Sample Teaching Sequence (Hong Kong Secondary 4 6 Chemistry)

Transcription

1 Revised (1 Sept 009 Sample Teahing Suene (Hong Kong Seondary 4 6 Chemistry Topi: Chemial Equilibrium Teahing Suene Content 1.1 Reversible reations Examples of reversible reation; forward reation; reverse reation; importane of reversible reations. 1. Charateristis of hemial uilibrium Dynami uilibrium; hanges in reatant and produt onentration with time; hanges in the rates of forward and reverse reations with time; uilibrium state. 1. The uilibrium law The uilibrium law; uilibrium onstant; uilibrium position; determination of uilibrium onstants. 1.4 Prediting the diretion of a reversible reation 1.5 Fator affeting the position of uilibrium Changing onentrations of reatants and/or produts Use the uilibrium onstant and reation quotient to predit the diretion in whih a reversible reation will proeed to reah uilibrium. Use the uilibrium onstant and reation quotient to analyze the effets of hanging onentrations: adding or removing reatants and produts; hanging volume or pressure in gaseous uilibria; adding inert gases to gaseous uilibria. Adding a atalyst Effet of atalysts on the rates of forward reation and reverse reation; effet of atalysts on uilibrium onstant and uilibrium position. Changing temperature Apply van t Hoff law qualitatively to predit the effet of hanging temperature on uilibrium onstant and uilibrium position. 1.6 Controlling reversible reations Maximum yield; ost-effetiveness; the Haber proess. For illustrative purposes, a sample of teahing and learning materials for Parts is presented on pp. -1. Derek Cheung 009 1

2 1.4 Prediting the Diretion of a Reversible Reation The uilibrium onstant, K, is 18 at a given temperature for the reation of iron(iii ions with thioyanate ions. Fe + (aq + NCS (aq Fe(NCS + (aq In a ertain experiment, mole of Fe + (aq, mole of NCS (aq, and mole of Fe(NCS + (aq are mixed in a.0-litre flask. Is the system at uilibrium? If it is not, in whih diretion will the system proeed to reah uilibrium? If the initial onentrations of Fe + (aq, NCS (aq, and Fe(NCS + (aq are substituted into the uilibrium onstant expression, the value obtained is greater than the uilibrium onstant [ Fe( NCS [ Fe [ NCS This indiates that the onentration of Fe(NCS + (aq is bigger than its onentration at uilibrium and the reversible reation is not at uilibrium. In order to reah uilibrium, the value must be restored to 18. This an be ahieved if some Fe(NCS + (aq ions reat (making the numerator smaller to produe Fe + (aq and NCS (aq ions (making the denominator bigger. Thus, although both forward and reverse reations our simultaneously, the reverse reation will predominate to derease the Fe(NCS + (aq onentration until uilibrium is established. When the reverse reation ours to a greater extent than the forward reation, we say that the reversible reation proeeds from right to left to reah uilibrium. The ratio of onentrations shown above is alled reation quotient,. The reation quotient expression has the same mathematial form as the uilibrium onstant expression but involves speifi onentrations that are not neessarily uilibrium onentrations. The subsript indiates that the reation quotient is a ratio of onentrations. We an apply the reation quotient to predit the diretion of a reversible reation. When a mixture ontains only reatants, is ual to zero. When only produts are present, is ual to an infinitely large value. When both reatants and produts are present and their onentrations remain onstant, is ual to K. The diretion in whih a reversible reation will proeed is shown in the following figure: Reatants only 0 At uilibrium K Produts only Reation proeeds to the right to reah uilibrium Reation proeeds to the left to reah uilibrium If is less than K, the reversible reation will proeed from left to right to form more produts until uilibrium is ahieved. If is greater than K, the reversible reation will proeed from right to left to form more reatants until uilibrium is ahieved. If is ual to K, the reversible reation is already at uilibrium and the onentrations of reatants and produts will not hange. Derek Cheung 009

