Chapter 18 - Rate of reactions

Question 1

How are reaction rates measured ?

Answer 1

By observing the changes in quantities of reactants or products over time

Question 2

What is the equation for the rate of reaction ?

Answer 2

Change in concentration / change in time

Question 3

Why is change in concentration and time used ?

Answer 3

For consistency throughout the reaction

Question 4

What is concentration measured in ?

Answer 4

Mol/dm^3/s

Question 5

What is the shorthand format for concentration ?

Answer 5

• [A] = concentration of reactant A

- It has the usual units of concentration

Question 6

How are rate of reaction and concentration of a particular reactant raised to a power related ?

Answer 6

They are proportional

Question 7

What is the equation for rate of reaction and concentration ?

Answer 7

rate is proportional to [A]^n

Question 8

What is the power ?

Answer 8

The order of the reaction

Question 9

What are the common orders of reaction ?

Answer 9

• Zero order
• First order
• Second order

Question 10

When is the order zero ?

Answer 10

When the concentration of a reactant has no effect on the rate

Question 11

What is the equation for zero order reactions ?

Answer 11

rate is proportional to [A]^0

Question 12

What happens in a zero order reaction ?

Answer 12

• Any number raised to the power of zero is 1

- Concentration does not influence the rate

Question 13

When is the order one ?

Answer 13

When the rate depends on its concentration raised to the power of one

Question 14

What is the equation for first order reactions ?

Answer 14

rate is proportional to [A]^1

Question 15

What happens in a first order reaction ?

Answer 15

If the concentration of A is doubled, the reaction rate increases by a factor of 2^1 = 2

Question 16

When is the order two ?

Answer 16

When the rate depends on its concentration raised to the power of two

Question 17

What is the equation for second order reactions ?

Answer 17

rate is proportional to [A]^2

Question 18

What happens in a second order reaction ?

Answer 18

If the concentration of A is doubled, the reaction rate increases by a factor of 2^2 = 4

Question 19

What does the rate equation give ?

Answer 19

It gives the mathematical relationship between the concentrations of the reactants and the reaction rate

Question 20

What is the rate equation ?

Answer 20

rate = k [A]^m [B]^n

Question 21

What is the rate constant also known as ?

Answer 21

• The proportionality constant

- It is the number that mathematically converts between the rate of reaction and the concentration of orders

Question 22

What does the overall order of a reaction give ?

Answer 22

The overall effect of the concentrations of all reactants on the rate of reaction

Question 23

How is the overall order of the reaction calculated ?

Answer 23

Overall order = sum of orders with respect to each reactant

Question 24

What do the units of the rate constant depend upon ?

Answer 24

The number of concentration terms in the rate equation

Question 25

How can you determine the units of k ?

Answer 25

- Rearranging the equation to make k the subject - Substitute units into the expression for k - Cancel common units and show the final units

Question 26

What are the units for overall order 0 ?

Answer 26

mol/dm^3/s

Question 27

What are the units for overall order 1 ?

Answer 27

/s

Question 28

What are the units for overall order 2 ?

Answer 28

dm^3/mol^1/s

Question 29

What is initial rate?

Answer 29

The instantaneous rate at the beginning of an experiment when t=0.

Question 30

In a concentration time graph, and a rate time graph, what would zero order look like?

Answer 30

- Concentration time - linear to 0 concentration | - Rate time - Horizontal line

Question 31

In a concentration time graph, and a rate time graph, what would first order look like?

Answer 31

- Concentration time - curve getting closer to zero | - Rate time - Linear proportional

Question 32

In a concentration time graph, and a rate time graph, what would second order look like?

Answer 32

- Concentration time - steeper curve | - Rate time - proportional to x^2

Question 33

What are the two ways of continuous monitoring when a gas is produced?

Answer 33

- Gas collection | - Mass loss

Question 34

What is a way of utilising light to monitor rate?

Answer 34

- Using a colorimeter. | - Measuring the amount of light absorbed by a solution.

Question 35

What is half life?

Answer 35

The time taken for half of a reactant to be used up.

Question 36

In a concentration time graph, how can you deduce whether the reactant is first or second order?

Answer 36

- Measure multiple half lives. - If the half life is constant, it is first order. - If the half life is increasing, it is second order.

Question 37

How do you calculate the rate constant from the rate of reaction ?

Answer 37

k = rate/concentration

Question 38

How do you calculate the rate constant from the half life ?

Answer 38

k = ln|2| / t(1/2)

Question 39

How can rate concentration graphs be plotted ?

Answer 39

From measurements of the rate of reaction at different concentrations

Question 40

Why are rate concentration graphs important ?

Answer 40

They offer a route into the direct link between the rate and concentration in the rate equation

Question 41

What graph does a zero order reactant produce ?

Answer 41

Horizontal straight line with zero gradient

Question 42

What does the rate equal in a zero order reaction ?

Answer 42

rate = k

Question 43

How do you find the rate constant from a zero order graph ?

