# All reversible reactions reach an dynamic equilibrium state. H 2 O+ CO H 2 +CO 2. Rate of reverse reaction (H 2 + CO 2 )

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1 4.2 Equilibria Many reactions are reversible + 3H 2 2NH 3 All reversible reactions reach an dynamic equilibrium state. Dynamic equilibrium occurs when forward and backward reactions are occurring at equal rates. The concentrations of reactants and products stays constant and the reaction is continuous. We use the expression position of equilibrium to describe the composition of the equilibrium mixture. If the position of equilibrium favours the reactants (also described as towards the left ) then the equilibrium mixture will contain mostly reactants. Reaction rate H 2 O+ CO H 2 +CO 2 Eventually the rates become equal and a state of equilibrium has been reached. Equilibrium Forward rate = backward rate Rate of forward reaction (H 2 O+ CO ) When H 2 O and CO are initially mixed the forward reaction is relatively fast and the backward reaction cannot occur. As the concentrations of H 2 O and CO decrease the forward reaction will slow down. Rate of reverse reaction (H 2 + CO 2 ) As the concentrations of H 2 and CO 2 increase the backward reaction will speed up. Le Chatelier s Principle We use Le Chatelier s principle to work out how changing external conditions such as temperature and pressure affect the position of equilibrium Le Chatelier s principle states that if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it). Effect of Changing Concentration A + B C + D If extra A is added to the equilibrium mixture the forward reaction occurs more to oppose the change and remove some of the extra A and form extra C and D. The equilibrium has moved to the right. If C is removed from the equilibrium mixture the forward reaction occurs more to oppose the change and replace some of the C removed; the amount of D therefore increases whilst A and B are used up. The equilibrium has moved to the right. N Goalby chemrevise.org 1

2 Effect of Concentration on equilibrium I 2 + 2OH - brown I - + IO - + H 2 O colourless Increasing the concentration OH - ions causes the equilibrium to shift to oppose this and move in the forward direction to remove OH - ions. The position of equilibrium will shift towards the right, giving a higher yield of I - and IO -. ( The colour would change from brown to colourless) Adding H + ions reacts with the OH - ions and reduces their concentration so the equilibrium shifts back to the left giving brown colour. Co(H 2 O) 2+ 6 (aq) + 4Cl - (aq) pink CoCl 2-4 (aq) + 6H 2 O(l) blue If the concentration of Cl - ions is increased by adding extra HCl to an equilibrium mixture, the position of equilibrium will shift towards the products and thereby removing some the of the extra Cl - ions added. So the colour will change from pink to blue. Effect of Temperature on equilibrium If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try and reduce the temperature by absorbing heat. And its reverse If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try and increase the temperature by giving out heat. Typical Exam question: What effect would increasing temperature have on the yield of ammonia? + 3H 2 2NH 3 H = -ve exo Exam level answer : must include bold points If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia. Low temperatures may give a higher yield of product but will also result in slow rates of reaction. Often a compromise temperature is used that gives a reasonable yield and rate Effect of Pressure on equilibrium Typical Exam question: What effect would increasing Increasing pressure will cause the equilibrium to shift pressure have on the yield of methanol? towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure. CO (g) + 2H 2(g) CH 3 OH (g) And its reverse Decreasing pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change and thereby increase the pressure. If the number of moles of gas is the same on both sides of the equation then changing pressure will have no effect on the position of equilibrium H 2 + Cl 2 2HCl Exam level answer : must include bold points If pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure. The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol. Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs for pumping the gases to make a high pressure) and the equipment is expensive (to contain the high pressures) N Goalby chemrevise.org 2

