Equilibrium

What does the equilibrium constant K describe?

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to their stoichiometric coefficients.

Write the general expression for K_c for aA + bB ⇌ cC + dD.

K_c = ([C]^c [D]^d) / ([A]^a [B]^b).

What is K_p?

The equilibrium constant written with partial pressures (in atm): K_p = (P_C)^c (P_D)^d / (P_A)^a (P_B)^b.

How do you convert between K_c and K_p?

K_p = K_c (RT)^{Δn}, where Δn = moles of gaseous products − moles of gaseous reactants.

If Δn = 0, how are K_p and K_c related?

They are equal: K_p = K_c.

Do equilibrium constants have units?

They can. The units depend on reaction stoichiometry, though many texts omit units by using activities.

What does K > 1 tell you?

Products are favored at equilibrium.

What does K < 1 tell you?

Reactants are favored at equilibrium.

What does K = 1 tell you?

Neither side is favored. Both reactants and products are present in comparable amounts at equilibrium.

When do we say a reaction 'goes to completion'?

When K is very large (≫ 1). Products dominate.

When do we say a reaction 'barely proceeds'?

When K is very small (≪ 1). Reactants dominate.

What is the reaction quotient Q?

The same form as K but using current concentrations or pressures, not necessarily at equilibrium.

How do you use Q vs K to predict direction?

If Q < K, the reaction shifts forward (toward products). If Q > K, it shifts reverse (toward reactants). If Q = K, it is at equilibrium.

What is Q_c vs Q_p?

Q_c uses concentrations. Q_p uses partial pressures.

How does reversing a reaction affect K?

K_reverse = 1 / K_forward.

How does multiplying a reaction by a factor n affect K?

raise it to the power of the factor

How do you combine steps to get a target equation?

Add the balanced steps and multiply their K values, applying inversion or powers for reversed or scaled steps.

What does 'product-favored' mean in plain words?

At equilibrium there is more product than reactant; the forward direction is stronger.

What does 'reactant-favored' mean in plain words?

At equilibrium there is more reactant than product; the reverse direction is stronger.

In which units should partial pressures be for K_p?

Atmospheres (atm), unless otherwise specified.

What are R and T in K_p = K_c (RT)^{Δn}?

R is the gas constant (0.08206 L·atm·mol⁻¹·K⁻¹) and T is temperature in kelvin.

If Q < K, what happens to product and reactant amounts as the system shifts?

Products increase and reactants decrease until Q = K.

What does it mean to shift a reaction to the right?

The system makes more products and uses up reactants until a new equilibrium is reached (products increase, reactants decrease).

What does it mean to shift a reaction to the left?

The system makes more reactants and uses up products until a new equilibrium is reached (reactants increase, products decrease).