6. Chemical equilibria, Le Chatelier's principle and Kc

• Define reversible reaction


  forward and reverse reactions proceed at equal rates the concentrations of reactants and products remain constant.also called backwards reactions-A + B ⇌ C + D
• what happens in the reversible reactions

  if it is exothermic in one direction then it must be endothermic in the opposite directionThe amount of energy transferred in either direction is the same
• example of reversible reactions


  can be seen in some hydrated saltsThese are salts that contain water of crystallisation which affects their shape and colour
• water of crystallisation define

  is the water that is included in the structure of some salts during the crystallisation processA common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO4.5H2OWater of crystallisation is indicated with a dot written in between the salt and the surrounding water molecules
• anhydrous salts

  lost their water of crystallisation, usually by heating, in which the salt becomes dehydratedeg -When anhydrous copper(II) sulfate is added to water, it turns blue and heat is given off so the reaction is exothermicWhen hydrated copper(II) sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube
• hydrates and anhydrous copper sulfate


  https://www.savemyexams.co.uk/notes/gcse-chemistry-aqa-new/chemical-change-rate-extent/reversibility-equilibrium/energy-changes-reversible-reactions/
• Equilibrium


  the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction
• Dynamic equilibrium


  the reactants and products are dynamic (they are constantly moving)the rate of the forward reaction is the same as the rate of the backward reaction in a closed system, and the concentrations of the reactants and products is constant
• graph to draw both of them


  https://www.savemyexams.co.uk/notes/a-level-chemistry-aqa/1-physical-chemistry-aqa/1-8-chemical-equilibria-le-chateliers-principle-kc/1-8-1-chemical-equilibria/
• 2 types of systems


  open and closed
• closed system


  none of the reactants or products escape from the reaction mixtureIf the reaction involves gases, equilibrium can only be reached in a closed system
• open system


  matter and energy can be lost to the surroundingsWhen a reaction takes place entirely in solution, equilibrium can be reached in open flasks as a negligible amount of material is lost through evaporation
• Never assume


  concentrations of the reactants and products are equal. However, they are not equal but they remain constant at dynamic equilibrium (i.e. the concentrations are not changing).The concentrations will change as the reaction progresses, only until the equilibrium is reached.
• Define position of equilibrium


  relative amounts of products and reactants in an equilibrium mixture
• what happens when the position shifts to the left


  the concentration of reactants increases
• when happens to the position shifts to the right

  the concentration of products increases
• Le Chatelier's Principle

  If a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this changeThe principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration
• effects of concentration on equilibrium- increase-decrease


  increase in concentration- equilibrium shifts to the right to reduce the effect of increase in the concentration of a reactantdecrease in concentration- equilibrium to the left to reduce the effect of a decrease in reactant (or an increase in the concentration of product)
• effecr of pressure


  Changes in pressure only affect reactions where the reactants or products are gasesincrease in pressure= equilibrium shifts in the direction that produces the smaller number of molecules of gas to decrease the pressure againdecrease in pressure= equilibrium shifts in the direction that produces the larger number of molecules of gas to increase the pressure againsame number of moles of gases on either side of the equation,--NO effect on the position of equilbrium when the presure is changed-Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilbrium is unchanged
• effect of temperature

  increase- equilibrium moves in the endothermic direction to reverse the changedecrease- equilibrium moves in the exothermic direction to reverse the change
• Effect of catalyst on equilibrium

  A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)Catalysts only cause a reaction to reach equilibrium fasterCatalysts therefore have no effect on the position of the equilibrium once this is reached
• an example ofa dynamic equilibrium


  is the reaction between H2 and N2 in the Haber process
• what happens att he beginning of harber process

  are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highestas the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease.However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen)
• what happens when 2 reactions are interlinked and none of the gas can escape


  the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached-graphhttps://www.savemyexams.co.uk/notes/gcse-chemistry-aqa-new/chemical-change-rate-extent/reversibility-equilibrium/equilibrium/
• what happens to equilibrium


  is only reached in a closed vessel.
• the haber process


  Ammonia made by reacting nitrogen and hydrogen at high pressure at 450°CAn iron catalyst is used .Unreacted N2 and H2 are reused
• harber process image


  https://www.savemyexams.co.uk/notes/gcse-chemistry-aqa-new/using-resources/haber-process-npk-fertilisers/the-haber-process/
• haber process


  The reactants are hydrogen and nitrogen which are extracted from methane and the air respectivelystage 1 -H2 and N2 gases are pumped into the compressor through pipesstage 2- The gases are compressed to about 200 atmospheres inside the compressorstage 3-The pressurised gases are pumped into a tank containing layers of catalytic iron beads at a temperature of 450°C. Some of the hydrogen and nitrogen react to form ammonia in the following reversible reaction:N2 (g) + 3H2 (g) ⇌ 2NH3 (g)stage 4- Unreacted H2 and N2 and product ammonia pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vesselsstage 5- unreacted H2 and N2 gases are recycled back into the system
• what effects haber process in the rate of reaction


  Reaction conditions such as temperature and pressure-if the reaction is reversible then the position of equilibrim is also affected-the graph-effect of changing pressure and temperature on the yield of ammonia obtained
• he graph-effect of changing pressure and temperature on the yield of ammonia obtained


  By following any of the curved lines on the graph it can be seen that as the pressure increases, so too does the yield at any given temperatureBy following any vertical line upwards from the x-axis, the graph shows that as the temperature decreases, the yield actually increasesThe actual conditions used must be chosen depending on a number of economical, chemical and practical considerations
• Dunamic equilibrium only apply


  to a reversible reaction occurring in a closed vessel.
• economic considerations

  In the Haber Process the raw materials are readily available and inexpensive to purify:Nitrogen - from the airHydrogen- from natural gasIf the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable.Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain.Production energy costs are also a factor to be considered carefully and alongside the raw materials issue.
• economic consideration for temperature, pressure and catalyst

  temperature- 450 degree celciuspressure- 200 atmcatalyst-iron
• temperature- 450 degree celcius


  A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made.If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made.However at a lower temperature the rate of reaction is very slow.So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly
• pressure 200 atm

  A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be madeA higher pressure would favour the forward reaction as it will try to decrease the pressure by creating fewer molecules (2 molecules of gaseous products) so a higher yield of products will be madeHowever, high pressures can be dangerous and very expensive equipment is neededSo 200 atm is a compromise pressure between a lower yield of products being made safely and economically
• Harber process- catalysts


  -not affect the position of equilibrium but it does increase the rate at which equilibrium is reachedThis is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.So a catalyst is used as it helps the reaction reach equilibrium quickerIt allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy requiredWithout it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH3 molecules
• compromise condition


  the reaction conditions chosen for the Haber process are not ideal in terms of the yield but do provide balance between product yield, reaction rate and production cost
• harber process image


  https://www.savemyexams.co.uk/notes/gcse-chemistry-aqa-new/using-resources/haber-process-npk-fertilisers/choosing-reaction-conditions/