2 Overview Quiz A What prevents reaction from going? Increasing the Rate ofreactionTemperatureQuiz BCatalysisCollisionsIndustryRates & Extents ofReactionsMeasuring ChangeTemperatureQuiz CQuiz GPressureConcn.Mass ofSystemLe Chatelier’s PrincipleThe Equilibrium ConstantEstablishing EquilibriumQuiz FQuiz EDynamicEqm.Quiz D
3 Quiz A- Common misconceptions When solid copper carbonate reacts with excess acid, carbon dioxide is produced.The curves shown were obtained under two different sets of conditions.XVolumeCO2oftimeYThe change from X to Y could be brought about by :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.
4 Quiz A- Common misconceptions 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 ofthe reverse reaction.D. the reaction goes to completion.E. the ratio of the products to reactants is equal to exactly 1.
5 Quiz A- Common misconceptions X(g) is placed in a flask and the following reaction proceeds to equilibriumX(g) ⇌ Y(g) ΔH = +veWhich 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 willalways be higher than the activation energy of the reverse reaction no matterthe 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 theinitial equilibrium.E. The forward reaction is completed before the reverse reaction begins.
6 Quiz A- Common misconceptions The graph below shows the variation of concentration of a reactant , X, with timeas the reaction proceeds.[X] / mol.dm-3Time /s30600.0200.005What is the average reaction rate (mol dm-3 s-1) during the first 30 seconds?AB 0.005C 0.015D 0.020E The slope of the tangent to the curve at t = 30s
7 Quiz A- Common misconceptions Excess marble chips (CaCO3) were added to 50 cm3 of 1 M HCl. The experimentwas repeated using the same mass of marbles chips and 50 cm3 of 1M CH3COOH.Which of the following would have been the same for both experiments?A The average rate of reaction.B The rate at which the first 2 cm3 of gas was evolved.C The time taken for the reaction to be complete.D The mass of the marble chips remaining after the reaction had stopped.E None of the above.
8 What prevents reactions from going? Potential EnergyReaction co-ordinateEaEnergy Profile2NO N2 + O22NOΔHN2 + O2
9 Rate = k [A]x [B]y Increasing the Rate of Reaction Orders of reactants Rate constantRate = k [A]x [B]yNon-examinable ! But useful for understanding.
10 Increasing the Rate of Reaction A. TemperatureEveryone knows that as we increase the temperature, the reaction rate goes up……But how do we explain this observation scientifically?QAThe answer lies in the Maxwell Distribution where the idea ofa range of molecular speeds is introduced.
11 What makes reactions go faster? Fraction of moleculesMolecular VelocityT2Maxwell Distribution of velocitiesT2 > T1KEmin = ½mv2minvminT2T1
12 Increasing the Rate of Reaction If we increase the temperature we will increase the fraction of the molecules withthe energy to overcome the activation energy.f = e–Ea/RTWhere f = fraction of molecularcollisions with energy greater orequal to EaIf T then f If f then k If k then RateNote : If Ea due to a catalyst then f and the Rate
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 alower activation energy, Ea.Catalysts can either be in the same phase as the reactant –homogeneous catalysts -or a different phase –heterogeneouscatalysts.
14 Increasing the Rate of Reaction Potential EnergyReaction co-ordinateEaEnergy ProfileN2(g) + O2(g) 2NO(g)Ea2NOΔHN2 + O2ONNONOCatalyzedNon – catalyzedNNONOO
15 Increasing the Rate of Reaction C. Increasing collisionsThe more collisions there are the more likely reactants are to react.We can influence the amount of collisions by:1. Increasing the concentration of reactants (pressure in the case of gases).2. Increasing the surface area of solid reactants (or solid catalysts).
16 Quiz B- Increasing reaction rates A catalyst lowers the activation energy of a reaction from 100 kJ/mol to 50 kJ/mol.What increase in f will result?The activation energy for a reaction is known to be 50 kJ/mol of reactant. If thetemperature is increased from 300 K to 310 K what increase in the rate of reactionwould you expect?
17 Q A Measuring Change A (g) B(g) Consider the following reaction: How could we measure the rate of this reaction?AWell let’s consider what might change during this reaction.Im bending your spoonwith my mind! ! ! ! !Mass of systemTemperaturePressureConcentration
18 Measuring Change A. Temperature * See Maxwell Distribution If the reaction is exothermic or endothermic then the temperature will change.Exothermic; ΔH is –ve; T Endothermic; Δ H is +ve; T But if the temperature changes then the rate of reaction will change! Why? *T Rate of reaction Therefore if we want to measure the rate of reaction we normally keep thetemperature of the reactor constant. This is called thermostatting the reactor.Therefore we cannot measure the rate of a reactionusing the change in temperature* See Maxwell Distribution
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:RTV× nP == const × nP nWill PA change during the reaction?QAOf course : PA as nA Will PB change during the reaction?QAOf course : PB as nB Will PTOTAL change during the reaction.QANO! PTOT nTOT and nTOT won’t change for this reaction .
20 Measuring Change Therefore we cannot measure the rate of this reaction using ptotal
21 Measuring Change 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 systemand the mass of the system will decrease.In this particular case we have a closed system.Therefore we cannot measure the rate of This reactionusing the mass of the system
22 Measuring Change D. Concentration This is by far the most common means of measuring rates, because concentrationsALWAYS change during a reaction. In this example :[A] with time while [B] We need to be able to measure concentrations and the most used techniqueis SPECTROSCOPY where a reactant or product selectively absorbs or emitselectromagnetic radiation, leaving it coloured.The intensity or amount of the radiation absorbed (A) is proportional to the concentration.A [ ]Normally chemists measure concentrations to determine reaction ratesA substance does not need to be coloured to absorb electromagnetic radiation.CO2 for instance is colourless as it does not absorb visible radiation but it doesabsorb infra-red radiation (the cause of global warming!). Therefore we coulduse infra-red spectroscopy to monitor [CO2].
