A catalyst provides an alternative pathway of lower activation energy. E A cat Energy E A uncat ΔHΔH Reactants Products

51 © DGMcC THE EFFECT OF CATALYSTS ON REACTION RATES A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction.

52 © DGMcC Catalysts and activation energy with a simple analogy. Suppose you have a mountain between two valleys so that the only way for people to get from one valley to the other is over the mountain. Only the most active people will manage to get from one valley to the other. Now suppose a tunnel is cut through the mountain. Many more people will now manage to get from one valley to the other by this easier route. You could say that the tunnel route has a lower activation energy than going over the mountain. But you haven't lowered the mountain! The tunnel has provided an alternative route but hasn't lowered the original one. The original mountain is still there, and some people will still choose to climb it.

53 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect -reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

54 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect -reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

55 © DGMcC The Maxwell-Boltzmann Distribution Because of the key role of activation energy in deciding whether a collision will result in a reaction, it would obviously be useful to know what sort of proportion of the particles present have high enough energies to react when they collide.

56 © DGMcC Because of the many collisions taking place between molecules, there is a spread of molecular energies and velocities. This has been demonstrated by experiment. It indicated that...no particles have zero energy/velocity some have very low and some have very high energies/velocities most have intermediate velocities. Maxwell-Boltzmann Distribution MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

57 © DGMcC The Maxwell-Boltzmann Distribution The area under the curve is a measure of the total number of particles present. The graph only applies to gases, but the conclusions that we can draw from it can also be applied to reactions involving liquids.

58 © DGMcC Increasing the temperature alters the distribution get a shift to higher energies/velocities curve gets broader and flatter due to the greater spread of values area under the curve stays constant - it corresponds to the total number of particles T1T1 T2T2 TEMPERATURE T 2 > T 1 INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

59 © DGMcC Decreasing the temperature alters the distribution get a shift to lower energies/velocities curve gets narrower and more pointed due to the smaller spread of values area under the curve stays constant - it corresponds to the total number of particles T1T1 T3T3 TEMPERATURE T 1 > T 3 DECREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

60 © DGMcC EaEa ACTIVATION ENERGY - E a The Activation Energy is the minimum energy required for a reaction to take place The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

61 © DGMcC The Maxwell-Boltzmann Distribution and activation energy Remember that for a reaction to happen, particles must collide with energies equal to or greater than the activation energy for the reaction:

62 © DGMcC Explanation increasing the temperature gives more particles an energy greater than E a more reactants are able to overcome the energy barrier and form products a small rise in temperature can lead to a large increase in rate T1T1 T2T2 TEMPERATURE T 2 > T 1 EaEa MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER

63 © DGMcC The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. If a catalyst is added, the Activation Energy is lowered - E a will move to the left. MOLECULAR ENERGY EaEa MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER ADDING A CATALYST NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

64 © DGMcC The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. Lowering the Activation Energy, E a, results in a greater area under the curve after E a showing that more molecules have energies in excess of the Activation Energy EaEa MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY ADDING A CATALYST EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

65 © DGMcC Notice that the large majority of the particles don't have enough energy to react when they collide. To enable them to react we either have to change the shape of the curve, or move the activation energy further to the left. You can change the shape of the curve by changing the temperature of the reaction. You can change the position of the activation energy by adding a catalyst to the reaction.

66 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) - catalysts and their effect -reaction profiles, collision theory, activation energy - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

67 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) -reaction profiles, collision theory, activation energy - catalysts and their effect - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant - determining order of reaction and rate equation from ……...data on initial rates;

68 © DGMcC Orders of reaction Orders of reaction are always found by doing experiments. You can't deduce anything about the order of a reaction just by looking at the equation for the reaction. So let's suppose that you have done some experiments to find out what happens to the rate of a reaction as the concentration of one of the reactants, A, changes. Some of the simple things that you might find are:

69 © DGMcC Orders of reaction One possibility: The rate of reaction is proportional to the concentration of A That means that if you double the concentration of A, the rate doubles as well. If you increase the concentration of A by a factor of 4, the rate goes up 4 times as well. You can express this using symbols as: Writing a formula in square brackets is a standard way of showing a concentration measured in moles per cubic decimetre (litre).

