C18 - Rates of reactions

Question 1

Equation of rate of reaction in terms of reactants / products

Answer 1

Rate = quantity reacted or produced / time

Question 2

Equation of rate of reaction in terms of concentration

Answer 2

Rate = change in concentration / change in time

Question 3

Units for concentration

Answer 3

Mol/dm^3/s

Question 4

What does [A] mean?

Answer 4

Concentration of A

Question 5

What is zero order?

Answer 5

Concentration of a reactant has no effect on the rate

Question 6

What is 1st order?

Answer 6

Rate is proportional to the concentration raised to the power of 1

Question 7

What is 2nd order

Answer 7

Rate depends on its concentration raised to the power of 2

Question 8

What is the rate equation

Answer 8

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

Question 9

What is ‘k’?

Answer 9

Rate constant

Question 10

What does the ‘m’ and ‘n’ mean?

Answer 10

The order of the reaction with respect to that reactant

Question 11

How do you calculate the units of ‘k’?

Answer 11

Rearrange formula
Substitute units into expression or k
Cancel common units

Question 12

How do you calculate the overall order of a reaction?

Answer 12

Add all orders

Question 13

How can you determine orders experimentally?

Answer 13

Measuring concentrations at equal time intervals from a titration or colorimeter practical
Compare concentrations
Determine relationship between concentration and rate

Question 14

When should rate be measured during experiment?

Answer 14

As close to the start of experiment
(Initial rate)

Question 15

What is the initial rate

Answer 15

Instantaneous rate at the beginning of an experiment when t=0

Question 16

What is continuous monitoring?

Answer 16

Continuous measurements taken over the course of a reaction

Question 17

What are the two methods of continuous monitoring

Answer 17

Monitory gas collection
Monitoring mass loss
Monitoring colour change

Question 18

How are concentration-time graphs plotted

Answer 18

Continuous monitoring of reaction

Question 19

How do you monitory colour change?

Answer 19

Using a colorimeter

Question 20

What does a colorimeter do?

Answer 20

Wavelength of light passing through coloured solution is controlled using a filter
Amount of light absorbed by solution is measured

Question 21

How do you use colorimetry

Answer 21

1) Prepare standard solutions
2) Select filter complimentary to colour of chemical
3) Zero the colorimeter with water
4) Measure the absorbance readings of the solutions
5) Plot a calibration curve
6) Carry out reaction to produce product that needs to be tested
7)Take absorbance readings of reacting mixture at measured time intervals
8) Use calibration curve to identify concentration at each absorbance reading
9) Plot concentration time graph to identify order

Question 22

Concentration-time graph shapes

Answer 22

Zero order - Straight line with negative gradient
First order - Downwards curve with decreasing gradient over time (constant half life)
Second order - Downward curve that’s steeper at start but tails off more slowly (non-constant half life)

Question 23

What is a half life?

Answer 23

Time taken for the concentration of a reactant to decrease by half
T(1/2)

Question 24

What is exponential decay

Answer 24

Concentration halves every half life (constant half life)

Question 25

How do you determine k of a first order reaction?

Answer 25

K = (ln2) / t(1/2)
Rate constant = (ln2) / half life

OR

Tangent to curve drawn at particular concentration
Gradient calculated giving rate of reaction
Rate constant determined is calculated by rearranging rate equation ( k = rate / (conc of reactants) )

Question 26

What are the shapes of rate-concentration graphs?

Answer 26

Zero order - Horizontal straight line, rate = k
First order - Straight line graph, rate = k[A]
Second order - Upward curve, rate = k[A]^2

Question 27

What reaction can you use to obtain the initial rate?

Answer 27

Clock reaction
Measure time to see visible change
1/t is proportional to initial rate
Repeated with different concentrations

Question 28

Describe the iodine clock reaction

Answer 28

Colour change delayed by aqueous sodium thiosulfate (Na2S2O3) which removes iodine as it forms, when it’s used up the colour change occurs
Blue black colour occurs when aqueous iodine forms
Concentration of one of the reactants changed

Time measured for colour change to occur
1/t proportional to initial rate
Graph of 1/t against conc drawn
Graph’s shape will tell you order of reaction

Question 29

How accurate are clock reactions

Answer 29

Approximate, but reasonably accurate given that less than 15% of the reaction has taken place

Question 30

What’s a reaction mechanism

Answer 30

Series of steps that make up an overall reaction

Question 31

What is the rate determining step

Answer 31

The slowest step in a reaction mechanism

Question 32

How can you predict reaction mechanism

Answer 32

Order and species involved in rate equation only involve those that are true for the rate determining step

Question 32

Hydrolysis if haloalkanes half equation

Answer 32

RBr + OH- —> ROH +Br-

Question 33

Conditions for hydrolysis of haloalkanes

Answer 33

Hot aqueous alkali

Question 34

How do you determine a rate mechanism for a reaction

Answer 34

Question 35

What happens to rate constant when temp increases

Answer 35

Shifts Boltzmann distribution to right, Inc proportion of particles exceed Ea
(Particles move faster and collide more frequently)

Question 36

What is the Arrhenius equation

Answer 36

Question 37

What does the exponential factor represent

Answer 37

Proportion of molecules that exceed Ea, have sufficient energy for reaction to take place

Question 38

What does the pre exponential factor represent

Answer 38

Frequency of collisions with correct orientation

Question 39

Logarithmic form of the Arrhenius equation

Answer 39

Question 40

How can the logarithmic form of the Arrhenius equation tell us the equation of a straight line

Answer 40