Answer 43

The intercept on the y axis gives the rate constant k

Question 44

What is the relationship between the rate and concentration in a zero order reaction ?

Answer 44

The reaction rate does not change with increasing concentration

Question 45

What graph does a first order reactant produce ?

Answer 45

Straight line graph through the origin

Question 46

What does the rate equal in a first order reaction ?

Answer 46

rate = k[A]

Question 47

How do you find the rate constant from a first order graph ?

Answer 47

The rate constant can be determined by measuring the gradient of the straight line of this graph

Question 48

What is the relationship between the rate and concentration in a first order reaction ?

Answer 48

Rate is directly proportional to the concentration

Question 49

What graph does a second order reactant produce ?

Answer 49

Upward curve with increasing gradient

Question 50

What does the rate equal in a second order reaction ?

Answer 50

rate =k[A]^2

Question 51

How do you find the rate constant from a second order graph ?

Answer 51

- Plot a second graph of rate against concentration squared - This will give u a straight line through the origin - The gradient of this straight line graph is equal to the rate constant

Question 52

What is the relationship between the rate and concentration in a second order reaction ?

Answer 52

Rate increases as concentration increases

Question 53

What is a clock reaction ?

Answer 53

A more convenient way of obtaining the initial rate of a reaction by taking a single measurement

Question 54

What do you look for/measure in a clock reaction ?

Answer 54

The time t from the start of an experiment is measured for a visual change to be observed

Question 55

What else are you measuring in a clock reaction ?

Answer 55

- An average rate of a change in reactant over time - The shorter the period of time over which an average rate is measured, the less the rate changes over that time period

Question 56

What is the initial rate proportional to ?

Answer 56

1/t

Question 57

As temperature increases, how does the rate of the reaction change ?

Answer 57

The rate increases

Question 58

As temperature increases, how does the rate constant increase ?

Answer 58

The rate constant increases

Question 59

For many reactions, what does a 10 degree rise in temperature result in ?

Answer 59

- Doubles the rate constant | - Doubles the rate of the reaction

Question 60

When temp increases, what two factors contribute to the increase in rate ?

Answer 60

- Boltzmann distribution shifts right, increasing the proportion of particles that exceed the activation energy - As the temp increases, particles move faster and collide more frequently

Question 61

What is the change in rate mainly determined by ?

Answer 61

Activation energy

Question 62

Why is the change in rate mainly determined by the activation energy ?

Answer 62

The increased frequency of collisions is comparatively small compared with the increase in the proportion of molecules that exceed Ea from the shift in the boltzmann distribution

Question 63

What is the Arrenhius equation ?

Answer 63

An exponential relationship between the rate constant and temperature

Question 64

What is the formula for the Arrenhius equation ?

Answer 64

k = A x e^(-E(a)/RT)

Question 65

What is A known as in the Arrenhius equation ?

Answer 65

Pre exponential factor

Question 66

What is E(a)/RT known as in the Arrenhius equation ?

Answer 66

Exponential factor

Question 67

What does the exponential factor represent ?

Answer 67

The proportion of molecules that exceed the activation energy and have sufficient energy for a reaction to take place

Question 68

What is the pre exponential term also called ?

Answer 68

The frequency factor

Question 69

What does the frequency factor take into account ?

Answer 69

It takes into account the frequency of collisions with the correct orientation

Question 70

When does the frequency factor increase ?

Answer 70

Increases slightly with temperature but it is essentially constant over a small temperature range

Question 71

What does the frequency factor essentially give us ?

Answer 71

The rate if there were no activation energy

Question 72

What is the logarithmic form of the Arrhenius equation?

Answer 72

ln|k| = -E(a)/RT + ln|A|

Question 73

What can the log form of the Arrhenius equation be modelled as?

Answer 73

- A straight line graph in y=mx + c - ln|k| is y - 1/T is x - E(a)/R is gradient - ln|A| is y intercept

Question 74

Define stoichiometry

Answer 74

- The balancing numbers | - The relative amounts of the species in the reaction

Question 75

Is it likely for more than two particles to collide at the same time ?

Answer 75

No it is unlikely that more than two particles will collide at the same time

Question 76

Why is a single step reaction unlikely ?

Answer 76

Multiple ions colliding simultaneously is an extremely unlikely event

Question 77

How are reactions likely to take place ?

Answer 77

In a series of steps

Question 78

Define reaction mechanism

Answer 78

The series of steps that make up an overall reaction

Question 79

Define rate determining step

Answer 79

The slowest step in a multi step reaction pathway

Question 80

How can you predict reaction mechanisms (RDS) ?

Answer 80

- The rate equation includes reacting species involved in the RDS - The orders in the rate equation match the number of species involved in the RDS

Question 81

Why is the RDS important ?

Answer 81

It provides important evidence in supporting or rejecting a proposed reaction mechanism

Question 82

What does investigating the hydrolysis of haloalkanes determine ?

Answer 82

- The overall order of reaction - The rate equation - Possible mechanism for the reaction