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Rates & Extents of Reactions. Rates & Extents of Rates & Extents of Reactions Quiz A Quiz A What prevents reaction from What prevents reaction from going?

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Presentation on theme: "Rates & Extents of Reactions. Rates & Extents of Rates & Extents of Reactions Quiz A Quiz A What prevents reaction from What prevents reaction from going?"— Presentation transcript:

1 Rates & Extents of Reactions

2 Rates & Extents of Rates & Extents of Reactions Quiz A Quiz A What prevents reaction from What prevents reaction from going? Increasing the Rate of Increasing the Rate of reaction Measuring Change Measuring Change Establishing Equilibrium Establishing Equilibrium The Equilibrium Constant The Equilibrium Constant Le Chatelier’s Principle Le Chatelier’s Principle Industry Temperature CatalysisCollisions Quiz B Temperature Pressure Mass of System Conc n. Quiz C Dynamic Eq m. Quiz D Quiz E Quiz F Quiz G Overview

3 Quiz A- Common misconceptions 1.When solid copper carbonate reacts with excess acid, carbon dioxide is produced. The curves shown were obtained under two different sets of conditions. The curves shown were obtained under two different sets of conditions. Volume of of CO 2 CO 2 time X Y A.increasing the concentration of the acid. B.decreasing the particle size. C.adding a catalyst. D.increasing the temperature. E.decreasing the mass of copper carbonate. The change from X to Y could be brought about by :

4 2. Chemical reactions are in a state of dynamic equilibrium only when: A. the rate of the reverse reaction is equal to the rate of the forward reaction. B. the reaction involves a zero enthalpy change. C. the activation energy of the forward reaction equals the activation energy of the reverse reaction. D. the reaction goes to completion. E. the ratio of the products to reactants is equal to exactly 1. Quiz A- Common misconceptions

5 3.X(g) is placed in a flask and the following reaction proceeds to equilibrium X(g) ⇌ Y(g) Δ H = +ve X(g) ⇌ Y(g) Δ H = +ve Which of the following statements is correct? A.The forward reaction rate increases as the reaction “gets going”. B.The forward reaction rate always equals the reverse reaction rate. C.The activation energy of the forward reaction of the system considered will always be higher than the activation energy of the reverse reaction no matter the reaction conditions. D.When equilibrium is re-established after a disturbance (e.g. adding more Y) the rate of the forward and reverse reactions will be equal to those at the initial equilibrium. E.The forward reaction is completed before the reverse reaction begins. Quiz A- Common misconceptions

6 4.The graph below shows the variation of concentration of a reactant, X, with time as the reaction proceeds. A B0.005 C0.015 D0.020 EThe slope of the tangent to the curve at t = 30s [X] / mol.dm -3 Time /s What is the average reaction rate (mol dm -3 s -1 ) during the first 30 seconds? Quiz A- Common misconceptions

7 5.Excess marble chips (CaCO 3 ) were added to 50 cm 3 of 1 M HCl. The experiment was repeated using the same mass of marbles chips and 50 cm 3 of 1M CH 3 COOH. Which of the following would have been the same for both experiments? AThe average rate of reaction. BThe rate at which the first 2 cm 3 of gas was evolved. CThe time taken for the reaction to be complete. DThe mass of the marble chips remaining after the reaction had stopped. ENone of the above. Quiz A- Common misconceptions

8 What prevents reactions from going? Potential Energy Reaction co-ordinate EaEaEaEa N 2 + O 2 2NO Energy Profile ΔHΔHΔHΔH 2NO  N 2 + O 2

9 Increasing the Rate of Reaction Rate = k [A] x [B] y Rate constant Orders of reactants Non - examinable ! But useful for understanding.

10 Increasing the Rate of Reaction A. Temperature Everyone knows that as we increase the temperature, the reaction rate goes up…… But how do we explain this observation scientifically? The answer lies in the Maxwell Distribution where the idea of a range of molecular speeds is introduced.

11 What makes reactions go faster? Fraction of molecules Molecular Velocity Maxwell Distribution of velocities KE min = ½mv 2 min T1T1T1T1 T1T1T1T1 T2T2T2T2 T2T2T2T2 v min T 2 > T 1

12 Increasing the Rate of Reaction If we increase the temperature we will increase the fraction of the molecules with the energy to overcome the activation energy. f = e –E a / RT –E a / RT If T  then f  If f  then k  If k  then Rate  Note : If E a  due to a catalyst then f  and the Rate  Where f = fraction of molecular collisions with energy greater or equal to E a

13 Increasing the Rate of Reaction B. Catalysis  Catalysts are employed to speed–up the attainment of equilibrium.  Catalysts do not change the position of equilibrium.  Catalysts offer a different mechanism for the reaction to occur.  For the catalyst to be successful it must offer a mechanism with a lower activation energy, E a. lower activation energy, E a.  Catalysts can either be in the same phase as the reactant – homogeneous catalysts - or a different phase –heterogeneous homogeneous catalysts - or a different phase –heterogeneous catalysts. catalysts.

