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REACTION RATES AND REVERSIBLE REACTIONS

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1 REACTION RATES AND REVERSIBLE REACTIONS
1/2/2019 WANYERA C

2 How Fast Does the Reaction Go?
Reaction Rate How Fast Does the Reaction Go? 1/2/2019 WANYERA C

3 For an industrial process, it is important to obtain the product as
Reaction rate is a measure of how much of the reactants are consumed or how much of the products are formed per unit time. The consideration of the rate or speed of the reaction is very important in chemistry particularly in industrial processes. For an industrial process, it is important to obtain the product as 1/2/2019 WANYERA C

4 Fast as possible for commercial and economic reasons.
Chemists, by studying factors that affect the rate of reactions, can find out the best conditions for any particular reaction. Some of the common properties considered are;- Rate of production of a gas. Changing mass of a reaction. Change in intensity of colour. Formation or disappearance of a precipitate 1/2/2019 WANYERA C

5 Factors affecting the rate of a reaction
1. CONCENTRATION Effect of concentration on rate of reaction. Flask Volume of acid(cm³) Volume of water (cm³) Concentration (Mol dm⁻³) Time taken (t sec) A 40 39 B 30 10 42 C 20 86 D 607 1/2/2019 WANYERA C

6 conclusion The higher the concentration of the acid, the faster the reaction. 1/2/2019 WANYERA C

7 When the number of particles reduces, the collision rate decreases.
When the number of particles increases, the rate of collision also increases. When the number of particles reduces, the collision rate decreases. How does the rate of reaction change as the reaction proceeds? Consider reaction between magnesium and HCl 1/2/2019 WANYERA C

8 Mg(s) + 2HCl(aq) MgCl₂(aq) + H₂(g)
The graph below shows shape of the graph obtained by plotting the volume of the gas produced against time. 1/2/2019 WANYERA C

9 Average rate = Volume of hydrogen gas produced Time taken
The rate of production of hydrogen gas is highest at the end of the experiment, then it slows down and eventually stops when one or all of the reactants is used up. The rate of reaction at any given time of the reaction can be calculated as Average rate = Volume of hydrogen gas produced Time taken 1/2/2019 WANYERA C

10 e.g. 1/2/2019 WANYERA C

11 Average rate of reaction between A and B is given by;
= (V₂ - V₁) cm³ (t₂ - t₁) sec The rate of a reaction at a particular time(t) is determined by drawing a tangent to the curve at a certain point and obtaining its gradient. 1/2/2019 WANYERA C

12 1/2/2019 WANYERA C

13 Since the rate of reaction is inversely proportional to time taken
The rate of reaction at point c on the curve when time is in seconds is given by;- rate ∆V = (V₄ - V₃) cm³ ∆t (t₄ t₃) sec Since the rate of reaction is inversely proportional to time taken rate α ⅟t 1/2/2019 WANYERA C

14 A graph of ⅟t(s⁻¹) against concentration of the acid is a straight line.
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15 20 cm³ of hydrogen peroxide solution was mixed with 20cm³ of sulphuric acid, 30cm³ of potassium iodide(excess), 5 cm³ of sodium thiosulphate and 1 cm³ of starch solution. The hydrogen peroxide reacts with potassium iodide at a reasonable rate, forming iodine. As soon as iodine is formed, it reacts rapidly with sodium thiosulphate untill all the sodium thiosulphate is used up. 1/2/2019 WANYERA C

16 The time taken for the blue colour to appear was recorded.
The iodine formed after the sodium thiosulphate is used up gives a blue colour with the starch. The time taken for the blue colour to appear was recorded. The experiment was repeated using hydrogen peroxide solutions of different concentration and the results are shown below. 1/2/2019 WANYERA C

17 Time (sec) for blue colour to appear ⅟t( sec⁻¹)
Hydrogen peroxide (mol ⁻¹) Time (sec) for blue colour to appear ⅟t( sec⁻¹) 0.30 20.0 0.050 0.25 24.5 0.041 0.20 30.3 0.033 0.15 40.0 0.0250 0.10 60.6 0.0165 1/2/2019 WANYERA C

