Rate equations

Question 1

Iodine Clock Experiment: State and explain why different volumes of water are added to each mixture. MS [2]

Answer 1

To make volumes constant for all mixtures.

So that volume of propanone is proportional to concentration.

Question 2

RP7a: How do we use results from an iodine clock experiment to determine order of reaction with respect to Iodide ions? MS [5]

Answer 2

in results, we have time taken to go blue
for different [A]
Rate = 1/t
Plot log 1/t vs log [A]
Gradient = Order of Reaction

Question 3

What is the difference between a continous rates and an initial rates method?

Answer 3

continuous involves following one experiment over time, recording the change in [ ] e.g using gas syringe
whereas initial rates involves timing different experiments to reach a fixed [ ] of product then doing 1/t to work out initial rate

Question 4

RP7a - Recall what occurs during an Iodine Clock Experiment [9]

Include:
Practical Procedure
Purpose of Thiosulfate
Independent, Dependent, Control Variables

Answer 4

KI, H2SO4, Starch and S2O3 all in 1 beaker
H2O2 in a separate beaker
Add H2O2 to the first beaker, then immediately start timer
H2O2 reacts with I- to form I2
Thiosulfate then immediately reacts with Iodine formed
Excess I2 remains after all Thiosulfate is used up
Reaction with starch suddenly forms blue-black colour
[I2] is varied, all others remain constant
In each experiment, time taken for reaction mixture to turn blue is measured

Question 5

What is meant by order of reaction?

Answer 5

Power of concentration term in the rate equation

Question 6

What does the rate equation show us about a mechanism?

Answer 6

shows species which appear in or before the rds

Question 7

How can you determine for the order of reaction for compound A from the a series of steps in a mechanism?

Answer 7

The order of reaction is the number of molecules of that
species involved in or before the rate determining step.

Question 8

What is the ‘rate-determining step’?

Answer 8

• Each step has a different rate of reaction
• The slowest step controls the rate of reaction
• This is called the RDS (rate determining step)

Question 9

What are the typical units for Activation Energy when calculated from gradient?

Answer 9

J mol-1

To conver to kJ / mol, divide by 1000

Question 10

Zero order

Answer 10

Time \
Concentration -

Question 11

First order

Answer 11

Time (
concentration /

Question 12

Second order

Answer 12

Concentration )

Question 13

What 2 factors does the Arrhenius constant take into consideration

Answer 13

frequency of collisions
orientation of molecules

Question 14

What’s the arrhenius equation

Answer 14

K = Ae ^-(Ea/RT)

Question 15

How can we relate temperature to the rate constant, k?

Answer 15

Increasing temperature increases K

Question 16

Different volumes of the same initial concentrations will have the …

Answer 16

same initial rate but endin different amounts

Question 17

Why is it preferable to do rate calculations with the initial rate?

Answer 17

To know conc of reactants at the start of the reaction

Question 18

If one reactant is in a muchlarger excess, what is it’s order of reaction?

Answer 18

• Order = 0
• No change on the rate of reaction
• Concentration is effectively constant

Question 19

What are the limitations of a typical gas syringe?

Answer 19

only 100ml of gas, so you don’t want a reaction to produce more than this volume

Question 20

What is important to note about k the rate constant, regarding temperature?

Answer 20

• k is only constant at a fixed temperature
• the value of k refers to a specific temperature and it increases if we increase temperature

Question 21

How do we work out total order?

Answer 21

add all the individual orders together (m+n+…)

Question 22

What is the generalised rate equation for reaction ‘‘aA + bB→’’?

Answer 22

r = K[A]^m[B]^n

Question 23

How do you find Ea with the arrhenius equation?

Answer 23

Ea = RT (LnA-LnK)