3 Effect of Catalysts on equilibrium A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved. It does not effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount. Catalysts speeds up the rate allowing lower temperatures to be used (and hence lower energy costs) but have no effect on equilibrium. Environmental benefits of Catalysts Catalysed reactions can occur at lower temperature so less fuel needed and fewer emissions from fuels. Catalysed reaction enables use of an alternative process with higher atom economy so meaning fewer raw materials needed and less waste products are produced Importance of equilibrium to industrial processes Both the rate of reaction and percentage yield must be considered. The key quantity to consider is mass of product produced per unit time. Having a fast rate of reaction does not mean necessarily that a good yield will be achieved. Sometimes a compromise is needed. Optimum conditions of temperature and pressure are used that give the lowest cost, rather than the fastest reaction or highest percentage yield. Industrial processes cannot exist in true equilibrium since the products are removed as they are formed to improve conversion of reactants. They are not closed systems Haber process + 3H 2 2NH 3 H = -ve exo The Haber process is used to manufacture ammonia, which can be used to produce nitrogen-based fertilisers. The raw materials for the Haber process are nitrogen and hydrogen. Nitrogen is obtained from the air and hydrogen may be obtained from natural gas or other sources. Conditions used T= 450 o C, P= atm, catalyst = iron Low temperature gives good yield as equilibrium reaction will shift in the exothermic reaction to counteract the low temperature. A low temperature will produce a slow rate of reaction. Therefore a compromise temperature of C is used that gives a fast reaction without giving a very small percentage yield A high pressure gives good yield because the reaction will shift to the products as there are fewer moles of gas on the products side. This opposes the low pressure. In general increasing pressure will increase reaction rate. It is worth considering that Increasing pressure has a limited effect on the rate of heterogeneous catalysed reactions because the reaction takes place on surface of the iron catalyst. The active sites on the iron catalyst surface are already saturated with reactant molecules so increasing pressure wont make a significant increase in rate. High pressures are expensive to generate. Too high a pressure would lead to too high electrical energy costs for pumps to produce the pressure. A moderately high pressure is used that balances the rate and yield with the cost of producing the pressure. In addition, the higher the pressure used, the higher the cost of the equipment needed to withstand the pressure. The catalysts speeds up the rate of reaction which allows a lower temperature to be used to get the required rate of reaction. This significantly lowers the energy costs. On leaving the reactor the gases enter a cooler. On cooling, the ammonia liquefies and is removed. Nitrogen and hydrogen stay as gases The unreacted nitrogen and hydrogen are recycled back in to the reactor, so no raw materials are wasted and improve their atom economy N Goalby chemrevise.org 3

4 Uses of ammonia Making fertilisers Reacting ammonia with acids can make fetilisers such as ammonium sulfate, ammonium nitrate or ammonium phosphate. Making nitric acid Ammonia is used to make nitric acid. Uses of nitric acid make fertilisers such as ammonium nitrate. make explosives (e.g. TNT ) making various dyes, drugs, polymers 2NH 3 + H 2 SO 4 (NH 4 ) 2 SO 4 Ammonia + sulfuric acid ammonium sulfate NH 3 + HNO 3 NH 4 NO 3 Ammonia + nitric acid ammonium nitrate Contact process Sulfuric acid is produced by the contact process. There are several stages in the process. Stage 1 Sulfur is burnt in air to produce sulfur dioxide S (s) + O 2 (g) SO 2 (g) Stage 2 Sulfur dioxide is reacted with oxygen to produce sulfur trioxide in a reversible reaction SO 2 (g) + ½ O 2 (g) SO 3 (g) H = -98 kj mol -1 Conditions used T= 450 o C, P= 10 atm, catalyst = V 2 O 5 Low temperatures gives higher yields as the equilibrium reaction will shift in the exothermic reaction to oppose the low temperature. Low temperatures will produce a slow rate of reaction. Therefore a compromise temperature of C is used that gives a fast reaction without giving a very small percentage yield The pressure used is just above atmospheric pressure. Higher pressure will not have much of an effect on the yield as there is only a slight increase in the moles of gaseous products compared to the moles of reactants. The vanadium (V) oxide catalyst used is a heterogeneous catalyst. Increasing pressure has a limited effect on the rate of heterogeneous catalysed reactions because the reaction takes place on surface of the catalyst. The active sites on the V 2 O 5 catalyst surface are already saturated with reactant molecules so increasing pressure wont make a significant increase in rate. The vanadium (V) oxides catalyses the reaction in a two step reaction. The V 2 O 5 changes oxidation number in the first step but reforms in the second step step 1 SO 2 + V 2 O 5 SO 3 + V 2 O 4 step 2 2V 2 O 4 + O 2 2V 2 O 5 Stage 3: Sulphur trioxide is dissolved in 98% sulfuric acid to produce fuming sulfuric acid (called Oleum). The Oleum is the diluted to produce concentration sulfuric acid. The sulfur trioxide is not dissolved directly in water because the reaction is too exothermic. The heat produced causes the sulfuric acid to evaporate SO 3 (g) + H 2 SO 4 (l) H 2 S 2 O 7 (l) H 2 S 2 O 7 (l) + H 2 O (l) H 2 SO 4 (l) N Goalby chemrevise.org 4

