Chem Unit 12 Test

AP Chemistry
October 10 2024

Chapter 12 Review

• Kinetics- study of the speed of a reaction

Factors that effect Rate:

1. Nature of Reactats - Larger molecules react slower and smaller molecules react faster

2. Temperature - high temperature - high kinetic energy - more collisions to speed up reaction

3. Concentration of Reactants - the greater the chance of collisions. for gases the higher the pressure - the faster the reaction rate

4. Physical State of Reactants - same states of matter makes collisions easier

5. Catalysts - provide an alternative pathway for collisions by lowering the activation energy that speeds up the reaction. Speed up the reaction w/o being consumed. Start as a reactant in the first step and then end as a product in the final step Homogenous Catalyst - Exist in the same state of matter as the reactants Heterogenous Catalyst - Exists in a different state of matter as the reactants. Most commonly it is a solid among gaseous reactants

Differential Rate Law = Rate Law

• Measures the rate of reaction vs. concentration

• Determined experimentally

• Follows the format: aA + bB = cC + dD; Rate = k[A]^m[B]ⁿ; Where K = rate constant; m and N = are experimentally determined order of reactants and only time where coefficients are used as exponents in kinetics is when analyzing a reaction mechanism/ step-wise function

Solving the Rate Law

• Cocmpare two experiments- where one reactant is changing and the others are remaining constant

• Place the experiment with the larger rate/ concentrations on top so that you get a whole number

• Solve for K by plugging in an experiments data set

• Sum of the exponents in the rate is equal to the overall order at the reaction ex: Rate = k overall order = 2+1=3

Intergrated Rate Law

• Compares the change in concentration to the change in time

• Linear line = order of the reactnt

Zero order

• Compares the concentration directly to time

• [A] = -kT +

• Slope of the graph = -k so k= -slope (There is a negative in the integrated rate law)

• Zero order reactants are not included in the overall rate law Changing the concentration does not affect the rate law in zero order

First order

• Comapres the log of concentration directly to time

• Slope of graph = -k so k=-slope (there is a negative in the integrated rate law)

• Rate law of a firs torder reactant

Second order

• Compares directly on time

• = kT +

• Slope = K (there is no negative in the integrated rate law)

• Rate law for 2nd order reactant

Half lifes

• Zero order the half life gets more rapid reach time ex: 1st 10min, 2nd 5 min, 3rd 2.5 min

• First order the half life is consistent every time ex: 1st 10min, 2nd 10min, 3rd 10min

• Second order the half life gets longer each time ex: 1st 10min, 2nd 20 min, 3rd 40min

Reaction Mechanisms

• A combination of elementary steps/ reactions that sum to become the overall reaction

• Intermediate are the species produced in one step and consumed in a subsequent step

• Slow step is the rate- determining step and it should match the overall rate law that was determined experimentally; if it does not match the rate law then the mechanism is not valid

• Rate laws of the elementary steps can be determined through reaction stoichiometry (Balanced equation) Coefficients become the exponents

• If an intermediate is in the rate determining step, you must substitute its concentration with the reactants that formed the intermediate this only works if the reaction that formed the intermediate is an equilibrium reaction

Molecularity of a Reaction

• unimolecular : when only one chemical species is reacting (decomposition or rearrangement) Typically an elementary not an overall

• bimolecular : when two chemical species are reacting can be either an elementary step or overall

• Termolecular when three chemical species is reacting very rare and typically overall reaction, not elementary

Tips for Kinetics

• K, rate constant, has units - writing out all of the units of everything is the rate law to make sure you get them right

• Overall order helps determine units: 0: Rate= K; 1: Rate = k [A]; 2: Rate = k; or 3: Rate = k ; 4: Rate = K;

• Rate law is determined experimentally

• First order is base not base10 (log)