Chapter 16 Chemical Equilibrium

Chemical equilibrium

The state where the forward and reverse reaction rates are equal; concentrations remain constant but not necessarily equal.

Dynamic equilibrium

Reactants and products continue to form, but their concentrations stay constant over time.

Equilibrium constant (K)

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

Expression for Kc

Products^coefficients / Reactants^coefficients using molar concentrations.

Expression for Kp

Products^coefficients / Reactants^coefficients using partial pressures.

Relationship between Kp and Kc

Kp = Kc(RT)^Δn where Δn = moles gas products – moles gas reactants.

What is Δn?

Change in moles of gas between products and reactants; used in relating Kp and Kc.

Large K (>1)

Products favored at equilibrium.

Small K (<1)

Reactants favored at equilibrium.

Reaction quotient (Q)

Calculated like K but using initial concentrations; predicts reaction direction.

If Q < K

Reaction shifts right toward products.

If Q > K

Reaction shifts left toward reactants.

If Q = K

System is already at equilibrium.

Le Chatelier’s principle

A system at equilibrium shifts to counteract a disturbance (stress).

Effect of adding reactant

Shifts reaction right (toward products).

Effect of adding product

Shifts reaction left (toward reactants).

Effect of removing reactant

Shifts reaction left.

Effect of removing product

Shifts reaction right.

Effect of increasing pressure

Shifts toward the side with fewer moles of gas.

Effect of decreasing pressure

Shifts toward the side with more moles of gas.

Effect of increasing volume

Shifts toward side with more gas moles.

Effect of decreasing volume

Shifts toward side with fewer gas moles.

Effect of temperature on exothermic reaction (heat is a product)

Increase temperature shifts left; decrease temperature shifts right.

Effect of temperature on endothermic reaction (heat is a reactant)

Increase temperature shifts right; decrease temperature shifts left.

What does NOT change K?

Adding reactants/products, changing pressure, changing volume.

What DOES change K?

Temperature only.

ICE table

Used to calculate equilibrium concentrations by tracking Initial, Change, and Equilibrium.

Common equilibrium mistake

Do NOT include solids or liquids in the K expression.

Why solids and liquids are excluded?

Their concentrations are constant and do not affect equilibrium.

When to use quadratic equation

When x is not negligible in ICE table.

5% rule

Assume x is small if (x / initial concentration) < 0.05; allows dropping x in denominator.

Reverse reaction K value

Kreverse = 1 / Kforward.

Adding coefficients (multiplying reaction)

Knew = (Kold)^n where n = multiplication factor.

Heterogeneous equilibrium

Equilibrium involving more than one phase (solid + gas, etc.).

Homogeneous equilibrium

Equilibrium where all species are in the same phase.

Catalyst effect on equilibrium

Catalyst speeds up the rate to reach equilibrium but does NOT change K or shift equilibrium.

Stress that has no effect on equilibrium

Changing pressure when both sides have equal moles of gas.

Shifting reaction to produce more product

Decrease product, add reactant, decrease volume (if fewer moles gas on product side).

Shifting reaction to consume more reactant

Add product, increase volume (if more moles gas on reactant side).

Temperature shift trick

Endothermic = heat is reactant → add heat shifts right; exothermic = heat is product → add heat shifts left.