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CHEMICAL EQUILIBRIUM

CHEMICAL EQUILIBRIUM. CHEMICAL EQUILIBRIUM. The dynamic nature of equilibria know that many chemical reactions are reversible

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CHEMICAL EQUILIBRIUM

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  1. CHEMICAL EQUILIBRIUM

  2. CHEMICAL EQUILIBRIUM • The dynamic nature of equilibria • know that many chemical reactions are reversible • understand that for a reaction in equilibrium, although the concentrations of reactants and products remain constant, both forward and reverse reactions are still proceeding at equal rates • Qualitative effects of changes of pressure, temperature and concentration on a system in equilibrium • be able to use Le Chatelier‟s principle to predict the effects of changes in temperature, pressure and concentration on the position of equilibrium in homogeneous reactions • know that a catalyst does not affect the position of equilibrium Importance of equilibria in industrial processes • be able to apply these concepts to given chemical processes • be able to predict qualitatively the effect of temperature on the position of equilibrium from the sign of ΔH for the forward reaction • understand why a compromise temperature and pressure may be used • know about the hydration of ethene to form ethanol and the reaction of carbon monoxide with hydrogen to form methanol as important industrial examples where these principles can be applied • know the importance of these alcohols as liquid fuels.

  3. CHEMICAL EQUILIBRIUM The dynamic nature of equilibria

  4. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependent on the concentration of reactants... the forward reaction starts off fastbut slows as the reactants get less concentrated FASTEST AT THE START SLOWS DOWN AS REACTANTS ARE USED UP TOTAL CONVERSION TO PRODUCTS THE STEEPER THE GRADIENT, THE FASTER THE REACTION In an ordinary reaction; all reactants end up as products; there is 100% conversion Since products form as reactants are used up there will be an inverse curve representing their formation

  5. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependent 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 Since products form as reactants are used up there will be an inverse curve representing their formation

  6. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependent on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated FASTEST AT THE START 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 Since products form as reactants are used up there will be an inverse curve representing their formation

  7. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependent on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated FASTEST AT THE START 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 Since products form as reactants are used up there will be an inverse curve representing their formation

  8. CONCENTRATION CHANGE IN A REACTION As the rate of reaction is dependent on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated FASTEST AT THE START 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 Since products form as reactants are used up there will be an inverse curve representing their formation

  9. EQUILIBRIUM REACTIONS Some reactions reach a point where there is no change in the amount of product formed even though some reactant remains. These are called EQUILIBRIUM REACTIONS. 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. AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  10. EQUILIBRIUM REACTIONS The reaction does NOT stop. Some of the product molecules collide and reform the reactant molecules - some of the reactant molecules collide to reform products. The concentrations of the reactants reach a point where their concentrations remain constant – but the reaction has NOT stopped 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. AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  11. EQUILIBRIUM REACTIONS The reaction does NOT stop. Some of the product molecules collide and reform the reactant molecules - some of the reactant molecules collide to reform products. The concentrations of the reactants and products remains constant because... The concentrations of the reactants reach a point where their concentrations remain constant – but the reaction has NOT stopped FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE = The rate of the FORWARD reaction (reactants  products) AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE The rate of the BACKWARD reaction (products  reactants) The reaction is said to be a REVERSIBLE REACTION

  12. EQUILIBRIUM REACTIONS In terms of the molecules present: FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  13. EQUILIBRIUM REACTIONS In terms of the molecules present: FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  14. EQUILIBRIUM REACTIONS In terms of the molecules present: FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  15. EQUILIBRIUM REACTIONS In terms of the molecules present: FASTEST AT THE START NO BACKWARD REACTION FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP BACKWARD REACTION STARTS TO INCREASE The reaction does NOT stop – it has reached a state of DYNAMIC EQUILIBRIUM AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE EQUAL AND OPPOSITE

  16. DYNAMIC EQUILIBRIUM Summary When a chemical equilibrium is established ... • both reactants and the products are present in the mixture • the same equilibrium can be set up by starting with just the reactants or just the products • the reaction is dynamic - it is moving forwards and backwards • the concentrations of reactants and products remain constant at equilibrium

  17. CHEMICAL EQUILIBRIUM Qualitative effects of changes of pressure, temperature and concentration on a system in equilibrium

  18. 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.’