3 Consider the reation: PCl 5 (g Cl (g + PCl (g K 1.5 at 60 o C If a.0-litre ontainer is found to ontain. moles of Cl (g, 1.5 moles of PCl (g, and.0 moles of PCl 5 (g, is the system at uilibrium? If it is not, in whih diretion will the system proeed to reah uilibrium? The onentration of a gas an be expressed as mol/l; that is, moles of the gas per litre oupied. The reation quotient expression for PCl 5 (g Cl (g + PCl (g is. 1.5 [ Cl [ PCl [ PCl Beause is less than K, the onentrations of Cl (g and PCl (g are not as large as they would be at uilibrium. Thus, the system is not at uilibrium. Although both forward and reverse reations our simultaneously, the forward reation will predominate until uilibrium is established. In other words, the reversible reation will proeed from left to right to reah uilibrium. 1. K 1.5 Reation proeeds to the right to reah uilibrium At 5 o C, K is 5 for the reation H (g + I (g HI(g. Predit how the reversible reation proeeds if [H 0.1 M, [I 0.14 M, and [HI 0.8 M. The reation quotient expression is [ HI [ H [ I ( (0.1(0.14 Beause is greater than K, the reversible reation is not at uilibrium and the reverse reation will predominate until uilibrium is established. Thus, the reversible reation will proeed from right to left to form more H and I moleules until uilibrium is ahieved. Derek Cheung 009

4 1.5 Fators Affeting the Position of Equilibrium One a reversible reation has reahed uilibrium, it remains at uilibrium until it is disturbed by hanges in onentration and/or temperature Changing Conentrations of Reatants and/or Produts Adding or Removing Reatants and Produts The way in whih the position of uilibrium alters by hanging onentration an be diretly predited from the properties of uilibrium onstant. Consider the following reversible reation in a test tube: H + (aq + CrO 4 (aq Cr O 7 (aq + H O(l yellow orange In aqueous solution, hromate ions, CrO - 4, and dihromate ions, Cr O - 7, exist. The uilibrium onstant expression for the reversible reation is K [ 7 4 [ Cr O + [ H CrO For a dilute solution, water onentration is not signifiantly hanged by the reation and does not appear in the uilibrium onstant expression. Imagine that the reation mixture is allowed to reah uilibrium at a given temperature. Suppose we now add a few drops of dilute H SO 4 (aq to the mixture and keep the temperature onstant. How will an inrease in H + (aq onentration affet the uilibrium position? Immediately after adding the extra H + (aq ions, before any hemial hanges our, is less than K due to an inrease in [H + (making the denominator of the expression bigger. [ CrO + [ H [ CrO 7 4 Beause is less than K, the reation mixture is no longer at uilibrium. The reversible reation will proeed from left to right to inrease the produt onentrations until uilibrium is re-established. Therefore, although both forward and reverse reations our, the forward reation will predominate until uilibrium is re-established (i.e., the forward reation and the reverse are ourring at ual rates. In the new uilibrium state, the onentration of dihromate ions, Cr O 7 -, are greater than it was initially. This indiates that the position of uilibrium has shifted to the produt side (i.e., to the right. The mixture beomes more orange in the new uilibrium state. (Carry out a demonstration experiment or insert a photo here Atmospheri arbon dioxide reats with water in rain to form arboni aid (H CO, whih ionizes into hydrogen ion and hydrogen arbonate ion: H CO (aq H + (aq + HCO (aq That is why normal rainwater is slightly aidi. The ph of natural, unpolluted rainwater is about 5.6, but rainwater with a ph below 5.6 is alled aid rain. The uilibrium onstant expression for the ionization reation is K + [ H [ HCO [ H CO Imagine that this reversible reation is allowed to reah uilibrium in a ontainer at a given temperature. Derek Cheung 009 4

5 Suppose we now add a few drops of NaOH(aq to the mixture and keep the temperature onstant. How will addition of NaOH(aq affet the uilibrium position? The reversible reation, H CO (aq H + (aq + HCO (aq, does not involve NaOH, but the OH (aq ions from NaOH an reat with H + (aq ions to form H O. Immediately after adding NaOH(aq, the H + (aq onentration in the mixture will derease. As a result, is less than K due to a derease in [H + (making the numerator of the expression smaller. + [ H [ HCO [ H CO Beause is less than K, the reation mixture is no longer at uilibrium. The reversible reation will proeed from left to right to inrease the produt onentrations until uilibrium is re-established. Therefore, although both forward and reverse reations our, the forward reation will predominate until uilibrium is re-established (i.e., the forward reation and the reverse are ourring at ual rates. In the new uilibrium state, the onentration of HCO - (aq ions is greater than it was initially. This indiates that the position of uilibrium has shifted to the produt side (i.e., to the right. Consider the uilibrium NO(g N (g + O (g in a sealed ontainer at a given temperature. Suppose that more oxygen gas is suddenly added to the uilibrium system at onstant temperature. How will an inrease in oxygen onentration affet the position of uilibrium? The uilibrium onstant expression for the reversible reation is [ N K [ [ O NO Immediately after adding the extra oxygen, before any hemial hanges our, is greater than K due to an inrease in [O (making the numerator of the expression bigger. [ N[ O [ NO Beause is greater than K, the reation mixture is no longer at uilibrium. The reverse reation will predominate until uilibrium is re-established. The position of uilibrium will shift to the reatant side (i.e., to the left and more NO moleules will be formed. Derek Cheung 009 5

