Chapter 3: 3.1 The Equilibrium Constant Flashcards
Define:
Equilibrium
The condition where the rate of forward reaction equals rate of reverse reaction
What is the symbol used for equilibrium constant?
K
True or False:
Equilibrium constants are temperature dependent
True
Define:
Homogeneous equilibrium
If all the components are in the same phase in a system
Define:
Heterogeneous equilibrium
If the components are in different phases in a system
Are solids included in the equilibrium constant expression?
No
How is the general equilibrium reaction written?
aA + bB ⇌ cC + dD
* A and B are reactants
* C and D are products
State:
The general form of the equilibrium constant expression for this reaction, using concentrations
K = [C]^c[D]^d/[A]^a[B]^b
True or False:
K is unitless
True, K is unitless and based on activities
List:
Subscripts and meanings for K
- (aq) = Concentration (M or mol/L)
- (g) = Partial Pressure (bar or atm)
- (sp) = Solubility
How is the proper thermodynamic definition of the equilibrium constant given?
Given in terms of properties called:
* Activity: For species in a solution
* Fugacity: For gas-phase species
What are activity and fugacity related to?
Concentration and partial pressure
True or False:
Activity and fugacity have units
False, they are dimensionless
What should be the only unit used in calculations of K?
M
State the significance of the magnitude of K when:
- K is greater than 1
- K is less than 1
- Equilibrium lies toward the right (product side) of the equation, indicating more products than reactants
- Equilibrium lies toward the left (reactant side) of the equation, indicating more reactants than products
State the new K value when:
Reversing an Equilibrium Reaction
1/K (original)
State the new K value when:
Multiplying/Dividing an Equilibrium Reaction
When multiplied by some coefficient, m, the new K value is K^m
State the new K value when:
Combining Equilibria
K (new) = K (1) * K (2)
How is K and Amounts determined?
- K: Can be found is equilibrium amounts of each species involved are known
- Amounts: Can be found if K is known via stoichiometry
Defineand state the notation for:
Reaction quotient
The number that indicates whether the system is at equilibrium, and the direction in which the reaction must proceed to attain equilibrium
* Q
What must be known in order for reaction quotient to be calculated?
Amounts of products and reactants
The expression for Q is the same as K except for what?
The concentrations used are not necessarily equilibrium concentrations
In terms of Q and K:
When is a system at equilibrium?
When Q=K
If Q > K…
- System not at equilibrium
- Must proceed in reverse direction
- Too much product OR not enough reactant
If Q < K…
- System not at equilibrium
- Must proceed in forward direction
- Too much reactant OR not enough product
Define:
Le Châtelier’s Principle
If a chemical system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of a participant in the equilibrium, the equilibrium will shift in such a way as to minimize the disturbance
Applying Le Châtelier’s Principle:
How does concentration/partial pressure change by addition or removal of a species change the equilibrium?
- If reactants are removed, then Q will be smaller than K, meaning the reaction will proceed forward until Q = K again
- If products are removed, then Q will be larger than K, meaning the reaction will proceed backwards until Q = K again
Applying Le Châtelier’s Principle:
Total Pressure Change by a Volume Change
Example: 4 moles of reactants, 2 moles of products
1. If volume is increased the reaction will proceed backwards until equilibrium is reached
2. If volume is decreased the reaction will proceed forwards until equilibrium is reached
True or False:
Le Châtelier’s Principle applies to systems with solids and liquids when volume changes
Depends. If the system is JUST solids and liquids, changing the volume will not affect anything
True or False:
If the moles of reactants and products are the same in a gaseous system, changing the volume will not affect the system (according to Le Châtelier’s Principle)
True
Applying Le Châtelier’s Principle:
Relationship Between Temperature and K Values
For an exothermic reaction, heat is released thus is a product
1. Increasing the temperature = Increasing amount of products, thus the K value will decrease as the equilibrium will be shifted to the left
For an endothermic reaction, heat is required thus is a reactant
1. Increasing the temperature = Increasing amount of reactants, thus the K value will increase as the equilibrium will be shifted to the right
Applying Le Châtelier’s Principle:
Gibbs Free Energy and the Equilibrium Constant
- If ΔG^o is positive, the equilibrium lies to the left (reactant) side and K is small
- If ΔG^o is negative, the equilibrium lies to the right (product) side and K is large
True or False:
A positive ΔG^o means the process is nonspontaneous under standard conditions, which means that it does not proceed at all
False, this means that it only proceeds to a very small extent
List
The process and relationship between ΔG^o and K
- ΔG = ΔG^o + RTlnQ
- As ΔG = 0 and Q = K at equilibrium: ΔG^o = -RTlnK
In the formula ΔG^o = -RTlnK:
State what each variable is
- ΔG^o is standard Gibbs free energy for the reaction in J/mol (standard conditions means 25 degrees Celsius, 1 atm pressure, and 1M concentration for everything in solution)
- R is gas constant (8.314 J/mol K)
- T = 298.15 K (standard conditions)
- K is equilibrium constant
State the formula for:
Standard free energy change using enthalpy and entropy changes
ΔG^o = ΔH^o - TΔS^o
True or False:
While standard free energy change varies over temperature, standard enthalpy and entropy change varies little over temperature
True
State the formula for:
Finding K values at temperatures other than standard (298.15 K)
lnK = -ΔH^o/RT + ΔS^o/R
(Substituting ΔG^o for -RTlnk and dividing by -RT)
State the formula and purpose of:
van’t Hoff Equation
ln K2/K1 = ΔH^o/R (1/T1 - 1/T2)
* Used to determine K at a second temperature (T2), given ΔH^o rxn, K1 and T1
What are alternative uses for van’t Hoff equation?
- If K is known at two different temperatures, the equation may also be used to calculate ΔH^o rxn
- Determining the appropriate temperature at which to carry out a reaction
