Chapter 7 Equilibrium
Terms in this set (25)
When a reaction takes place at the same rate as its reverse reaction, so no net change is observed.
A state of balance between continuing processes.
In studies of equilibria we are dealing with reversible reactions - those that occur in both directions.
The convention is to describe the reaction from left to right (reactants to products) as the forward reaction, and the reaction from right to left (products to reactants) as the backward or reverse reaction.
The symbol is used to show that the reaction is an equilibrium reaction.
Features of Equilibrium state
1. Equilibrium is dynamic - The reaction has not stopped but both forward and backward reactions are still occurring at the same rate.
2. Equilibrium is achieved in a closed system - A closed system has no exchange of matter with the surroundings, so equilibrium is achieved where both reactants and products can react and recombine with each other.
3. The concentrations of reactants and products remain constant at equilibrium - They are being produced and destroyed at an equal rate.
4. At equilibrium there is no change in macroscopic properties - Macroscopic properties are observable properties such as colour and density. These do not change as they depend on the concentrations of the components of the mixture.
5. Equilibrium can be reached from either direction - The same equilibrium mixture will result under the same conditions, no matter whether the reaction is started with all reactants, all products, or a mixture of both.
The proportion of reactant and product in the equilibrium mixture
Reactions where the mixture contains predominantly products are said to 'lie to the right' and reactions with predominantly reactants are said to 'lie to the left'.
Has a fixed value for a particular reaction at a specified temperature.
The only thing that changes the value of Kc for a reaction is the temperature.
The value for Kc can then be determined by substituting the equilibrium concentrations of all reactants and products into this equation.
• The equilibrium constant expression has the concentrations of products in the numerator and the concentrations of reactants in the denominator.
• Each concentration is raised to the power of its coefficient in the balanced equation. (Where it is equal to one it does not have to be given.)
• Where there is more than one reactant or product the terms are multiplied together.
The magnitude of the equilibrium constant
Gives information about how far a reaction goes at a particular temperature, but not about how fast it will achieve the equilibrium state.
- if Kc ⪢ 1, More products are present and favoured. Numerator is bigger (the top of the fraction, which has products in Kc expression). Equilibrium shifts to the right.
- if Kc ⪡ 1, More reactants are present and favoured. Denominator is bigger (the bottom of the fraction, which has reactants in Kc expression). Equilibrium shifts to the left.
A measure of the relative amounts of reactants and products present in a reaction
at a particular time.
Its value is determined from substituting concentrations of reaction components, all measured at the same time, into the equilibrium expression
• If Q = Kc, reaction is at equilibrium, no net reaction occurs
• If Q < Kc, reaction proceeds to the right in favour of products
• if Q > Kc, reaction proceeds to the left in favour of reactants.
Kc '= 1/Kc or Kc′ = Kc^-1.
Kc^x = Kc^2.
Halving the reaction coefficients
Square root of Kc
Adding two reactions
Multiplies the two expressions Kc i × Kc ii
Le Chatelier's Principle
A system at equilibrium when subjected to a change will respond in such a way as to minimize the effect of the change.
Change in concentration
Increase in the concentration of one of
the reactants = rate of the forward reaction to increase while the backward reaction will not be affected = When equilibrium re-establishes itself, the mixture will have new concentrations of all reactants and products, and the equilibrium will have shifted in favour of products = The value of Kc will be unchanged.
Thus this means, Le Chatelier's principle: addition of reactant causes the system to respond by removing reactant - this favours the forward reaction and so shifts the equilibrium to the right.
Decrease in the concentration of product = the rate of the backward reaction is now decreased = there will be a shift in the equilibrium in favour of the products = A different equilibrium position will be achieved = value of Kc will be unchanged.
Le Chatelier's principle: removal of product causes the system to respond by making more product - this favours the forward reaction and so shifts the equilibrium to the right.
Change in Pressure
An increase in pressure favours the side
of an equilibrium reaction that has the smaller number of gas molecules.
A decrease in pressure will cause a shift in the equilibrium position to the side with the larger number of molecules of gas.
This is because there is a direct relationship between the number of gas molecules and the pressure exerted by a gas in a fixed volume.
A different equilibrium position will be achieved but the value of Kc will be unchanged
Change in temperature
Kc is temperature dependent, so changing the temperature will change Kc.
