Unit 1-Part/Test Two-Chemical Equilibrium Flashcards
Chemical Equilibrium
Occurs when two opposing chemical reactions occur at equal rates
What are the types of Equilibria
- Phase Equilibria
- Solubility Equilibria
- Chemical Equilibrium
Phase Equilibria
-equilibrium may be established between different phases of a compound in a sealed container
Ex. H20 (l) in a sealed container
Initially: H20(l)changes to H2O(g)
gradually: H20(g) changes to H2O(l)
Note that the reverse rate is never faster than the forward rate. Forward is either faster than or equal to the reverse rate
Solubility Equilibria
- Occur in saturated solutions
- (ex) when NaCl(s) is placed in water, the initial rate of dissolving is fast.
- as more solid dissolves, the rate of dissolving slows and recrystallization begins
- when the solution is saturated there are NO VISIBLE CHANGES
- at equilibrium, the rate of dissolving and the rate of recrystallization are equal
Chemical Equilibria
Chemical reactions that are reversible may result in a chemical equilibrium.
Ex NO2 gas changing to N2O4
Initially the forward rate is high
- as more products form the reverse reaction begins and increases in rate
- eventually the forward rate slows and the reverse rate increases such that the FORWARD AND REVERSE RATES ARE EQUAL
What are the conditions for equilibrium
- Macroscopic properties are constant( NO OBSERVABLE CHANGE)
- Forward and reverse rates must be equal
- A closed system is required for equilibrium
- Equilibrium may occur from either direction
La Châtelier’s Principle
(The principle used to predict changes in concentration when a stress is applied to a system at equilibrium)
- when a stress is applied to a system at equilibrium, the system will adjust or shift to relieve the stress
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What are the types of stress
- Changes in concentration
- Temperature
- Pressure/volume
- Catalyst
Stress: Change in Concentration
An increase in concentration on one side of an equation favors or drives the reaction to the opposite side
- an equilibrium shifts away from a substance that increase in concentration, or toward a substance that decreases in concentration
IMPORTANT NOTES
- adding a solid or liquid does not change molar concentration
- changing the amount of the solid or liquid in an equilibrium does not cause a shift
Stress: change in temperature
Raising the temperature of an exothermic equilibrium favours the formation of reactants
- raising the temperature of an endothermic equilibrium favours formation of products
OR
-raising the temperature shifts the equilibrium from the energy term/heat
- decreasing the temperature shifts equilibrium toward the energy term(the heat)
TEMPERATURE IS THE ONLY THING THAT WILL CHANGE Keq
Stress: Pressure/Volume
An increase in pressure of a system at equilibrium has the same effect as a decrease in the volume of a system(inverse relationship)
- increasing the pressure of a system at equilibrium by reducing volume causes the equilibrium to shift in the direction with fewer molecules of gas
Stress: Catalyst
Does NOT cause a shift in equilibrium.
Increase BOTH rates equally so equilibrium is reacher faster
Stress: Surface area
(same as catalyst)
It does NOT cause a shift in equilibrium
- Increase/or decrease in surface area increases/or decreases BOTH rates equally so equilibrium is reached faster/or slower
Dynamic equilibrium
When 2 opposing processes occur at the same rate
Equilibrium (Keq)
(Pg495) For any system at equilibrium ,there is a mathematical relationship between reactants and products
(K/Keq/Kc- is the equilibrium constant expression-the ratio between the conc of products and conc of reactants)
Kc is sometimes used instead of keq when units are molar concentrations
NOTE: solids and liquids ARE NOT included in keq or kc expressions because their concentration is constant
Interpreting K:
if K is larger than 1 products are favored
if K is smaller than 1 reactants are favored
For any equilibrium The only way to change the actual value of k is to change the temperature