5 The Equilibrium law: Writing an expression for Equilibrium constant Kc For a generalised reaction Kc = equilibrium constant ma + nb pc + qd m,n,p,q are the stoichiometric balancing numbers Kc= A,B,C,D stand for the chemical formula [ C] p [D] q [ A] m [B] n [ ] means the equilibrium concentration Example 1 (g) + 3H 2 (g) Kc= [NH 3 (g) ] 2 2 NH 3 (g) [ (g) ] [H 2 (g) ] 3 Working out the unit of Kc The unit of Kc changes and depends on the equation. Put the unit of concentration (mol dm -3 ) into the Kc equation [NH 3 (g) ] 2 Cancel out Kc= [mol dm-3 ] 2 units 1 Unit = Unit = [ (g) ] [H 2 (g)] 3 [mol dm -3 ] [mol dm -3 ] 3 [mol dm -3 ] 2 Unit = [mol dm -3 ] -2 Unit = mol -2 dm +6 Example 2: writing Kc expression Working out the unit H 2 (g) +Cl 2 (g) 2HCl [HCl (g) Kc= (g) ] 2 [mol dm -3 ] Unit Kc= 2 [H 2 (g) ] [Cl 2 (g)] [mol dm -3 ] [mol dm -3 ] = no unit Calculating Kc Most questions first involve having to work out the equilibrium moles and then concentrations of the reactants and products. Usually the question will give the initial amounts (moles) of the reactants, and some data that will help you work out the equilibrium amounts. Calculating the moles at equilibrium moles of reactant at equilibrium = initial moles moles reacted moles of product at equilibrium = initial moles + moles formed N Goalby chemrevise.org 5

6 Example 3 For the following equilibrium H 2 (g) +Cl 2 (g) 2HCl (g) In a container of volume 600cm 3 there were initially 0.5mol of H 2 and 0.6 mol of Cl 2. At equilibrium there were 0.2 moles of HCl. Calculate Kc H 2 Cl 2 HCl Initial moles Equilibrium moles 0.2 It is often useful to put the mole data in a table. Using the balanced equation if 0.2 moles of HCl has been formed it must have used up 0.1 of Cl 2 and 0.1 moles of H 2 (as 1:2 ratio) Work out the moles at equilibrium for the reactants moles of hydrogen at equilibrium = = 0.4 H 2 Cl 2 HCl Initial moles Equilibrium moles moles of reactant at equilibrium = initial moles moles reacted moles of chlorine at equilibrium = = 0.5 If the Kc has no unit then there are equal numbers of reactants and products. In this case you do not have to divide by volume to work out concentration and equilibrium moles could be put straight into the kc expression Equilibrium concentration (M) 0.4/0.6 = /0.6 = /0.6 =0.33 Kc= [HCl (g) ] 2 [H 2 (g) ] [Cl 2 (g)] Work out the equilibrium concentrations conc = moles/ vol (in dm 3 ) Finally put concentrations into Kc expression Kc = x0.83 = no unit Example 4 For the following equilibrium (g) + 3H 2 (g ) 2 NH 3 (g) Initially there were 1.5 moles of and 4 mole of H 2, in a 1.5 dm 3 container. At equilibrium 30% of the Nitrogen had reacted. Calculate Kc H 2 NH 3 Initial moles Equilibrium moles 30% of the nitrogen had reacted = 0.3 x1.5 = 0.45 moles reacted. Using the balanced equation 3 x 0.45 moles of H 2 must have reacted and 2x 0.45 moles of NH 3 must have formed Work out the moles at equilibrium for the reactants and products moles of reactant at equilibrium = initial moles moles reacted moles of nitrogen at equilibrium = = 1.05 moles of hydrogen at equilibrium = x3 = 2.65 moles of product at equilibrium = initial moles + moles formed moles of ammonia at equilibrium = 0 + (0.45 x 2) = 0.9 H 2 NH 3 Initial moles Finally put concentrations into Kc expression Equilibrium moles Equilibrium concentration (M) 1.05/1.5 = /1.5 = /1.5 =0.6 Kc= [NH 3 (g) ] 2 [ (g) ] [H 2 (g)] 3 Work out the equilibrium concentrations conc = moles/ vol (in dm 3 ) Kc = x = mol -2 dm +6 N Goalby chemrevise.org 6