14 Increasing the Rate of Reaction Potential Energy Reaction co-ordinate EaEaEaEa N 2 + O 2 2NO EaEaEaEa Energy Profile Non – catalyzed Catalyzed N 2 (g) + O 2 (g)  2NO(g) ΔHΔHΔHΔH

15 Increasing the Rate of Reaction C. Increasing collisions The more collisions there are the more likely reactants are to react. 1. Increasing the concentration of reactants (pressure in the case of gases). 2. Increasing the surface area of solid reactants (or solid catalysts). We can influence the amount of collisions by:

16 1. A catalyst lowers the activation energy of a reaction from 100 kJ/mol to 50 kJ/mol. What increase in f will result? What increase in f will result? Quiz B- Increasing reaction rates 2.The activation energy for a reaction is known to be 50 kJ/mol of reactant. If the temperature is increased from 300 K to 310 K what increase in the rate of reaction temperature is increased from 300 K to 310 K what increase in the rate of reaction would you expect? would you expect?

17 Measuring Change Consider the following reaction: How could we measure the rate of this reaction? Well let’s consider what might change during this reaction. Temperature Pressure Mass of system Concentration A (g)  B(g)

18 Measuring Change A. Temperature If the reaction is exothermic or endothermic then the temperature will change. But if the temperature changes then the rate of reaction will change! Why? * * See Maxwell Distribution Therefore if we want to measure the rate of reaction we normally keep the temperature of the reactor constant. This is called thermostatting the reactor. Exothermic; Δ H is –ve; T  Endothermic; Δ H is +ve; T  T  Rate of reaction 

19 Measuring Change B. Pressure We know from Grade 11 that : PV = nRT Therefore if we keep the volume of the thermostatted reactor constant then: RT RT V × n× n× n× n P = = const × n P  n P  n Will P A change during the reaction? Of course : P A  as n A  Will P B change during the reaction? Of course : P B  as n B  Will P TOTAL change during the reaction. NO! P TOT   n TOT and n TOT won’t change for this reaction.

20 Measuring Change

21 C. Mass of the system This one is easy – if the reactor is a closed system then the mass cannot change. Mass must be conserved. If the reaction is taking place in an open system then gases might leave the system and the mass of the system will decrease. In this particular case we have a closed system.

22 Measuring Change D. Concentration This is by far the most common means of measuring rates, because concentrations ALWAYS change during a reaction. In this example : We need to be able to measure concentrations and the most used technique is SPECTROSCOPY where a reactant or product selectively absorbs or emits electromagnetic radiation, leaving it coloured. The intensity or amount of the radiation absorbed (A) is proportional to the concentration. A  [ ] A  [ ] Normally chemists measure concentrations to determine reaction rates A substance does not need to be coloured to absorb electromagnetic radiation. CO 2 for instance is colourless as it does not absorb visible radiation but it does absorb infra-red radiation (the cause of global warming!). Therefore we could use infra-red spectroscopy to monitor [CO 2 ]. [A]  with time while [B] 

23 1. Which of the following reactions could we use P TOTAL for the measurement of the reaction rate? of the reaction rate? a) 4NH 3 (g) + 5O 2 (g)  4NO(g) + 6H 2 O(g) Quiz C- Measuring change b) O 3 (g) + NO(g)  NO 2 (g) + O 2 (g) c) 2 CaO(s) + 5C(s)  2CaC 2 (l) + CO 2 (g) 2. Predict the necessary experimental conditions to be able to measure the rate of the following reaction by monitoring the mass of the system. the following reaction by monitoring the mass of the system. 2CaO(s) + 5C(s)  2CaC 2 (l) + CO 2 (g) 3.Sulfur dioxide gas can be oxidized to sulfur trioxide gas by oxygen gas. In a particular experiment a stoichiometric amount of sulfur dioxide and oxygen are allowed to react in a glass vessel. If the initial pressure in the flask is 3 atm, are allowed to react in a glass vessel. If the initial pressure in the flask is 3 atm, what would be the predicted final pressure if the reaction goes to completion and the temperature remains constant.

24 Quiz C- Measuring change 4. Calculate the rate of CO 2 production (moles of CO 2 / dm 3 sec) from an industrial process given the data in the two graphs shown below. process given the data in the two graphs shown below.

25 Quiz C- Measuring change

26 Closed System Open System Establishing Equilibrium

27 Solution A at t = 0 sec A ⇌ B Molecules /cm 3 time Establishing Equilibrium K =K =K =K =[B][A] = 3

28 Molecules /cm 3 time Dynamic Equilibrium

29 Quiz D- Establishing Equilibrium 1.Which of the following systems are at equilibrium ?(Assuming they have been left long enough to establish equilibrium.) a) A boiling kettle. b)A sealed thermostatted test-tube containing a drop of water. c)A completely flat battery. d)A water tank under the following conditions: H2OH2OH2OH2O H2OH2OH2OH2O 5 L s L min -1

30 The Equilibrium Constant The Law of Mass action, which is independent of kinetic theory, states that for a reaction - aA + bB ⇌ cC + dD the ratio the ratio [C] c [D] d [A] a [B] b will be a constant when the system is at equilibrium. This constant is known as the equilibrium constant, K c. One must remember that the value of K c is specific to a particular reaction equation(i.e. one in which the stoichiometry is fixed) and that it is specific to a given temperature. and that it is specific to a given temperature.