If forward reaction is exothermic = it releases heat.
- If subjected to a decrease in temperature = the system will respond by producing heat = it does this by favouring the forward exothermic reaction = This means that the equilibrium will shift to the right, in favour of the product = value of Kc will increase = give a higher yield of products at a lower temperature.
- If subjected to increase in temperature = favours the backward endothermic reaction = shifts the equilibrium to the left = decreasing the value of Kc = concentration of reactants increase
If forward reaction is endothermic = it absorbs heat.
- If subjected to decrease in temperature = will be to favour the backward exothermic reaction = the equilibrium will shift to the left, in favour of reactants = Kc will decrease.
- If subjected to higher temperatures = the forward endothermic reaction is favoured = so the equilibrium shifts to the right = Kc will increase.
Kc value is never affected by any factor except temperature:
- An increase in temperature increases the value of Kc for an endothermic reaction and decreases the value of Kc for an exothermic reaction.
- This is because changes in temperature have a different effect on the rates of the forward and backward reactions, due to their different activation energies
Addition of Catalyst
A catalyst lowers the activation energy by the same amount for the forward and backward reactions.
So the rate of both these reactions will be increased by the same factor
The catalyst will therefore have no effect on the position of equilibrium, or on the value of Kc.
Can increase rate of production
Haber process, the production of ammonia, NH3
The Haber process is based on the reaction
N2(g) + 3H2(g) s 2NH3(g) ∆H = −93 kJ mol-1
The following information can be derived from this equation:
• all reactants and products are gases
• there is a change in the number of gas molecules as the reaction proceeds: four gas
molecules on the left and two on the right
• the forward reaction is exothermic so releases heat; the backward reaction is endothermic so absorbs heat.
1. Concentration: The product ammonia is removed as it forms, thus helping to pull the equilibrium to the right and increasing the yield.
2. Pressure: as the forward reaction involves a decrease in the number of gas molecules, it will be favoured by a high pressure.
The usual pressure used in the Haber process is about 2 × 107 Pa.
3. Temperature: as the forward reaction is exothermic, it will be favoured by a lower temperature.
However, too low a temperature would cause the reaction to be uneconomically slow, and so a moderate temperature of about 450 °C is used.
4. Catalyst: a catalyst will speed up the rate of production and so help to compensate for the moderate temperature used
A catalyst of finely divided iron is used, with small amounts of aluminium and magnesium oxides added to improve its activity.
After separation of the NH3 product, the unconverted reactants are recycled to the reactor to obtain an overall yield of about 95%.
This recycling of unconverted reactants is commonly used in industrial processes, and allows processes with low equilibrium yield to be made commercially viable.
Contact Process: the production of sulfuric acid H2SO4
The Contact process involves a series of three simple reactions:
(i) the combustion of sulfur to form sulfur dioxide;
(ii) the oxidation of sulfur dioxide to sulfur trioxide:
2SO2(g) + O2(g) s 2SO3(g) ∆H = -196 kJ mol-1
(iii) The combination of sulfur trioxide with water to produce sulfuric acid.
rate of reaction depends on step 2
1. Pressure: forward reaction involves reduction in the number of molecules of gas from three molecules reactant to two molecules product: high pressure will favour product - 2 × 105 Pa (gives high equilibrium yield)
2. Temperature: Forward reaction is exothermic: low temperature will increase the equilibrium yield, but decrease the rate - 450 °C
3. Catalyst: Increases the rate of reaction - vanadium(V) oxide
Production of methanol
CO(g) + 2H2(g) s CH3OH(g)
1. Pressure: forward reaction involves reduction in the number of molecules of gas from three molecules reactant to one molecule product: high pressure will favour product - 5-10 × 106 Pa
2. temperature: forward reaction is exothermic: low temperature will increase the equilibrium yield, but decrease the rate - 250 °C
3. catalyst: increases the rate of reaction -
If the following reaction increases pressure, what way does the equilibrium shift?
If the following reaction decreases pressure, how does the equilibrium shift?
When cooling (taking heat away), shifts in what direction?
Shifts right - direction that produces heat
When adding heat (increasing the temperature) shifts in what direction?
Shifts left - direction that absorbs heat