18 What does ⅟time represent?
Plot a graph of ⅟time against concentration of hydrogen peroxide. Explain why this graph shows the rate of the reaction is proportional to the concentration of hydrogen peroxide solution. Use the graph to determine the time that would have been taken for the blue colour to appear if the concentration of hydrogen peroxide had been; 1/2/2019 WANYERA C

19 0.05M 0.18M 1/2/2019 WANYERA C

20 2. TEMPERATURE Generally a rise in temperature tends to increase the rate of a reaction. When a mixture of substances is heated, the particles move faster. This has two effects: 1/2/2019 WANYERA C

21 As temperature rises, the particles of the reacting substances move faster hence they collide with each other more often. The rate of reaction therefore increases. When two molecules collide, they will only react with each other if they have sufficient energy- activation energy. 1/2/2019 WANYERA C

22 The reaction rate will therefore depend upon the proportion of molecules which have this or higher energy. KLB PG 80 1/2/2019 WANYERA C

23 Things that Affect Rate
Temperature Higher temperature, faster particles. More and harder collisions. Faster Reactions. Concentration More concentrated molecules closer together Collide more often. Faster reaction. 1/2/2019 WANYERA C

24 Collision theory and activation energy
Collision theory states that for a reaction to occur between reactants A and B to product C, portions of A and B must collide with sufficient energy to react. Many collisions occur between reactant particles but only those with sufficient energy will result in a reaction. 1/2/2019 WANYERA C

25 The sufficient energy is the activation energy.
The minimum amount of energy that the particles or reactants must have in order to react is called the activation energy. Activation energy is an energy barrier which must be overcome before a reaction can take place. The collisions which succeed in producing a reaction are called effective collisions. 1/2/2019 WANYERA C

26 Non active particles can be activated by absorbing some energy.
The particles having activation energy(Ea) are known as activated particles. Non active particles can be activated by absorbing some energy. During a reaction, increase in temperature results in an increase in kinetic energy of particles making particles to move faster and collide more frequently with sufficient energy to cause more effective collisions per unit time. 1/2/2019 WANYERA C

27 High concentration of particles of the reactants increases the number of collisions which increases the number of successful collisions and hence increase in rate of a reaction. 1/2/2019 WANYERA C

28 Energy Reactants Products Reaction coordinate 1/2/2019 WANYERA C

29 Activation Energy - Minimum energy to make the reaction happen.
Reactants Products Reaction coordinate 1/2/2019 WANYERA C

30 Activated Complex or Transition State
Energy Reactants Products Reaction coordinate 1/2/2019 WANYERA C

31 3. Surface area Is significant in heterogenous reaction where the reactants are not in the same phase. Increasing surface area of a solid reactant increases reaction rate. The reaction CaCO₃ (s) + 2HCl (aq) CaCl₂ (aq) +H₂O (l)+ CO₂ (g) 1/2/2019 WANYERA C

32 Increase in the surface area of particles increases the area of contact of the reacting particles.
The greater the surface area of contact between reacting particles, the higher the rate of a reaction. The rate of a reaction of powdered marble chips with acid reaches completion earlier than unground marble. 1/2/2019 WANYERA C

33 NB: Both curves flatten out at the same volume.
CO₂ produced is the same in both experiment. 1/2/2019 WANYERA C

34 1/2/2019 WANYERA C

35 4. Catalysts substances that alters the rate of a chemical reaction but remains chemically unchanged at the end of a reaction. Catalysts provide an easier route for the reactants to attain the minimum energy required for a reaction to take place. The new path has a lower activation energy. Most catalysts work by providing a surface on which particles can react hence solid catalysts are effective in powder form e.g. in contact process. 1/2/2019 WANYERA C