31 The Equilibrium Constant For a gaseous reaction the equilibrium constant can be expressed as a ratio of the partial pressures of the products and reactants. For example the equilibrium constant for the reaction : aA(g) + bB(g) ⇌ cC(g) + dD(g) could be expressed as (P C ) c (P D ) d (P A ) a (P B ) b KP =KP =KP =KP = Note that if one uses this expression the pressures must be quoted in atmospheres. Also note that the numerical value quoted in atmospheres. Also note that the numerical value of K P might not be the same as the numerical value of K C. of K P might not be the same as the numerical value of K C.

32 Quiz E- Equilibrium Constants 1.The reaction between nitrogen gas and oxygen gas to form nitrogen dioxide gas is shown below. N 2 (g) + 2O 2 (g) ⇌ 2NO 2 (g) a) Write down an expression for the equilibrium constant, K c, for this reaction. b) K c is equal to about 2.6 × for this reaction at 25 o C. In a 1 L flask at 25 o C there are 1.0 × molecules of N 2, 3.0 × molecules of O 2 and 1.0 × there are 1.0 × molecules of N 2, 3.0 × molecules of O 2 and 1.0 × molecules of NO 2. i) Is this system at equilibrium? ii) If this system is not at equilibrium, in what direction will the reaction proceed? c)What is the numerical value of the equilibrium constants for each of the following reactions? i) ½N 2 (g) + O 2 (g) ⇌ NO 2 (g) ii) 2NO 2 (g) ⇌ N 2 (g) + 2O 2 (g)

33 Quiz E- Equilibrium Constants 2.At a given constant temperature, a 1 L flask initially containing mol of SO 2 and mol of O 2, is allowed to come to equilibrium. 80% of the SO 2 is found to have reacted to form SO 3. Calculate the equilibrium constant for the reaction - O 2 (g) + 2SO 2 (g) ⇌ 2SO 3 (g) 3. For the endothermic reaction : 2SO 3 (g) ⇌ O 2 (g) + 2SO 2 (g) state the effect on the equilibrium constant that the following disturbances will have. a) Increasing the concentration of SO 3 (g). b) Decreasing the concentration of SO 2 (g). c) Doubling the size of the reaction flask. d) Decreasing the temperature.

34 Le Chatelier’s Principle If a change is imposed upon a system at equilibrium the position of the equilibrium will shift in a direction that tends to reduce that change. Note : By changing the position of equilibrium we are not changing the equilibrium constant if the temperature remains constant. Equilibrium constants can only change if one changes the temperature of the reaction vessel.

35 Quiz F- Influencing the Equilibrium 1.Consider one of the gaseous equilibria involved in the industrial preparation of nitric acid by the Ostwald process. 2NO(g) + O 2 (g) ⇌ 2NO 2 (g) Δ H = –ve 2NO(g) + O 2 (g) ⇌ 2NO 2 (g) Δ H = –ve What qualitative effect would the following disturbances have on the position of the equilibrium? a) An increase in P NO. b) An increase in temperature. c) A decrease in reactor volume. d) An increase in pressure via the addition of an inert gas e.g. Ar.

36 Quiz F- Influencing the Equilibrium 2.Consider the following equilibria below : BaCO 3 (s) + (aq) ⇌ Ba 2+ (aq) + CO (aq) CO (aq) + H 2 O(l) ⇌ CO 2 (g) + 2OH - (aq) What qualitative effect would the following disturbances have on the position of the equilibrium? a) Making the particle size of the BaCO 3 smaller. b) Decreasing the pH of the aqueous solution.

37 Industry They want to do things as quickly and as efficiently as possible. However, sometimes, doing something quickly might not mean doing the same thing efficiently. Thermodynamic (i.e. enthalpy ) considerations might sometimes clash with kinetic (i.e. rate) considerations. Industries want to make money!

38 Quiz G- Equilibrium & rates in industry 1.Predict the conditions of temperature and pressure required to increase productivity in the following industrial processes. productivity in the following industrial processes. a) N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g) Δ H = - ve b) 4NH 3 (g) + 5O 2 (g) ⇌ 4NO (g) + 6H 2 O(g) Δ H = - ve c) 2CaO(s) + 5C(s) ⇌ 2CaC 2 (l) + CO 2 (g) Δ H = +ve

39 Quiz G- Equilibrium & rates in industry 2. An industrial process to convert X into Y has the following stoichiometry 2X(g) ⇌ Y(g) Δ H = - ve The reaction is catalysed by a solid heterogenous catalyst. Which of the following set of experimental conditions would an industrial chemist choose to optimize the reaction? X Y + X T = 300K P = 10 atm A)X Y + X T = 600K P = 1 atm B) X Y + X T = 300K P = 10 atm C)X Y + X T = 300K P = 1 atm D)


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