Redox Chemistry

Question 1

What are oxidising and reducing agents?

Answer 1

Substance reduced is oxidising agent

Question 2

Titration of potassium manganate seven with iron two ions

Answer 2

KMnO4 in burrette
Acidified FeSO4 in conical
Solution in the conical flask remains colourless until all Iron two ions oxidised
Addition turns solution pink

Question 3

Titration of potassium manganate seven with ethanedioic acid

Answer 3

Reaction is very slow because it’s catalysed by manganese ions which aren’t present at the beginning
Heated up to 60°C to speed up reaction
Autocatalysis- as Mn ions increase reaction speeds up

Question 4

What is the equation for the reaction between sodium thiosulphate and iodine solution?

Answer 4

2(S2O3)2- + I2 –> (S4O6)2- + 2I-

Question 5

Titration of sodium thiosulfate and iodine solution

Answer 5

Endpoint when changes from yellow – brown to colourless
Difficult to tell so a small amount of starch is added which reacts with free iodine to form a deep blue black colour
It’s the start is added to whirly, the iodine is absorbed strongly into the starch which reduces accuracy

Question 6

How does bleach work?

Answer 6

Contains sodium chlorate I Which acts as an oxidising agent in acidic solution
ClO- + 2H+ +2e- –> Cl- + H2O
Effectiveness depends on ability to oxidise coloured compounds in stains

Question 7

How can the concentration of bleach be determined?

Answer 7

Reacted with excess iodide in acidic solution
ClO- + 2H+ + 2I- –> I2 + Cl- + H20
Determine the quantity of iodine produced by back titrating with sodium thiosulfate
Work backwards to determine the [ClO-]

Question 8

Titration of copper two ions

Answer 8

Add Excess KI to known solution of copper to ions
Titrate the iodine solution with Na2S2O3 to find [I-]
Work backwards to find the concentration of copper two ions

Use to find a percentage of copper in an alloy

Question 9

What can electric potential to be used for?

Answer 9

To determine if a reaction will take place between two systems, and which direction it will go in

Question 10

What is an electrochemical cell?

Answer 10

A reaction can be separated into two half cells which can be used to harness energy through the flow of electrons between the half cells

Question 11

How can the potential difference of half cells be shown?

Answer 11

Lamp can be replaced with a volt meter to show electromotive force (e.m.f.)

Question 12

What happens in an electrochemical cell between zinc and copper?

Answer 12

As copper ions reduced, The copper electrode slowly decreases
Flow of electrons can be shown by lamp

Question 13

What is half cell notation?

Answer 13

Zn(s) | Zn2+ (aq) // Cu2+ (aq) | Cu (s) E=+1.10V
Dashed line represents salt bridge
Most negative on the left

Question 14

What does a positive e.m.f. show?

Answer 14

A change which occurs spontaneously
A negative value may occur in the other direction
Strong oxidising agent

Question 15

What does a more positive electrode potential tell you?

Answer 15

The greater its tendency to attract electrons

Question 16

What is a reference half cell used for?

Answer 16

Impossible to find electrode potentials for individual half-cells
Used to compare against
Assigned electrode potential of OV under standard conditions of 1moldm-3

Question 17

What is the oxidation number of an element?

Answer 17

The charges they would have if the electrons in each bond of the molecule or ion the belonged to the more electronegative element

Question 18

What is the standard hydrogen electrode?

Answer 18

Hydrogen gas at 1atm pressure and 298K bubbling around a platinum electrode in 1.00moldm-3 H+ ions
2H+(aq, 1moldm-3) +2e- H2(298K, 1tm) E=0V
Platinum electrode coated with finely divided platinum to act as catalyst

Question 19

How must reactions which uses inert electrodes be displayed?

Answer 19

2H+(aq) , H2 (g) | Pt(s)

Least oxidised species is next to electrode

Question 20

What is the convention for redox reactions for half-cells? What does this mean?

Answer 20

Species with higher oxidation number must appear on left

Ecell =E(RH electrode) - E(LH electrode)

Question 21

What is the e.m.f. independent of? What must you do when you reverse an equation?

Answer 21

Number of electrons being transferred

Reverse the sign of E standard

Question 22

How can you tell if a reaction is feasible and positive from Estandard cell?

Answer 22

X> +0.6V completion

0

Question 23

How does Ecell relate to entropy?

Answer 23

Directly proportional to total entropy change

More positive value, more energetically favourable

Question 24

What limitations do standard electrode potentials have?

Answer 24

Concerned with energetic stability, not kinetic stability so gives no indication of visible change
Reaction might not be under standard conditions

Question 25

How does concentration affect electrode potentials?

Answer 25

Increasing conc Cu2+ move equilibrium to right, reducing compounds and removing electrons from the system
Makes electrode potentials more positive

Question 26

What is a hydrogen fuel cell?

Answer 26

Two electrodes separated by membrane which allows H+ to pass through but not H2 and O2
Electrons released at hydrogen electrode are used up at oxygen electrode
Potential difference set up pushes the electrons through external circuit where they do work

Question 27

What are the half-equations for a hydrogen fuel cell?

Answer 27

2H2(g) –> 4H+(aq) + 4e-

02(g) + 4H+(aq) + 4e- –> 2H20 (l)

Question 28

How can methanol be made?

Answer 28

Non-renewable fossil fuels or agricultural waste such as straw and biomass
Chemical recycling of CO2

Question 29

What advantages and disadvantages does it have compared to hydrogen?

Answer 29

Easier to store (no high pressures or low temps)
Gives off more energy that highly compressed H2
Efficiency is low as alcohol can pass through proton exchange membrane

Question 30

What happens at the anode and cathode in methanol-based fuel cells?

Answer 30

Anode: H2O + CH3OH –> CO2
Cathode: O2 –> H2O

Question 31

Ethanol-based fuel cells

Answer 31

Ethanol can be used instead of methanol as it is less toxic and more easily produced
Currently, an expensive platinum catalyst is used
Polymer effectively acts as electrolyte

Question 32

What reaction do dichromate 6 breath analysers use?

Answer 32

3C2H5OH(g) + 2(Cr2O7)2-(aq) + 16H+ (aq) –> 3CH3COOH(aq) + 4Cr3+(aq) + 11H2O(l)
Ethanol in breath oxidised to ethnic acid which reduced orange dichromate to green
Second tube for comparison
Electric current causes needle in meter to move

Question 33

How do fuel-cell breath analysers work?

Answer 33

Breath flows along one side of fuel cell and any alcohol is reduced to ethanoic acid, H+ and electrons
H+ move through lower part of fuel cell and combine with oxygen and electrons on other side to form water
More ethanol is oxidised, the larger the electric current is

Question 34

Why are inferred breath analysers used?

Answer 34

Both dichromate and fuel-cell analysers are preliminary testing so further analysis must be made
Used to measure concentration
Gives very accurate results

Question 35

What are the problems with breath analysers?

Answer 35

Preliminary analysers can’t be used in court and must be calibrated regularly
Test the air breathed out so alcohol may come from mouth, throat or stomach
Often officers have to wait to give another test