lecture 4 - chemical kinetics

Question 1

arrhenius equation

Answer 1

k = A exp(- Ea / RT )

Question 2

heating a reaction causes it to

Answer 2

go faster
increases its rate

temp has an exponential effect on k

Question 3

what k in arrhenius equation

Answer 3

rate constant - varies with temp

Question 4

what is A in arrhenius equation

Answer 4

pre exponential factor
same units as k

Question 5

what is Ea

Answer 5

activation energy
J mol-1

Question 6

what is R

Answer 6

gas constant

Question 7

what is T

Answer 7

temperature
in K elvin

Question 8

what is the arrhenius equation based on

Answer 8

empirical observations

Question 9

another equation for arrhenius

Answer 9

k= A exp (-Ea/kB T)

Ea now has J units - Ea of an individual molecules, not a mole

Question 10

activation energy

Answer 10

energy barrier that must be overcome when molecules collide in order for a reaction to occur

Question 11

Ea units

Answer 11

kJ/mol

Question 12

kinetics depends on

Answer 12

the Ea
not how exo/endothermic smt is

Question 13

area under the curve

Answer 13

amount of particles

Question 14

area under the curve after the Ea point

Answer 14

probability of a particle having the Ea.

shows the amount of particles that have the Ea or more

Question 15

if temp stays the same but the Ea increases, what happens to the amount of molecules that have the Ea

Answer 15

it decreases

Question 16

if the temp changes what happens to the graph

Answer 16

it changes
the distribution of energy changes

Question 17

low temp graph

Answer 17

tall and skinny

Question 18

high temp graph

Answer 18

wide and short

Question 19

when temp is increased, what happens to the amount of molecules that have the Ea

Answer 19

it increases

Question 20

exp (-Ea/RT) is proportional to what part of the curve

Answer 20

area under the curve past the Ea bit

this is where the temp dependence of the rate constant comes from

Question 21

what 2 things does the pre exponential factor depend on. A

Answer 21

frequency of collisions (determined using collision frequency)

probability of correct geometry of collisions for the reaction to occur

eg: frequency and orientation

Question 22

probability a collision will occur with the correct geometry is also known at the

Answer 22

steric factor

Question 23

what value do steric factors usually have

Answer 23

a very small number or 1

Question 24

what does steric factors having a small value mean/ represent

Answer 24

a very specific geometry is required

Question 25

what does a steric factor having the value of 1 mean

Answer 25

that any collision will lead to a reaction if it has the Ea required.
orientation/geometry doesnt matter

Question 26

what do we do to the steric factor and collision frequency to get the pre exponential factor, A

Answer 26

we multiply them together

Question 27

some reactions have multiple steps and the Ea and A correspond to 1 individual step however for k

Answer 27

may be a combo of individual rate constants ( combo of Ks)
or correspond to the K of the slowest step

Question 28

ln(k) = ln(A) - Ea/RT

Answer 28

plotting ln(k) against 1/T
gives straight line
gradient = -Ea/R
Ea = gradient x -R
intercept = ln(A)
then calculate A and Ea

Question 29

in some reactions, Ea changes with

Answer 29

temperature

due to changes in reactant structure + quantum mechanical tunnelling

Question 30

temp dependent Ea equation

Answer 30

Ea = -R d(ln((k)) / d(1/T)

Question 31

what does the arrhenius equation define

Answer 31

the rate constant ks temperature dependence