Collision Theory

For a rx to occur reacting molecules must collide so 1 - The old bonds “loosen up”, or start

to break apart 2 - The correct orientation to allow new

bonds to form e- clouds of atoms trying to bond together

must touch each other

Collision Theory

NO + NO3 2NO2

2 requirements for a chem reaction to occurfrom a collision between molecules

2 – proper alignment of molecules so new bonds can form

1 – enough energy to break the old bonds

Reaction rates How fast reactants products Rates are determined

experimentally Only by experiment

R = - [reactants] = + [products] t t

3 Types of Reaction Rates Average rate

Slope of line between 2 data points Instantaneous rate

Slope of tangent line at that moment Initial rate

Instantaneous rate at start of the reaction Fast rate

Rates slow down with time Decreased conc of reactants

Rate Law

aA dD r = k[A]m

nature reactants fast rx = large k slow rx = small k effects of temp

t k therefore direct relationship

Rate Law

aA + bB cC + dD r = k[A]m x [B]n nature reactants

fast rx = large k slow rx = small k effects of temp

t k therefore direct relationship

Reaction order 0, 1st, or 2nd

definition r = k[A]m x [B]n m is order for A

Order is usually whole # 0, 1, or 2

n is order for B m + n is overall order

Rx Mechanisms Terms:

Reaction mechanism series of steps (rxs) from original reactants to the

final product Elementary step

Each single step (reaction) in the pathway Rx intermediate

Subst. formed during the rx mechanism that is used up in subsequent steps and doesn’t show up as a reactant or product in the overall rx

Rate determining step Slowest step in the reaction mechanism

Rx mechanisms

Reactions probably occur in a series of steps Add individual steps together = overall

equation Rate determining step:

The slowest step in the reaction mechanism A B C D E fast slow fast fast

Factors that Influence Reaction Rates

Nature reactants Conc. Of reactants

Surface area Catalyst/inhibitor Temp

Nature of Reactants and Products

How extensive is the rearrangement of the e- clouds of reactants and products Precipitation reactions

No rearrangement of e- clouds Iron rusting (oxidizing)

A lot of rearranging of e- clouds

Energy Diagrams

Activation energy Energy required to start a chem rx Activated complex

High energy, short lived particle formed by the collision of reactant molecules

Can from products or reform reactants Depends on whether the collision causes a rx or

not

Effects of Catalyst

Reduce the activation energy

Reaction Mechanism

Each step in reaction pathway is called an elementary (intermediate) step

Elem step can contain reaction intermediates (or intermediate products) Products of elementary step that is used up in a

subsequent elementary step They don’t show up in the overall reaction

A ---> B ---> C ---> D ---> E ---> F

fast slowfast fast fast

Overall reaction