6 For gaseous uilibrium systems, we need to pay speial attention to the onditions under whih the hange in onentration of reatants and/or produts is made beause volume of the ontainer will also affet onentration of a gaseous speies. However, the ontainer volume is not important if the number of moles of gaseous produts in the balaned uation is ual to the number of moles of gaseous reatants in the balaned uation. An example is N (g + O (g NO(g. Consider the reation CO(g + H (g CH 4 (g + H O(g at uilibrium in a ontainer fitted with a movable piston (i.e., the piston an move inwards and outwards. If a small amount of CO(g is suddenly added to the uilibrium mixture at onstant pressure and temperature, what will happen to the number of H O(g moleules when uilibrium is re-established? Atmospheri pressure Temperature regulator Mixer Constant temperature fluid The expression for this reversible reation is [ CH [ H n O CH 4 H O 4 [ CO[ H n n CO H n CH 4 CO H O H ( where [CH 4 is the onentration of CH 4, n CH4 is the number of moles of CH 4, is the ontainer volume, and so on. Immediately after adding the extra CO(g, before any hemial hanges our, the number of moles of CO, n o, will inrease. The ontainer volume,, also inreases beause the piston is movable and the hange is made at onstant pressure. But the numbers of moles of CH 4, H O, and H remain unhanged. Therefore, the new position of uilibrium will depend upon the ratio /n o in the above reation quotient expression. If the new ratio in the expression > the original ratio in the K expression, then > K ; the uilibrium position will shift to the reatant side (i.e., to the left. If the new ratio < the original ratio, then < K ; the uilibrium position will shift to the produt side (i.e., to the right. Thus, the position of uilibrium an be shifted to the left or right, depending upon the initial amount of CO in the uilibrium mixture. Derek Cheung 009 6

7 At a partiular temperature, the reation N (g + H (g NH (g is at uilibrium in a sealed ontainer with fixed volume. The uilibrium mixture ontains 0.45 M N, 1.1 M H, and 0.8 M NH. (a Calulate K. (b Calulate and predit what will happen if 0.0 mol/dm N is suddenly injeted into the ontainer at onstant temperature. [ NH (0.8 (a K 0. 1 [ N [ H (0.45(1.1 (b Immediately after adding the extra N gas, before any hemial hanges our, the onentration of N beomes ( M. The reation quotient expression is [ NH [ N [ H (0.8 (0.75( Beause < K, the uilibrium position will shift to the produt side (i.e., to the right to form more NH moleules. The reation N (g + H (g NH (g is at uilibrium in a ontainer fitted with a movable piston. Predit the diretion of the shift in uilibrium position if more N gas is suddenly added to the mixture at onstant pressure and temperature. The piston is movable. We need to onsider the hange in ontainer volume,, when writing the reation quotient expression. [ NH n [ N [ H nn nh NH NH N ( H Immediately after adding the extra N gas, before any hemial hanges our, the number of moles of N and the ontainer volume must inrease. The number of moles of H and the number of moles of NH remain unhanged. Therefore, the new position of uilibrium will depend upon the ratio, /n N, in the above reation quotient expression. If the new ratio, /n N, in the expression is greater than the original ratio in the K expression, then is greater than K and the position of uilibrium must shift to the left. If the new ratio is less than the original ratio, then is less than K and the position of uilibrium must shift to the right. Thus, the position of uilibrium an be shifted to the left or right, depending upon the initial amount of N in the uilibrium mixture. Derek Cheung 009 7

8 Changing olume or Pressure in Gaseous Equilibria The onentrations of reatants and produts in a gaseous uilibrium system an be hanged by hanging the volume of the reation ontainer, without adding or removing any reatants and produts. Imagine that the following reversible reation is at uilibrium in a ontainer. How will an inrease in ontainer volume affet its uilibrium position? SO (g + O (g SO (g We need to onsider the hange in ontainer volume,, when writing the reation quotient expression. The reation quotient expression is nso [ SO [ SO [ O n SO n O SO SO ( O The effet of an inrease in ontainer volume on this system an be diretly predited from the properties of uilibrium onstant and reation quotient. Immediately after the ontainer volume is inreased and before any hemial hanges our, the value of beomes bigger. Beause inreases, the numerator of the expression beomes bigger. is greater than K. The uilibrium position will shift to the left to form more SO and O moleules. However, onsider the following reversible reation: H (g + Br HBr(g Imagine that this reversible reation is at uilibrium in a ontainer. How will an inrease in ontainer volume affet its uilibrium position? The expression is nhbr [ HBr HBr [ H [ Br nh n ( ( Br n H n Beause ual numbers of moles of gaseous substanes appear on both sides of the balaned uation, the ontainer volume,, in the numerator and denominator anels out. Thus, hanging the ontainer volume does not affet the position of uilibrium. When the ontainer volume inreases, is still ual to K. That is, the system is still at uilibrium, and no shift in uilibrium position ours. Therefore, hanging ontainer volume will shift an uilibrium system only if the sum of the oeffiients for gaseous reatants in the balaned uation is different from the sum of the oeffiients for gaseous produts. Br The reation Cl (g + PCl (g PCl 5 (g is at uilibrium in a syringe. If the volume is dereased at onstant temperature by foring the plunger inwards, will the uilibrium shift to the left or right? Derek Cheung 009 8

