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Topic 4b - Reversible reactions - Coggle Diagram
Topic 4b - Reversible reactions
Dynamic equbilibrium
If a reversible reaction takes place in a closed system, then a state of dynamic equilbirium will be reached
Dynamic equilibrium is when the forward rate equals the backward and the concentration of reacting substances remains constant
A closed system means that none of the reactants or products can escape and nothing else can get in
Origionally, in a reversibe reaction, the reactants react and their conentrations fall, so the forwards reaction slows
As more products are made, their concentrations rises and the backward reaction speeds up
After a while, the forward and backward will be going at the same rate, the system reaches dynamic equilibrium
Both reactions still happen, but the overall effect is nil as the forward and backward reactions cancel out
Equal
A reaction at equilibruim doesn't meant the amount of reactants equal the amount of products
If equilibrium lies to the right, concentration of products is greater
If equilibrium lies to the left, concentration of reactants is greater
Haber process
Nitrogen and hydrogen gas are used to make ammonia
Industrial conditions
Temperature - 450 degrees
Pressure - 200 atm
Catalyst - iron
Temperature
Forward reaction is exothermic
Increasing the temperature will move the equilibrium the wrong way
Yield of ammonia would be greater at lower temperatures
Lower temperature - lower rate of reaction
450 is a compromise between maximum yield and speed of reaction
N2 + 3H2 = 2NH3
Pressure
High pressure favours the forward reaction, 4 moles on left and 2 on the right
Higher pressures increases ammonia yield
Pressure is as high as possible to give the best yield, pressures like 1000 would be far too expensive
Factors
Increasing the rate of reaction increases the rate at which you reach equilibrium
Higher temperature and pressure means that means the particles will collide more frequently and more molecules collide with enough energy for a reaction to occur
Catalysts make the reaction fater, but doesn't change the position of equilibrium
Without a cataylst, the temperature would have to be raised even further, which would reduce the yield, and cost more
Le Chatelier's principle
Basics
If you change the conditions of a reversible reactions at equilibrium, the system will try to counter the change
You can alter the conditions to alter the yield
Temperature
All reversible reactions are exothermic one way and endothermic the other
Raising the temperature will move the equilibrium to the endothermic direction, endothermic yield will increase
Decreasing tempratue favours the exothermic direction, so the yield of the exothermic will increase
Pressure
Changing the pressure affects the reactions when both products and reactants are gases
Many of these reactions have a greater volume one side, more molecules of gas
Raising the pressure favours the reaction that produces a products that occupies less volume
Reducing the pressure favours the reaction that produces a product that occupies a greater volume
Concentration
Changing the concentration will mean the system is no longer at dynamic equilibrium, so it will try to reach it again
Increasing the concentration of reactants moves the position of the equilibrium towards the right so more products are produced
Increasing the concentration of products moves the positoin of equilibrium towards the reactants, so more reactant is produced