ch 6 - Equilibrium Flashcards
irreversible reaction
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
Reversible reaction
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
dynamic equilibrium
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
static equilibrium
reaction has stopped
entropy
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
law of mass action
for generic reversible reaction aA + bB ->
equilibrium constant
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 ->
forward and reverse reaction rates (k sub f and k sub r) in reference to K sub c
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)
reaction quotient, Q
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)
If Q < K sub eq
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
Q = K sub eq
delta G = 0; the reaction is in dynamic equilibrium; reactants and products are present in equilibrium proportions; forward and reverse reactions are equal
Q> K sub eq
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
things to remember about the law of mass action
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
Le Chatelier’s principle
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
changes in pressure (and volume) to reversible systems
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
