Equilibrium

What does the eq’m constant (K) represent

What does the eq’m constant (K) represent

The ratio between [ ] of reactants and products

How is K evaluated (for aA + bB <- → dD + eE)

K = [D]^d [E]^e / [A]^a [B]^b

What does it mean if K is equal to one

Then [products] = [reactants]

What does it mean if K is greater than 1

• [Products] is greater than [reactants] in the mixture

• The fwd rxn is more favourable (eq’m favours the products)

What does it mean if K is less than 1

• [Products] is smaller than [reactants] in the mixture

• The rev rxn is more favourable (eq’m favours the reactants)

What does ICE stand for in an ICE table

I - initial

C - change

E - equilibrium

What is the equation used for calculating K(p) (using partial pressures)

K(p) = (P(D))^d (P(E))^e / (P(A))^a (P(B))^b

How to convert between K(c) and K(p)

K(p) = K(c) (RT)^Δn

What is the 500 rule

• Says if the initial [ ] of reactant divided by K is greater than 500, then the change in [ ] for the reactant is insignificant and can be ignored

  • 500 rule = [initial reactant]/K

If an eq’m is written in reverse, what would happen to the new K value

new K = 1/original K

What does it mean if Q = K

The reacting system is already at eq’m

What does it mean if Q > K

• [products] is too large & [reactants] is too small

  • Reacting system must proceed in the reverse direction (to the left/reactants)

What does it mean if Q < K

• [products] is too small & [reactants] is too large

  • Reacting system must proceed in the forward direction (to the right/products)

What does LeChatelier’s Principle state

If a system is subjected to some form of ‘stress’ the system will shift in a way that reduces stress and re-establishes eq’m

What are the four type of stresses should be considered in LeChatelier’s Principle

1. A change in concentration

2. Change in temperature

3. A change in pressure

4. Introduction of a catalyst

How does concentration affect the shift

• If a reactant is increased, the fwd rxn will temporarily be favoured to decrease [B] and re-establish eq’m, and system will shift right

• If a product is increased, the rev rxn will temporarily be favoured to decrease [c] and re-establish eq’m, and system will shift left

How does change in temperature affect K and the shift for an exothermic rxn

• If T is increased, product is being added and system shifts left

  • K = [products]/[reactants], so K must decrease

• If T is decreased, product is being removed and system shifts right

  • K = [products]/[reactants], so K must increase

How does change in temperature affect K and the shift for an endothermic rxn

• If T is increased, reactant is being added and system shifts right

• If T is decreased, reactant is being removed and system shifts left

  • K = [products]/[reactants], so K must decrease as T decreases

How does pressure affect the shift

• If volume decreases, the reacting system will favour the side w fewer moles of gas, so the system shifts right

• If volume increases, the reacting system will side w more moles of gas, so system shifts left

• This is bc if V decreases, P increases

How does a catalyst affect the shift

It doesn’t

What is the equation for K(a) with the equation HA + H2O <- → A- + H3O+

K(a) = [A-] [H3O+] / [HA]

How can pH be used to calculate Ka

With pH, H3O can be calculated, then letting Ka be calculated

What is percent ionization

Expresses the fraction of molecules that ionized as a percentage of the initial concentration of acid

What is the equation for K(b) with the equation B + H2O <- → HB+ + OH-

K(b) = [HB+] [OH-] / [B]

What is the K(w) constant

• K(a) X K(b) = K(w)

• K(w) = 1.00 X 10^-14

What are the strong acids (and therefore weak conjugate bases)

HCl, HBr, HI, H2SO4, HNO3, HCLO3, HCLO4

What are the strong bases (and therefore weak conjugate acids)

Strong bases: All alkali metal hydroxides and heavy earth metal hydroxides

Conjugate acids: Li, Na, K, Rb, Cs, Fr, Sr, Ba, and Ra (appropriate charge on each)

What is the equivalence point

When moles acid = moles base (sol’n in flask is a salt sol’n)

Why is it important to know the pH of a salt sol’n

So we can select an indicator to signal the endpoint (when the indicator changes colour) of a titration. We need to know what the pH of the sol’n will be when we reach the equivalence point to choose said indicator. The pH where the indicator changes colour needs to be as close as possible to the equivalence point pH.