3/6 SI Session

What change would we expect to see once the reaction reaches equilibrium?

How do we know how quickly the reaction reaches equilibrium?

• By watching it

• By the rate law

• By looking it up online

How do we write the forward and backward elementary reaction for the following reaction?

Answer explanation

forward: R = k₁[NO₂]²

backward: R = k_-1[N₂O₄]

at equilibrium:

k₁[NO₂]² = k_-1[N₂O₄]

k₁/k_{-1 =} [N₂O₄]/[NO₂]²

Identify the variables of the equilibrium expression.

Answer explanation

K_c molar equilibrium constant

[A], [B], [C], [D] concentrations of A, B, C, D

in mol/L

a, b, c, d

stoichiometric coefficients

Identify the variables of the equilibrium expression.

Answer explanation

K_p molar equilibrium constant

P_A, P_B, P_C, P_D

partial pressures of A, B, C, D

in atm

a, b, c, d

stoichiometric coefficients

Can we use K_P and K_c interchangeably?

• Yes

• No

• Maybe?

Answer explanation

Yes!

K_P = K_c(RT)^Δn

Δn = moles gas product - moles gas reactant

R = 0.082 atm L/mol K

T in kelvin

General rules for writing equilibrium constants:

Answer explanation

1. do not include pure solids/liquids

2. do not include water when it is the solvent

3. K_c uses molarities, K_p uses partial pressures

4. K_c and K_p are dimensionless

What would be the equilibrium constant for the following reaction?

• K_c = [O₃]/R

• K_p = P_O2³/P_O3²

• K_c = [O₃]²/[O₂]³

• K_p = P_O2/P_O3

Answer explanation

the reaction has both gas products and reactants so we will use K_p

K_p = P_O2³/P_O3²

What would be the equilibrium expression for the following reaction?

• K_c = [CO₂]

• K_p = (P_CaO)(P_CO2)/P_CaCO3

• K_p = P_CO2

• K_c = 0

Answer explanation

we don't consider solids wen writing equilibrium constants

so for this case,

K_p = P_CO2

or

K_c = [CO₂]

We can use K_c values to find out whether or not a reaction will proceed. If K_c > 1, then the reaction favors

Answer explanation

we can use K_c values to find out whether or not a reaction will proceed

aka if it favors products or reactants

K_c > 1 rxn favors products

K_c < 1 rxn favors reactants