9 The expression for the reation is npcl 5 [ PCl 5 [ Cl[ PCl ncl n PCl Cl PCl 5 ( PCl When the volume,, is dereased at onstant temperature, is less than K and thus the uilibrium must shift to the right to form more PCl 5 moleules. The reation CH 4 (g + Cl (g CH Cl(g + HCl(g is at uilibrium in a ontainer. Predit whether this uilibrium system will shift to the right, left, or not be affeted by an inrease in the ontainer volume. Changing the ontainer volume will not affet the uilibrium position beause the sum of the oeffiients for gaseous reatants in the balaned uation is ual to the sum of the oeffiients for gaseous produts. This predition is onfirmed by the expressions; the volume,, anels out in the expression. nch Cl nhcl [ CH Cl[ HCl [ CH 4[ Cl nch n 4 Cl CH Cl CH 4 HCl Cl The reation N O 4 (g NO (g is at uilibrium in a syringe. If the volume is dereased at onstant temperature by foring the plunger inwards, will the onentration of NO (g be higher or lower than the original onentration when uilibrium is re-established? The K and expressions are K [ NO [ N O 4 n NO [ NO NO [ N O n ( 4 N O 4 N O 4 When the volume,, is dereased at onstant temperature, is greater than K and thus the uilibrium must shift to the left. As a result, the number of N O 4 moleules will inrease. Beause the syringe volume is redued, the onentration of N O 4 in the new uilibrium mixture must inrease. To keep K onstant, the onentration of NO in the new uilibrium mixture must be higher than that in the initial uilibrium mixture. (Carry out a demonstration experiment or insert a photo here Derek Cheung 009 9

10 Adding Inert Gases to Gaseous Equilibria If the sum of the oeffiients for gaseous reatants in the balaned uation for a reversible reation is different from the sum of the oeffiients for gaseous produts, the position of uilibrium an be shifted by adding an inert gas at onstant pressure and temperature. As an example, onsider the following reversible reation: CO(g + Cl (g COCl (g Suppose that this reation is at uilibrium in a ontainer fitted with a movable piston. What will happen if some argon gas is added to the mixture at onstant pressure and temperature? The expression for the reation is ncocl n ncl CO COCl CO ( Cl Beause the piston is movable, the ontainer volume,, will inrease after argon gas is added. Thus, is greater than K. The uilibrium position will shift to the left to form more CO and Cl moleules. If the reation CO(g + Cl (g COCl (g is at uilibrium in a sealed ontainer with fixed volume, then adding an inert gas will not affet the value of any of the terms in the above expression. Thus, adding an inert gas at onstant volume and temperature will not affet the position of uilibrium. The reation CO(g + O (g CO (g is at uilibrium in a ontainer fitted with a movable piston at room temperature. What will happen if some nitrogen gas is added to the uilibrium mixture at onstant pressure and temperature? The expression for the reation is nco n n CO O CO CO ( O Nitrogen does not reat with any of the three gases at room temperature. Beause the piston is movable, the ontainer volume,, will inrease after nitrogen gas is added. Thus, is greater than K. The uilibrium position will shift to the left to form more CO and O moleules Adding a Catalyst A reversible reation an reah uilibrium more quikly in the presene of a atalyst. But addition of a atalyst to an uilibrium system does not affet its uilibrium onstant and uilibrium position beause the atalyst affets the ativation energy of both forward and reverse reations ually. Derek Cheung

