topic 16: kinetics II

Question 1

what is the measurement of the change in volume of a gas

Answer 1

this works if there is a change in the number of moles of gas in the reaction. using a gas syringe is a common way of following this

Question 2

what is the measurement of change of mass

Answer 2

this works if there is a gas produced which is allowed to escape. works better with heavy gases such as CO2
- the mass loss provides measure for reactants
- gas must be sufficiently dense or change in mass is too small to measure

Question 3

how do you titrate samples of reaction mixture with acid, alkali, sodium thiosulfate etc

Answer 3

small samples are removed from the reaction mixture, quenched (which stops the reaction) and the titrated with a suitable reagent
HCOOCH3(aq) + NaOH(aq) —> HCOONa(aq) + CH3OH(aq)
- the NaOH could be titrated with an acid
BrO3–(aq) + 5Br–(aq) + 6H+(aq) —> 3Br2(aq) + 3H2O(l)
- the H+ could be titrated with an alkali
CH3COCH3(aq) + I2(aq) → CH3COCH2I(aq) + H+(aq) + I–(aq)
- the I- can be titrated with sodium
thiosulfate

Question 4

what is the rate of reaction

Answer 4

the change in concentration of a substance in unit time
Its usual unit is mol dm-3s-1

Question 5

what is a rate equation

Answer 5

the rate equation relates mathematically the rate of reaction to the concentration of the reactants
r = k[A]m[B]n

Question 6

what is order with respect to a substance in a rate equation

Answer 6

orders are usually integers 0,1,2
0 means the reaction is zero order with respect to that reactant
1 means first order
2 means second order

Question 7

what is the overall order of reaction

Answer 7

the total order for a reaction is worked out by adding all the individual orders together (m+n)

Question 8

what is the rate constant

Answer 8

rate=k[A][B]

Question 9

what is the rate - determining step

Answer 9

the slowest step will control the overall rate of reaction

Question 10

what does the graph for the 1st order with initial rate

Answer 10

a straight diagonal line

Question 11

what does 1st order mean

Answer 11

the rate of reaction is directly proportional
to the concentration of A r = k[A]1

Question 12

what does the graph for the 0 order with initial rate

Answer 12

straight line parallel to x axis

Question 13

what does 0 order mean

Answer 13

the concentration of A has no effect on the
rate of reaction r = k[A]0 = k

Question 14

what does the graph for the 2nd order with initial rate

Answer 14

exponential graph

Question 15

what does 2nd order mean

Answer 15

the rate of reaction is proportional to the
concentration of A squared r = k[A]2

Question 16

how would the initial rate may have been
calculated

Answer 16

from taking gradients from concentration
/time graphs

Question 17

what happens to the half life for 1st order

Answer 17

independent of the concentration and is constant

Question 18

what happens to the half life for 0 order

Answer 18

the rate stays constant as the reactant is used up. this means the concentration has no
effect on rate

Question 19

what happens to the half life for 2nd order

Answer 19

rapidly increase

Question 20

what is the unit for k for 1st order

Answer 20

s-1

Question 21

what is the unit for k for 2nd order

Answer 21

mol-1dm3s-1

Question 22

what is the unit for k for 3rd order

Answer 22

mol-2dm6s-1

Question 23

what happens to the rate constant if the temperature increases

Answer 23

increases the value of the rate constant k

Question 24

what is Arrhenius equation

Answer 24

k = Ae-EA/RT
A = Arrhenius constant
Ea = activation energy
R = gas constant
T = temp

Question 25

how is Arrhenius equation rearranged

Answer 25

ln k = ln A – EA/(RT)

Question 26

what are the units of the components of Arrhenius equation

Answer 26

Temperature uses the unit K
R = 8.31 J mol-1K-1
activation energy will need to be in J mol-1
to match the units of R
the unit of the Arrhenius constant A will be the same as the unit of the rate constant k

Question 27

what is k directly proportional to

Answer 27

the rate of reaction
- so ln k = ln (rate)

Question 28

what is on the x and y axis of the graph

Answer 28

x = 1/T
y = ln rate

Question 29

what is the gradient of the line on the graph

Answer 29

-Ea/R

Question 30

what is the molecularity (number of moles of each substance) of the molecules in the slowest
step the same as

Answer 30

the order of reaction for each substance