Redox and electrode potentials

Question 1

What is reduction?

Answer 1

Gain of electrons.
Decrease in oxidation number.

Question 2

What is oxidation?

Answer 2

Loss of electrons.
Increase in oxidation number.

Question 3

What is a reducing agent?

Answer 3

It adds electrons to the species that is being reduced.
It contains the species that is oxidised.

Question 4

What is an oxidising agent?

Answer 4

It takes electrons from the species being oxidised.
The oxidising agent contains the species that is reduced.

Question 5

How do you write a redox equation from half-equations?

Answer 5

Balance the electrons by multiplying to get a lowest common multiple.
Add and cancel the electrons.
Cancel any species that are on both sides of the equation.

Question 6

How do you use oxidation numbers to write equations?

Answer 6

Assign oxidation numbers to all the atoms and calculate the change in oxidation number.
Balance the species that contain the elements that have changed oxidation number.
Then balance any remaining atoms - can use H+, OH- and H2O to balance.

Question 7

What is an example of using oxidation numbers to write equations?

Answer 7

S + HNO3 –> H2SO4 + NO2 + H2O
S has increased by +6.
N has decreased by -1.
So N needs balancing so that it decreases by 6.
S + 6HNO3 –> H2SO4 + 6NO2 + 2H2O
Water has been balanced too.

Question 8

How do you predict the products of redox reactions?

Answer 8

In aqueous redox reactions, H2O is often formed, H+ and OH- as well depending on conditions.

Question 9

What are manganate titrations?

Answer 9

Manganate (VII) titrations can be used for analysis of different reducing agents. E.g.
Iron (II) ions.
Ethanedioic acid (COOH)2

Question 10

What is the basic method of manganate (VII) titrations?

Answer 10

KMnO4 is added to the burette.
Using a pipette, the solution being analysed is added to the conical flask.
Excess H2SO4 is added to provide H+ ions for reduction of MnO4- ions.
The solution is decolourised as MnO4- is added, the end point is when there is a permanent pink colour.
The burette readings are from the top, not bottom of meniscus, because of the dark colour.

Question 11

How do you do redox titration calculations?

Answer 11

Write out the balanced redox equation.
Work out the moles of the MnO4- reacted, then the other species, using the equation to see how to multiply it.
If a 25cm^3 pipette used, then times by 10 to see how much in 250cm^3.

Question 12

What are iodine/thiosulfate titrations redox equations?

Answer 12

2S2O3^2- –> S4O6^2- + 2e-
I2 + 2e- –> 2I-
overall: 2S2O3^2- + I2 –> 2I- + S4O6^2-

Question 13

How is iodine thiosulfate used in analysis of oxidising agents?

Answer 13

Add Na2S2O3 to the burette.
Using a pipette, add the testing solution to the conical flask, and an excess of potassium iodide.
The oxidising agent reacts with I- to produce I2, which is a yellow-brown colour.

Question 14

How is starch used in iodine thiosulfate titrations?

Answer 14

Iodine is reduced to black iodide ions, which fades gradually, so the end point is hard to see.
A starch indicator is added when the iodine has faded to a pale yellow (near the end point).
A deep blue colour forms and shows the end point when it turns colourless.

Question 15

What are examples of iodine thiosulfate titrations?

Answer 15

Analysis of oxidising agents:
Chlorate (I) ions, ClO-
Copper (II) ions, Cu2+
These oxidise I- to I2.

Question 16

What is the analysis of copper?

Answer 16

For copper (II) salts, Cu2+ ions are produced by dissolving the compound in water.
Insoluble copper 2 compounds can be reacted with acids to form Cu2+ ions.
For alloys - bronze or brass, the alloy is reacted and dissolved in conc HNO3, then neutralisation, to form Cu2+ ions.

Question 17

What is a voltaic cell?

Answer 17

A type of electrochemical cell that converts chemical energy into electrical energy.
This takes place in modern cells and batteries.
It involves movement of electrons.

Question 18

What are metal|metal ion half-cells?

Answer 18

Consists of a metal rod dipped into a solution of its aqueous metal ion, shown as a line between the solution and metal.
E.g. Zn2+(aq)|Zn(s)

Question 19

What are half cells?

Answer 19

The equilibrium in a half cell is written so the forward reaction shows reduction.
In an isolated half cell, there is no net transfer of electrons.
When two half cells are connected, the direction of electron flow depends on the relative tendency of each electrode to release electrons.

Question 20

What is an ion|ion half cell?

Answer 20

It contains ions of the same element in different oxidation states.
E.g. a half cell containing a mixture of Fe3+ and Fe2+ ions.
Fe3+ (aq) + e- –> Fe2+ (aq)
An inert metal electrode made out of platinum is used.

