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
What are the three types of Keq calculations
- Given equilibrium concentrations, find k
- Find a missing concentration given k and other concentrations
- Given initial concentrations and Equilibrium data, find k ( ICE TABLES)
ICE Tables
Keq may be calculated given initial concentration and the change in concentration we can find the missing equilibrium concentrations and calculate keq
macroscopic
observable
The law of chemical equilibrium
the law stating that there is a constant ratio,at equilibrium,between the concentrations of products and reactants in any change
Percent reaction?
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