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AN INTRODUCTION TO CHEMICAL EQUILIBRIUM

AN INTRODUCTION TO CHEMICAL EQUILIBRIUM. 2008 SPECIFICATIONS. KNOCKHARDY PUBLISHING. KNOCKHARDY PUBLISHING. CHEMICAL EQUILIBRIUM. INTRODUCTION

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AN INTRODUCTION TO CHEMICAL EQUILIBRIUM

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  1. AN INTRODUCTION TO CHEMICAL EQUILIBRIUM 2008 SPECIFICATIONS KNOCKHARDY PUBLISHING

  2. KNOCKHARDY PUBLISHING CHEMICAL EQUILIBRIUM INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available. Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at... www.knockhardy.org.uk/sci.htm Navigation is achieved by... either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard

  3. CHEMICAL EQUILIBRIUM • CONTENTS • Concentration change during a chemical reaction • Dynamic equilibrium • Equilibrium constants • Le Chatelier’s Principle • Haber process • Check list

  4. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependant on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated FASTEST AT THE START THE STEEPER THE GRADIENT, THE FASTER THE REACTION SLOWS DOWN AS REACTANTS ARE USED UP TOTAL CONVERSION TO PRODUCTS In an ordinary reaction; all reactants end up as products; there is 100% conversion

  5. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependant on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated FASTEST AT THE START THE STEEPER THE GRADIENT, THE FASTER THE REACTION SLOWS DOWN AS REACTANTS ARE USED UP TOTAL CONVERSION TO PRODUCTS In an ordinary reaction; all reactants end up as products; there is 100% conversion

  6. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependant on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated FASTEST AT THE START THE STEEPER THE GRADIENT, THE FASTER THE REACTION SLOWS DOWN AS REACTANTS ARE USED UP TOTAL CONVERSION TO PRODUCTS In an ordinary reaction; all reactants end up as products; there is 100% conversion

  7. EQUILIBRIUM REACTIONS Initially, there is no backward reaction but, as products form, it speeds up and provided the temperature remains constant there will come a time when the backward and forward reactions are equal and opposite; the reaction has reached equilibrium. FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion. BUT IT DOESN’T MEAN THE REACTION IS STUCK IN THE MIDDLE AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  8. EQUILIBRIUM REACTIONS Initially, there is no backward reaction but, as products form, it speeds up and provided the temperature remains constant there will come a time when the backward and forward reactions are equal and opposite; the reaction has reached equilibrium. FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion. BUT IT DOESN’T MEAN THE REACTION IS STUCK IN THE MIDDLE AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  9. EQUILIBRIUM REACTIONS Initially, there is no backward reaction but, as products form, it speeds up and provided the temperature remains constant there will come a time when the backward and forward reactions are equal and opposite; the reaction has reached equilibrium. FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion. BUT IT DOESN’T MEAN THE REACTION IS STUCK IN THE MIDDLE AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  10. EQUILIBRIUM REACTIONS Initially, there is no backward reaction but, as products form, it speeds up and provided the temperature remains constant there will come a time when the backward and forward reactions are equal and opposite; the reaction has reached equilibrium. FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion. BUT IT DOESN’T MEAN THE REACTION IS STUCK IN THE MIDDLE AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  11. DYNAMIC EQUILIBRIUM IMPORTANT REMINDERS • a reversible chemical reaction is a dynamic process • everything may appear stationary but the reactions are moving both ways • the position of equilibrium can be varied by changing certain conditions Trying to get up a “down” escalator gives an excellent idea of a non-chemical situation involving dynamic equilibrium.

  12. DYNAMIC EQUILIBRIUM IMPORTANT REMINDERS • a reversible chemical reaction is a dynamic process • everything may appear stationary but the reactions are moving both ways • the position of equilibrium can be varied by changing certain conditions Trying to get up a “down” escalator gives an excellent idea of a non-chemical situation involving dynamic equilibrium. SummaryWhen a chemical equilibrium is established ... • both the reactants and the products are present at all times • the equilibrium can be approached from either side • the reaction is dynamic - it is moving forwards and backwards • the concentrations of reactants and products remain constant

  13. THE EQUILIBRIUM LAW Simply states “If the concentrations of all the substances present at equilibrium are raised to the power of the number of moles they appear in the equation, the product of the concentrations of the products divided by the product of the concentrations of the reactants is a constant, provided the temperature remains constant” There are several forms of the constant; all vary with temperature. Kcthe equilibrium values are expressed as concentrations of mol dm-3 Kpthe equilibrium values are expressed as partial pressures The partial pressure expression can be used for reactions involving gases

