Equilibrium

Question 1

What is dynamic equilibrium?

Answer 1

• When the decomposition rate equals the synthesis reaction rate.
  
  • Aka when the rate of the forward reaction is equal to the rate of the reverse reaction.
  • The concentrations of reactants and products remain constant, but they are not equal to each other.
• If Q=K, then the system is at equilibrium
• How quickly a reaction gets to equilibrium is dependent on kinetics

Question 2

What is K_eq ?

What does it mean if K>1 or K<1.

Answer 2

• K_eq = generic equilibrium constant
• The equilibrium constant depends on the ratio at equilibrium of products and reactants
• If the rxn is product favored, the product concentration is higher, and K>1
• If the rxn is reactant favored, the reactant concentration is higher, and K<1
• If K=1, the concentrations are about equal.
• Keq = k_forward / k_reverse
• Keq = Concentrations of products at eq raised to their stoichiometric coefficients / Concentrations of reactants at eq raised to their stoichiometric coefficients
• The equilibrium constant determines the direction of the rxn (not the speed)
• Unitless (but in terms of molarity for Kc, atm for Kp)

Question 3

What do you include in equilibrium constant expressions?

Answer 3

• Include aqueous phase reactants (K_c in terms of molarity)
• Include gas phase reactants (K_p in terms of partial pressure)
• NOT solids
• NOT pure liquids (ie solvents, like water)

Question 4

Kp vs Kc

Answer 4

• K_c is in terms of molarity, and K_p is in terms of partial pressures
• They are different only if there are changes in the number of moles of gas in the reaction
• If they’re different, then they can be related through this equation:
  
  • K_p=K_c(RT)^Δ​n
    
    • Δn can be negative

Question 5

Relate Q and K

Answer 5

• Q (the reaction quotient) uses initial concentrations, K uses equilibrium concentrations
• If Q = K, then the system is at equilibrium
• If Q < K, there are more reactants than products compared to equilibrium
  • Reaction is headed to the right
• If Q > K, there are more products than reactants compared to equilibrium
  
  • Reaction is headed to the left
• Hint: K <– Q. K–> Q. Put in alphabetical order and make an arrow out of the sign to show which way the rxn is headed.

Question 6

What’s an ICE table?

Answer 6

• Initial Change Equilibrium.
  
  • In terms of moles when the volume might change. Easier to use molarity if volume doesn’t change.
• Use this to determine Keq by looking at changes in concentration at the beginning, middle and end of a rxn, solving for x, and plugging the molarity into the equation for Keq
• Do not include pure liquids or solids

3 categories of problems:

1. Start with all reactants. In ICE chart, the products get a 0. Change: + products, - reactants.
2. Start with all products. In ICE chart, the reactants get a 0. Change: + reactants, - products.
3. Some of each. Solve for Q and compare to K to decide which way the equilibrium is going to go
   
   • If K<q>Q, move right.</q>

Question 7

When When K * 100 < [A]_0….

Answer 7

• Then the equilibrium constant is very small. Very little of the product reacts, assume that it didn’t actually change
• Don’t use the quadratic formula when solving for K

Question 8

Relate K’s of related reactions

Answer 8

• If the coefficients of a reaction are multiplied by “n”:
  
  • K_new=(K_old)ⁿ
• If the reaction is reversed:
  
  • K_new=(K_old)^-1
• When equations are added to make overall rxn:
  
  • K_new=K₁*K₂*K₃….

Question 9

How can equilibrium be disturbed?

(3 ways)

Answer 9

1. Temperature
   
   • Composition AND K_eq will change
   • K ↑ with T
2. Addition or removal of reactant or product
   
   • _{<span>Composition will change, but K</span>eq} will stay the same
3. Volume
   
   • _{<span>Composition will change for gaseous reactions, but K</span>eq} will stay the same
   • (↑ V favors side with more moles of gas, ↓V favors side with fewer moles gas)

Question 10

What is Le Chatelier’s principle?

Answer 10

• The system will shift to reestablish equilibrium, get back from Q to K

Question 11

What is the effect of volume on gas phase equilbria?

Answer 11

• ↑ volume of a container favors the side with the greatest number of gas moles.
• ↓ volume favors the side with the least number of moles.
• When the number of gas moles on either side is the same, there is no effect.

Question 12

What is the effect of temperature on equilibria?

Answer 12

• The only time composition AND Keq will change

Composition changes:

• Hint: Think of heat as a product or reactant
• Endothermic: Increasing the temperature favors the products, equilibrium shifts to the right.
• Exothermic: Increasing the temperature favors the reactants, equilibrium shifts to the left.
• Lowering temperature results in the reverse effects.

Rate constant changes:

• According to Arrhenius equation. Rate constant increases with temperature

Question 13

If you add a catalyst or an inert gas,

what is the effect on the equilibrium?

Answer 13

None