18) Rates of reactions

Question 1

Define order

Answer 1

the power to which the concentration of a reactant is raised in the rate equation

Question 2

For zero order reactants, change in concentration _?

Answer 2

doesn’t influence rate

Question 3

For first order reactants, change in concentration _?

Answer 3

results in the same change in rate

Question 4

For second order reactants, change in concentration _?

Answer 4

results in the change^2 in rate

Question 5

Give a generalised rate equation

Answer 5

k [A]^m [B]^n

Question 6

Overall order = ?

Answer 6

sum of orders with respect to each reactant

Question 7

Define initial rate

Answer 7

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

Question 8

Give 3 methods of continuous monitoring of rate

Answer 8

by gas collection
by mass loss
with a colorimeter

Question 9

How does a colorimeter work?

Answer 9

the wavelength of light passing through a coloured solution is controlled using a filter, and the amount of light absorbed by a solution is measured

Question 10

Give an 8 step method for continuous monitoring of rate with a colorimeter

Answer 10

1. prep standard sols. of known conc. of the coloured chemical
2. select filter w/ complementary colour
3. zero w/ water
4. measure absorbance readings for each conc.
5. plot calibration curve of absorbance against conc.
6. carry out the reaction with the coloured chemical + take absorbance readings of the reacting mixture at regular / measured time intervals
7. use the calibration curve to measure concentration at each absorbance reading
8. plot a conc.-time graph to determine order of reaction with respect to the coloured chemical e.g. iodine

Question 11

Define rate constant k

Answer 11

the constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation; proportionality constant

Question 12

For concentration-time graphs rate of reaction = ?

Answer 12

gradient

drawing a tangent at a specific conc. enables rate at that conc. to be calclulated

Question 13

For concentration-time graphs, a zero order reactant is shown as ? k = ?

Answer 13

a straight line

gradient = k

Question 14

For concentration-time graphs, a first order reactant is shown as ?

Answer 14

a curve with a constant half life (exponential decay)

Question 15

Define half-life

Answer 15

the time taken for the concentration of a reactant to decrease by half

Question 16

Give 2 ways which k can be determined for a first order reaction

Answer 16

1. calculate rate constant from rate (gradient) - rearrange the rate equation and sub. in values of rate (gradient of tangent) and conc. at the position where the tangent has been drawn
2. calculate rate constant from half-life
   k = ln2 / t 1/2

Question 17

For rate-concentration graphs, a zero order reactant is shown as ? k = ?

Answer 17

horizontal, straight line

y-intercept = k

Question 18

For rate-concentration graphs, a first order reactant is shown as ? k = ?

Answer 18

straight, diagonal line up from origin

gradient = k

Question 19

For rate-concentration graphs, a second order reactant is shown as ?

Answer 19

curved line up from origin

Question 20

How can k for a second order reactant on a rate-concentration graph be calculated?

Answer 20

plot a 2nd graph of rate against conc.^2

the gradient of this graph = k

Question 21

How can initial rate be found?

Answer 21

measuring the gradient of a tangent drawn at t = 0 on a conc.-time graph

Question 22

Describe a clock reaction (5)

Answer 22

• obtains initial rate by taking a single measurement
• time from start is measured for a visual change (colour or ppt.)
• provided there is no sig. change in rate it can be assumed average rate = initial rate (proportional to 1/t)
• the reaction is repeated with dif. conc. and values are calculated for each
• graph of 1/t against conc. plotted and order with respect to each reactant determined

Question 23

Describe an iodine-clock procedure (5)

Answer 23

• relies on the formation of iodine (usually orange-brown but starch is added)
• separate experiments are carried out using dif. conc. of one of the reactants whilst all other conc. are kept constant
• the solution begins colourless and time measured for the blue-black colour of the starch-iodine complex to appear
• colour change is delayed by a small amount of aqueous sodium thiosulfate which removes iodine as it forms (once it is used up, the colour appears)
• further series of experiments are then carried out in which the conc. of one of the other reactants is changed e.g. hydrogen peroxide

Question 24

Give the equation for the reaction of iodine with thiosulfate ions

Answer 24

2 S2O3 2- (aq) + I2 (aq) -> S4O6 2- (aq) + 2I- (aq)

Question 25

The initial rate measured during a clock reaction is ?

Answer 25

an approximation

reasonably accurate provided less than 15% of the reaction has taken place

Question 26

Define reaction mechanism

Answer 26

the sequence of bond-breaking and bond-forming steps that shows the path taken by e- during a reaction

Question 27

Define rate-determining step

Answer 27

the slowest step in the reaction mechanism of a multi-step reaction

Question 28

The rate equation only includes _?

Answer 28

reacting species involved in the rate-determining step

Question 29

The orders in the rate equation match _?

Answer 29

the no. of species involved in the rate-determining step

Question 30

As temp. increases and rate increases - what will also increase?

Answer 30

the value of k

Question 31

The increased frequency of collisions is _ compared with the increase in the proportion of molecules that exceed Ea from the _, therefore rate is mainly determined by _?

Answer 31

comparatively small
shift in the Boltzmann distribution
Ea

Question 32

Give the Arrhenius equation

Answer 32

k = A e ^(-Ea/RT)

Question 33

What is the pre-exponential term / frequency factor in the Arrhenius equation?

Answer 33

A

frequency of collisions with the correct orientation; constant over a small temp. range

Question 34

What is the exponential factor in the Arrhenius equation?

Answer 34

e ^(-Ea/RT)

proportion of molecules that exceed Ea + have sufficient energy for a reaction to take place

Question 35

Give the logarithmic arrangement of the Arrhenius equation

Answer 35

ln k = - Ea/RT + lnA

ln k = -Ea/RT x 1/T + lnA

Question 36

Describe a graph of the logarithmic form of the Arrhenius equation

Answer 36

y axis = lnk
x axis = 1/T / k^-1
gradient = - Ea/R
y-intercept = lnA

Question 37

How can Ea and A be calculated from the logarithmic form of the Arrhenius equation?

Answer 37

1. calc. values of lnk and 1/T from data
2. plot graph
3. calc. Ea from gradient
   Ea = R x -gradient (in Jmol^-1 so x10^-3)
4. calc lnA from y-intercept
5. A = e^lnA (exponential value on calculator)