Equilibria
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What can be said about about the rates of forwards and reverse reactions at equilibrium?
They are equal.
What makes a dynamic equilibrium?
• Rate of reaction in both directions is equal • Concentrations of reactants and products remains unchanged • Closed system
If concentration is increased, how does position of equilibrium change?
It shifts to the side favouring the reaction that uses that reactant up.
What does the position of equilibrium show? What affects it?
• Extent of a reaction • Temperature, pressure and concentration
State Le Chatelier’s Principle.
When a system in dynamic equilibrium is subject to an external change, the system adjusts itself to minimise the change.
What phase must the reactants be in for changes in pressure to affect equilibrium?
All reactants must be gaseous.
How do changes in temperature affect position of equilibrium?
Increase: shifts to favour endothermic process (positive delta-T) Decrease: shifts to favour exothermic process (negative delta-T)
How do changes in pressure affect equilibrium?
Increase: shifts to favour side of fewer gas moles Decrease: shifts to favour side of more gas moles
How can we use Le Chatelier’s principle to evaluate the best conditions for the Haber process?
• Forward reaction is exothermic, so we use a low temperature to favour this. • High pressure increases rate (more successful collisions) and yield • Balance of expense and yield
How does a catalyst affect the position of equilibrium?
It doesn’t!
If the ratio of products : reactants is <1, where is the position of equilibrium?
Shifted to the left
How do we determine Kc for a reverse reaction?
Find the inverse of Kc = 1/Kc
When would we apply the equilibrium constant Kc?
For the aqueous and gaseous phases only.
Why doesn’t Kc/Kp change when we introduce a change to a system?
Only the ratio of reactant:products changes, in order to maintain a constant value of Kc. Kc only refers to this rate at dynamic equilibrium.
Given the equilibria aA + bB <-> cC + dD, state the formula for the equilibrium constant.
[C]^c * [D]^d / [A]^a * [B]^b
If the ratio of products : reactants is >1, where is the position of equilibrium?
Shifted to the right
If the ratio of products : reactants is =1, where is the position of equilibrium?
Central
What does Kc represent?
The equilibrium constant considering concentration - the ratio of concentrations of products and reactants at dynamic equilibrium.
How do we calculate partial pressure?
We must calculate the mole fraction of each species, then multiply by the total pressure of the system.
What phase must all reactants and products be in when considering Kp?
Gaseous phase only.
What do all partial pressures sum to?
The total pressure of the system. Similarly, all mole fractions should add up to 1 - otherwise something has gone wrong.
How do we calculate Kp?
We use the same method as we do for Kc, but using partial pressures rather than concentrations.
How do we use RICE tables?
• Row 1: reaction equation • Row 2: initial moles (products at 0) • Row 3: change to equilibrium (this is calculated for each species using the stoichiometric ratio) • Row 4: moles at equilibrium
State the basic process for controlling equilibrium. Take the example of a constant temp., increased pressure and considering Kc.
Say that temperature is constant, but pressure increases. 1. Kc is constant for a given temperature 2. Pressure increases so concentration of all gaseous species increases 3. State Kc equation 4. The ratio of reactants:products changes, in order to maintain a constant Kc.