Chemistry Unit 5

Kinetics

The study of the rate or speed of chemical reactions

UNITS: measure of some variable/time

Reaction Rate

The change in concentration of a reactant or product with time

UNITS: M/s (Molarity/seconds)

Empirical - comes from experimenting instead of theory

Rate Reactants

DONT BE REDUNDANT WITH RATE OF DISAPPERANCE

-delta (M)/ delta (time)

Rates can be defined in terms of changes in ——-

pressure as well as changes in concentration

UNITS: M= mol/L

Not all substances disappear or appear at the same rate…it depends on ___

coefficients

HIGHER CONCENTRATION, INCREASED RATE OF REACTION, individual rate

Rate Law

Expresses the relationship of the rate of a reaction to the rate constant and the concentration to the reactants raised to some power

TWO THINGS THAT IMPACT: TEMP. AND CONCENTRATION

Orders can be..

-0

-Fractions

-Negative

Overall order

add order of reactants

To find the order of reactants from a Rate and Molarity

-Make sure one M is constant, observe the other’s change

-(concentration ratio)^x = rate ratio

Integrated Rate Law

Rate Laws can be used to determine the concentration of the reactants at any point in the equation

ln[A] vs. t

first order, straight line with slope - k, PARTIAL PRESSURE CAN BE USED TO REPLACE

Half life

The required time for the concentration of a reactant to decrease to half its initial concentration

first order reactions: t1/2 =

0.693/k, INDEPENDENT OF THE CONCENTRATION OF 3, CONVERT MIN TO SECONDS

With half life reactions, how do you calculate time required or k based on percentage

ln(% left) = -kt

ln(Ai/Af) cancels, then go from there, CONSTANT

2nd order reactions

will give straight line starting from origin, not downward sloping like first

t1/2 = 1/k(Ai)

second order half life

NOT CONSTANT< doubles per each

t1/2 = 1/k(Ai)

1/A vs. t FOR GRAPH

zero order

rate is constant, straight line downward sloping **A vs. t

UNITS FOR K

0th - m/s or m s^-1

1st - s^-1 or 1/s

2nd- 1/m*s or M^-1 s^-1

Half life of zero order

t1/2 = (A)0/2k

Collision Theory

explains why reactions happen at the rate they do and what factors affect them

Collision Theory summary

-high temp = increase in rate, as there is MORE KE TO OVERCOME ACTIVATION ENERGY BARRIER

-higher concentration = increased rate, as there is a higher probability of the particles colliding with the correct orientation

Activation Energy

The minimum amount of energy required to initiate a chemical reaction

Exothermic and Endothermic

respectively -H and +H

WHAT IS MORE LIKELY TO HAVE A HIGHER ACTIVATION ENERGY

endo

Activated Complex

A temporary species found in molecules as a result of collisions before they form a product, jumbled mess of atoms, LOWER PE= MORE STABLE

Why do large reactants tend to have slower reaction rates?

the probability of the collusion occuring at the proper reactive site is lower, MORE AREA = LESS ACCURACY IN HITTING CORRECT SPOT

Physical State

increased surface area =increased rate of reaction

Solids react slowest

Gases + Liquids fastest

Gases react slower than liquids due to space between particles

Catalyst

Lowers the activation energy, allowing particles with lower KE to react, increasing rate of the reaction

Reaction Mechanism

The sequence of elementary steps leading up to the product formation

Activation energy is always

a positive value

Intermediate

a species that is produced in one step then consumed in another

Uni, bi, ter (VERY RARE, sufficient energy and proper orientation)

just counting reactants

NOTE: Rate law of elementary step can be determined by coeff. as the order

ONLY TRUE FOR ELEMENTARY STEPS

review

the —— step is the rate determining step

slowest

Catalysts in terms of elementary steps

consumed in the first, reproduced in the next

MECHANISM MUST SATSIFY THESE 4 REQ.

• sum of elementary steps must give overall balanced chemical equation

• reasonable

• rate low should include all stes up to and including and must agree with experiment (AKA slowest step)

• rate law SHOULD NOT CONTAIN INTERMEDIATES BUT CAN HAVE CATALYSTS

H is unaffected by the

catalysts

3 types of catalysts

Homogeneous

Heterogeneous

Enzymes - biological

WHere is H graph?

under first bar,

EA is above but under peak

ACTIVATED COMPLEX IS AT PEAK,

LOWER PEAK INDICATES CATALYST

the right labelled side indicates reverse order reaction