CHEM 122 Ch. 13.1.-4

Reaction quotient symbol

Q

Qc

uses molar concentrations

Qp

uses partial pressures

Q units

NONE

equilibrium constant symbol

K

K =

Q at equilibrium

Law of mass action

At a given temperature, the reaction quotient for a system at equilibrium is constant.

A reaction exhibiting a ______ K will reach equilibrium when most of the reactant has been converted to product, whereas a _____ K indicates the reaction achieves equilibrium after very little reactant has been converted.

large, small

If Q < K, the reaction proceeds in the ______ direction.

forward

If Q > K, the reaction proceeds in the ______ direction.

reverse

Relationship between Kp and Kc

Kp​=Kc​(RT)^Δn

Δn =

moles of products - moles of reactants

Homogeneous equilibrium

All reactants and products are in the same phase (usually all gases or all aqueous). Include all gases and aqueous species in the equilibrium expression.

Hetergeneous equilibrium

Reactants and products are in different phases. Only include gases and aqueous species in the equilibrium expression; omit pure solids and liquids.

What gets included/removed in K expressions

keep gases (g) and aqueous species (aq), remove pure solids (s) and pure liquids (l) because their concentrations are constant.

How to write equilibrium expression

Products over reactants, with coefficients used as exponents.

If a reaction is reversed, what happens to K?

1/K (take the reciprocal)

If all coefficients are multiplied by a number x, what happens to K?

K^x (Raise K to the power of x)

If all coefficients are divided by a number, what happens to K?

Raise K to the reciprocal power (ex. if all divided by 2, raise K to ^1/2, OR take square root so 2√K)

If reactions are added together, what happens to K?

Multiply K’s (K1 x K2)

If reactions are subtracted, what happens to K?

Divide K’s (K1/K2)

Le Châtelier’s principle

if an equilibrium system is stressed, the system will experience a shift in response to the stress that re-establishes equilibrium

Changes in ________ and ________ are the two stresses that can shift an equilibrium.

concentration, temperature

Kc is the ratio of

the rate constant for the forward reaction, kf, to the rate constant for the reverse reaction, kr.

Kc > 1

products favored at equilibrium

Kc < 1

reactants favored at equilibrium

K_ uses [x] and K_ uses (x)

c, p

For questions asking when Kp = Kc, look for equations where

Δn = 0

When solving for Δn, ONLY COUNT

GASES (g)

For a chemical reaction at equilibrium,

the rates of the forward and reverse reactions are equal

T/F: All chemical reactions go to completion

false

Rate of rxn depends on ______ and k (________ dependent)

concentration, temperature

Equilibrium does NOT mean that individual __________ are equal.

concentrations

Equilibrium = ________ are constant

products

If Kc < 1, the reaction will be _____-favored and will proceed in the ________ direction.

reactant, reverse

If Kc > 1, the reaction will be _____-favored and will proceed in the ________ direction.

product, forward

If Kc = __, the reaction will favor neither products nor reactants strongly.

1

Kc is defined as the ratio of the rate of _____ reaction to the rate of _____ reaction.

forward, reverse

Reactants —> products means the reaction proceeds in the ______ direction.

forward

Products —> reactants means the reaction proceeds in the ______ direction.

reverse

ONLY _____ and ______ are included in k!

gases (g), aqueous solutions (aq)

P₀₂ =

partial pressure of oxygen

Kₚ is specifically for ___________ reactions.

gas phase

Write the expression for Kp for the following chemical reaction:

M units =

concentrations

atm units =

partial pressures

Shifts towards products = shifts _____.

right

Shifts towards reactants = shifts _____.

left

Consequently, changes in __________ and __________ are the two stresses that can shift an equilibrium.

concentration, temperature

Adding a reactant shifts equilibirum ______ (_______ direction).

right, forward

Removing a product shifts equilibrium ______ (_____ direction).

right, forward

Adding a product shifts equilibrium ______ (______ direction).

left, reverse

Removing a reactant shifts equilibrium _____ (______ direction).

left, reverse

Per the law of mass action, the value of the ____________ will be the same after the stress as it was before.

equilibrium constant (k)

If you add a reactant, Q ______.

decreases

If you remove a reactant, Q _______.

increases

If you add a product, Q _______.

increases

If you remove a product, Q ________.

decreases

When volume decreases/pressure increases, shift toward side w/ _____ moles of gas.

fewer

When volume increases/pressure decreases, shift toward side w/ ______ moles of gas.

more

If the reaction is exothermic, treat heat like a _______.

product

If the reaction is endothermic, treat heat like a _________.

reactant

In an exothermic reaction, if temperature increases, equilibrium will shift ____.

left

In an exothermic reaction, if temperature decreases, equilibrium will shift ____.

right

In an endothermic reaction, if temperature increases, equilibrium will shift ____.

right

In an endothermic reaction, if temperature decreases, equilibrium will shift ____.

left

Equilibrium shifts right/forward direction = ______ favored.

products

Equilibrium shifts left/reverse direction = _______ favored.

reactants

State Le Chatelier’s principle for predicting what happens to a system when something momentarily takes it away from equilibrium.

When a stress or disturbance is applied to a system at equilibrium, the system will respond by shifting in the direction that minimizes the effect of the stress.

T/F: Only gases and aqueous species affect equilibrium shifts, while pure solids and liquids do not shift equilibrium when added, removed, or changed in amount.

true

For pressure/volume problems with shifting, ONLY count moles of ___.

gas (g)

If ΔH < 0, the reaction is _________.

exothermic

f ΔH > 0, the reaction is _________.

endothermic

T/F: Temperature is the ONE stress that changes the value of K

TRUE

temperature and K change in the SAME direction

endothermic

temperature and K change in OPPOSITE directions

exothermic

T/F: Catalysts impact K

FALSE

Recognize that the use of a catalyst affects the rate at which a system reaches equilibrium but does not change the value of the equilibrium constant.

Catalyst increases the rate of reaction by decreasing the activation energy, but has no effect on the value of K. Catalysts allow reactions to achieve equilibrium faster.

For a given rxn, predict whether higher or lower temp + pressure conditions will favor the formation of desired substance

What conditions will shift equilibrium toward the side that makes more of the desired substance?

T/F: Adding an inert gas causes equilibrium to shift

FALSE

Leading zeros before a . (ex. 400.0) are

NEVER SIGNIFICANT

“What is the composition of the reaction mixture”

Find molar concentrations of reactants/products