11 1.5. Changing Temperature Although the values of K are unaffeted by hanges in onentration or by adding atalysts, hemists have found that when temperature hanges, the values of most K also hange. The values of K at different temperatures for two reversible reations are shown in the table below. The orresponding enthalpy hanges for the forward reations are also shown. N O 4 (g NO (g H 0 57 kj mol -1 NO(g N (g + O (g H kj mol -1 Temperature/K K Temperature/K K x x x x x x 10 In the uilibrium system N O 4 (g NO (g, olourless dinitrogen tetraoxide (N O 4 moleules dissoiate to form brown nitrogen dioxide (NO moleules and this forward reation is endothermi ( H 0 > 0. The intensity of brown olour an be used to measure the onentration of nitrogen dioxide in the uilibrium mixture. The uilibrium onstant inreases as the temperature inreases from 98 K to 600 K. This indiates that inreasing temperature an shift the uilibrium position to the right to form more NO moleules. Thus, if we allow this reversible reation to reah uilibrium in a sealed flask, inreasing the reation temperature will make the uilibrium mixture appear darker. (Carry out a demonstration experiment or insert a photo here In the uilibrium system NO(g N (g + O (g, the forward reation is exothermi ( H 0 < 0. The uilibrium onstant dereases as the temperature inreases from 98 K to 00 K. This indiates that inreasing temperature an shift the uilibrium position to the left to form more NO moleules. The uilibrium mixture will ontain more NO moleules at a higher temperature. Jaobus Henrius van t Hoff, a Duth hemist and the winner of the first Nobel Prize in hemistry, disovered the effets of hanging temperature on uilibrium position: Inreasing temperature will shift the uilibrium position of a reversible reation in the endothermi diretion, whilst dereasing temperature will shift the uilibrium position of a reversible reation in the exothermi diretion. van t Hoff ( The reation below is at uilibrium in a beaker. What will happen to the uilibrium position if the temperature of the mixture is inreased? Co(H O + 6 (aq + 4 Cl - (aq CoCl - 4 (aq + 6 H O(l H 0 > 0 pale pink deep blue Due to the presene of pink and blue ions, the uilibrium mixture is usually violet in olour. However, an inrease in temperature will shift the uilibrium position of the reversible reation in the endothermi diretion. The above thermohemial uation indiates that the forward reation is endothermi. Thus, Derek Cheung

12 the uilibrium position will shift to the right when temperature is inreased. The uilibrium mixture will appear dark blue at a higher temperature. (Carry out a demonstration experiment or insert a photo here The reation below is at uilibrium in a sealed flask. What will happen to the uilibrium position if the temperature of the mixture is inreased? SO (g + O (g SO (g H kj mol -1 The thermohemial uation indiates that the reverse reation is endothermi. Thus, the uilibrium position will shift to the left when temperature is inreased. The uilibrium mixture will ontain more SO and O moleules at a higher temperature. 1.6 Controlling Reversible Reations: The Haber Proess In 009, the world population is 6.79 billion. To make the large quantities of fertilizers needed for present-day agriulture, hemists synthesize ammonia from its elements: N (g + H (g NH (g H 0-9 kj mol -1 This famous reversible reation is known as the Haber proess. How an hemists obtain the maximum yield of ammonia in a reasonable time and at a reasonable ost? The forward reation is exothermi. Therefore, dereasing temperature an shift the uilibrium position to the right to yield more NH moleules. However, the reation is slow at low temperatures. To inrease the reation rate, a atalyst may be used. An effetive atalyst for the Haber proess is Fe O 4 mixed with KOH, Al O, and SiO. Beause the atalyst does not work below 400 o C, the operating temperature is usually about 450 o C. Should hemists use low or high pressure to inrease the yield of NH? We an apply the onept of reation quotient to predit the pressure that we should use. The reation quotient expression for the Haber proess is [ NH n [ N [ H nn nh NH NH N ( H To give the maximum yield of NH, we need to make smaller than K so that the uilibrium position shifts to the right. Sine the total volume,, is in the numerator, an inrease in pressure of the gaseous system will redue and thus make smaller than K. Therefore, the Haber proess is arried out at high pressure. But the higher the pressure, the greater the ost will be. The operating pressure is usually about 00 atm. The boiling point of NH (- o C is muh higher than that of N (-196 o C and H (-5 o C. To further shift the uilibrium position to the right, the produt gas is hilled to liquefy and remove NH gas from the reator. As a result, the number of moles of NH, n NH, dereases and is less than K. Derek Cheung 009 1

13 To summarize, the following onditions are used by hemists to produe ammonia by the Haber proess: Fe O 4 is used as a atalyst A relatively low temperature (450 o C High pressure (00 atm NH is ontinuously liquefied and removed ompressed N and H heating oil hot N and H reator with atalyst NH and unreated N and H reyling pump unreated N and H ooling oil Liquid NH storage tank Derek Cheung 009 1