70 © DGMcC Orders of reaction You can also write this by getting rid of the proportionality sign and introducing a constant, k.

71 © DGMcC Orders of reaction Another possibility: The rate of reaction is proportional to the square of the concentration of A This means that if you doubled the concentration of A, the rate would go up 4 times (2 2 ). If you tripled the concentration of A, the rate would increase 9 times (3 2 ). In symbol terms:

72 © DGMcC Orders of reaction Generalising this By doing experiments involving a reaction between A and B, you would find that the rate of the reaction was related to the concentrations of A and B in this way: This is called the rate equation for the reaction. The concentrations of A and B have to be raised to some power to show how they affect the rate of the reaction. These powers are called the orders of reaction with respect to A and B.

73 © DGMcC Orders of reaction If the order of reaction with respect to A is 0 (zero), this means that the concentration of A doesn't affect the rate of reaction. Mathematically, any number raised to the power of zero (x 0 ) is equal to 1. That means that that particular term disappears from the rate equation.

74 © DGMcC Overall order of the reaction The overall order of the reaction is found by adding up the individual orders. For example, if the reaction is first order with respect to both A and B (a = 1 and b = 1), R= k[A] [B] the overall order is 2. We call this an overall second order reaction.

75 © DGMcC Some examples Each of these examples involves a reaction between A and B, and each rate equation comes from doing some experiments to find out how the concentrations of A and B affect the rate of reaction.

76 © DGMcC Example 1 Rate = k[A] [B] In this case, the order of reaction with respect to both A and B is 1. The overall order of reaction is 2 - found by adding up the individual orders. Note: Where the order is 1 with respect to one of the reactants, the "1" isn't written into the equation. [A] means [A]1.

77 © DGMcC Example 2 This reaction is zero order with respect to A because the concentration of A doesn't affect the rate of the reaction. The order with respect to B is 2 - it's a second order reaction with respect to B. The reaction is also second order overall (because = 2). Rate = k [B] 2

78 © DGMcC Example 3 This reaction is first order with respect to A and zero order with respect to B, because the concentration of B doesn't affect the rate of the reaction. The reaction is first order overall (because = 1). Rate = k[A]

79 © DGMcC Orders of reaction What if you have some other number of reactants? It doesn't matter how many reactants there are. The concentration of each reactant will occur in the rate equation, raised to some power. Those powers are the individual orders of reaction. The overall order of the reaction is found by adding them all up.

80 © DGMcC For the reaction A+2B ⇄ C+D We know Order of Reaction rate α [A] x and rate α [B] y x is called the order of reaction with respect to A y is called the order of reaction with respect to B they are small whole numbers (1,2,3,) or 0. x+y is called the order of reaction. 0 = zero order, 1 = first order, 2 = second order etc. rate  [A] x [B] y or rate = k[A] x [B] y where k = rate constant

81 © DGMcC N.B. ! For the equilibrium constant you use the stoichiometry of the equation but for rate equations you can’t. Orders can only be determined experimentally. for A+2B ⇄ 3C+4D rate = k[A][B] 2   K c =[C] 3 x [D] 4 [A] x [B] 2

82 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) -reaction profiles, collision theory, activation energy - catalysts and their effect - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant

83 © DGMcC To understand how reaction conditions affect reactions you should understand: • RATE OF REACTION - qualitative effects of concentration, temperature and pressure changes (including those used on an industrial scale) and the effect of particle size (although relatively unusual on an industrial scale) -reaction profiles, collision theory, activation energy - catalysts and their effect - the effect of a change of temperature on energy distribution (Maxwell–Boltzmann distribution curves) - use of energy distribution curves to explain the effect of a change in temperature and the use of a catalyst - rate equations - order of reaction (zero, first, second) and the rate constant

84 © DGMcC The END Thank-you for staying awake


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