Rates of Reaction

Question 1

How do you calculate the initial rate of reaction? Why is this possible?

Answer 1

Calculate the rate (concentration/time) at t=0, where the graph is roughly linear, and where the other reactants are in excess.

Question 2

Define first order reaction w.r.t [A]

Give a more general definition for orders of reaction

Answer 2

A reaction where doubling [A] means the rate of reaction is doubled

The power to which the concentration of a reactant is raised in the rate equation

Question 3

Define second order reaction w.r.t [A]

Answer 3

doubling the concentration of [A] means the rate is increased by 2^

2=4 times

Question 4

Define zero order reaction w.r.t [A]. Give an example

Answer 4

The concentration of [A] does not affect the reaction, for example decomposition of H2O2

Question 5

What is the rate equation?

Answer 5

An equation that links to the rate of reaction to the concentration of each reactant, raised to their order, linked by the rate constant.

Question 6

How can we determine the order based on C-T graph?

Answer 6

Innitial rates method - Assuming the concentration of the reactant is roughly linear and has a small amount of change, the rate is proportional to 1/time.

gradient of C-T graph is rate of reaction

repeat for several different concentrations,

plotting rate / [A] graph to show order of reactant A

Question 7

How do you measure the half life of reactions with a first order conc-time graph?

Answer 7

Measure the time for the concentration to halve, and repeat this process three times. By comparing the half lives of three separate divisions, we can see if it is constant. If so, it is first order.

Question 8

What is characteristic of half life of zero order reaction?

Answer 8

It decreases with time

Question 9

What is characteristic of the half life of second order reactions?

Answer 9

It increases with time.

Question 10

What is the effect of termperature on the rate of reaction? Does anything else affect the rate of reaction?

Answer 10

For every 10oC increase, the rate doubles.

No.

Question 11

Define molecularity

Answer 11

The number of molecules in the rate determining step.

Question 12

How do you determine order of reaction

by inspection?

Answer 12

Fine two experiments where there is a change in only one reactant. Find the effect this has on the initial rate, and hence the proportionality: 0,1,2 order. Then state that between exp.1 and 2, the concentration of A doubled, and the rate doubled also, so rate is proportional to [A], hence first order wrt A.

Question 13

Define the half life of a reaction

Answer 13

The time taken for the concentration of a reactant to halve.

Question 14

What is characteristic of the half life of first order graphs?

Answer 14

The half life is constant. This is a defining feature as it is an exponential decrease in terms of e.

Question 15

When working out a half life of a first order graph, what must you do at the end to prove the half life is constant?

Answer 15

Find the average of at least 3 half lives. If the average is close to the three values, the half life can be said to be constant, so the graph is first order.

Question 16

If paracetamol has a biological half life of 380 seconds, how long does it take for the concentration to fall to 1/16 its original concentration?

Answer 16

1/1 –> 1/2 –> 1/4 –> 1/8 –> 1/16

4 half lives = 4*380 = 1520

Question 17

What are we comparing when we plot a clock reaction for various concentrations of a reactant?

Answer 17

the proportionality of 1/T vs concentration. Since 1/T is proportional to the rate, we can determine the order of the graph, if not the actual rate at these points.

Question 18

What measures can we use for clock reactions to determine the time taken?

Answer 18

Time taken for:
Precipitate to form
Solid to disappear
sudden colour change
change in pH

Question 19

What are the species in the rate equation all involved in>

Answer 19

The rate determining step

Question 20

What is the rate determining step?

Answer 20

The slow step of a reaction which determines the rate, in a multi step reaction.

Question 21

In this example, is the rate determining step mono or bimolecular?
r=k[NO2]^2

Answer 21

Bimolecular, because there are 2 molecules of NO2 reacting.

Question 22

Suggest an overall reaction mechanism for NO2 + CO = NO + CO2 using the rate equation:
r=k[NO2]^2

Identify any intermediates in the reaction

Answer 22

Slow step:

2NO2 –> 2NO + O2

Fast step

2CO + O2 –> 2CO2

O2 is the intermediate

Question 23

What does k express in terms of the overall reaction?

Answer 23

How quickly the reaction occurs. A small value of k means a slow reaction, whilst a large value of k means a fast reaction.

Question 24

Can the speed of a reaction be determined mathematically?

Answer 24

No, because the rate equation only concerns the slow step. This means k only refers to the slow step, and not the overall reaction.

Question 25

What equation could you use to calculate the overall speed of the reaction?

Answer 25

Arrhenious equation.

Question 26

How can you tell if the reaction is 1st order given a rate concentration graph? What two things must you say?

Answer 26

Straight line AND goes through origin.

Question 27

If the rate equation is r=k[H2][NO]^2, what happens if both H2/NO both increase in concentration by 4x?

Answer 27

increase by 64x

Question 28

What is the rate of disappearance of I- if the rate of formation of I2 is 1E-5 moldm-3s-1

Answer 28

Since two moles of I- formed from 1 mole of I2, the rate is 2E-5

Question 29

What is the half life in a first order reaction if the pressure is halved?

Answer 29

The same, as it is independent of initial concentration. Only dependent on temperature.

Question 30

What does molecularity refer to?

Answer 30

The number of molecules in an ELEMENTARY REACTION.

For a multi step reaction, this is the rate determining step