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13 Cards in this Set

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reversible reactions
can occur in both the forward and reverse directions

most reactions do not go to completion-- appear to stop

the rate of the reaction depends on the concentration of the reactants

the rate of the forward reactions continues to decrease as the amount of reactants decreases
and the rate of the reverse reaction increases
until the 2 rates become equal
chemical equilibrium
the state in which the forward and reverse reactions balance each other because they take place at equal rates

doesn’t mean that the amounts of the reacts and the products are equal

a state of action, not inaction
Law of chemical equilibrium
at a given temperature, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a constant value
equilibrium constant
aA + bB == cC + dD

[C]c[D]d /
Keq = [A]a[B]b

square brackets = molar concentrations
exponents = corresponding coefficient
Keq constant only at specified temp
Keq >1: more products than reactants at equilibrium
Keq <1: less products than reactants at equilibrium
Keq will always be the same for a given reaction at a given temp, regardless of the concentrations of reactants and products
homogeneous equilibrium
all reactants and products are in the same physical state
Heterogeneous equilibrium
reactants and products are present in multiple physical states

solids and liquids have unchanging concentrations so they can be combined with K
Le Chatelier’s Principle
if a stress is applied to a system at equilibrium, the system shifts in the directions that relieves the stress
stress
any change in a system that upsets the equilibrium

3 types:
-concentration
-volume (pressure)
-temp
concentration
adding or removing a reactant or product shifts the equilibrium in the direction that removes the stress
volume (pressure)
changing the volume only shifts the equilibrium if # moles of product does not = the # moles of reactant

when you decrease the volume of a reaction vessel, you increase the pressure causing the reaction to shift to the side with the SMALLEST number of moles
-opposite is also true
temperature
changing the temp of a reaction at equilibrium alters both the equilibrium constant and the equilibrium position

when a reaction is exothermic (releases energy), lowering the temp shifts the equilibrium to the RIGHT b/c the forward reaction liberates heat and removes the stress

TEMP DECREASE: shift TOWARDS the added energy

TEMP INCREASE: shift AWAY the added energy
Keq
large Keq – equilibrium mixture contains more products than reactants

small Keq – equilibrium mixture contains more reactants than products
Calculating the concentration of one substance in a reaction
* the concentrations of the other substances must also be known

1. write balanced equation
2. write equilibrium constant expression
3. isolate the substance you are solving for
4. substitute all known concentrations and the value of Keq