ch 6 - Equilibrium

Question 1

irreversible reaction

Answer 1

the reaction proceeds in one direction only; reaction goes to completion; and the max amount of product formed is determined by the amount of limiting reagent initially present

Question 2

Reversible reaction

Answer 2

reaction can proceed in one of two ways: forward (toward the products or “to the right”) and reverse (toward the reactants or toward the left); do not usually proceed to completion because the products react to form the reactants

Question 3

dynamic equilibrium

Answer 3

forward and reverse reactions are still occurring but reaction has settled into a state of in which the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants remain constant; no change in concentration of products or reactants

Question 4

static equilibrium

Answer 4

reaction has stopped

Question 5

entropy

Answer 5

the measure of the distribution of energy throughout a system or between a system and its environment; for a reversible reaction at a temp, the reaction will reach equilibrium when the system’s entropy - or energy distribution - is at a max and the Gibbs free energy of the system is at a min

Question 6

law of mass action

Answer 6

for generic reversible reaction aA + bB ->

Question 7

equilibrium constant

Answer 7

K sub c; subscript c indicates that it is in terms of concentration; when dealing with gases the equilibrium constant is referred to as K sub p and subscript p indicates that it is in terms of pressure; for dilute solutions, K sub c and K sub eq are used interchangeably and new equation in reference to generic reversible 2A ->

Question 8

forward and reverse reaction rates (k sub f and k sub r) in reference to K sub c

Answer 8

K sub c = K sub eq = k sub f/k sub r; if it proceeds in more than one step (say 3) it will look like this: K sub c = (k sub 1 k sub 2 k sub 3)/ (k sub -1 k sub -2 k sub -3) = ([C]^c[D]^d)/([A]a[B]^b)

Question 9

reaction quotient, Q

Answer 9

serves as a timer to indicate how far the reaction has proceeded toward equilibrium: Q sub c = ([C]^c[D]^d)/[A]^a[B]^b)

Question 10

If Q < K sub eq

Answer 10

delta G < 0 (reaction proceeds in forward direction); the forward reaction has not yet reached equilibrium; there is greater concentration of reactants (and smaller concentration of products) than at equilibrium; the forward rate of reaction is increased to restore equilibrium

Question 11

Q = K sub eq

Answer 11

delta G = 0; the reaction is in dynamic equilibrium; reactants and products are present in equilibrium proportions; forward and reverse reactions are equal

Question 12

Q> K sub eq

Answer 12

delta G > 0 (reaction proceeds in reverse direction); forward reaction has exceeded equilibrium; there is a greater concentration of products (and smaller concentration of reactants) than at equilibrium; the reverse rate of reaction is increased to restore equilibrium

Question 13

things to remember about the law of mass action

Answer 13

equilibrium constant expression refers to activities of compounds and not concentrations so it does not apply to pure solids or pure liquids (for MCAT there is negligible difference between concentration and activity); 2. K eq is temp-dependent; 3. the larger the value of K sub eq, the farther to the right the equilibrium position; 4. if equilibrium constant for reaction written in one direction is K sub eq, the reverse is 1/K sub eq

Question 14

Le Chatelier’s principle

Answer 14

states that if a stress is applied to a system, the system shifts to relieve the applied stress. If reactants are added (or products removed), Q sub c < K eq, and the reaction will spontaneously react in the forward direction, increasing the value of Q sub c

Question 15

changes in pressure (and volume) to reversible systems

Answer 15

only systems that contain at least one gas will be affected. If pressure is increased system will move toward the direction of less moles (whichever side of the equation has less moles of something being produced) and if the pressure is decreased (volume increased) will move toward the side with more moles

Question 16

changes in temp to reversible systems

Answer 16

changing temp will cause system to react; though it is not a change in reaction Q sub c or Q sub p but a change in K sub eq; does not cause change to actual system immediately but K eq is changed and Q no longer is equal to it. must move based on enthalpy: if rxn is endothermic (delta H > 0) heat is a reactant and exothermic vice versa

Question 17

kinetic product

Answer 17

reactants at lower temps with smaller heat transfer produce this; double bond is located between C-1 and C-6; less energy needed to reach transition state; base can more easily reach C-6 to remove a proton, and resulting enolate can form; less substituted double bond and less stable

Question 18

thermodynamic product

Answer 18

reactants form this at higher temps with larger heat transfer: the double bond is located between C-1 and the methyl group. Requires more energy to form transition state of this reaction because the base must overcome the steric hindrance created by the methyl group; but this product is more stable and less likely to react further

Question 19

difference between K sub c and K sub p

Answer 19

K sub c is concentration constant; K sub p is partial pressure

Question 20

note on liquid and solid in equilibrium expression

Answer 20

they do not appear. Only gases appear