AP Chemistry

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13. Chemical equilibrium

Equilibrium

equilibrium: concentrations of the reactants and products are not changing

chemical reaction is balanced

reversible reaction: arrow points both directions (⇋)

e.g.

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reaction can go in either direction

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forward reaction: up to down, left to right

reverse reaction: down to up, right to left

at equilibrium:

concentrations are constant

reaction rates are the same

Equilibrium constant

equilibrium constant: K or K_{eq}

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[x] means concentration

example:

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K > 1: more products than reactants at equilibrium

K < 1: more reactants than products at equilibrium

can also use with pressure

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still use powers, etc. for ratios

example:

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concentrations at equilibrium:

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calculate K

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if the equation is flipped, use the reciprocal

if the coefficients all change by the same ratio, take K to that power (e.g. multiply all coefficients by 2 → K = K²)

example: find K for

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cannot do concentration or pressure for a solid or liquid

assume the concentration or pressure is 1

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Determining equilibrium

reaction quotient (Q): like K but not at equilibrium

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Q = K: at equilibrium

Q > K

more products than at equilibrium

reaction shifts left

reaction wants to go right to left to make more reactants

Q < K:

more reactants than at equilibrium

reaction shifts right

reaction wants to go left to right to make more products

example:

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at 500℃:

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what direction will the reaction shift if:

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Q > K so reaction shifts left

a really high concentration of one reactant will break equilibrium

Le Châtelier’s Principle

le Châtelier’s Principle: if you “stress” a reaction at equilibrium, the reaction will adjust to return to equilibrium

analogy: seesaw

stays equal if both sides are equally weighted

if one side has more children, it will tip down

can only add children, not take away

fulcrum: arrows

children: reactants/products

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add H₂: reaction shifts right (products)

add H₂O: reaction shifts left (reactants)

remove O₂: reaction shifts left (reactants)

remove H₂O: reaction shifts right (products)

add heat: reaction shifts right (endothermic)

reduce heat: reaction shifts left (endothermic)

increase pressure: reaction shifts right (fewer molecules on right; try to decrease pressure)

decrease pressure: reaction shifts left (more molecules on left; try to increase pressure)

add catalyst: nothing happens (at equilibrium)

Equilibrium

Equilibrium constant

Determining equilibrium

Le Châtelier’s Principle

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