Redox

Question 1

<p>Equation for reduction of MnO4- in acidic conditions and colour changes</p>

Answer 1

<p>MnO4- + 8H+ + 5e- --> Mn2+ +4H2O

| Purple to very pale pink/almost colourless</p>

Question 2

<p>MnO4- reaction with Fe2+</p>

Answer 2

<p>MnO4- + Fe2+ + 8H+ --> Mn2+ +4H2O + 5Fe3+</p>

Question 3

<p>How to carry out titration with Manganate and iron ions </p>

Answer 3

<p>Solution containing Fe2+ is acidified with sulphuric acid. (HCl would react with manganate ions). Titrate like normal, end point is when excess MnO4- ions are present and this is when there is the first hint of permanent pink colour </p>

Question 4

<p>Reaction of iodine and thiosulfate ions </p>

Answer 4

<p>2S2O32- + I2 -> 2I- + S4O62-</p>

Question 5

<p>Titration using iodine/thiosulfate</p>

Answer 5

<p>Without an indicator, pale yellow colour of iodine disappears, leaving colourless solution. Starch is added near end point, a dark blue solution will form which will then disappear</p>

Question 6

<p>Estimating the copper content of solutions and alloys

| Reaction</p>

Answer 6

<p>If a solution containing Cu2+ ions is mixed with I- a light brown/yellow solution and white precip (which appears light brown due to iodine in solution) are formed.
2I- -> I2 + 2e-
Cu2+ + e- -> Cu+
2Cu2+ + 4I- -> 2CuI + I2</p>

Then titrated against a known conc. of sodium thiosulfate

Question 7

<p>What is the basis of all cells and batteries? </p>

Answer 7

<p>Controlled electron transfer in redox reaction, producing electrical energy</p>

Question 8

<p>What is a copper half cell compromised of? (plus equation)</p>

Answer 8

<p>A solution containing Cu2+ ions into which a strip of copper metal is placed
Cu2+ + 2e- Cu</p>

Question 9

<p>How is electrochemical cell made?</p>

Answer 9

<p>Through combining two half cells with different electrode potentials where one releases electrons and the other gains them</p>

Question 10

<p>Breathalyzer relies on ethanol fuel cell in which ethanol is oxidised by oxygen to form ethanoic acid and water. a) Write balanced equation for fuel cell equation</p>

Answer 10

<p>CH3CH2OH + O2 --> CH3COOH + H2O</p>

Question 11

<p>b) ethanol/ethanoic acid redox system has electrode potential +0.06 V and oxygen + H+/H20 system has electrode potential +1.23 V. Why does current produced increase with ethanol concentration?</p>

Answer 11

<p>Ethanol equilibrium shifts to left/more negative electrode
More electrons are released so current produced is greater</p>

Question 12

Potassium manganite (VII) preparation through two-step synthesis starting from manganese (IV) oxide. In step 2, equilibrium is set up:
3MnO42- + 2H2O 2MnO42- + MnO2 + 4OH-
How is equilibrium position shifted when carbon dioxide gas is bubbled through the mixture?

Answer 12

CO2 reacts with H2O to form carbonic acid,
H2CO3 + OH- –> H2O + HCO3-
Equilibrium shifts to right to restore OH-

Question 13

Purpose of wire in electrochemical cell

Answer 13

Electrons are transferred between the half cells through the wire

Question 14

Purpose of salt bridge in electrochemical cell

Answer 14

Allows ions to be transferred between the half cells, connects the two solutions

Question 15

Diagram of copper and zinc cell

Answer 15

Solution of Cu2+ 1 mol dm^3
Copper strip
Wire and voltmeter
Salt bridge
Zinc strip
Solution of Zn2+ 1 mol dm^3
298 K

Question 16

Non metal/non metal ion half cell

Answer 16

No electrode to connect the wire to (because gas??) so a platinum electrode is placed in the solution so that it is in contact with both the aqueous ions and the gas coming through.

Question 17

Metal ion/ metal ions half cell, e.g Fe2+/Fe3+

Answer 17

Solution with equal concentrations of the two ions. Platinum electrode.

Question 18

E~ Standard electrode potential of a half cell definition

Answer 18

The e.m.f of a half cell compared with a standard hydrogen half cell, measured under standard conditions (must be stated)

Question 19

What is e.m.f?

Answer 19

Electromotive force, the voltage produced by a cell when no current flows

Question 20

How are standard electrode potentials measured?

Answer 20

A standard half cell for thing being measured is connected to a standard hydrogen half cell.

Question 21

What is standard cell potential and how is it calculated?

Answer 21

The e.m.f between the two half cells making up the cell under standard conditions (state)
Standard electrode potential at positive terminal - standard electrode potential at negative terminal

Question 22

What is the cell reaction?

Answer 22

The overall chemical reaction taking place in the cell, the sum of the oxidation and reduction half equations taking place in each cell

Question 23

What does sign of electrode potential tell us?

Answer 23

If it is negative, the half cell is more likely to give up electrons than the standard hydrogen half cell and vice versa

Question 24

Standard electrode potentials of oxidising and reduction agents

Answer 24

Oxidising agents: gain electrons and cause other species to lose them, they have more positive standard electrode potentials
Reducing agents: lose electrons so they have more negative standard electrode potentials

Question 25

Criteria for a feasible reaction

Answer 25

Half equation forming oxidising agent must have a more positive standard redox potential than the half equation for the reducing agent

Question 26

Limitations of predicting feasibility using standard electrode potentials

Answer 26

• Non-standard conditions alter the value of an electrode potential, (equilibrium is shifted)
• Activations energy may be very high so rate of reaction may be very slow
  If the standard cell potential is less than 0.4 V a reaction is unlikely to take place

Question 27

How do electrochemical cells produce electricity?

Answer 27

My means of a redox reaction, including two redox equations with different electrode potentials

Question 28

Characteristics of non-rechargeable cells

Answer 28

Provide electricity until the chemicals have reacted to such an extent that the voltage falls. The cell is then flat and discarded (high charge density)

Question 29

Characteristics of rechargeable cells

Answer 29

Chemicals in the cells react, providing electrical energy, the cell reaction can be reversed during recharging when the chemicals in the cell are regenerated and the cell can be used again. (Examples: Ni-Cad batteries, lithium ion and lithium polymer batteries)

Question 30

Characteristics of fuel cells

Answer 30

The cell reaction uses external supplies of a fuel and an oxidant, which are consumed and need to be provided continuously. The cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant. It is clean

Question 31

Advantages of fuel cells using hydrogen rich fuels

Answer 31

Methanol: liquid fuel is easier to store than hydrogen gas. Methanol can be generated from biomass
Less pollution and less CO2
Greater efficiency

Question 32

Storage of hydrogen, pros and cons

Answer 32

Can be stored as liquid under low pressure but a very low temp. is required and liquid hydrogen will need to be stored in giant flask to prevent boiling
Can be adsorbed onto the surface of a solid material in a similar way that a catalyst is able to hold gases to its surface
Can be absorbed within some solid materials

Question 33

Limitations of hydrogen fuel cells

Answer 33

Large scale storage and transportation of hydrogen poses problems
Feasibility of storing a pressurized liquid
Current adsorbers and absorbers have a limited life time
Use toxic chemicals in the production
Have limited lifetimes

Question 34

How is hydrogen produced?

Answer 34

Electrolysis of water or reacting methane with system

Question 35

Characteristics of ethanol fuel cell

Answer 35

Oxygen reacts at one end as the oxidant, produces water

Ethanol at other electrode, produces CO2 and H+ s and e- s