Ch 6 - Equilibrium

Question 1

What are reversible reactions?

Answer 1

they eventually reach a state in which energy is minimized and entropy is maximized

Question 2

What does the chemical equilibria being dynamic mean?

Answer 2

the reactions are still occurring, just at a constant rate

Question 3

Why do the concentrations of reactants and products remain constant?

Answer 3

because they rate of the forward reaction equals the rate of the reverse reaction

Question 4

What does the law of mass action give?

Answer 4

• the expression for the equilibrium constant, Keq
  for aA + bB cC + dD
  Keq = ([C]^c[D]^d)/([A]^a[B]^b)

Question 5

What is the reaction quotient and how is it calculated?

Answer 5

• calculated value that relates the reactants and product concentrations at any given time during a reaction
• has same form as Keq, but can be calculated at any concentrations of reactants and products

Question 6

What is Keq?

Answer 6

• the ratio of products to reactants at equilibrium, with each species raised to its stoichiometric coefficient
• constant at constant temperature

Question 7

What phases appear in the law of mass action?

Answer 7

• pure solids and liquids do not appear

- only gases and aqueous species

Question 8

What does comparing Q and Keq provide?

Answer 8

information about where the reaction is with respect to equilibrium state

• if Q < Keq, G < 0, rxn proceeds in forward direction (G<0)
• if Q = Keq, G = 0, reaction in dynamic equilibrium
• if Q > Keq, G > 0, rxn proceeds in reverse direction (G>0)

Question 9

What does the magnitude of Keq determine?

Answer 9

• the balance of a reaction and whether the amount that has reacted can be treated as negligible when compared to other concentrations
• if K > 1, products are present in greater concentration at equilibrium
• if K = 1, products and reactants are present at equilibrium at reasonably similar levels
• if K < 1, reactants are present in greater concentration at equilibrium
• if K «< 1, the amount of reactants that have been converted to products can be considered negligible in comparison to the initial concentration of reactants (if 2+ orders less than the initial concentration of reactant)

Question 10

What is Le Chatelier’s principle?

Answer 10

• states that when a chemical system experiences a stress, it will react so as to restore equilibrium

Question 11

What are the 3 main types of stress applied to a system?

Answer 11

• changes in concentration, pressure and volume, or temperature

Question 12

How does changing concentrations affect a system?

Answer 12

• increasing the concentration of reactants or decreasing concentration of products will shift the reaction to the right
• increasing the concentration of products or decreasing the concentration of reactants will shift the reaction to the left

Question 13

How does changing the pressure or volume affect a system?

Answer 13

• increasing pressure on a gaseous system (decreasing its volume) will shift the reaction toward the side with fewer moles of gas
• decreasing pressure on a gaseous system (increasing its volume) will shift the reaction toward the side with more moles of gas

Question 14

How does changing the temperature affect a system?

Answer 14

• increasing the temperature of an endothermic reaction or decreasing the temperature of an exothermic reaction will shift the reaction to the right
• decreasing the temperature of an endothermic reaction or increasing the temperature of an exothermic reaction will shift the reaction to the left

Question 15

What are kinetic products and how do they compare to thermodynamic products?

Answer 15

• higher in free energy than thermodynamic products and can form at lower temperatures (favor low temperatures with low heat transfer)
• sometimes referred to as “fast” products because they can form more quickly under such conditions

Question 16

What are thermodynamic products and how do they compare to kinetic products?

Answer 16

• lower in free energy than kinetic and are therefore more stable
• favor high temperature with high heat transfer
• despite proceeding more slowly, they are more spontaneous (more negative G)

Question 17

What is the difference between static and dynamic equilibirum?

Answer 17

in dynamic, the forward and reverse reactions are still occurring, unlike when they stop in static, by they are going at the same rate, so there is no net change in the concentrations of the products or reactants

Question 18

How are reaction rates, entropy, and Gibbs free energy affected at equilibrium?

Answer 18

• rate of forward reaction = rate of reverse reaction
• entropy is at a maximum
• gibbs free energy at a minimum

Question 19

In the reaction A (aq) + 2B (g) C (g) + heat, when will the reaction shift to the right?

Answer 19

• if A or B is added
• if C is removed
• if the pressure is increased or volume is reduced
• if the temperature is reduced

Question 20

In the reaction A (aq) + 2B (g) C (g) + heat, when will the reaction shift to the left?

Answer 20

• if A or B is removed
• if C is added
• if the pressure is reduced or volume is increased
• if the temperature is increased

Question 21

On a reaction coordinate diagram, how would the kinetic pathway appear as compared to the thermodynamic pathway?

Answer 21

• kinetic requires a smaller gain in free energy to reach the transition state
• kinetic also have a higher free energy of the procuts, with a smaller difference in free energy between the transition state and the products

Question 22

What is the difference between Kc and Kp?

Answer 22

• Kc the equilibrium constant for concentrations

- Kp, dealing with gases in terms of pressure

Question 23

What is the explanation for a reaction that stops just before all reactants are converted to products?

Answer 23

• the reaction is reversible and the forward rate is equal to the reverse rate
• likely the reaction has reached equilibrium very far to the right (with high product concentration and low reactant concentration)
• must be reversible since the reaction did not proceed all the way to the right
• any rxn in equilibrium has forward = reverse rate