chem 3.8 equilibrium constants

what state will reversible reactions eventually reach?

dynamic equilirbium

meaning of dynamic equilibrium?

rates of the forward and back reactions are equal and the composition of the mixture remains constant

what does le chatelier’s principle state?

if a system at equilibrium is changed then the position of equilibrium shifts to oppose the change

what are the two equilibrium constants?

Kc

Kp

what does Kc measure?

the amounts of reactants and products in terms of their concentrations

usually used for reactions in solutions

can also be used for gaseous reactions

what does Kp measure?

the amounts of reactants and products in terms of their partial pressures

used for reactions involving gases

what is the expression for Kc?

What conditions is Kc affected by?

affected by changes in temperature → causes increase or decrease depending on the sign of ΔH

unaffected by changes in pressure

unaffected by addition of catalyst

What are the units of Kc for this expression?

moldm^-3

What are the units of Kc for this expression?

mol^-2dm⁶

what are the units of Kc for this expression?

no units

what are the 2 types of systems that dynamic equilibria can exist in?

homogenous system

heterogenous system

meaning of homogeneous equilibrium system

has all the substances involved in the equilibrium in the same phase e.g. gaseous reactions or solution reactions

meaning of heterogeneous equilibrium system

has the substances involve in the equilibrium reaction present in more than one phase

what would the Kc be in this homogenous equilibria?

CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)

What would the Kc be in this hetereogenous equilibria?

H₂O(g) + C(s) ⇌ H₂(g) + CO(g)

What is the total partial pressure?

the sum of all the partial pressure of the gases present

what is the partial pressure of any gas?

a fraction of the total pressure determined by its mole fraction

what is the expression for Kp?

what are the units of Kp?

kPa

Pa

atm

(same unit must be used for all gases)

If a question gives you the MOLES of all the reactants and products, how would you calculate Kc?

use moles to find concentrations → concentration = moles/volume

sub concentrations into Kc expression

If a question gives you the MOLES of all the reactants and products, how would you calculate Kp?

find total number of moles (add all moles together)

calculate mole fraction of each gas in mixture → moles of each component/total moles

calculate partial pressure of each gas in mixture → mole fraction of each component x total partial pressure

sub into Kp expression

what does the size of Kc or Kp indicate?

how far a reversible reaction has proceeded

does not tell us how fast it goes (nothing to do with rate)

If Kc or Kp = 1, what does this indicate?

equilibrium is half way between reactants and products

If Kc or Kp > 100, what does this indicate?

equilibrium lies well to RHS → favours products

If Kc or Kp < 0.01, what does this indicate?

equilibrium lies well to LHS → favours reactants

what will happen to Kc and Kp when temperature increases in exothermic reactions? (ΔH -ve)

Kc and Kp decrease

equilibrium shifts to LHS

decreases amount of products

increases amount of reactants

what will happen to Kc and Kp when temperature increases in endothermic reactions? (ΔH +ve)

Kc and Kp increase

equilibrium shifts to RHS

increases amount of products

decreases amount of reactants

Describe what happens to Kc when concentration changes?

Kc doesn’t change

initially the value of the expression for Kc changes as the system is no longer at equilibrium

position of equilibrium changes to convert reactants to products or vice versa

new equilibrium reached

Value of Kc restored

describe what happens to Kp when the pressure changes?

Kp does not change

initially the value of the expression for Kp changes as system is no longer at equilibrium

position of equilibrium moves changing the proportions of reactants and products

new equilibrium reached

value of Kp is restored

If a question gives you incomplete information about a mixture, how would you calculate the amount of each substance?

RICE method

what does the RICE method stand for?

R - ratio (big numbers from the equation)

I - initial (initial numbers of moles, if not given, write 0)

C - changes in moles (always - for reactants, always + for products)

E - equilibrium (moles at equilibrium)