Reactions and Equilibrium In a forward reaction, the reactants collide to make products as the reactions continues towards the right side In a reverse reaction, the products collide to make reactants as the reactions continues towards the left side

As the reaction goes to the right, the reactant concentration decreases causing the forward rate to decrease as there are fewer collisions between reactant particles. As equilibrium is approached, the reverse rate increases because the concentration of products increases as there are more collisions between product particles.

At equilibrium, the forward rate is equal to the reverse rate At equilibrium, the forward rate is equal to the reverse rate. The concentration of reactants and products are both constant (BUT NORMALLY NOT EQUAL TO EACH OTHER…) When the forward rate is equal to the reverse rate, the reaction is said to be in dynamic equilibrium because particles are still colliding and reacting.

A System at Equilibrium At equilibrium, concentrations (M, mol, P, etc.) of reactant and product both remain constant. N2O4(g) 2 NO2(g)

(forward & reverse rxns occur simultaneously) Dynamic Equilibrium: (forward & reverse rxns occur simultaneously) N2O4(g) 2 NO2(g) double arrow

Dynamic Equilibrium: N2O4(g) 2 NO2(g) Initially, forward & reverse rxns occur at different rates. (based on collisions, one slows down; other speeds up) At equilibrium: Rateforward = Ratereverse N2O4(g) 2 NO2(g) Ratef Rater

At Equilibrium… N2O4(g) 2 NO2(g) …concentrations are ________ …forward and reverse rates are ________ constant equal N2O4(g) 2 NO2(g)

H2O(l) ⇄ H2O(g) Phase Change Equilibrium Example: evaporation in a closed container H2O(l) ⇄ H2O(g) Phase Change Equilibrium

Rate of Crystallization Saturated Solution Equilibrium Rate of Solvation = Rate of Crystallization Example: At the end of making rock candy Animation: http://www.kentchemistry.com/links/Kinetics/Equilibrium.htm total moles of dissolved NaCl remains constant total moles of undissolved NaCl(solid) remains constant

Quick Quiz! 1) When a reaction has reached equilibrium, reactants and products … A) have stopped reacting. B) continue forming. C) have equal concentrations. D) decrease in concentration. 10

A B ⇄ Quick Quiz. 100 2) A reaction starts with 100 molecules of reactant and no product. As it approaches equilibrium, the forward rate will ________ and the reverse rate will ________. A) remain constant , increase B) decrease , remain constant C) increase , decrease D) decrease , increase 11

A B ⇄ Quick Quiz. 100 3) A reaction starts with 100 molecules of product and no reactant. At equilibrium, the amount of reactant A will be… A) greater than B B) less than B C) equal to B D) constant 12