Chapter 18 - Rates of Reaction

Question 1

What are the equations for rate?

Answer 1

Rate = quantity reacted or produced/time
Rate = change in concentration/change in time

Question 2

How can you rearrange the rate equation to work out k?

Answer 2

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

Question 3

What is concentration measured in?

Answer 3

mol dm^-3

Question 4

What is zero order?

Answer 4

When the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant
Concentration does not influence rate

Question 5

What is first order?

Answer 5

When the rate depends on a reactant’s concentration raised to the power of 1
If concentration doubles, rate also doubles

Question 6

What is second order?

Answer 6

When rate depends on a reactant’s concentration raised to the power of 2
If concentration doubles, reaction rate increases by 2 to the power of 2 = 4

Question 7

What is the rate equation?

Answer 7

Gives the mathematical relationship between the concentrations of the reactants and the reaction rate
rate = k [A]m [B]n
Where k = rate constant, m= order of reaction for A and n= order of reaction for B

Question 8

What is the overall order?

Answer 8

The overall effect of all reactants on the rate of reaction.
Overall order = sum of orders with respect to each reactant

Question 9

How does the rate constant change with concentration and pressure?

Answer 9

It does not change

Question 10

What is the initial rate?

Answer 10

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

Question 11

What are the methods for continuous monitoring of reactions to determine rate?

Answer 11

Monitoring by gas collection
Monitoring by mass loss
Monitoring rate with a colorimeter

Question 12

What shape concentration time graph does zero order reaction produce?

Answer 12

A straight line with a negative gradient

Question 13

What shape concentration time graph does a first order reaction produce?

Answer 13

A downward curve with a decreasing gradient over time

Question 14

What shape concentration time graph does a second order reaction produce?

Answer 14

A downward curve, steeper at start but tailing off more slowly

Question 15

What is half life?

Answer 15

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

Question 16

How can a first order relationship be determined from a concentration time graph?

Answer 16

By measuring successive half lives. If they are the same, then the reaction is first order with respect to the reactant. HALF LIFE IS CONSTANT FOR FIRST ORDER

Question 17

How can you determine k from a first order reaction?

Answer 17

• calculating rate constant from a curve
  Find gradient, then use that in k = rate (gradient)/reactant
• calculating rate constant from half life
  k = ln2/ half life

Question 18

What shape rate concentration graph do zero order reactions produce?

Answer 18

A horizontal straight line with zero gradient

Question 19

What shape rate concentration graph does a first order reaction produce?

Answer 19

A straight line graph through the origin

Question 20

What shape rate concentration graph does a second order reaction produce?

Answer 20

An upward curve with increasing gradient

Question 21

How can the initial rate be found?

Answer 21

By measuring the gradient of a tangent drawn at t = 0 on a concentration-time graph

Question 22

What is a clock reaction, and how can this be used to determine initial rate?

Answer 22

The time from the start of an experiment is measured for a visual change to be observed (usually a colour or precipitate). Initial rate is then proportional to 1/time.

Question 23

What is an iodine clock?

Answer 23

Aqueous iodine is coloured orange brown, so the time from the start of the reaction and the appearance of the iodine colour can be measured. Starch is usually added as it forms a complex with iodine which is an intense dark blue black colour

Question 24

How accurate are clock reactions?

Answer 24

You are measuring the average rate during the first part of reaction (assumption that average rate is constant and same as initial rate). Initial rate is therefore an approximation, but still reasonably accurate provided that less than 15% of reaction has taken place

Question 25

What is the rate determining step?

Answer 25

When there is a multi-step reaction, they take place at different rates. The slowest step is called the rate determining step

Question 26

How can you predict reaction mechanisms?

Answer 26

The rate equation only includes reacting species involved in the rate determining step.
The orders in the rate equation match the numbers of species involved in the rate determining step.

Question 27

Why is it unlikely that some reaction mechanisms only take place in one step?

Answer 27

Stochiometry in rate equation does not always match stochiometry in the overall equation
Collisions are unlikely with more than 2 ions/species

Question 28

What 2 factors affect the rate constant when temperature increases?

Answer 28

• an increase in temp shifts Boltzmann distribution to right to increase proportion of particles exceeding Ea
• as temp increases, particles move faster and collide more frequently

Question 29

What is the Arrhenius equation?

Answer 29

An exponential relationship between the rate constant k and temperature T
k = A e^ -Ea/RT
GIVEN ON DATA SHEET

Question 30

What are the different parts of the Arrhenius equation, and what units are used?

Answer 30

k = rate constant
A = pre-exponential factor, always same units as k
Ea = activation energy, J mol-1
R = gas constant, J K-1 mol -1
T = temperature, Kelvin
e^ -Ea/RT = exponential factor

Question 31

What is the logarithmic form of the Arrhenius equation?

Answer 31

lnk = -Ea/RT + lnA

Question 32

How can Ea and A be determined graphically?

Answer 32

Via a plot of lnk againsts 1/T which gives a straight line graph with
- gradient -Ea/R
- y intercept of lnA

Question 33

How does the logarithmic form of the Arrhenius equation match up to y = mx+c?

Answer 33

lnk = -Ea/R x 1/T + lnA
y = m x + c

Question 34

How do you go from lnA to A?

Answer 34

If lnA = x, A = e^x