36 Other catalysts act by adsorption.
Catalysts can also increase the rate of a reaction by having a short lived intermediate compound or complex(activated complex) which involves the catalyst and which breaks up to release the catalyst and the products. Other catalysts act by adsorption. 1/2/2019 WANYERA C

37 NB: Energy of the reactants and products remains the same but the activation energy is lowered. Since catalysts are unchanged by the end of the reaction, they can be re used. Some common catalysts 1/2/2019 WANYERA C

38 The heat change for the reaction (∆H) remains the same
catalyst Industrial process Iron Manufacture of ammonia in haber process Vanadium(V) oxide; platinum Manufacture of sulphuric (VI) acid. Enzymes (yeast) Production of alcoholic drinks by fermentation of sugar The heat change for the reaction (∆H) remains the same 1/2/2019 WANYERA C

39 Energy Reactants Products Reaction coordinate 1/2/2019 WANYERA C

40 Catalyst 1/2/2019 WANYERA C

41 Catalysts Hydrogen bonds to surface of metal. Break H-H bonds
Pt surface 1/2/2019 WANYERA C

42 Catalysts H H C C H H H H H H Pt surface 1/2/2019 WANYERA C

43 Catalysts C C The double bond breaks and bonds to the catalyst.
Pt surface 1/2/2019 WANYERA C

44 Catalysts C C The hydrogen atoms bond with the carbon Pt surface H H H
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45 Catalysts H H C C H H H H H H Pt surface 1/2/2019 WANYERA C

46 5. Light Some reactions speed up considerably when exposed to sunlight. In such reactions, molecules absorb energy in form of light energy rather than heat energy and so they are called photochemical reactions. The ultra violet part of the sunlight provides energy that speeds up the reaction. The influence of light on a chemical reaction is called photocatalysis 1/2/2019 WANYERA C

47 Examples Reaction between chlorine and hydrogen does not occur under ordinary temperature and pressure in darkness. When exposed to direct light, the reaction is explosive. Cl₂(g) + H₂(g) HCl(aq) U.V light 1/2/2019 WANYERA C

48 ii. Reaction between bromine and methane CH₄ (g) + Br₂ (g) CH₃Br (g) + HBr (g) iii. Photosynthesis
U.V light (g) (l) (g) (aq) 1/2/2019 WANYERA C

49 NB Photographic plates are normally coated with silver bromide.
Processing of films is done in a dark room to avoid decomposition of the silver bromide. 1/2/2019 WANYERA C

50 Effect of pressure of a gas on the rate of reactions
Pressure is significant in reactions involving gaseous reactants. Increase in pressure reduces the volume means the concentration of particles increases, the collision rate therefore increases consequently increasing the rate of reaction. When pressure decreases, the volume increases hence reducing concentration of particles and in effect reducing the rate of a reaction. 1/2/2019 WANYERA C

51 1/2/2019 WANYERA C

52 Reversible physical changes
Reversible reactions are those reactions which can be made to go either direction by changing conditions such as temperature. Reversible physical changes These involve change of state only e.g. from solid to liquid which a solid melts or from liquid to gas when a liquid evaporates. Ice liquid steam Heat cool Heat cool 1/2/2019 WANYERA C

53 I₂ (s) I₂ (g) (sublimation) where no liquid is formed.
Heat cool 1/2/2019 WANYERA C

54 Reversible chemical reactions (changes)
Expt klb pg 92 Heating hydrated copper (II) sulphate. Reversible chemical reactions are those in which products, under certain conditions can be combined to reform to reactants. When hydrated copper (II) sulphate crystals are heated, a white powder is formed. 1/2/2019 WANYERA C

55 CuSO₄.5H₂O (s) CuSO₄ (s) + 5H₂O (l)
blue crystals white powder colourless liquid When the colourless liquid is poured back into the tube containing the white solid( anhydrous copper(II)sulphate turns blue and crystalline structure of hydrated copper (II) sulphate is restored. CuSO₄.5H₂O (s) CuSO₄ (s) + 5H₂O (l) Heat cool 1/2/2019 WANYERA C

