Ch 16: Chemical Equilibrium

What does it mean for a process to be reversible?

The products of the reaction can react to form reactants

In regard to the rate, what does it mean for a system to be in equilibrium?

The rates of the forward reaction and reverse reaction are the same

What does it mean when we say equilibrium is a "dynamic state"?

Both forward and reverse reactions continue to occur, but there's no net change in reactant or product concentrations over time.

True or False: At equilibrium, chemical reactions stop occurring.

False. Reactions continue in both directions, but concentrations remain constant.

What remains unchanged at equilibrium?

The concentrations of reactants and products.

In what ways can equilibrium be established?

Starting with only reactants, only products, or any mixture of both.

True or False: Equilibrium can only be reached if both reactants and products are present initially.

False. It can be reached from reactants only, products only, or a mix.

What is the reaction quotient?

• Q_c = a fraction with product concentrations in the numerator and reactant concentrations in the denominator 

  • each conc. is raised to a power = to its stoichiometric coefficient in the balanced chemical equation

What is the law of mass action?

the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants, each raised to the power of their respective stoichiometric coefficients.

What is the equilibrium constant?

• K_c  = (Conc of product/s raised to stoichiometric coefficient)/(Conc of reactant/s raised to stoichiometric coefficient)

What is the reaction quotient of Ag⁺(aq) + 2NH₃(aq)to Ag(NH₃)₂⁺(aq)?

What is the reaction quotient for N₂(𝑔) + 3H₂(g) ⇄ 2NH₃(𝑔)?

*if we were using partial pressure (atm): Q_c = (P_NH3)²/(P_N2)(P_H2)³ where P = pressure

If K_c > 1 then _____ are favored

If K_c < 1 then _____ are favored

• Products are favored; there are more products present 

• Reactants are favored; there are more reactants present

When is it better to use Q_c vs K_c?

• Only use K_c when the system is at equilibrium

• Q_c is used when the system is not known to be at equilibrium

What does it mean for a chemical reaction to be heterogeneous?

• When the species in a reversible chemical reaction are not all in the same phase

  • ie some are gas, some are solid, etc.

If you are asked to find the K_c of a heterogeneous reaction, what are the rules you need to follow?

• Only gas and aqueous appear in equilibrium

• Pure solids and pure liquids do NOT appear in  equilibrium

(True/False): Reversing a chemical equation inverts the equilibrium constant.

True, og K_c becomes 1/K_c

What is the new equilibrium constant for:
2C(g) to A(g) + B(g) if the original is A(g) + B(g) to 2C(g) and K_c = 4.39 × 10⁻³

1. 4.39 × 10⁻³

2. 1 / (4.39 × 10⁻³)

3. √(4.39 × 10⁻³)

2. 1 / (4.39 × 10⁻³)

What happens to Kc when a reaction is multiplied by a factor n?

Kc is raised to the power of n, (Kcⁿ)

For the reaction:
2A(g) + 2B(g) ⇌ 4C(g)
What is the relationship to the original Kc if the original reaction is A(g) + B(g) to 2C(g).

K_c²

True or False: Dividing a reaction by 2 results in taking the square root of the original Kc.

True

What is the new Kc when two reactions are added together?

Multiply the individual Kc values: Kc₁ × Kc₂

Given:
A + B ⇌ C (Kc₁ = 4.39 × 10⁻³)
C ⇌ D + E (Kc₂ = 1.15 × 10⁴)
What is Kc for the overall reaction: A + B ⇌ D + E?

50.5

Which manipulation results in Kc = √(original Kc)?

Dividing the reaction by 2

What is the relationship between K_c and K_p?

• K_p = K_c(RT)^Δn

  • where R=0.08206 L(atm)/K(mol) and Δn is the moles of products - moles of reactants

What is K_p?

The equilibrium constant that expresses the ratio of the partial pressures of products to reactants in a gaseous chemical equaiton. 

How are K and ΔG° related?

• They relay the same information:

  • K>1 then -ΔG° (spontaneous reaction)

  • K<1 then +ΔG° (nonspontaneous reaction)

What does it mean if Q_c < K_c?

• The ratio of initial concentrations of products to reactants is too small.

• To reach equilibrium, reactants must be converted to
  products.

• The system proceeds in the forward direction

What does it mean if Q_c = K_c?

• The initial concentrations are equilibrium concentrations.
  The system is at equilibrium

What does it mean if Q_c > K_c?

• The ratio of initial concentrations of products to reactants is too large.

• To reach equilibrium, products must be converted to
  reactants.

• The system proceeds in the reverse direction.

What is the relationship between ΔG and ΔG°?

• ΔG = ΔG° + RT In Q

  • R is the gas constant(8.314 J/K mol).

  • T is the Kelvin temperature.

  • Q is the reaction quotient.
    

Which determines spontaneity, ΔG or ΔG°?

ΔG 

What is the relationship between ΔG° and K?

ΔG° = −RT In K

• *at equilibrium, ΔG=0 and Q=K

What does a graph look like when the G° of the reactants is less than the G° of the products? Draw a graph and include the sign of the ΔG°, when Q<K, when Q=K, when Q>K, when ΔG <0, when ΔG=0, and when ΔG>0.

What does a graph look like when the G° of the reactants is more than the G° of the products? Draw a graph and include the sign of the ΔG°, when Q<K, when Q=K, when Q>K, when ΔG <0, when ΔG=0, and when ΔG>0.

If products are favored in a reaction at equilibrium, what does that mean for the K, lnK, and ΔG° values?

• K is >1

• lnK is postive

• ΔG° is negative

If reactants are favored in a reaction at equilibrium, what does that mean for the K, lnK, and ΔG° values?

• K < 1

• lnK is negative

• ΔG° is positive

If neither products nor reactants are favored in a reaction at equilibrium, what does that mean for the K, lnK, and ΔG° values?

• K = 1

• lnK is 0 

• ΔG° is 0

What is an ICE Table?

• A table used to help determine initial conc. change in conc. and equilibrium conc. of all parts of a reaction. 

*Practice how to use this, there will be an extra credit ? on this

What does the Le Châtelier’s principle state?

• that when a stress is applied to a system at equilibrium, the system, will respond by shifting in the direction that minimizes the effect of the stress.

• ie substances are added or removed from a system to reach equilibrium

What causes the equilibrium to shift to the left (towards the reactants)?

• The removal of reactants or the addition of products.

What causes a reaction to shift to the right (towards the products)?

• reactants are added or products are removed

What happens to the equilibrium position when the volume of a gaseous system is decreased? Increased?

• when V is decreased, the equilibrium shifts toward the side with fewer moles of gas to reduce pressure

• When V is increased, the equilibrium shifts toward the side with more moles of has to increase pressure

What factor can change the value of the equilibrium constant in a chemical reaction?

temperature changes can alter the equilibrium constant.

In an endothermic reaction, is heat a reactant or a product?

Heat is a reactant.

What happens to the equilibrium position and K_c when heat is added to an endothermic reaction?

The equilibrium shifts toward the products, and K_c increases.

What happens to the equilibrium position and K_c when heat is removed from an endothermic reaction?

The equilibrium shifts toward the reactants, and K_c decreases.

In an exothermic reaction, is heat a reactant or a product?

Heat is a product.

What happens to the equilibrium position and K_c when heat is added to an exothermic reaction?

The equilibrium shifts toward the reactants, and K_c decreases.

What happens to the equilibrium position and K_c when heat is removed from an exothermic reaction?

The equilibrium shifts toward the products, and K_c increases.