  19. 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.’ i.e. 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.

  20. LE CHATELIER’S PRINCIPLE 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. ...the equilibrium position will change... ...by adjusting the relative amounts of reactants and products.

  21. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE Equilibrium between [Co(H2O)6]2+(aq) and CoCl42-(aq) pinker it is the more Co(H2O)6(aq) is present bluer it is the more Co(H2O)6(aq) is present equilibrium mixture is a purple colour if 50% of each

  22. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE Equilibrium between Co(H2O)6(aq) and CoCl4(aq) Colour is found to be temperature dependent, so... ...the position of equilibrium must be temperature dependent.

  23. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE The forward and backward reactions have the following enthalpy changes: -ΔH +ΔH Le Chatelier’s Principle suggests that if we cause a change in the surroundings, eg, of temperature, the system responds to cancel out the change and restore the original conditions. The system can only respond by moving the equilibrium in one direction or the other.

  24. LE CHATELIER’S PRINCIPLE SURROUNDINGS Reaction goes in direction to give out heat to the surroundings SYSTEM EXOTHERMIC direction (ΔH = -ve) Surroundings get colder Surroundings get warmer

  25. LE CHATELIER’S PRINCIPLE SURROUNDINGS Reaction absorbs heat from the surroundings SYSTEM ENDOTHERMIC reaction (ΔH = +ve) Surroundings get warmer Surroundings get colder

  26. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE Le Chatelier predicts... -ΔH +ΔH the eqbm will move in the EXOTHERMIC direction if the temperature is DECREASED the eqbm will move in the EXOTHERMIC direction if the temperature is DECREASED the eqbm will move in the ENDOTHERMIC direction if the temperature is INCREASED the eqbm will move in the ENDOTHERMIC direction if the temperature is INCREASED

  27. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE So; does the system move in the ENDOTHERMIC direction if the temperature is INCREASED?

  28. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE So; does the system move in the ENDOTHERMIC direction if the temperature is INCREASED? blue colour has intensified... amount of CoCl4(aq) has increased can only have happened if eqbm moved in the endothermic direction

  29. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE So; does the system move in the EXOTHERMIC direction if the temperature is DECREASED? pink colour has intensified... amount of Co(H2O)6(aq) has increased can only have happened if eqbm moved in the exothermic direction

  30. REACTION TYPE (left to right) 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 The direction of movement depends on the sign of the enthalpy change! TEMPERATURE

  31. 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 in a given volume, the greater the pressure they exert. 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.

  32. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE The system can only respond by moving the equilibrium in one direction or the other. N2O4(g) Ý 2NO2(g) Dinitrogen tetraoxide is a colourless gas that forms by the dimerization of 2 nitrogen dioxide molecules (brown).

  33. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE Increase the pressure: Direction of fewer molecules N2O4(g)Ý 2NO2(g) The eqbm must have moved to turn some brown NO2 molecules into colourless N2O4 molecules.

  34. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE Decrease the pressure: Direction of more molecules N2O4(g)Ý 2NO2(g) The eqbm must have moved to turn some colourless N2O4 molecules into brown NO2 molecules.

  35. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE 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

  36. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM PRESSURE Predict the effect of an increase of pressure on the equilibrium position of.. H2(g) + CO2(g) CO(g) + H2O(g) NO CHANGE:- equal number of gaseous molecules on both sides

  37. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE TEMPERATURE Predict the effect of an increase of temperature on the equilibrium position of.. 2SO2(g) + O2(g) 2SO3(g) DH = - 40 kJ mol-1 MOVES TO LHS

  38. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM TEMPERATURE TEMPERATURE Predict the effect of an increase of temperature on the equilibrium position of.. H2(g) + CO2(g) CO(g) + H2O(g) DH = + 60 kJ mol-1 MOVES TO RHS

  39. 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 Catalysts work by providing an alternative reaction pathway involving a lower activation energy. 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.

  40. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM CATALYSTS An increase in temperature speeds 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 industrial processes a compromise temperature is used . 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. CATALYSTS

  41. FACTORS AFFECTING THE POSITION OF EQUILIBRIUM END

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