18 - Rates

Question 1

Two ways of determining rate

Answer 1

• Measuring the decrease in the concentration of a reactant over time

• Measuring the increase in the concentration of a product over time

Question 2

How can rate equations be determines

Answer 2

can only be determined experimentally not by mole ratios

Question 3

Rate-concentration = 0 order

Answer 3

o Changing the concentration of the chemical has no effect on the rate of the reaction

Question 4

Is the 0 order included in rate equation

Answer 4

No

Question 5

Rate from 0 order rate-concentration graph

Answer 5

o Rate = K // y-intercept

Question 6

Rate - concentration 1st order

Answer 6

o The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction

Question 7

Rate from a first order rate-concentration graph

Answer 7

o Rate = gradient

Question 8

Rate - concentration 2nd order

Answer 8

o The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four

Question 9

Rate from 2nd order rate-concentration graph

Answer 9

o Upward curve = plot rate against conc squared = straight line = k = gradient

• gradient of straight line

Question 10

Overall order of a rate equation

Answer 10

sum of the powers of the reactants in a rate equation

Question 11

Order with respect to [(CH3)3CBr]

Answer 11

• From the above table, that is experiments 1 and 2
o The [(CH3)3CBr] has doubled, but the [OH-] has remained the same
o The rate of the reaction has also doubled
o Therefore, the order with respect to [(CH3)3CBr] is 1 (first order)

Question 12

Order with respect to [OH-]

Answer 12

• From the above table, that is experiments 1 and 3
o The [OH-] has doubled, but the [(CH3)3CBr] has remained the same
o The rate of reaction has increased by a factor of 4 (i.e. increased by 22)
o Therefore, the order with respect to [OH-] is 2 (second order)

Question 13

Rate equation of this

Answer 13

Rate = k [(CH3)3CBr] [OH-]2

Question 14

Draw 0 order rate concentration graph

Answer 14

Question 15

Draw second order rate-concentration graph

Answer 15

Question 16

Draw third order rate-concentration graph

Answer 16

Question 17

How to find k

Answer 17

Use one row from table

Question 18

Units of k

Answer 18

Question 19

Monitoring rate with a colorimeter

Answer 19

• In colorimeter – wavelength of light passing through a coloured solution is controlled using a filter

• Amount of light absorbed is measures

Question 20

How to use a colorimeter - 6 marks

Answer 20

Question 21

Concentration - time graph 0 order

Answer 21

• In a zero-order reaction, the concentration of the reactant is inversely proportional to time

• When the order with respect to a reactant is 0, a change in the concentration of the reactant has no effect on the rate of the reaction

Question 22

Rate constant of concentration time graph

Answer 22

gradient= rate constant k

Question 23

Draw 0 order concentration time graph

Answer 23

Question 24

1st order concentration-time graph

Answer 24

• In a first-order reaction, the concentration of the reactant decreases with time

o The graph is a curve going downwards and eventually plateaus:

Question 25

Draw a first order concentration time graph

Answer 25

Question 26

How to deduce order of reaction from graph

Answer 26

Half life

Question 27

Half life is only for…

Answer 27

Concentration - time graph

Question 28

Zero order half life

Answer 28

successive half-lives decrease with time

Question 29

First order half life

Answer 29

half-life of a first-order reaction remains constant throughout the reaction

Question 30

Second order half life

Answer 30

half-life increases with time

Question 31

Define half life

Answer 31

the amount of time taken for the amount (or concentration) of the limiting reactant in a reaction to decrease to half its initial value

Question 32

Determination of K for a first order reaction - two ways - concentration-time

Answer 32

- rate -half life

Question 33

1st order reaction has… (concentration-time)

Answer 33

Constant half life

Question 34

Calculating the rate constant from the rate - first order concentration - time

Answer 34

Question 35

Calculating the rate constant from half-life - first order concentration - time

Answer 35

Question 36

Clock reaction

Answer 36

Idk

Question 37

What is the rate determining step

Answer 37

Reactant in rate-determining step is in the rate equation

Question 38

Feature of rate determining step

Answer 38

Slowest step of

Question 39

Example of rate determining step

Answer 39

Question 40

Increasing temp on rate

Answer 40

• Increasing temp increases rate of reaction (may not impact yield due to la chateliers principle) • Therefore increasing temp increases the value of the rate constant, k, assuming that the concentration of the reactants remains unchanged

Question 41

relationship between the rate of reaction and temperature

Answer 41

exponential

Question 42

When temp of a reaction mixture increases,

Answer 42

particles to move around faster resulting in more frequent collisions o proportion of successful collisions increases, meaning a higher proportion of the particles possess the activation energy to cause a chemical reaction

Question 43

Boltzmann distribution

Answer 43

Question 44

Arrhenius equation

Answer 44

Question 45

Calculate the activation energy of a reaction which takes place at 400 K, where the rate constant of the reaction is 6.25 x 10-4 s-1. A = 4.6 x 1013 and R = 8.31 J mol-1 K-1.

Answer 45

Take natural logs of both sides

Question 46

Arrhenius plot:

Answer 46

• A graph of ln k against 1/T can be plotted, and then used to calculate Ea

Question 47

How to draw Arrhenius plot - remember equation (look at it in form y=mx+c)

Answer 47

Question 48

How to find rate constant from concentration time graph - method 2

Answer 48

Question 49

what is the rate constant

Answer 49

a constant of proportionality in the rate equation

Question 50

how would you prove that the gradient of the graph gives you the rate constant

Answer 50

For a first-order reaction of a reactant A: rate = k[A] Therefore, the gradient of a rate–concentration graph gives the value of the rate constant, k.

Question 51

if you equate the rate to be 1/t what assumption is made + how valid is this

Answer 51

The assumption is that the reaction proceeds at constant rate to the measured point. The assumption is reasonable so long as the point is soon after the start of the reaction.

Question 52

how could you follow the progress of a reaction between gaseous hydrogen and gaseous bromine to make gaseous hydrogen bromide

Answer 52

A colorimeter could be used to detect the loss of colour of the bromine as it formed hydrogen bromide

Question 53

when predicting equations what should you focus on

Answer 53

the number of moles in the rate equation ( the power) and the number of moles in the overall equation

Question 54

does the order of the reactant in the rate equation match up to the number of moles in the rate determining step

Answer 54

?

Question 55

what is the pre-exponential factor

Answer 55

A

Question 56

units of the parts of the Arrhenius equation ???

Answer 56

R - Jmol−1K−1 A - K - Ea - Jmol-1 T - K