56 Where is the sign for the reaction which goes both ways
The left to right reaction is referred to as forward reaction. The right to left reaction is the reverse reaction. 1/2/2019 WANYERA C

57 Question Hydrated cobalt(II)chloride exist as pink crystals and anhydrous cobalt(II) chloride is a blue powder. Describe a laboratory experiment that can be used to show that the action of heat on hydrated cobalt(II)chloride is a reversible reaction.(KCSE 2010) 1/2/2019 WANYERA C

58 Answer Heat the hydrated salt, the pink substance changes to blue, collect vapour and cool, add liquid to solid, it turns pink. Heat the hydrated salt in a sealed container, the pink substances changes to blue; allow the blue substance to cool.It changes to pink substance; 1/2/2019 WANYERA C

59 Characteristics of reversible reactions
At the beginning of a reaction. The rate of forward reaction is faster The concentration of reactants is greater. The concentration of products is lowest. At zero time, the rate of reverse reaction is zero, because no products are formed. 1/2/2019 WANYERA C

60 The rate of forward reaction decreases with time .
The rate of reverse reaction and the concentration of the products increases with time. A time is reached when forward and reverse rates are equal. Other examples of reversible reactions. 1/2/2019 WANYERA C

61 COCl₂.6H₂O (s) COCl₂ (s) + 6H₂O (g) pink blue 3Fe (s) + 4H₂O (g) Fe₃O₄ (s) + 4H₂ (g) grey brown CaCO₃ (s) CaO (s) + CO₂ (g) 2SO₂ (g) + O₂ (g) 2SO₃ (g) N₂ (g) + 3H₂ (g) 2NH₃ (g) 1/2/2019 WANYERA C

62 2HI (g) H₂ (g) + I₂ (g) NH₄Cl (s) NH₃ (g) + HCl (g) N₂ (g) + O₂ (g) 2NO (g) 2HgO (s) 2Hg (l) + O₂ (g) 1/2/2019 WANYERA C

63 EQUILIBRIUM In a reversible chemical reaction the two opposing reactions(forward and reverse reactions) eventually reach a point where no changes in the reactant mixture seem to be occuring. Rate of forward reaction = rate of reverse reaction. At this stage, both forward and reverse reactions are taking place at the same rate but in opposite directions. 1/2/2019 WANYERA C

64 The reaction is said to have attained “ a state of balance”
Microscopic (invisible) processes continue but in a balance that yields macroscopic(visible) changes. The reaction is said to have attained “ a state of balance” Forward and reverse reactions are therefore in a state of equilibrium. A reaction in equilibrium is represented by the sign 1/2/2019 WANYERA C

65 When you first put reactants together the forward reaction starts.
Since there are no products there is no reverse reaction. As the forward reaction proceeds the reactants are used up so the forward reaction slows. The products build up, and the reverse reaction speeds up. 1/2/2019 WANYERA C

66 This is dynamic equilibrium.
Eventually you reach a point where the reverse reaction is going as fast as the forward reaction. This is dynamic equilibrium. The rate of the forward reaction is equal to the rate of the reverse reaction. The concentration of products and reactants stays the same, but the reactions are still going on 1/2/2019 WANYERA C

67 Examples of dynamic equilibrium. Acid base equilibrium
Chromate dichromate equilibrium 1/2/2019 WANYERA C

68 Factors affecting equilibrium
The balance or position of equilibrium may be altered by most of the factors that affect the rate of reaction. They include: Concentration of the components Temperature Pressure Use of catalysts Volume of the vessel 1/2/2019 WANYERA C

69 When a system at equilibrium is suddenly disturbed, it will respond in such a way that the equilibrium is eventually re-established. This is in accordance with the le chartelier’s principle which states that; 1/2/2019 WANYERA C