  14. THE EQUILIBRIUM CONSTANT Kc for an equilibrium reaction of the form... aA + bB cC + dD then (at constant temperature)[C]c . [D]d= a constant,(Kc) [A]a . [B]b where[ ]denotes the equilibrium concentration in mol dm-3 Kcis known as the Equilibrium Constant

  15. THE EQUILIBRIUM CONSTANT Kc for an equilibrium reaction of the form... aA + bB cC + dD then (at constant temperature)[C]c . [D]d= a constant,(Kc) [A]a . [B]b where[ ]denotes the equilibrium concentration in mol dm-3 Kcis known as the Equilibrium Constant ExampleFe3+(aq) + NCS¯(aq) FeNCS2+(aq) Kc = [ FeNCS2+ ] withunits of dm3 mol-1 [ Fe3+ ] [ NCS¯ ]

  16. THE EQUILIBRIUM CONSTANT Kc for an equilibrium reaction of the form... aA + bB cC + dD then (at constant temperature)[C]c . [D]d= a constant,(Kc) [A]a . [B]b where[ ]denotes the equilibrium concentration in mol dm-3 Kcis known as the Equilibrium Constant VALUE OF Kc AFFECTEDby a change of temperature NOTAFFECTEDby a change in concentration of reactants or products a change of pressure adding a catalyst

  17. LE CHATELIER’S PRINCIPLE ”When a change is applied to a system in dynamic equilibrium, the system reacts in such a way as to oppose the effect of the change.” Everyday example A rose bush grows with increased vigour after it has been pruned. Chemistry example If you do something to a reaction that is in a state of equilibrium, the equilibrium position will change to oppose what you have just done

  18. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CONCENTRATION The equilibrium constant is not affected by a change in concentration at constant temperature. To maintain the constant, the composition of the equilibrium mixture changes. If you increase the concentration of a substance, the value of Kc will theoretically be affected. As it must remain constant at a particular temperature, the concentrations of the other species change to keep the constant the same.

  19. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CONCENTRATION example CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) the equilibrium constant Kc = [CH3COOC2H5] [H2O] = 4 (at 298K) [CH3CH2OH] [CH3COOH]

  20. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CONCENTRATION example CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) the equilibrium constant Kc = [CH3COOC2H5] [H2O] = 4 (at 298K) [CH3CH2OH] [CH3COOH] increasing [CH3CH2OH] - willmakethebottomlinelarger so Kc will be smaller - to keep it constant, some CH3CH2OH reacts with CH3COOH - this reduces the value of the bottom line and increases the top - eventually the value of the constant will be restored

  21. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CONCENTRATION example CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) the equilibrium constant Kc = [CH3COOC2H5] [H2O] = 4 (at 298K) [CH3CH2OH] [CH3COOH] increasing [CH3CH2OH] - will make the bottom line larger so Kc will be smaller - to keep it constant, some CH3CH2OH reacts with CH3COOH - this reduces the value of the bottom line and increases the top - eventually the value of the constant will be restored decreasing [H2O] - willmakethetop line smaller - some CH3CH2OH reacts with CH3COOH to replace the H2O - more CH3COOC2H5 is also produced - this reduces the value of the bottom line and increases the top

  22. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CONCENTRATION example CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l) the equilibrium constant Kc = [CH3COOC2H5] [H2O] = 4 (at 298K) [CH3CH2OH] [CH3COOH] increasing [CH3CH2OH] - willmakethebottomlinelarger so Kc will be smaller - to keep it constant, some CH3CH2OH reacts with CH3COOH - this reduces the value of the bottom line and increases the top - eventually the value of the constant will be restored decreasing [H2O] - willmakethetop line smaller - some CH3CH2OH reacts with CH3COOH to replace the H2O - more CH3COOC2H5 is also produced - this reduces the value of the bottom line and increases the top

  23. THE EFFECT OF CHANGING THE CONCENTRATION ON THE POSITION OF EQUILIBRIUM INCREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE RIGHT DECREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE LEFT INCREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE LEFT DECREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE RIGHT FACTORS AFFECTING THE POSITION OF EQUILIBRIUM SUMMARY REACTANTS PRODUCTS

  24. THE EFFECT OF CHANGING THE CONCENTRATION ON THE POSITION OF EQUILIBRIUM INCREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE RIGHT DECREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE LEFT INCREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE LEFT DECREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE RIGHT FACTORS AFFECTING THE POSITION OF EQUILIBRIUM SUMMARY REACTANTS PRODUCTS Predict the effect of increasing the concentration of O2 on the equilibrium position 2SO2(g) + O2(g) 2SO3(g) Predict the effect of decreasing the concentration of SO3 on the equilibrium position