23 Quiz C- Measuring change 1. Which of the following reactions could we use PTOTAL for the measurementof the reaction rate?a) 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)b) O3(g) + NO(g) NO2(g) + O2(g)c) 2CaO(s) + 5C(s) 2CaC2(l) + CO2(g)2. Predict the necessary experimental conditions to be able to measure the rate ofthe following reaction by monitoring the mass of the system.2CaO(s) + 5C(s) 2CaC2(l) + CO2(g)Sulfur dioxide gas can be oxidized to sulfur trioxide gas by oxygen gas. In aparticular experiment a stoichiometric amount of sulfur dioxide and oxygenare 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 completionand the temperature remains constant.
24 Quiz C- Measuring change 4. Calculate the rate of CO2 production (moles of CO 2 / dm3 sec) from an industrialprocess given the data in the two graphs shown below.
29 Quiz D- Establishing Equilibrium Which of the following systems are at equilibrium ?(Assuming they have been left longenough 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:WaterH2O5 L s-1300 L min-1
30 aA + bB ⇌ cC + dD The Equilibrium Constant The Law of Mass action, which is independent of kinetic theory,states that for a reaction-aA + bB ⇌ cC + dDthe ratio[C]c [D]d[A]a [B]bwill be a constant when the system is at equilibrium. Thisconstant is known as the equilibrium constant, Kc.One must remember that the value of Kc is specific to a particularreaction equation(i.e. one in which the stoichiometry is fixed)and that it is specific to a given temperature.
31 aA(g) + bB(g) ⇌ cC(g) + dD(g) The Equilibrium ConstantFor a gaseous reaction the equilibrium constant can beexpressed as a ratio of the partial pressures of the productsand reactants. For example the equilibrium constant for thereaction :aA(g) + bB(g) ⇌ cC(g) + dD(g)could be expressed as(PC)c (PD)d(PA)a (PB)bKP =Note that if one uses this expression the pressures must bequoted in atmospheres. Also note that the numerical valueof KP might not be the same as the numerical value of KC.
32 Quiz E- Equilibrium Constants The reaction between nitrogen gas and oxygen gas to form nitrogen dioxide gasis shown below.N2(g) + 2O2(g) ⇌ 2NO2(g)a) Write down an expression for the equilibrium constant, Kc, for this reaction.b) Kc is equal to about 2.6 × for this reaction at 25 oC. In a 1 L flask at 25 oCthere are 1.0 × molecules of N2, 3.0 × 1015 molecules of O2 and 1.0 × 1012molecules of NO2.i) Is this system at equilibrium?ii) If this system is not at equilibrium, in what direction will thereaction proceed?What is the numerical value of the equilibrium constants for each of the followingreactions?i) ½N2(g) + O2(g) ⇌ NO2(g)ii) 2NO2(g) ⇌ N2(g) + 2O2(g)
33 Quiz E- Equilibrium Constants At a given constant temperature, a 1 L flask initially containing mol of SO2 and0.150 mol of O2, is allowed to come to equilibrium. 80% of the SO2 is found to havereacted to form SO3. Calculate the equilibrium constant for the reaction-O2(g) + 2SO2(g) ⇌ 2SO3(g)3. For the endothermic reaction :2SO3(g) ⇌ O2(g) + 2SO2(g)state the effect on the equilibrium constant that the following disturbances will have.a) Increasing the concentration of SO3(g).b) Decreasing the concentration of SO2(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 theposition of the equilibrium will shift in a directionthat tends to reduce that change.Note : By changing the position of equilibrium we are not changing theequilibrium constant if the temperature remains constant. Equilibriumconstants can only change if one changes the temperature of the reactionvessel.
35 Quiz F- Influencing the Equilibrium Consider one of the gaseous equilibria involved in the industrial preparationof nitric acid by the Ostwald process.2NO(g) + O2(g) ⇌ 2NO2(g) ΔH = –veWhat qualitative effect would the following disturbances have on the position ofthe equilibrium?a) An increase in PNO.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 :BaCO3(s) + (aq) ⇌ Ba2+(aq) + CO32- (aq)CO32- (aq) + H2O(l) ⇌ CO2(g) + 2OH-(aq)What qualitative effect would the following disturbances have on the positionof the equilibrium?a) Making the particle size of the BaCO3 smaller.b) Decreasing the pH of the aqueous solution.
37 Industries want to make money! IndustryIndustries want to make money!They want to do things as quickly and as efficiently as possible.However, sometimes, doing something quickly might not meandoing the same thing efficiently.Thermodynamic (i.e. enthalpy ) considerations might sometimesclash with kinetic (i.e. rate) considerations.
38 Quiz G- Equilibrium & rates in industry Predict the conditions of temperature and pressure required to increaseproductivity in the following industrial processes.a) N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -veb) 4NH3(g) + 5O2(g) ⇌ 4NO (g) + 6H2O(g) ΔH = -vec) 2CaO(s) + 5C(s) ⇌ 2CaC2(l) + CO2(g) ΔH = +ve
39 Quiz G- Equilibrium & rates in industry 2. An industrial process to convert X into Y has the following stoichiometry2X(g) ⇌ Y(g) ΔH = -veThe reaction is catalysed by a solid heterogenous catalyst. Which of the following setof experimental conditions would an industrial chemist choose to optimize the reaction?XY + XT = 300K P = 10 atmA)XY + XT = 600K P = 1 atmB)XY + XT = 300K P = 10 atmC)XY + XT = 300K P = 1 atmD)Catalyst