Equilibrium
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Contents |
Basic principles
- In all reactions, there are in fact two processes occuring: A forward reaction where reactions are turned into products, and a reverse reaction where products become reactants.
- When the reaction reaches equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.
- Students should be familiar with Concentration/time graphs for reactants and products, as well as reaction rate/time graphs.
- Concentration/time graphs: these show the concentration of reactants, products, or both, over time.
- Concentration of reactants will decrease at a negative rate until dynamic equilibrium is reached.
- Concentration of products will increase at a positive, decreasing rate until dynamic equilibrium is reached.
- At dynamic equilibrium, concentration of reactants and products will not change.
- Reaction rate/time graphs: these show the rate of the forward and/or reverse reaction over time.
- forward rate will decrease until equilibrium is reached
- This is because the forward rate decreases as the concentration of reactants decreases
- reverse rate will increase from zero until equilibrium is reached
- This is because the reverse rate increases as the concentration of products increases
- forward rate will decrease until equilibrium is reached
- Concentration/time graphs: these show the concentration of reactants, products, or both, over time.
Types of dynamic equilibrium
Dynamic equilibrium occurs in closed systems. It's called dynamic because the forward and backward processes are still occurring, but since they're happening at the same rate there is no overall change.
- physical reactions - equilibrium reached between substances of different states.
- eg. evaporation and condensing of water
- Because some water molecules have enough energy to melt, while others don't. (think of Maxwell-Boltzman distribution)
- eg. evaporation and condensing of water
- chemical reactions - equilibrium reached between reactants and products
- concentration of reactants and products reaches a constant (Kc)
Le Chatelier's Principle
Equilibrium will always 'try to compensate' for any changes to the system.
Example: A + B <-> C + D(g)
- increased concentration of A
- leads to higher reaction rate towards the right
- therefore more C and D created to counterbalance the A added.
- equilibrium shifts to the right.
- therefore more C and D created to counterbalance the A added.
- leads to higher reaction rate towards the right
- decreased concentration of D
- rate of backwards reaction decreases
- equilibrium shifts to the right.
- rate of backwards reaction decreases
Factors that effect equilibrium
Change in Concentration
See Le Chatelier's Principle example, above.
Temperature Change
If the forward reaction is exothermic (and thus the reverse reaction is endothermic),
- Increasing the temperature of the system
- -> the endothermic reaction happens more easily
- -> equilibrium shifts left.
- -> the endothermic reaction happens more easily
- Decreasing the temperature of the system
- -> the endothermic reaction doesn't happen as fast
- -> equilibrium shifts right.
- -> the endothermic reaction doesn't happen as fast
If the forward reaction is endothermic, the opposite happens.
If the reactions aren't exothermic or endothermic, changing temperature has no effect on equilibrium.
Pressure Change
If the reactants take up more space than the products (eg. in 2NO2(g) <-> N2O4(g) there are two moles of gas in the reactants, but only one in the products, so the reactants take up more space),
- Increasing the pressure of the system
- -> the gas that takes up more space is more difficult to create
- -> backwards reaction happens less
- -> equilibrium shifts right
- -> backwards reaction happens less
- -> the gas that takes up more space is more difficult to create
- Decreasing the pressure of the system
- -> the gas that takes up more space is easier to create
- -> equilibrium shifts left
- -> the gas that takes up more space is easier to create
If the reactants take up less space than the products, the opposite happens.
If the reactants and products have the same volume characteristics, changing pressure has no effect on equilibrium
And I thought I was the sensible one. Thanks for setting me srtiaght.
Applications
Questions involving the practical implications of equilibrium law mostly focus on the Haber Process and the Contact Process.
Haber Process
manufacture of ammonia (NH3)
- volume of reactants is higher than volume of product
- therefore a high pressure aids the forward reaction
- reaction is exothermic
- low temperature would mean equilibrium is further to the right (more ammonia) but the rate of reaction would be slow.
- an optimum temperature must be found
- low temperature would mean equilibrium is further to the right (more ammonia) but the rate of reaction would be slow.
- Iron powder catalyst is used
- In the end, the yield of ammonia per cycle is about 15%.
Contact Process
manufacture of sulphuric acid (SO3)
Similar elements to Haber process:
- volume of reactants is higher than volume of product
- therefore a high pressure aids the forward reaction
- reaction is exothermic
- low temperature would mean equilibrium is further to the right (more sulphuric acid) but the rate of reaction would be slow.
- an optimum temperature must be found
- low temperature would mean equilibrium is further to the right (more sulphuric acid) but the rate of reaction would be slow.
However,
- a different catalyst is used (vanadium oxide)
- yield is already high at a pressure of 2atm, so it's unneccesary to conduct the reaction at a higher pressure.