17 HL Equilibrium
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What does Le chatelier’s principle state?
Le Châtelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
What can Le Chatelier’s principle be used to predict?
The principle can be used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration
What can the equilibrium law explain?
The equilibrium law can explain and quantify the effect of changes in concentration at a particular temperature
What are equilibrium explanations based on?
These explanations are based on the idea that Kc is not affected by a change in concentration
For a reaction, aA + bB ⇌ cC + dD, how does increasing concentration of A affect Kc?
No change! If the concentration of A increases The position of equilibrium shifts to the right as the forward reaction works to remove excess A The concentrations of C and D increase to minimise this change The concentration of B decreases because it is being used up to minimise the change Therefore, the value of Kc remains unchanged
For a reaction, aA + bB ⇌ cC + dD, how does decreasing concentration of A affect Kc?
If the concentration of A decreases The position of equilibrium shifts to the left as the backward reaction works to replace A The concentrations of C and D decrease to minimise this change The concentration of B increases because it is also being produced when C and D react Therefore, the value of Kc remains unchanged
How does increasing nitrogen affect the Kc? N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
N2 (g) + 3H2 (g) ⇌ 2NH3 (g) An increase in the amount of nitrogen will cause the following: The equilibrium to shift to the right An increase in the amount of ammonia A decrease in the amount of hydrogen Kc will remain unchanged
How does increasing nitrogen for the haber reaction affect the rate of reaction?
An increase in the amount of nitrogen causes the rate of the forward reaction to increase This means that more ammonia is produced, causing the rate of the backward reaction to increase This process of increasing forward and backward reactions continues until a new equilibrium is established The rate at this newly established equilibrium will be higher than the original rate
What does the unit of Kc depend on?
In the equilibrium expression, each term inside a square bracket represents the concentration of that chemical in mol dm-3 Therefore, the units of Kc depend on the equilibrium expression
Steps for calculating Kc of ethanoic acid?
Step 1: Calculate the concentrations of the reactants and products: Step 2: Write out the balanced symbol equation with the concentrations of each chemical underneath: Step 3: Write out the equilibrium constant for the reaction: Step 4: Substitute the equilibrium concentrations into the expression and calculate the answer: Step 5: Deduce the correct units for Kc:
What is Kc independent of?
The equilibrium constant, Kc, gives no information about the individual rates of reaction It is independent of the kinetics of the reaction
What is the equation that connects Kc and Gibbs free energy?
The equilibrium constant, Kc, is directly related to the Gibbs free energy change, ΔGꝊ, according to the following (van’t Hoff’s) equation: ΔGꝊ = -RT lnK ΔGꝊ= Gibbs free energy change (kJ mol–1) R = gas constant (8.31 J K-1 mol-1) T = temperature (Kelvin, K) K = equilibrium constant This equation is provided in section 1 of the data booklet
What can the relationship between gibbs and Kc be used to determine?
This relationship between the equilibrium constant, Kc, and Gibbs free energy change, ΔGꝊ, can be used to determine whether the forward or backward reaction is favoured
What must Kc and Delta G be at for the products to be favoured?
K > 1 (greater)] G < 0 (negative)
What must Kc and Delta G be at for neither reactants or products to be favoured i.e. equilibrium?
k = 1 G = 1
What must Kc and Delta G be at for the reactants to be favoured?
K < 1 (lower) G > 0 (positive)
At a given temperature, what does negative gibbs indicate?
At a given temperature, a negative ΔG value for a reaction indicates that: The reaction is feasible / spontaneous The equilibrium concentration of the products is greater than the equilibrium concentration of the reactants The value of the equilibrium constant is greater than 1
What happens as gibbs becomes more negative?
As ΔG becomes more negative: The forward reaction is favoured more The value of the equilibrium constant increases