Crane Chemistry Equillibrium

Equilibrium is reached when …

rate of forward reaction = rate of reverse reaction

concentrations of reactants and products are constant

Law of Mass Action

For reaction: e A + f B →← g C + h D

Equilibrium expression:

K = ([C]^g [D]^h) / ([A]^e * [B]^f)

K = equilibrium constant

Brackets unit is mol/L (M) (Molarity)

Magnitude of K

K>1

more products at equilibrium

Equilibrium lies to the right

K=1

No! check your math

K < 1

more reactants at equilibrium

equilibrium lies to the left

Calculating K_p from K_c

K_p = K_c (RT)^∆n

where:

R = 0.0821

T = temperature in kelvin

∆n = moles of products - moles of reactants

Homogeneous Equilibria

all reactants / products are in one phase

Heterogeneous Equilibria

reactants / products are in more than one phase

Solids and liquids are

not included in K calculations

Manipulating K

Swapping products and reactants

take the reciprocal of K

example:

PCl₅(s) →/← PCl₃(L) + Cl₂(g) ——- K_c=0.0239

turns into

PCl₃(L) + Cl₂(g) →/← PCl₅(s) ——- K_c= 1 / 0.0239

Changing Coefficients

put K to the coefficient power

example:

PCl₅(s) →/← PCl₃(L) + Cl₂(g) ——- K_c= 0.0239

turns into

2PCl₅(s) →/← 2PCl₃(L) + 2Cl₂(g) ——- K_c = 0.0239²

Adding reactions

multiply k’s

example:

2CO₂(g) + H₂O(g) →← 2O₂(g) + CH₂CO(g)

K = 6.1 E8

CH₄(g) + 2O₂(g) →← CO₂(g) + 2H₂O(g)

K = 1.2 E14

6.1E8 * 1.2 E14 = 7.3 E22

Reaction Quotient (Q)

To determine if a reaction is at equilibrium, compare Q to a given K

For reaction: e A + f B →← g C + h D

Q = ([C]^g [D]^h) / ([A]^e * [B]^f)

Q = K

reaction is at equilibrium

Q < K

too many reactants, reaction must go forward / shift right

Q > K

too many products, must go in reverse / shift left

Lechatelier’s Principle

if a stress is applied to a system at equilibrium, the position of teh equilibrium will shift to alleviate that stress

3 ways to apply stress

change in amount

change in pressure (volume)

change in temperature

Change in Amount

1N₂(g) + 3H₂(g) →/← 2NH₃(g)

add N₂ - shifts right

add NH₃ - shifts left

remove N₂ - shifts left

remove NH₃ - shifts right

if a component is added, the equilibrium shifts to lower the concentration of that component

Change in Pressure (Volume)

1N₂(g) + 3H₂(g) →/← 2NH₃(g)

1. add an inert gas - no effect

2. change the volume of the container

   1. Increase pressure → shifts toward the side with fewer moles of gas

      1. in the reaction above, would shift right

   2. Decrease pressure → shifts toward the side with more moles of gas

      1. in the reaction above, would shift left

   3. If both sides have equal moles of gas → no shift

Change in Temperature

1N₂(g) + 3H₂(g) →/← 2NH₃(g) + 92 KJ

if heat is a product, it is exothermic (heat is produced) [like in the reaction above]

if heat is a reactant, it is endothermic (heat is required)

for exothermic:

1. If you add heat, it shifts left

2. If you remove heat, it shifts right

for endothermic:

1. if you add heat, it shifts right

2. if you remove heat, it shifts left

ICE Charts

Approximation Method

First, check that the K value is E-3 or smaller

If that is true, you can ignore subtracting the x from the reactants at equillibrium

Then, calculate x

Lastly, use this formula to check

x / |initial| *100% < 5%