Further kinetics

Question 1

Calculate the time it takes for the mass of A to decrease from 600 - 35g if the decompositions has a constant half liffe of 14 minutes (1)

Answer 1

• 600-300-150-75-37.5
• 14x4
• 56 minutes

Question 2

IO3-(aq)+5I-(aq)+6H+(aq)->3I2(aq)+3H2O(l)
state why the order of reaction with repect to iodide ions cannot be five even though 5 mol of iodide ions are shown in the equation (1)

Answer 2

• the chance of five or more ions colliding in the rate determinitng step is neglibile

Question 3

State why the volume must be kept the same for the iodine clock experiment (1)

Answer 3

volume of iodate(v) ions is proportional to the concentration

Question 4

In the iodine clock experiment explain the purpose of the starch present in the reaction mixture when the starch is neither in the rate or reaction equation (2)

Answer 4

• starch is an indicator
• time taken for formation of blue black can be used to calculate reaction rate

Question 5

Explain how using a catalyst increase the rate constant k (2)

Answer 5

• a catalyst provides an alternative pathway for a lower activation energy
• so a greater proportion of molecules have an energy greater than the activation energy

Question 6

The heterogenous catalyst palladium was suggesteed for use in this reaction
Explain how impurities in the gaaseous reactants could make the catalyst less effective (3)

Answer 6

• impurities bind/bond to catalyst surface
• therfore less active sites available for the reaction
• impurities form strong bonds to the surface

Question 7

Describe how a sample of a solid such as gentian violet is purified by recrystilisation (4)

Answer 7

• dissolve impure product in a minimum volume of hot solvent
• cool in ice/leave to recrystalllize
• filter using vacuum filtration/ filtration with suction
• dry solid in desiccator/between filter paper

Question 8

Explain why the student uses a large excess of methanol in this experiement (1)

Answer 8

• zero order with respect to CH3OH
• to keep [CH3OH] constant

Question 9

Aqueous solutions of hydrogen peroide H2O2 decompose as in equation beloo
2H2O2 ->2H2O +O2
a student investigate by measuring the volume of oxygen gas produced over time
suggest a differnt experimental method that would allpw the rate of this reaction to be followed over time (1)

Answer 9

measure mass lost

Question 10

the rate constant k, for this reaction is determined at different temperatures T
xplain how the student could fetermine the activation energy for the reaction graphicaly using values of k and T (3)

Answer 10

• plot graph with k and 1/T
• measure the gradient
• Ea=-R x gradient

Question 11

Explain why the aborbance of iodine decreases during the experiment (1)

Answer 11

Iodine has a yellow brown colour and concentration of I2 decreases

Question 12

The strudent concluded H+ acts as a catalyst explain why the student conclusion is not correct (1)

Answer 12

• H+ ions are used up / are in the overall equation

Question 13

What is meant by the rate determining step (1)

Answer 13

slowest step in a reaction

Question 14

State how the concentration of the bromine could have been monitored when it reacts with a large excess of HCOOH (1)

Answer 14

more reduction of colour of bromine

Question 15

Suggest a different experimental method that would allow the rate of this reaction to followed over time Br2 +HCOOH->H+ +2Br-+CO2 (1)

Answer 15

measure the volume of CO2 produced

Question 16

Why would you use an exces HCOOH ensure that the order with respect to HCOOH is effectivley zero (1)

Answer 16

concentration of HOOH would be constant

Question 17

Suggest a balanced equation for the overall reaction between H2O2. I- and H+ (2)

Answer 17

H2O2 + 2H+ + 2I- -> 2H2O

Question 18

How could the intial rate of methanal formation be measured from a concentration time graph

Answer 18

calculating the gradient when t=0

Question 19

O3 +C2H4->2CH2O+1/2O2
the intial methanal formation is different from that of oxygen formation
explain why (1)

Answer 19

ration of second equation is 1:1
2 mol of 2CH2O forms for every 1/2 mol of O2 so ratios are different

Question 20

This experiment was repeated but at a higher temoerature what would be the effect of this change on the rate and the rate constant of the reaction (2)

Answer 20

the rate of the reaction and rate constant would increase
as there directly proportional

Question 21

Suggest why oxygen free radicals are often called diradicals

Answer 21

• 2 unparied electrons

Question 22

What is meant by the half life of a reaction (1)

Answer 22

time for the concentration of a reactant to fall by half of it’s original concentration

Question 23

How can you tell if concentration is first order by half life (1)

Answer 23

if the half life is constant

Question 24

Suggest the role of H+ in the mechanism Explain your reason (2)

Answer 24

• it is used as a catalyst
• and not used up in the actual reaction

Question 25

what is meant by the term rate of reaction (1)

Answer 25

the change in concentration of a rectant or product per unit of time