Chemical Equilibria Flashcards
What is dynamic equilibrium
in which the rate of the forward reaction equals that of the reverse reaction, and there is no net change in composition
What is the link between Thermodynamics/Kinetics and Chemical Equilibrium
Thermodynamics: can be used to find the equilibrium composition
Kinetics: can be used to assess how quickly equilibrium is achieved
Why does Gibbs free energy of a reaction mixture change with composition
Because the Gibbs free energy of each reactant or product changes with concentration
What is effective concentration
Effective concentration of a species refers to the concentration of that species, considering any interactions or derivations from ideal behaviour that may occur in a solution or gas
What is the equation for Gibbs Free energy of a reaction when the composition is changing
Where a = effective concentration
G(A) = Gibbs Free energy of pure A
For an ideal gas, what is the equation for activity
Where p∘ = 1bar
What is an ideal solution
No forces between the molecules in a solution
For an ideal solution, what is the equation for activity
Where c∘ = 1mol dm⁻³
What is concentration and activity like in a pure solid or liquid
For pure solids or pure liquids concentration is fixed and the activity is 1
For a spontaneous reaction to occur
ΔG products < ΔG reactants
ΔG < 0
How does Gibbs free energy relate to equilibrium
- For a reaction that reaches equilibrium, the formward reaction is spontaneous until equilibrium is reached
- At equilibrium the Gibbs free energy of the reaction mixture is a minimum
- Beyond equilibrium the production of more products would result in an increase in Gibbs free energy
- At equilibrium the Gibbs free energy of the products equals that of the reactants, so ΔrG = 0
Describe the values of ΔrG for the forward and backwards reactions and equilibrium
Forwards = ΔrG < 0
Backwards = ΔrG > 0
Equilibrium = ΔrG = 0
The composition of the equilibrium mixture is given by
the equilibrium constant K
Defined by the activities of the reactants and products at equilibrium
Equilibrium constant is
A dimensionless concept
What is the expression for equilibrium constant for the following reaction in terms of partial pressure
What is equilibrium constant for the following reaction, in terms of partial pressure
What is the equilibrium constant for the following reaction in terms of concentration
What is reaction quotient
The favoured direction for reaction at any stage can be assessed from the reaction quotient which is calculated using the same expressuin for equilibrium constant but substituting non-equilibrium values
The difference in Gibbs free energy between the products and reactants at any stage is measured through which equation
If Q<K then ΔrG<0 = forward reaction proceeds
If Q>K then ΔrG>0 = backwards reaction proceeds
What is the link between Gibbs Free energy and equilibrium constant
When
the products are favoured in the equilibrium mixture
When
the reactants are favoured in the equilibrium mixture
What is Le Chatelier’s Principle
When a change is made to a sysmtem in dynamic equilibrium, the equilibrium responds so as to minimise the effect of the change
Explain in terms of equilibrium constant, why removing products causes more product to form
- This is a way of increasing yield from a reaction which has only a small amount of products in th equilibrium mixture
- For example in the Haber process (N₂(g) + 3H₂(g) → 2NH₃(g)) if ammonia is removed from the reaction mixture p(NH₃) decreases so Q<K and the forwards reaction is favoured
In terms of the equilibrium constant, why does increasing the pressure, shift equilibrium in the direction of the fewest moles of gas
- Composition of equilibrium does depend on overall pressure
- If K is to remain constant when the total pressure is increase, then NH₃ must increase relative to N₂ and H₂
For an endothermic reaction, what change favours the products
Increasing the temperature as heat is absorbed
K becomes bigger
For an exothermic reaction, what change favours the products
decreasing temperature, as heat is released
K becomes bigger
What is the Van Hoff Equation and how does it relate to exothermic and endothermic reactions
The Van Hoff equation takes the form of a straight line, so that a graph of Ln(k) over 1/T will have the gradient -ΔrH/ R
For an exothermic reaction the slope is +ve
For an endothermic reaction the slope is -ve
How do catalyst affect reactions
A catalyst increases the rate at which a reaction mixture reaches equilibrium, but does not alter the composition of equilibrium mixture
A catalyst can be used to lower the temperature required for a significant rate of reaction
For a reaction with a negative ΔrH value, what can be said
a reaction releases more energy than it absorbs
the reaction is exothermic
A reaction with a positive ΔrH value, what can be said
a reaction absorbs or uses more energy than it releases
the reaction is endothermic
The decomposition reaction has a ΔrH [700K] = +161 kJ mol⁻¹
Would K increase or decrease if the temperature was increease
This is an endothermic reaction, so the slop from the Van Hoff equation is negative
So if Ln(K) increases as 1/T decreases, therefore K increases as T increases
Therefore the products are favoured
