Equilibrium Basics
What type of reaction can occur in both the forward and reverse directions?
A reversible reaction
What does Le Chatelier’s Principle state happens when stress is applied to a system at equilibrium?
The system shifts in the direction that relieves the stress
In the reaction H₂ + I₂ ⇌ 2HI, what direction will the reaction shift if HI is added?
Left (toward reactants)
In an exothermic reaction, heat behaves like a reactant or a product?
A product
What does the equilibrium constant (K) represent?
The ratio of product concentrations to reactant concentrations at equilibrium
What is chemical equilibrium?
A state where the forward and reverse reaction rates are equal
What is a “stress” in a chemical equilibrium system?
Any change that disturbs equilibrium (such as concentration, temperature, or pressure changes)
In the reaction PCl₅ ⇌ PCl₃ + Cl₂, what happens if Cl₂ is removed?
The reaction shifts right
If temperature increases in an exothermic reaction, which direction does the reaction shift?
Left (away from heat)
In the equation K = CˣDʸ / AⁿBᵐ, what do the exponents represent?
The coefficients from the balanced equation
At equilibrium, do reactions stop or continue?
They continue; equilibrium is dynamic
If more product is added to a system at equilibrium, which direction will the reaction shift?
Toward the reactants (left)
In CO + 3H₂ ⇌ CH₄ + H₂O, what direction will the reaction shift if CO is added?
Right (toward products)
If temperature decreases in an exothermic reaction, which direction does the reaction shift?
Right (toward heat)
For the reaction
H₂ + I₂ ⇌ 2HI
At equilibrium the concentrations are:
[H₂] = 0.50 M
[I₂] = 0.50 M
[HI] = 2.00 M
Calculate the equilibrium constant K.
K = [HI]² / ([H₂][I₂])
K = (2.00)² / (0.50 × 0.50)
K = 4 / 0.25
K = 16
In chemical equilibrium, concentrations remain constant because the forward and reverse reactions occur at what relationship to each other?
The same rate
If a reactant is removed from a system at equilibrium, which direction will the reaction shift?
Toward the reactants (left)
In A + B ⇌ C + D, the solution becomes more yellow and C is yellow. Which direction did the reaction shift?
Right
When can pressure changes affect equilibrium?
Only when gases are involved
For the reaction
PCl₅ ⇌ PCl₃ + Cl₂
The equilibrium concentrations are:
[PCl₅] = 0.200 M
[PCl₃] = 0.600 M
[Cl₂] = 0.600 M
Calculate the equilibrium constant K.
K = ([PCl₃][Cl₂]) / [PCl₅]
K = (0.600 × 0.600) / 0.200
K = 0.360 / 0.200
K = 1.8
Where on the graph is equilibrium achieved?
15-20 seconds
Consider the following reaction, where the reactants are yellow and the products are orange.
2CrO4²⁻ (aq) + 2H⁺ (aq) ⇌ Cr2O7²⁻ (aq) + H2O (l)
The solution turns from yellow to orange after acid is added. What direction did the reaction shift and why?
The reaction shifted right because adding acid increased H⁺ concentration, so the system shifted to use up the extra H⁺ and produce more Cr₂O₇²⁻ (orange).
In the reaction A + B ⇌ C + D, if B is removed and the system shifts left, what is the system trying to do?
Replenish or replace the missing B
In the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what happens if pressure increases?
The reaction shifts right (toward fewer molecules)
For the reaction
N₂O₄ ⇌ 2NO₂
At equilibrium:
[N₂O₄] = 0.040 M
[NO₂] = 0.080 M
Calculate the equilibrium constant K.
K = [NO₂]² / [N₂O₄]
K = (0.080)² / 0.040
K = 0.0064 / 0.040
K = 0.16