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Dynamic equilibria
If a reversible reaction is carried out in a closed container so that the reactants and products cannot escape, a state of dynamic equilibrium can be established.
A + B C + D
This state is dynamic because both the forward and reverse reactions are ongoing.
It is an equilibrium because:
the net concentrations of the components of the reaction mixture remain constant.
the rates of the forward and reverse reactions are the same
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Position of equilibrium
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[C]c[D]d
[B]b[A]aKc =
The equilibrium constant
At equilibrium, the ratio of the concentrations of products to reactants is constant. The ratio value is called the equilibrium constant, Kc. It is always the same for a particular reaction under fixed conditions.
aA + bB cC + dD
the equilibrium constant (Kc) is given by:
For the general reaction:
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Units of Kc
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Work out the units of Kc
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Calculating Kc
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The value of Kc
The size of Kc tells us about how the equilibrium mixture is made up.
KC =[products]
[reactants]
If Kc > 1, the concentration of products is greater than the concentration of reactants and we say that the equilibrium lies to the right hand side.
If Kc < 1, then the concentration of reactants is greater than the concentration of products and we say that the equilibrium lies to the left hand side.
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Kc calculations
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Gas equilibria
For reactions involving gases, it is difficult to measure the concentration of a particular gas. Instead the quantity of each gas in an equilibrium mixture is described in terms of the pressure that it exerts – the partial pressure.
The mole fraction is given by:
partial pressure = mole fraction × total pressure (pA) (xA) × (ptot)
mole fraction of A =no. moles of species A
total no. moles of all species
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Calculating partial pressures
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Calculating partial pressures
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The equilibrium constant, Kp
The equilibrium constant for gas phase reactions is called Kp.
where p(A) means the partial pressure of species A
aA(g) + bB(g) cC(g) + dD(g)
For this general equilibrium, Kp is given by:
p(C)cp(D)d
p(A)ap(B)bKp =
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(kPa) (kPa)
(kPa)2(kPa)Kp =
Units of Kp
Like Kc, the units for Kp must be worked out for each reaction.
So units are calculated as follows:
For the gas phase equilibrium:
The equilibrium constant, Kp, is given by:
A(g) + 2B(g) C(g) + D(g)
p(C)p(D)p(A)2p(B)
Kp =
= kPa–11
kPa=
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Kp expressions and units
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Calculating Kp
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Calculating Kp
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Le Chatelier’s Principle
Le Chatelier’s principle is used to determine what effect a change will have on a mixture at equilibrium.
Le Chatelier’s principle states that:
If a factor affecting the position of an equilibrium is altered, the position of the
equilibrium shifts to oppose the effect of the change.
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Effect of temperature on Kc
Changing the temperature changes the value of the equilibrium constant, Kc.
The effect on Kc depends on whether the reaction is exothermic or endothermic.
If the reaction is exothermic, increasing the temperature shifts the equilibrium to the left and the value of Kc decreases.
If the reaction is endothermic, increasing the temperature shifts the equilibrium to the right and the value of Kc increases.
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Effect of concentration on Kc
If the concentration of a reactant or product changes, the equilibrium shifts to oppose the change.
The value of the equilibrium constant is only affected by temperature.
If the concentration of a product is decreased, the equilibrium shifts to convert more of the reactants into products.
If the concentration of a reactant is decreased, the equilibrium shifts to favour the backward reaction.
The ratio of products to reactants is restored and the value of Kc remains the same.
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The effect of pressure
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The effect of a catalyst
A catalyst increases the rate of reaction without being used up itself.
time
ener
gy
with catalyst
without catalyst
A catalyst increases the rate of the forward and backward reactions equally.
It does not affect the position of equilibrium, the yield or the value of the equilibrium constant.
It does, however, increase the speed at which equilibrium is achieved.
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Le Chatelier’s principle: true or false?
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Industrial equilibria
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Maximizing atom economy
Industrial processes are continually being made more economical and efficient.
Unreacted reagents are recycled. In the Haber process, hydrogen and nitrogen that pass out of the reaction vessel with ammonia are separated out and fed back in.
Research is ongoing for alternative reactions that require lower temperatures and pressures and cheaper, more effective catalysts.
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Glossary
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What’s the keyword?
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Multiple-choice quiz