7 Calculating the amounts of the equilibrium mixture from Kc Using algebra it is possible to work out the amounts of each component in an equilibrium mixture using the value of Kc Example 5 For the following equilibrium H 2 (g) + Br 2 (l) 2HBr (g) If mol of H 2 and mol of Br 2 are mixed and allowed to reach equilibrium. If Kc = what are the equilibrium amounts of each substance. Kc= [HBr (g) ] 2 [H 2 (g) ] [Br 2 (g)] Make x = moles of H 2 that have reacted at equilibrium V = volume of container 0.21 = 0.21 = (2x/V) 2 (0.2-x)/V. (0.2-x)/V (2x) 2 (0.2-x). (0.2-x) This reaction is equimolar (same number of moles of reactant as products) so it is possible to cancel out the volume V 0.21 = (2x) 2 (0.2-x) 2 Square root both sides 2x 0.21 = 0.2-x Rearrange to give x 0.458(0.2-x) = 2x x = 2x = 2x x x = /2.458 x = This example has been carefully set up to avoid the formation of a quadratic equation. It is not on the A-level syllabus to solve quadratic equations. So at equilibrium Moles of H 2 = = mol Moles of Br 2 = = 0.163mol Moles of HBr = 2x = mol N Goalby chemrevise.org 7

8 Practical: Working out equilibrium constant Kc A common experiment is working out the equilibrium constant for an esterification reaction. Ethanol and ethanoic acid are mixed together with a sulphuric acid catalyst. CH 3 CO 2 H + CH 3 CH 2 OH CH 3 CO 2 CH 2 CH 3 + H 2 O Ethanoic acid Ethanol Ethyl Ethanoate Method Part 1 Preparing the equilibrium mixture 1 Use burettes to prepare a mixture in boiling tube of carboxylic acid, alcohol, and dilute sulfuric acid. 2 Swirl and bung tube. Leave the mixture to reach equilibrium for one week Part 2 Titrating the equilibrium mixture 1 Rinse a 250 cm 3 volumetric flask with distilled water. Use a funnel to transfer the contents of the boiling tube into the flask. Rinse the boiling tube with water and add the washings to the volumetric flask. 2 Use distilled water to make up the solution in the volumetric flask to exactly 250 cm 3. Stopper the flask, then invert and shake the contents thoroughly. 3 Use the pipette to transfer 25.0 cm 3 of the diluted equilibrium mixture to a 250 cm 3 conical flask. 4 Add 3 or 4 drops of phenolphthalein indicator to the conical flask. 5 Set up the burette with sodium hydroxide solution.. 6 Add the sodium hydroxide solution from the burette until the mixture in the conical flask just turns pink. Record this burette reading in your table. 7 Repeat the titration until you obtain a minimum of two concordant titres. The sodium hydroxide will react with the sulphuric acid catalyst and any unreacted carboxylic acid in the equilibrium mixture There are many different calculations that can be based on this experiment. Let s look at general stages. Not all calculations will use all the stages. Working out initial amount of moles of reactants The amount of moles of alcohol and carboxylic acid can be calculated from the densities and volumes of liquids added Mass = density x volume then Moles = mass x Mr The initial amount of moles of acid catalyst used is usually determined by titrating a separate sample of catalyst with sodium hydroxide Working out equilibrium amount of moles of acid present from the titre results 39.0 cm 3 of mol dm -3 sodium hydroxide was used in the above titration. The initial moles of sulphuric acid was 5x10-4 mol. Calculate the moles of ethanoic acid present at equilibrium Amount of NaOH = vol X conc = x = mol So total amount of H + present in 25cm 3 = mol So total amount of H + present in 250cm 3 = mol Total mol acid present = moles of carboxylic acid + moles of acid catalyst So Amount of carboxylic acid at equilibrium = (5x10-4 x 2) = mol X 2 because H 2 SO 4 has 2 H + The pink colour of the phenolphthalein in the titration can fade after the end-point of the titration has been reached because the addition of sodium hydroxide may make the equilibrium shift towards the reactants N Goalby chemrevise.org 8

