Dynamic equilibria

What is a closed system?

a system where none of the products or reactants can escape - isolated from its surroundings

What happens at dynamic equilibrium?

• the rate if the forwards reaction is equal to the rate of the backwards reaction

• concentration of reactants and products do not change

What is Le Chatelier’s principle?

If the conditions of a reaction at equilibrium are changed, then the position of the equilibrium moves to oppose that change

What is the effect of catalysts on an equilibrium?

• increases the rate of both the forward and reverse reactions equally

• it does not change the position of the equilibrium but it reaches equilibrium faster

What happens to the position of equilibrium when temp is increased?

equilibrium moves in the endothermic direction

What happens to the position of equilibrium when temp is decreased?

equilibrium moves in the exothermic direction

What happens to the position of equilibrium when pressure is increased?

moves in the direction of the smallest number if molecules of gas

What happens to the position of equilibrium when pressure is decreased?

moves in the direction of the largest number if molecules of gas

What happens to the position of equilibrium when concentration is increased?

shifts in the opposite direction to the substance - to use up the added substance

What happens to the position of equilibrium when concentration is decreased?

shifts towards the substance with a lower concentration

What are the features of a system in dynamic equilibrium?

1. backwards and forwards reactions are happening at the same rate

2. both reactions take place simultaneously

3. all reactants and products are present

4. concentration of reactants and products remain constant

How can the exact position of equilibrium be calculated?

using the equilibrium constant (Kc)

What is the equilibrium law?

What do the square brackets mean in the Kc equation?

concentration of

What does a Kc value of 1 indicate?

equilibrium position is halfway between products and reactants

What does a Kc value greater than 1 indicate?

• equilibrium is to the right

• [products] > [reactants]

What does a Kc value of less than 1 indicate?

• equilibrium is to the left

• [reactants] > [products]

What is the only thing Kc is affected by?

changes in temperature

How are equilibrium quantities calculated - acronym?

Initial

Change

Equilibrium

How is the ICE acronym used to calculate equilibrium quantities?

• moles at start are calculated for each reactant and product

• the moles at equilibrium are calculated

• the change in moles of calculated

any missing data can be found using the other two pieces of data for each substance

How can the stoichiometry of an equation be used when creating ICE tables to calculate equilibrium quantities?

use ratios for change in moles

How is Kp calculated?

Kp = pressure of products / pressure of reactants

How does the equilibrium constant change when the reaction is reversed?

inverse of the expression = 1/Kc

How does the equilibrium constant change when the balancing coefficients are halved?

square root of Kc

How does the equilibrium constant change when the balancing coefficients are doubled?

square the expression = Kc²

What is the effect of increasing temperature on the equilibrium position in exothermic reactions?

K decreases

What is the effect of decreasing temperature on the equilibrium position in endothermic reactions?

K increases

What does it mean when the ratio of products/reactants is greater than Kc?

• the concentration of products is greater than at equilibrium

• the reverse reaction is favored until equilibrium is re-established

• then Kc is restored

What does it mean when the ratio of products/reactants in less than Kc?

• the concentration of products is less than at equilibrium

• the forwards reaction is favored until equilibrium is re-established

• then Kc is restored

What is a mole fraction?

sum of all mole fractions in a mixture add up to 1

mole fraction of gas A in mixture of gases = moles of gas A / total moles of gas in the mixture

What is partial pressure?

the contribution that each gas makes to the total pressure of the gas

partial pressure of gas A = mole fraction of gas A x total pressure in a mixture of gases

How is Kp calculated using mol fractions and pp?

• initial mol

• mol change

• equilibrium mol change

• mol fraction

• partial pressure

Kp = pp products / pp reactants