5.2.3 - Redox And Electrode Potentials

Question 1

What is an oxidising agent?

Answer 1

Causes something to be oxidised but is in itself reduced.

Question 2

What is a reducing agent?

Answer 2

Causes something to be reduced but is in itself oxidised.

Question 3

When assigning oxidation numbers what is first?

Answer 3

Elements and then metals.

Question 4

What is the order of preference for non-metals?

Answer 4

Hydrogen, fluorine, oxygen, chlorine, bromine, iodine.

Question 5

When balancing half equations what is used in acidic conditions?

Answer 5

H+ and H2O

Question 6

In alkali conditions what is used to balance equations?

Answer 6

OH- and H2O

Question 7

What is the overall equation of manganate and iron?

Answer 7

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

Question 8

What colour are manganate ions in aqueous potassium manganate?

Answer 8

Purple

Question 9

How do you calculate error on a pipette?

Answer 9

(0.06/25) x 100

Question 10

How do you calculate the error on a burette?

Answer 10

(0.05x2/titre) x 100

Question 11

What are the rows down on a titration table?

Answer 11

Final reading (cm3), initial reading (cm3), volume used (cm3)

Question 12

What are the columns across on a titration table?

Answer 12

Rough, 1, 2, 3

Question 13

What dp are titration values recorded to?

Answer 13

2dp ending in 5 or 0

Question 14

What are concordant results?

Answer 14

Results within 0.1 of one another.

Question 15

What dp is the average titre recorded to?

Answer 15

1dp

Question 16

What is the standard electrode potential of a half cell?

Answer 16

The emf of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentration 1moldm-3 and a gas pressure of 1atm.

Question 17

Describe a metal/metal ion half cell?

Answer 17

Metal dipping into a solution of metal ions. (the same metal)

Question 18

Describe a metal ion / metal ion half cell?

Answer 18

Platinum electrode dipping into solution of both metal ions.

Question 19

Describe a non metal gas / non metal ions half cell?

Answer 19

Gas enters through container which leads to platinum electrode. All in non-metal ions in solution.

Question 20

What connects the two cells?

Answer 20

High resistance voltmeter.

Question 21

Why is a high resistance voltmeter used?

Answer 21

So no current can flow.

Question 22

What connects the two solutions?

Answer 22

Salt bridge.

Question 23

What does the salt bridge do?

Answer 23

Completes the circuit, allowing ions to flow but not electrons.

Question 24

What is the salt bridge made of?

Answer 24

A strip of filter paper soaked in potassium nitrate solution. KNO3 is used as it does not react with any of the half cell solutions.

Question 25

When 2 cells are joined together which is there negative terminal and what does this mean?

Answer 25

The one with the more negative electropotential value, the reaction goes backwards, releasing more electrons.

Question 26

When 2 cells are joined together which is the positive terminal, and what does that mean?

Answer 26

The one with the more positive electropotential value, the reaction goes forwards gaining electrons.

Question 27

How do you calculate the the cell potential of 2 half cells?

Answer 27

Ecell = most positive - most negative

Question 28

When calculating the overall equation which half equation do you flip?

Answer 28

The one with the more negative electropotential value.

Question 29

Which way do the electrons flow in the cell?

Answer 29

From the negative terminal to positive terminal.

Question 30

Why does I- react with MnO4-?

Answer 30

I2/I- reaction has the more negative E value so this equilibrium will shift to the left, releasing electrons. MnO4/Mn2+ reaction has the more positive E value so this equilibrium will move to the right and gain electrons.

Question 31

What are the limitations of using the electrochemical series to predict reactions?

Answer 31

• They are based on values under standard conditions.

- They predict equilibrium position not rate which may be too slow because of high activation energy.

Question 32

What are the 3 types of electrochemical cells?

Answer 32

Non-rechargable, rechargeable and fuel cells.

Question 33

What is a fuel cell?

Answer 33

A fuel cell uses the energy from the reaction of a fuel (H2 or hydrogen rich fuels such as methanol) with an oxidant (O2) to create a voltage.

Question 34

How does a fuel cell work?

Answer 34

The reactant flows in and the product flows out while the electrolyte remains in the cell so they can operate continuously as long as there is a supply of fuel and oxidant.

Question 35

What are the two half equations for the hydrogen-oxygen fuel cell?

Answer 35

2H2O + 2e- H2 + 2OH-

1/2O2 + H2O + 2e- 2OH-

Question 36

What is the overall equation for the hydrogen-oxygen fuel cell?

Answer 36

H2 + 1/2O2 -> H2O

Question 37

What are the benefits of the fuel cell?

Answer 37

Works continuously as long as there is a constant supply of oxidant and fuel. Doesn’t have to be recharged.

Question 38

What are the risks of fuel cells?

Answer 38

Toxic if ingested (e.g lithium cells). Rapid discharge can cause fires/explosions.

Question 39

What are the disadvantages of fuel cells?

Answer 39

A lot of energy is needed to make fuel cells. Difficulty in disposing of them. Making hydrogen uses electricity and storing H2 under pressure as liquid is flammable and explosive.