9 Working out equilibrium amount of moles of other substances Calculate the equilibrium amount of ethanol, ethyl ethanoate and water if there were initially mol of ethanol and mol of ethanoic acid and at equilibrium there were mol of ethanoic acid. Amount of ethanoic acid that reacted = initial amount equilibrium amount = = 0.344mol Amount of ethanol at equilibrium = initial amount - amount that reacted = = mol Amount of ethyl ethanoate at equilibrium = initial amount + amount that formed = = mol Amount of water at equilibrium = initial amount + amount that formed = = mol The amount of water at equilibrium would not really be 0 as there would be water present in the acid catalyst Calculating the equilibrium constant Finally calculate the equilibrium constant. To work out equilibrium concentrations divide the equilibrium amounts by the total volume. Then put in Kc expression Kc = [CH 3 CO 2 CH 2 CH 3 ] [H 2 O] [CH 3 CO 2 H] [CH 3 CH 2 OH] In order to confirm that one week was sufficient time for equilibrium to be established in the mixture from Part 1, several mixtures could be made and left for different amount of time. If the resulting Kc is the same value then it can be concluded the time is sufficient N Goalby chemrevise.org 9

10 Partial Pressures and Kp If a reaction contains gases an alternative equilibrium expression can be set up using the partial pressures of the gases instead of concentrations. Partial Pressure The partial pressure of a gas in a mixture is the pressure that the gas would have if it alone occupied the volume occupied by the whole mixture. If a mixture of gases contains 3 different gases then the total pressure will equal the 3 partial pressure added together P =p 1 + p 2 + p 3 partial pressure = mole fraction x total pressure of gas 1 of gas 1 p 1 = x 1 P mole fraction mole fraction = number of moles of a gas total number of moles of all gases For a 3 part mixture x 1 = y1 y 1 +y 2 +y 3 Example 6 : A mixture contains 0.2 moles, 0.5 moles O 2 and 1.2 moles of CO 2. If the total pressure is 3kPa. What are the partial pressures of the 3 gases? Total moles of gas = = 1.9 mole fraction of = 0.2/1.9 =0.105 Partial pressure =0.105x 3 of =0.315 mole fraction of O 2 = 0.5/1.9 =0.263 Partial pressure of O 2 = x 3 =0.789 mole fraction of CO 2 = 1.2/1.9 =0.632 Partial pressure of CO 2 = x 3 =1.896 Writing an expression for K p (g) + 3H 2 (g) Kp= p 2 NH 3 p p 3 H 2 2 NH 3 (g) p means the partial pressure of that gas Kp = equilibrium constant Only include gases in the K p expression. Ignore solids, liquids, and aqueous substances. N Goalby chemrevise.org 10

11 Working out the unit of Kp Put the unit of pressure(kpa) into the Kp equation Kp= p 2 NH 3 (g) p (g) p 3 H 2 (g) Cancel out units Unit = Unit = Unit = kpa -2 kpa 2 kpa kpa 3 1 kpa 2 However, if the equation is written the other way round, the value of Kp will be the inverse of above and the units will be kpa 2. It is important therefore to write an equation when quoting values of Kp. Heterogeneous equilibria for Kp Kp expressions only contain gaseous substances. Any substance with another state is left out CaCO 3 (s) CaO (s) + CO 2 (g) Kp =p CO 2 Unit kpa Example 7 For the following equilibrium (g) + 3H 2 (g ) 2 NH 3 (g) 1 mole of and 3 moles of H 2 are added together and the mixture is allowed to reach equilibrium. At equilibrium 20% of the has reacted. If the total pressure is 2kPa what is the value of Kp? H 2 NH 3 Initial moles Equilibrium moles 20% of the nitrogen had reacted = 0.2 x1.0 = 0.2 moles reacted. Using the balanced equation 3 x 0.2 moles of H 2 must have reacted and 2x 0.2 moles of NH 3 must have formed Work out the moles at equilibrium for the reactants and products moles of reactant at equilibrium = initial moles moles reacted moles of nitrogen at equilibrium = = 0.8 moles of hydrogen at equilibrium = x3 = 2.40 moles of product at equilibrium = initial moles + moles formed moles of ammonia at equilibrium = 0 + (0.2 x 2) = 0.4 H 2 NH 3 Initial moles Finally put concentrations into Kp expression Equilibrium moles Mole fractions 0.8/3.6 = /3.6 = /3.6 =0.111 Kp= p 2 NH 3 (g) p (g) p 3 H 2 (g) Partial pressure x2 = x2 = x2 = Kp = x = kpa -2 N Goalby chemrevise.org 11

