14: Redox II

Question 1

Which electrode does reduction occur at?

Answer 1

Cathode
(small cat)

Question 2

Which electrode does oxidation occur at?

Answer 2

Anode
(An Ox)

Question 3

What is the positive electrode?

Answer 3

Anode

Question 4

What is the negative electrode?

Answer 4

Cathode

Question 5

What electrode does the more reactive metal become?

Answer 5

Anode
Gives up its electrons more readily to become oxidised

Question 6

What electrode does the less reactive metal become?

Answer 6

Cathode

Question 7

Which side does the half-cell being oxidised go?
(Anode)

Answer 7

Left

Question 8

What salts can be used in a salt bridge?

Answer 8

KCl , KNO3

Question 9

Standard electrode potential definition

Answer 9

the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode

Question 10

What are the standard conditions for half-cells?

Answer 10

Solutions of ions have a concentration of 1M
298K
100kPa

Question 11

When using the conventional method of drawing cells, which goes on the left?

Answer 11

The half-cell with the more negative potential
Hydrogen if used
ROOR

Question 12

More reactive metals have ____ standard electrode potentials?

Answer 12

More negative

Question 13

More reactive non-metals have ____ standard electrode potentials?

Answer 13

More positive

Question 14

If Ecell is positive?

Answer 14

The reaction is thermodynamically feasible

Question 15

What membrane is used in an alkaline hydrogen-oxygen fuel cell?
What can and cant pass through?

Answer 15

Anion-exchange membrane
Allows anions (OH-) and water to pass through but not hydrogen and oxygen gas

Question 16

What electrolyte is used in an alkaline hydrogen-oxygen exchange membrane?

Answer 16

Aqueous alkaline solution (KOH)

Question 17

What electrodes do the electrons flow from and to?

Answer 17

positive to negative

Question 18

What is fed in at the positive electrode (Anode) of an alkaline fuel cell?
What reaction occurs?

Answer 18

Hydrogen gas
2H2 + 4OH- —> 4H2O + 4e-

Question 19

What is fed in at the negative electrode (Cathode) of an alkaline fuel cell?
What reaction occurs?

Answer 19

Oxygen gas
O2 + H2O + 4e- —> 4OH-

Question 20

What membrane is used in an acidic hydrogen-oxygen fuel cell?
What can and cant pass?

Answer 20

Polymer electrolyte membrane (PEM)
Only allows H+
Doesn’t let e- through

Question 21

What is fed in at the anode of an acid fuel cell?
What reaction occurs?

Answer 21

Hydrogen gas
H2 —> 2H+ + 2e-

Question 22

What is fed in at the cathode of an acid fuel cell?
What reaction occurs?

Answer 22

Oxygen gas
O2 + 4H+ + 4e- —> 2H2O

Question 23

How do hydrogen-rich fuel cells work?

Answer 23

Methanol/ ethanol converted to hydrogen by a reformer
Oxidised at the anode in the presence of water
e.g. CH3OH + H2O —> 6H+ + CO2 + 6e-
H+ ions pass through the electrolyte and are oxidised to water

Question 24

Why is acid added to the conical flask in a redox titration?

Answer 24

To ensure that there are enough H+ for the oxidising agent to be reduced

Question 25

Where is the substance of unknown concentration added in a redox titration?

Answer 25

Conical flask

Question 26

What titrations are used to find the concentration of an oxidising agent?

Answer 26

Iodine-sodium thiosulfate titrations

Question 27

Step 1 in an iodine-sodium thiosulfate titration?

Answer 27

Use a sample of the oxidising agent to oxidise as much iodide as possible Add a known volume of the oxidising agent to an excess of potassium iodide (KI) - to oxidise some of the iodide ions to iodine

Question 28

Step 2 in an iodine-sodium thiosulfate titration?

Answer 28

Find out how many moles of iodine have been produced Titrate the solution from step 1 with sodium thiosulfate (NasS2O3) of know concentration - once the iodine colour fades to a pale yellow, add some starch solution and continue the titration until the blue-black colour disappears

Question 28

How to calculate the concentration of copper in an alloy?

Answer 28

Dissolve a weighted amount of the alloy in concentrated nitric acid Pour into a volumetric flask and make up to the mark with deionised water Pipette out a portion and add sodium carbonate solution until a precipitate forms Add ethanoic acid until the precipitate dissolves Add an excess of potassium iodide solution A white precipitate of Copper (I) Iodide forms Titrate the mixture against sodium thiosulfate solution

Question 29

Sources of error in iodine-sodium thiosulfate titrations?

Answer 29

Starch indicator must be added at the right point - when most of the iodine has reacted, or the blue-black colour will be very slow to disappear Starch solution needs to be freshly made Precipitate of copper (I) iodide makes seeing the colour change hard Iodine can evaporate - keep solution cool