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equilibrium: concentrations of the reactants and products are not changingchemical reaction is balancedreversible reaction: arrow points both directions (⇋)e.g. 
reaction can go in either direction
forward reaction: up to down, left to rightreverse reaction: down to up, right to leftat equilibrium:concentrations are constantreaction rates are the same
equilibrium constant: K or K_{eq}
[x] means concentrationexample: 
K > 1: more products than reactants at equilibriumK < 1: more reactants than products at equilibriumcan also use with pressure
still use powers, etc. for ratiosexample: concentrations at equilibrium:
calculate K
if the equation is flipped, use the reciprocalif 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 
cannot do concentration or pressure for a solid or liquidassume the concentration or pressure is 1
reaction quotient (Q): like K but not at equilibrium
Q = K: at equilibriumQ > Kmore products than at equilibriumreaction shifts leftreaction wants to go right to left to make more reactantsQ < K:more reactants than at equilibriumreaction shifts rightreaction wants to go left to right to make more productsexample: at 500℃: 
what direction will the reaction shift if:
Q > K so reaction shifts lefta really high concentration of one reactant will break equilibrium
le Châtelier’s Principle: if you “stress” a reaction at equilibrium, the reaction will adjust to return to equilibriumanalogy: seesawstays equal if both sides are equally weightedif one side has more children, it will tip downcan only add children, not take awayfulcrum: arrowschildren: reactants/products
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)
AP Chemistry
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13. Chemical equilibrium
Equilibrium
Equilibrium constant
Determining equilibrium
Le Châtelier’s Principle
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