chapter 18- rates of reaction

Continuous

One experiment with different readings being taken as the reaction proceeds

Discontinuous

Involves lots of separate experiments with different starting concentrations and one reading being recorded each experiment

Rate equation

Change in concentration/change in time

Units for rate

Molsdm^-3sec^-1

The rate equation

Rate= K(A)^x(B)^y

What does k stand for in the rate concentration

rate constant, increases with temp, therefore faster rate but lower activation energy=lower value of K- connects rate to concentration

0 order 

rate is not affected by (A)

1st order

Rate is proportional to (A)

2nd order

rate is proportional (A)²

Half-life

Time taken for the concentration of a reactant to half t_1/2

Describe a time and concentration graph for zero order

straight line, negative gradient

Gradient= K

Rate doesn’t change

Describe a time concentration graph for first order

downwards exponential decay curve- decreasing gradient

Half-life t_1/2 is constant

Describe time concentration graph for second-order

downwards exponential decay curve 

Half-life is not constant 

Two methods to determine K using half-life and first order reaction graphs

1. Draw a tangent to calculate rate, Rearrange rate equation to determine K

2. There is an exponential relationship for a constant half-life K= Ln2/t_1/2 (only can use for first order

Rate determining step

slowest step in the reaction sequence

In regards to the rate determining step, if a species is in the rate question, will it be in the RDS

Up to and including RDS

In regards to RDS, if a species has a partial order of 0, where will it be on the mechanism

after the RDS

In regards to RDS, if a species has an order of 1, where will it be in the mechanism

one molecule/particle will appear up to and including the RDS

In regards to RDS, if a species has a partial order of 2 where in the ,mechanism will it be

two molecules/particles of it appears in the mechanism and up to and including the RDS

Two types of reactions for hydrolysis of haloalkanes

1. Substitution nucleophilic 1

2. Substitution nucleophilic 2

SN1

Mechanism 2- two step route

Hydrolysis of tertiary haloalkanes Initial bond breaking, formation of carbocation intermediate, the nucleophilic attack

SN2

hydrolysis of secondary haloalkane

Mechanism 1- singe step route

Simultaneous nucleophilic attack and bond breaking 

SN of 2 particles in RDS

The Arrhenius equation

K= A e ^-Ea/RT

What does the A in then Arrhenius equation stand for?

Pre-exponential factor

How can the Arrhenius equation be expressed in a logarithmic form 

Ln K= -Ea/R 1/T + ln A

Gradient m= -Ea/R(gas constant)

Intercept on y axis= ln A