  25. THE EFFECT OF CHANGING THE CONCENTRATION ON THE POSITION OF EQUILIBRIUM INCREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE RIGHT DECREASE CONCENTRATION OF A REACTANT EQUILIBRIUM MOVES TO THE LEFT INCREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE LEFT DECREASE CONCENTRATION OF A PRODUCT EQUILIBRIUM MOVES TO THE RIGHT FACTORS AFFECTING THE POSITION OF EQUILIBRIUM SUMMARY REACTANTS PRODUCTS Predict the effect of increasing the concentration of O2 on the equilibrium position 2SO2(g) + O2(g) 2SO3(g)EQUILIBRIUMMOVES TO RHS Predict the effect of decreasing the concentration of SO3 on the equilibrium position EQUILIBRIUMMOVES TO RHS

  26. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE When studying the effect of a change in pressure, we consider the number of gaseous molecules only. The more particles you have in a given volume, the greater the pressure they exert. If you apply a greater pressure they will become more crowded (i.e. they are under a greater stress). However, if the system can change it will move to the side with fewer gaseous molecules - it is less crowded. No change occurs when equal numbers of gaseous molecules appear on both sides.

  27. THE EFFECT OF PRESSURE ON THE POSITION OF EQUILIBRIUM INCREASE PRESSURE MOVES TO THE SIDE WITH FEWER GASEOUS MOLECULES DECREASE PRESSURE MOVES TO THE SIDE WITH MORE GASEOUS MOLECULES FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE When studying the effect of a change in pressure, we consider the number of gaseous molecules only. The more particles you have in a given volume, the greater the pressure they exert. If you apply a greater pressure they will become more crowded (i.e. they are under a greater stress). However, if the system can change it will move to the side with fewer gaseous molecules - it is less crowded. No change occurs when equal numbers of gaseous molecules appear on both sides.

  28. THE EFFECT OF PRESSURE ON THE POSITION OF EQUILIBRIUM INCREASE PRESSURE MOVES TO THE SIDE WITH FEWER GASEOUS MOLECULES DECREASE PRESSURE MOVES TO THE SIDE WITH MORE GASEOUS MOLECULES FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE When studying the effect of a change in pressure, we consider the number of gaseous molecules only. The more particles you have in a given volume, the greater the pressure they exert. If you apply a greater pressure they will become more crowded (i.e. they are under a greater stress). However, if the system can change it will move to the side with fewer gaseous molecules - it is less crowded. No change occurs when equal numbers of gaseous molecules appear on both sides. Predict the effect of an increase of pressure on the equilibrium position of.. 2SO2(g) + O2(g) 2SO3(g) H2(g) + CO2(g) CO(g) + H2O(g)

  29. THE EFFECT OF PRESSURE ON THE POSITION OF EQUILIBRIUM INCREASE PRESSURE MOVES TO THE SIDE WITH FEWER GASEOUS MOLECULES DECREASE PRESSURE MOVES TO THE SIDE WITH MORE GASEOUS MOLECULES FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE When studying the effect of a change in pressure, we consider the number of gaseous molecules only. The more particles you have in a given volume, the greater the pressure they exert. If you apply a greater pressure they will become more crowded (i.e. they are under a greater stress). However, if the system can change it will move to the side with fewer gaseous molecules - it is less crowded. No change occurs when equal numbers of gaseous molecules appear on both sides. Predict the effect of an increase of pressure on the equilibrium position of.. 2SO2(g) + O2(g) 2SO3(g)MOVES TO RHS :- fewer gaseous molecules H2(g) + CO2(g) CO(g) + H2O(g) NO CHANGE:- equal numbers on both sides

  30. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE • temperature is the only thing that can change the value of the equilibrium constant. • altering the temperature affects the rate of both backward and forward reactions • it alters the rates to different extents • the equilibrium thus moves producing a new equilibrium constant. • the direction of movement depends on the sign of the enthalpy change.

  31. REACTION TYPE DECREASE TEMP DH INCREASE TEMP EXOTHERMIC - TO THE LEFT TO THE RIGHT ENDOTHERMIC + TO THE RIGHT TO THE LEFT FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE • temperature is the only thing that can change the value of the equilibrium constant. • altering the temperature affects the rate of both backward and forward reactions • it alters the rates to different extents • the equilibrium thus moves producing a new equilibrium constant. • the direction of movement depends on the sign of the enthalpy change.

