C.W. BAKER HIGH SCHOOL

STUDY GUIDE FOR KINETICS

a. Kinetics is the branch of chemistry which is involved in studying the factors which influence the speed or rate of a reaction.

b. The basic idea behind kinetics is that chemical reactions take place when the reactant molecules collide with each other.

c. However, not all collisions between reactant molecules result in a reaction. Therefore, chemists have developed the idea of an effective collision. Effective collisions are those collisions between molecules which may lead to the formation of product. In order for a collision to be an effective collision it must meet two criteria:

1. Sufficient energy - effective collisions must have sufficient energy to provide for the breaking of bonds in the reactant molecules.

2. Proper orientation - effective collisions must allow the reactant molecules to come together in such a way as to allow for formation of the new bonds that will hold the atoms together in the products.

d. When reactant molecules undergo effective collisions, they form a high energy reaction intermediate called activated complex (A*). In this intermediate the old bonds are breaking and new bonds are forming. The activated complex can then break down to more stable products (or can also break back down into the original reactants.).

e. Factors which affect the rate of reaction:

1. Concentration (Pressure for gases) - an increase in concentration of the reactant molecules increases the probability of collisions between the reactant molecules, thus increasing the formation of activated complex, and causing the rate of the reaction to increase. This is analogous to comparing the rates of auto accidents in midtown Manhattan, NY with the rate of accidents in midtown Fargo, ND.

2. Temperature - an increase in temperature (the measure of the average KE of a system) means that the average speed of the reactant molecules has increased. This increases the probability of effective collisions occurring in two ways:

1. Since the molecules are moving faster, there is a greater probability of collisions between the molecules.

2. At higher temperatures there is a greater percentage of "fast" molecules which have sufficient energy to undergo effective collisions. Thus, a higher percentage of the total collisions will be effective collisions.

3. Nature of the Reactants - different reactants will react at different rates. This is related to the amount of activation energy required to form the activated complex.

4. Degree of Subdivision (Surface Area) - the greater the degree of subdivision, the faster the rate of the reaction. Example: a solid log will burn much more slowly than a log that has been split into a number of pieces. This is because the split log has a much greater surface area where oxygen molecules in the air can collide with the wood.

5. catalyst - a catalyst is a substance that speeds up a reaction without being used up in the reaction. A catalyst works by combining with the reactants to form an entirely new activated complex with a lower energy than the original activated complex. The catalytic converter in automobile exhaust system (see Howstuffworks "How Catalytic Converters Work" )speeds up the reaction of CO and unburned hydrocarbons in the auto's exhaust with oxygen to produce less toxic CO₂. Although drivers must continually replace the gasoline (the reactant), it generally is not necessary to replace catalytic converters because they are not used up in the reaction.

EQUILIBRIUM

a. System in which the rate of some forward change is equal to the rate of the reverse reaction.

b. Equilibrium systems only take place in closed systems.

c. Since the rates of the forward and reverse reactions are equal it appears that no change is taking place in the reaction and the amounts of reactants and products remain constant.

d. A treadmill is a good example of a equilibrium system: the runner runs forward at the same speed as the tread moves backwards and the runner stays in the same position.

e. Le Chatlier’s Principle – if a stress is applied to a system at equilibrium the system will respond in such a way as to alleviate the stress.

1. Equilibrium systems act to counter-act any changes which are made to them. Three common changes and their effect on equilibrium systems are:

a. Concentration – adding additional reactants causes the reaction to “shift” (speed up) in the forward direction. Such a shift is said to “favor” the products because it causes more products to be produced. Likewise, addition of additional products “shifts” (speeds up) the reaction in the reverse direction. Such a shift is also said to “favor” the reactants since it results in the increase in the reactants. The Common Ion Effect is an example of the effect that concentration has on equilibrium systems.

b. Temperature – an increase in temperature adds energy to an equilibrium system. The equilibrium, in an attempt to get rid of the extra energy, shifts (speeds up) in the endothermic direction.

c. Pressure – an increase in pressure favors the side of the reaction with the fewest number of gas molecules. Since equilibriums tend to “undo” any changes made to them, an equilibrium system will respond to an increase in pressure by “shifting” (speeding up) in the direction that produces fewer gas molecules. This has the effect of reducing the number of gas molecules, and thus the pressure.

d. Catalyst – a catalyst works by forming a new, lower energy activated complex. This has the effect of speeding up both the forward and reverse reactions equally. Since both forward and reverse reactions are speeded up to the same extent, the addition of a catalyst does not favor either the reactants or products. That is it will not affect the relative amounts of reactants and products present.

ENTROPY

1. Entropy measure the disorder of a system, the greater the disorder the higher of the entropy. (A messy desk would have a higher entropy than a neat and orderly desk).

2. In nature, there is a tendancy for things to go to a higher state of disorder. This tendency can be used to explain why ice melts, and water vaporizes.

1.