70 Le Châtelier’s Principle
States that 1/2/2019 WANYERA C

71 If a system at equilibrium is subjected to change, the processes occur which tend to counteract the change; Or if the stress is applied to a system at equilibrium, the system re-adjusts, if possible to reduce the effect of stress. 1/2/2019 WANYERA C

72 Three types of stress are applied. Changing concentration
Changing temperature Changing pressure 1/2/2019 WANYERA C

73 Effect of change of concentation on equilibrium
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74 Changing Concentration
If you add reactants (or increase their concentration). The forward reaction will speed up. More product will form. Equilibrium “Shifts to the right” Reactants ® products 1/2/2019 WANYERA C

75 consider Br₂ (g) +H₂O (l) OBr⁻ (aq)+ Br⁻ (aq) + 2H⁺ (aq) When NaOH is added to bromine water(above reaction), it provides hydroxide ions which react with hydrogen ions in solution to form water. OH⁻ (aq) H⁺ (aq) H₂O (l) 1/2/2019 WANYERA C

76 Consequently hydrogen ions are removed from the mixture.
The reaction proceeds to the right( forward reaction is favoured). Colour changes from yellow –orange to colourless. 1/2/2019 WANYERA C

77 Changing Concentration
If you add products (or increase their concentration). The reverse reaction will speed up. More reactant will form. Equilibrium “Shifts to the left” Reactants ¬ products 1/2/2019 WANYERA C

78 Consider When hydrochloric acid is added to bromine water, hydrogen ions are introduced which react with colorless bromide and hypobromite ions to form yellow orange aqeuous bromine. The reaction proceeds more to the left hence the increase in intensity of the colour( reverse reaction is favoured) 1/2/2019 WANYERA C

79 chromate(VII) dichromate(VI) equilibrium
2K₂CrO₄ (aq) + H₂SO₄ (aq) K₂Cr₂O₇ (aq) + K₂SO₄ (aq) + H₂O (l) yellow orange Potassium chromate(VII) potassium dichromate(VI) 2CrO₄²ˉ (aq)+ 2H⁺(aq) Cr₂O₇²ˉ (aq) + H₂O (l) yellow orange 1/2/2019 WANYERA C

80 Cr₂O₇²ˉ (aq)+ OHˉ (a) 2CrO₄²ˉ (aq)+H₂O (l) Orange yellow
Addition of acid causes more chromate (CrO₄²ˉ) to change to dichromate(Cr₂O₇²ˉ) ions making the solutions to become orange. Addition of hydroxide ions reverses the reaction leading to the formation of yellow chromate solution. Cr₂O₇²ˉ (aq)+ OHˉ (a) CrO₄²ˉ (aq)+H₂O (l) Orange yellow 1/2/2019 WANYERA C

81 Changing Concentration
If you remove products (or decrease their concentration). The reverse reaction will slow down. More product will form. Equilibrium reverse“Shifts to the right” Reactants ® products 1/2/2019 WANYERA C

82 Changing Concentration
If you remove reactants (or decrease their concentration). The forward reaction will slow down. More reactant will form. Equilibrium “Shifts to the left”. Reactants ¬ products Used to control how much yield you get from a chemical reaction. 1/2/2019 WANYERA C

83 Effect of temperature on equilibrium
if the temperature is increased, energy is added. To release the stress of extra energy, the equilibrium will shift to produce substances which are formed by absorbing energy. If the temperature of the system in equilibrium is increased, the reaction will move in away that heat is absorbed i.e. the endothermic reaction is favoured. 1/2/2019 WANYERA C

84 ( Dinitrogen tetraoxide) Nitrogen(IV)oxide
If the temperature of the system in equilibrium is lowered, the reaction will move in a way to produce more heat i.e. the exothermic reaction is favoured. E.g. N₂O₄ (g) NO₂ (g) pale yellow dark brown ( Dinitrogen tetraoxide) Nitrogen(IV)oxide 1/2/2019 WANYERA C

