16. Kinetics II

Question 1

What is the rate of reaction defined as?

Answer 1

The change in conc. of a substance in unit time

Question 2

What is the rate of reaction usual units?

Answer 2

mol dm-3 s-1

Question 3

When a graph of conc. of reactant is plotted vs time, what does the gradient represent?

Answer 3

the rate of reaction

Question 4

What is the initial rate?

Answer 4

The rate at the start of the reaction where its fastest

Question 5

What is the initial rate equal to on the graph?

Answer 5

the gradient of the tangent

Question 6

What are some methods to investigate rates of reaction?

Answer 6

measurement of the change in volume of gas, measurement of the change of mass, titrating samples of reaction mixture with acid alkali sodium thiosulfate, colorimetry, measuring change in electrical conductivity, measurement of optical activity

Question 7

what is the rate equation for aA + bB

Answer 7

r = k[A]m [B]n

Question 8

What do the m, n stand for?

Answer 8

The reaction orders

Question 9

What is 0 order?

Answer 9

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

Question 10

What is first order?

Answer 10

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

Question 11

What is second order?

Answer 11

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

Question 12

what does r stand for in the rate equation?

Answer 12

rate

Question 13

what is the usual unit for rate equations?

Answer 13

mol dm-3

Question 14

what do the square brackets represent? []

Answer 14

concentration of the element

Question 15

What does k stand for?

Answer 15

rate constant

Question 16

How is the total order for a reaction calculated?

Answer 16

by adding all the individual orders together (m+n)

Question 17

How can a rate concentration graph show the order of a particular reactant?

Answer 17

The concentration of the reactant must be varied whilst the concentrations of the other reactants should be kept constant.

Question 18

What is continuous rate data?

Answer 18

Data from one experiment where the concentration of one substance is followed throughout the experiment.

Question 19

What needs to happen for the continuous rate data method to work?

Answer 19

The conc. of the reactants not being followed must be in large excess in the experiment so their conc. stay virtually constant and don’t affect the rate.

Question 20

How is the continuous rate data processed?

Answer 20

by plotting the data and calculating successive half lives

Question 21

If half-lives are constant what is their order?

Answer 21

1st order

Question 22

What is the half-life of a first order reaction?

Answer 22

is independent of the conc. and is constant

Question 23

What does it mean if half-lives RAPIDLY increase?

Answer 23

the order is 2nd order

Question 24

What does it mean if the rate stays constant as the reactant is used up?

Answer 24

It’s zero order, the conc. has no effect on rate

Question 25

What does the value of k refer to?

Answer 25

to a specific temperature. it increases if we increase temp.

Question 26

What is the value of k independent from?

Answer 26

Conc. and time.

Question 27

When is k constant?

Answer 27

at a fixed temperature

Question 28

How do you calculate the units of k?

Answer 28

1. rearrange rate equation to give k as subject k = Rate/ [A] 2. Insert units and cancel k = mol dm-3 s-1/ mol dm-3 = s-1

Question 29

what is the unit for k for an overall 1st order reaction?

Answer 29

s-1

Question 30

what is the unit for k for an overall 2nd order reaction?

Answer 30

mol-1 dm3 s-1

Question 31

what is the unit for k for an overall 3rd order reaction?

Answer 31

mol-2 dm6 s-1

Question 32

What is the unit of k for r =k [A][B]2

Answer 32

1. k= Rate/ [A][B]2 2. k = mol dm-3 s-1/ mol dm-3 .(mol dm-3)2 3. k = s-1/ mol2 dm-6 (simplify the fraction) 4. k= mol-2 dm6 s-1 OVERALL ORDER IS 3 (1+2= 3)

Question 33

How do you normally work out the rate equation in experiments?

Answer 33

the initial concentrations of reactants are changed (one at a time) and measure the initial rate each time.

Question 34

With experimental data (in a table) how do you calculate the order for a particular reactant?

Answer 34

compare two experiments where only that reactant is being changed

Question 35

What order is it if the conc. is doubled and rate stays the same?

Answer 35

0 order. (2)0

Question 36

What order is it if the conc. is doubled and rate doubles?

Answer 36

1st order. (2)1

Question 37

What order is it if the conc. is doubled and rate quadruples?

Answer 37

2nd order. (2)2

Question 38

What is the overall rate equation and overall order if... A = 1st order B = 2nd order C = 0 order include units of rate constant

Answer 38

r = k [A] [B]2 unit of rate constant = mol-2 dm6 s-1 overall 3rd order (1+2=3)

Question 39

How do you work out orders when two reactant conc. are changed simultaneously?

Answer 39

the effect of both individual changes on conc. are multiplied together to give the effect on rate.