Unit 14 Rates of reactions & Equilibrium

The Speed of a Chemical Reaction Depends on: 1. The nature of the reactants. Reactions in which bonds need to be broken are slow. 4 Fe + 3 O 2  2 Fe 2 O 3 (slow) Reactions in which no bonds need to be broken are fast. Pb I - 1  PbI 2 (s) (fast) 2. Increasing the concentration of a reactant increases the speed of the chemical reaction because the chemicals collide more often. 3. Increasing the temperature makes it more probable that the reactant chemicals have enough energy to break bonds and form products. The average speed of a molecule increases as the temperature increases. 4. Addition of a catalyst speeds up a reaction.

When molecules do not have the same temperature, the higher the temperature (K E ), the faster the molecules.

All molecules do not travel at the same speed! It is dependent on the mass of the molecule. Kinetic energy depends only on temperature. If gases have an equal number of moles and their temperature is the same, they have equal amount of Kinetic energy. K E = ½ m v 2

Catalyst 1. Chemical added to a reaction in small amounts 2. Provides a site for the reaction to occur at 3. Lowers the activation energy 4. Does NOT change the heat of the reaction 5. Does NOT get used up in the reaction 6. Can be recovered after the reaction

Chemical Equilibrium occurs when an enclosed chemical reaction has NO observable change. The chemical reaction is still happening in both directions at the same rate, when equilibrium occurs. A saturated solution of PbI 2 PbI 2 PbI 2 (s)   Pb + 2 (aq) + 2 I - 1 (aq) A sealed tube with the gaseous mixture NO 2 2 NO 2 (g)  N 2 O 4 (g) kcal (red-brown) (clear)

Le Chatelier’s Principles: if you take a reaction that has reached equilibrium, and exert a force on it, the reaction will shift to counter act the force. A + B  C + D 1. The addition of any solid chemical does not change the equilibrium. 2. If A or B are ions or gases, an increase in concentration of either A or B causes a change in the equilibrium to produce more of C and D. A decrease in concentration of either A or B causes a change in the equilibrium to produce more A and B. An increase in concentration of either C or D will produce more of A and B. A decrease in concentration of either C or D will produce more of C or D.

Heat + A + B  C + D 3.An increase in heat will produce more of C and D. A decrease In heat will produce more of A and B. A + B  C + D + Heat An increase in heat will produce more of A and B. A decrease in heat will produce more of C and D.

a A (g) + b B (g)  c C (g) + d D (g) 4. If you increase the pressure of an equilibrium reaction containing gases, the reaction will produce the chemicals with the lowest combined coefficients of gases; a + b or c + d If you decrease the pressure of an equilibrium reaction containing gases, the reaction will produce the chemicals with the highest combined coefficients of gases; a + b or c + d 5. A catalyst does not change an equilibrium reaction

Equilibrium Constant: There is a number called the equilibrium constant which indicates how far a chemical reaction has gone to completion. K A large equilibrium constant means there are more product chemicals than reactants at equilibrium. K > 1000 A small equilibrium constant means there are more reactant chemicals than products at equilibrium. K <.001 If the equilibrium constant is approximately equal to one, there are equal amounts of reactants and products at equilibrium. *The equilibrium constant does not change except if there is a change in the temperature of the reaction. * K has no unit! It is just a number.

a A + b B  c C + d D K = [ C ] c [ D ] d [ A ] a [ B ] b Include the concentrations of ions and the atmospheric pressure of gases. If solids or liquids are in the reaction, use 1 as a place holder in the reaction.

2 H 2 (g) + O 2 (g)  2 H 2 O (g) K c = 2.0 x 10 9 Cu (s) + 2 Ag + 1 (aq)  2 Ag (s) + Cu + 2 (aq) K c = 2.0 x AgI (s)  Ag + 1 (aq) + I - 1 (aq) K c = 8.5 x

N 2 O 4 (g)  2 NO 2 (g) K c = 0.83 H 2 O (L)  H + 1 (aq) + OH - 1 (aq) K w = 1.0 x H + 1 (aq) + 2 CrO (aq)  Cr 2 O (aq) + OH - 1 (aq) K c = 3.0 x 10 14

Find the equilibrium constant for the following: N 2 (g) + 3 H 2 (g)  2 NH 3 (g) + 22 kcal [N 2 ] = 0.50 M ; [H 2 ] = 0.40 M ; [NH 3 ] = 0.42 M

The equilibrium constant K eq for AgCl is 1.4 x at 100 o C. Calculate [Ag + 1 ] AgCl (s)  Ag + 1 (aq) + Cl - 1 (aq)

Review Kj9. change heat, Kj pressure or 3. Peak of curve concentration Kjb. shift left, [A]up, Kj [B]up, [C]down, 6. Teacher will draw less D 7. Speeds up the reactionc. Teacher will write 8. Curve will shift to the left d. No effect, solid e. 1.62