85 The equilibrium shifts from left to right.
The favoured reaction in the above case is endothermic hence is favoured by increase in temperature i.e. heating tends to more dark-brown colour. The equilibrium shifts from left to right. The reverse reaction is exothermic thus it will be favoured by decrease in temperature i.e. cooling the mixture shifts the equilibrium to the left and more pale yellow colour is observed. 1/2/2019 WANYERA C

86 Summary of effect of temperature on equilibrium.
Consider the reaction A + B C + D………….. ∆H ± Ve Summary of effect of temperature on equilibrium. 1/2/2019 WANYERA C

87 Effect of change in temperature on equilibrium
Reaction Effect of change in temperature on equilibrium Increase Decrease Exothermic ∆H -Ve Shifts to the left and more of A and B would be formed. Shifts to the right and more C and D would be formed. Endothermic ∆H +Ve Shifts to the right and more of C and D would be formed. 1/2/2019 WANYERA C

88 Effect of pressure on equilibrium.
Change in pressure will only affect equilibrium involving gases. If the pressure of a system in equilibrium is increased, the system will favour the direction that has fewer molecules. A decrease in pressure favours the direction of the reaction that forms more molecules. 1/2/2019 WANYERA C

89 2NO₂ (g) N₂O₄ (g) dark brown pale yellow
when the volume of the gases is reduced by increasing the pressure, a pale yellow colour is observed. This shows increase in pressure favours forward reaction which forms fewer molecules. When the volume is increased by lowering the pressure, a dark brown colour is observed. 1/2/2019 WANYERA C

90 The reverse reaction occurs which forms more molecules by lowering the pressure.
If the number of molecules of reactants equals number of molecules of the product, a change in pressure would have no effect on the equilibrium. H₂ (g) I₂ (g) HI (g) 1 mol mol mol 1/2/2019 WANYERA C

91 In the above reaction, change in pressure will not affect the equilibrium position but will help in quick attainment of the equilibrium. 1/2/2019 WANYERA C

92 Effect of pressure change on equilibrium mixture
Reaction Effect of pressure change on equilibrium mixture N₂O₄(g) NO₂(g) Increase Decrease More N₂O₄ formed ( reverse rxn favoured) More NO₂ formed (forward rxn favoured) N₂(g) + 3H₂(g) NH₃(g) 2SO₂(g)+ O₂(g) SO₃(g) 4NH₃(g) + 5O₂(g) NO(g)+ 6H₂O (g) H₂(g) + Cl₂(g) 2HCl(g) 1/2/2019 WANYERA C

93 some industrial applications of chemical equilibrium.
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94 Interests of a manufacturer
Maximum quantity of products Minimum cost Shortest time possible. Optimum conditions are conditions required to obtain the greatest yield of the products in the shortest time and minimum cost. 1/2/2019 WANYERA C

95 1. Haber process N₂ (g) + 3H₂ (g) 2NH₂ ∆H = - 92kj/mol
Temperature 450˚C Pressure 200 atm Catalyst – iron (platimum is expensive) Reducing the concentration of ammonia as soon as it is formed. 1/2/2019 WANYERA C

96 2. Contact process 2SO₂ (g) + O₂ (g) 2SO₃ (g) temperature 450˚C
Pressure – atmospheric pressure which gives 96% yield Catalyst – vanadiun(V)oxide. 1/2/2019 WANYERA C

97 Effect of catalyst on equilibrium
A catalyst has no effect on the position of equilibrium but is useful if it is required to reach the equilibrium quickly. 1/2/2019 WANYERA C

98 Note Formation of gases and precipitates.
Ordinarily a reversible reaction does not go to completion as it takes place both in the forward and reverse directions. However, if one of the products formed during the reaction is removed, reverse reaction will not take place but the forward reaction will go to completion. 1/2/2019 WANYERA C

99 A reaction in which a gas evolved and allowed to escape or in which a precipitate is formed, will proceed in one direction only even though a reversible reaction might be expected. 1/2/2019 WANYERA C

100 End 1/2/2019 WANYERA C


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