  32. REACTION TYPE DECREASE TEMP DH INCREASE TEMP EXOTHERMIC - TO THE LEFT TO THE RIGHT ENDOTHERMIC + TO THE RIGHT TO THE LEFT FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE • temperature is the only thing that can change the value of the equilibrium constant. • altering the temperature affects the rate of both backward and forward reactions • it alters the rates to different extents • the equilibrium thus moves producing a new equilibrium constant. • the direction of movement depends on the sign of the enthalpy change. Predict the effect of a temperature increase on the equilibrium position of... H2(g) + CO2(g) CO(g) + H2O(g)DH = + 40 kJ mol-1 2SO2(g) + O2(g) 2SO3(g)DH = - ive

  33. REACTION TYPE DECREASE TEMP DH INCREASE TEMP EXOTHERMIC - TO THE LEFT TO THE RIGHT ENDOTHERMIC + TO THE RIGHT TO THE LEFT FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE • temperature is the only thing that can change the value of the equilibrium constant. • altering the temperature affects the rate of both backward and forward reactions • it alters the rates to different extents • the equilibrium thus moves producing a new equilibrium constant. • the direction of movement depends on the sign of the enthalpy change. Predict the effect of a temperature increase on the equilibrium position of... H2(g) + CO2(g) CO(g) + H2O(g)DH = + 40 kJ mol-1moves to the RHS 2SO2(g) + O2(g) 2SO3(g)DH = - ive moves to the LHS

  34. MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER NUMBER OF MOLECUES WITH A PARTICULAR ENERGY Ea MOLECULAR ENERGY FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CATALYSTS Catalysts work by providing an alternative reaction pathway involving a lower activation energy.

  35. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CATALYSTS An increase in temperature is used to speed up chemical reactions but it can have an undesired effect when the reaction is reversible and exothermic. In this case you get to the equilibrium position quicker but with a reduced yield because the increased temperature moves the equilibrium to the left. In many industrial processes a compromise temperature is used (see Haber and Contact Processes). To reduce the problem one must look for a way of increasing the rate of a reaction without decreasing the yield i.e. with a catalyst.

  36. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CATALYSTS An increase in temperature is used to speed up chemical reactions but it can have an undesired effect when the reaction is reversible and exothermic. In this case you get to the equilibrium position quicker but with a reduced yield because the increased temperature moves the equilibrium to the left. In many industrial processes a compromise temperature is used (see Haber and Contact Processes). To reduce the problem one must look for a way of increasing the rate of a reaction without decreasing the yield i.e. with a catalyst. Adding a catalyst DOES NOT AFFECT THE POSITION OF EQUILIBRIUM. However, it does increase the rate of attainment of equilibrium. This is especially important in reversible, exothermic industrial reactions such as the Haber or Contact Processes where economic factors are paramount.

  37. HABER PROCESS N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1 Conditions Pressure 20000 kPa (200 atmospheres) Temperature 380-450°C Catalyst iron

  38. HABER PROCESS N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1 Conditions Pressure 20000 kPa (200 atmospheres) Temperature 380-450°C Catalyst iron Equilibriumtheory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules

  39. HABER PROCESS N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1 Conditions Pressure 20000 kPa (200 atmospheres) Temperature 380-450°C Catalyst iron Equilibriumtheory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules Kinetictheory favours high temperature greater average energy + more frequent collisions high pressure more frequent collisions for gaseous molecules catalyst lower activation energy

  40. HABER PROCESS N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1 Conditions Pressure 20000 kPa (200 atmospheres) Temperature 380-450°C Catalyst iron Equilibriumtheory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules Kinetictheory favours high temperature greater average energy + more frequent collisions high pressure more frequent collisions for gaseous molecules catalyst lower activation energy Compromiseconditions Which is better? A low yield in a shorter time or a high yield over a longer period. The conditions used are a compromise with the catalyst enabling the rate to be kept up, even at a lower temperature.

  41. HABER PROCESS IMPORTANT USES OF AMMONIA AND ITS COMPOUNDS MAKING FERTILISERS80% of the ammonia produced goes to make fertilisers such as ammonium nitrate (NITRAM) and ammonium sulphate NH3 + HNO3 ——> NH4NO3 2NH3 + H2SO4 ——> (NH4)2SO4 MAKING NITRIC ACIDammonia can be oxidised to nitric acid nitric acid is used to manufacture... fertilisers (ammonium nitrate) explosives (TNT) polyamide polymers (NYLON)

  42. REVISION CHECK What should you be able to do? Understand how concentration changes during chemical reactions Recall the the characteristics of dynamic equilibrium Construct an expression for the equilibrium constant Kc Recall the factors which can affect the position of equilibrium Apply Le Chatelier’s Principle to predict changes in the position of equilibrium Recall and explain the conditions used in the Haber process Recall the importance of ammonia and its compounds CAN YOU DO ALL OF THESE? YES NO

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  45. AN INTRODUCTION TO CHEMICAL EQUILIBRIUM THE END © 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

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