12 Application of rates and equilibrium Effect of changing conditions on value of Kc or Kp The larger the Kc the greater the amount of products. If Kc is small we say the equilibrium favours the reactants Effect of Temperature on position of equilibrium and Kc Kc and Kp only change with temperature. It does not change if pressure or concentration is altered. A catalyst also has no effect on Kc or Kp Both the position of equilibrium and the value of Kc or Kp will change it temperature is altered In this equilibrium which is exothermic in the forward direction (g) + 3H 2 (g ) 2 NH 3 (g) If temperature is increased the reaction will shift to oppose the change and move in the backwards endothermic direction. The position of equilibrium shifts left. The value of Kc gets smaller as there are fewer products. Effect of Temperature on rate As the temperature increases a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases Effect of Concentration on position of equilibrium and Kc Changing concentration would shift the position of equilibrium but the value of Kc would not change. H 2 (g) +Cl 2 (g) 2HCl (g) Increasing concentration of H 2 would move equilibrium to the right lowering concentration of H 2 and Cl 2 and increasing concentration of HCl. The new concentrations would restore the equilibrium to the same value of Kc Effect of Concentration and pressure on rate At higher concentrations(and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions. Effect of Pressure on position of equilibrium and Kp The position of equilibrium will change it pressure is altered but the value of Kp stays constant as Kp only varies with temperature In this equilibrium which has fewer moles of gas on the product side (g) + 3H 2 (g ) 2 NH 3 (g) If pressure is increased the reaction will shift to oppose the change and move in the forward direction to the side with fewer moles of gas. The position of equilibrium shifts right. The value of Kp stays the same though as only temperature changes the value of Kp. Increasing pressure does not change Kp. The increased pressure increases the pressure terms on bottom of Kp expression more than the top. The system is now no longer in equilibrium so the equilibrium shifts to the right increasing mole fractions of products and decreases the mole fractions of reactants. The top of Kp expression therefore increases and the bottom decreases until the original value of Kp is restored Kp= Kp= Kp= p 2 NH 3 p p 3 H 2 x 2 NH 3. P 2 x.p x 3 H 2.P 3 x 2 NH 3. P 2 x x 3 H 2. P 4 Where P is total pressure and x mole fraction N Goalby chemrevise.org 12

13 Equilibrium constants and feasibility Values of equilibrium constants can all be used to predict the extent a reaction might occur. Reaction does not go Reactants predominate in an equilibrium equal amounts of products and reactants Products predominate in an equilibrium Reaction goes to completion K c < K c 0.1 K c = 1 K c 10 K c > ΔS total increases the magnitude of the equilibrium constant increases since ΔS = R lnk Where K is equilibrium constant Kc or Kp This equation shows a reaction with a Kc >1 will therefore have a positive ΔS total N Goalby chemrevise.org 13

14 Partition coefficients Solvent extraction splits a solute between two immiscible liquid solvents. The solute will be split between the two solvents in an equilibrium. Solute in aqueous solvent Solute in organic solvent Separating funnel organic solvent. Aqueous solvent The partition coefficient Kpc can be calculated from the equilibrium expression Kpc = [ solute in organic] [ solute in aqueous] The concentration term is usually mol dm -3. As the expression has the same substance in both terms it is also possible to use g dm -3 or gcm -3 K pc is defined as equilibrium constant for the solution of a solute between two immiscible solvents Example g of phenylamine was produced in an experiment. It was present in 30cm 3 of an alkaline solution. The phenylamine was extracted by addition of 50cm 3 of an organic solvent dichloromethane. After the extraction, the dichloromethane layer contained 3.01g of phenylamine. Calculate the partition coefficient, Kpc, of phenylamine between dichloromethane and water. mass left in water layer = = 0.55 g Kpc = (3.01/50) (0.55/30) = 3.28 Example 9 The partition coefficient of substance Y between ether and water is 35.0 A solution contains 3.50 g of Y dissolved in dm 3 of water. Calculate the mass of Y that can be extracted from this aqueous solution by mixing it with dm 3 of ether. mass left in water layer = 3.50 x Kpc = (x/0.05) ((3.5-x)/0.50) = 35 10x (3.5-x) = 35 x=2.72g N Goalby chemrevise.org 14

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