Topic 16 - Kinetics 2

Question 1

What a the rate of reaction?

Answer 1

the change in the amount of reactants/products per unit time

Question 2

What is a rate equation?

Answer 2

an equation that shows the relationship between the concentrations of reactants and the rate of reaction
eg. rate = k[A][B]

Question 3

What is an order?

Answer 3

a power in the rate equation that tells us how any change in concentration of a reactant affects the rate

Question 4

What is zero order in respect to a reactant?

Answer 4

when the concentration of the reactant has no effect on rate
eg. [X] doubles, rate stays the same

Question 5

What is first order in respect to a reactant?

Answer 5

when the change in magnitude of the concentration of a reactant has the same impact on the rate
eg. [X] doubles, rate doubles

Question 6

What is second order in respect to a reactant?

Answer 6

when the change in magnitude of the concentration of a reactant has the squared effect on the magnitude of the reactant concentration on the rate of reaction
eg. [X] doubles, rate quadruples (as 2²=4)

Question 7

What is the overall order of a reaction?

Answer 7

the sum of all the orders within the rate equation

Question 8

What method can be used to obtain a rate of reaction when there is a change in volume?

Answer 8

by inversion or with a gas syringe

-measure the volume of gas produced

Question 9

What method can be used to obtain a rate of reaction when there is a change in mass?

Answer 9

place conical flask where the reaction is occurring on a high resolution balance and measure the change in mass
(when a gas is being produced, mass decreases but when a gas is being used as a reactant, mass increases)

Question 10

What method can be used to obtain a rate of reaction when there is a change in colour?

Answer 10

disappearing cross experiment (when there is a change from colourless to coloured or vice versa) -measure the time taken for cross to disappear/appear

colourimetry (measure percentage transmission/absorbance at fixed time intervals and plot a graph of time against transmission/absorbance or use a calibration curve to determine concentrations)

Question 11

What method can be used to obtain a rate of reaction when there is a change in the conductivity of the reaction mixture?

Answer 11

condumetric analysis

usually used for inorganic reactions as conductivity depends on the number and types of ions present (ie. size and charge of ions)

Question 12

What method can be used to obtain a change in concentration of a particular substance at a particular time?

Answer 12

titration

remove aliquots at different time intervals and quench so that titrations can be done of all the different times

Question 13

What is colorimetry?

Answer 13

a method which uses percentage transmission/absorption of light during a reaction which involves a change in colour to determine the concentration of a reagent

Question 14

What shape is a graph of concentration against time when the reactant is at zero order?

Answer 14

straight line (going down) 
-constant gradient

Question 15

What shape is a graph of concentration against time when the reactant is at first order?

Answer 15

downwards curve

• gradient gets less and less steep
• half lives are constant

Question 16

What shape is a graph of concentration against time when the reactant is at second order?

Answer 16

downwards curve

• gradient gets less and less steep but is steeper than first order
• half lives increase

Question 17

What shape is a graph of rate against concentration when the reactant is at zero order?

Answer 17

horizontal line

Question 18

What shape is a graph of rate against concentration when the reactant is at first order?

Answer 18

straight line (going up)
-constant gradient

Question 19

What shape is a graph of rate against concentration when the reactant is at second order?

Answer 19

upwards curve

Question 20

What are tangents drawn at t=0 equal to?

Answer 20

initial rate

Question 21

What is the rate determining step of a reaction?

Answer 21

the slowest step which determines the rate of reaction

-only species in the RDS will appear in the rate equation

Question 22

What order are solid catalysts always at?

Answer 22

zero order

∴ they never appear in the rate equation

Question 23

Why is the rate determining step equivalent to the overall rate of reaction?

Answer 23

RDS is always the slowest so rate can only go as quick as this step

Question 24

What is the Arrhenius equation?

Answer 24

k=Ae^(-Ea/RT)

rearranged: lnk = lnA -Ea/RT

Question 25

What conditions affect the rate constant?

Answer 25

only temperature (higher temp = higher rate)

Question 26

How is the Arrhenius equation in the format of y=mx+c?

Answer 26

lnk = lnA -Ea/RT is like y=mx+c when:
y = lnk
m = -Ea/R
x = 1/T
c = lnA

Question 27

How can activation energy be found from a gradient of a graph of lnk against 1/T?

Answer 27

Ea = gradient × -R

R = -8.14

Question 28

What is the overall order of a hydrolysis of a primary halogenoalkane?

Answer 28

second order

Question 29

What is the overall order of a hydrolysis of a tertiary halogenoalkane?

Answer 29

first order

Question 30

What is the mechanism of the hydrolysis of a primary halogenoalkane? And how does the rate equation act as evidence for this?

Answer 30

SN2 mechanism

-2 species in RDS/rate equation

Question 31

What is the mechanism of the hydrolysis of a tertiary halogenoalkane? And how does the rate equation act as evidence for this?

Answer 31

SN1 mechanism

-only 1 species in RDS/rate equation

Question 32

In the iodine clock reaction, why should the total volume be kept the same?

Answer 32

so that volume of ions is proportional to concentration