Question 21

What does an electrochemical cell look like?

Answer 21

Remember:
The concentrations of the aqueous solution (1 mol dm-3)
Liquid lines.
Salt bridge.
Direction of electron flow.
To label the positive and negative half cells.

Question 22

What is standard electrode potential?

Answer 22

Eθ is the e.m.f. of a half cell connected to a standard hydrogen half cell under standard conditions of 298k, solution concentrations of 1 mol dm^-3, and 100kPa.
A standard hydrogen electrode is 0v.

Question 23

What is the tendency of electrodes?

Answer 23

In a cell with two metal/metal ion half cells connected, the more reactive metal releases electrons more readily and is oxidised, this is the negative electrode.
The electrode with the less reactive metal gains electrons and is reduced, this is the positive electrode.

Question 24

What does the hydrogen half cell look like?

Answer 24

Glass tube with holes in to allow H2 bubbles to escape.
Acid solution 1 mol dm^-3 H+.
Pt electrode.
H2(g) at 298k and 100kPa.

Question 25

What is a salt bridge?

Answer 25

It connects the two solutions by allowing ions to flow.
It contains a concentrated solution of an electrolyte that does not react with either solution.
E.g. a strip of filter paper soaked in aqueous potassium nitrate.

Question 26

What do the Eθ values mean?

Answer 26

The more negative the value, the greater the tendency to lose electrons and undergo oxidation.
The more positive the value, the greater the tendency to gain electrons and undergo reduction.

Question 27

What do the Eθ values mean for metals?

Answer 27

Metals tend to have negative E values and lose electrons.
Non-metals tend to have positive E values and gain electrons.
The more negative the E value, the greater the reactivity of a metal in losing electrons.
The more positive, the greater the reactivity of a non-metal in gaining electrons.

Question 28

What is the order of the reactivity series?

Answer 28

Most reactive:
Please Potassium
Stop Sodium
Calling Calcium
Me Magnesium
A Aluminium
Careless Carbon
Zebra Zinc
Instead Iron
Try Tin
Learning Lead
How Hydrogen
Copper Copper
Saves Silver
Gold Gold

Question 29

How do you calculate Standard Cell Potential from standard electrode potential?

Answer 29

Eθcell = Eθ (positive electrode) - Eθ (negative electrode)
The positive electrode is the one being reduced, and will go right.

Question 30

What is predicting the feasibility of redox reactions?

Answer 30

If the standard cell potential is positive then the reaction should take place and is feasible.

Question 31

What are the limitations of predictions using Eθ values?

Answer 31

Reaction rate - Reactions with a large activation energy have a slow rate, so are not feasible.
The actual conditions may be different from the standard conditions used to calculate Eθ values.
The values apply to aqueous equilibria, but many reactions take place that are not aqueous.

Question 32

What are the limitations of predictions using Eθ values - concentration?

Answer 32

SEP are measured using 1 mol dm^-3 concentration.
If the concentration is different, the electrode potential will be different.
Zn2+ + 2e- –> Zn
If the conc of Zn2+ is increased, equilibrium will shift to the right, removing electrons and making the electrode potential less negative.

Question 33

What are primary cells?

Answer 33

Non-rechargable, single use only.
The reaction is non-reversible.
Eventually, the chemicals will be used up, and the cell discarded or recycled.
Mainly used in clocks and smoke detectors.

Question 34

What are primary cells made from?

Answer 34

Mainly alkaline based on zinc and manganese dioxide Zn|MnO2, and a potassium hydroxide alkaline electrolyte.

Question 35

What are secondary cells?

Answer 35

Rechargeable and the reaction is reversible.
E.g. Lead-acid batteries in cars.
Nickel-cadmium (NiCd) cells and nickel-metal hydride (NiMH), used in radios and torches etc.
Lithium ion and lithium-ion polymer cells used in laptops, tablets, cameras, and electric cars.

Question 36

What are the risk and benefits of Li based cells?

Answer 36

Lithium is a light metal, which means a very high energy density for lithium-ion batteries.
Can be a regular shape or flexible to fit around other components.
But they can become unstable at high temperatures, and can ignite.
Li is very reactive, so needs care in recycling.

Question 37

What are fuel cells?

Answer 37

Fuel and oxygen flow into the fuel cell and the products flow out.
They can operate continuously provided that fuel and oxygen are supplied into the cell.
They do not need recycling.
Hydrogen is the most common fuel, they don’t produce CO2 in combustion.

Question 38

What is an acid hydrogen fuel cell?

Answer 38

Hydrogen gas enters at the anode and forms H+ ions, the electrons travel to the cathode.
O2 gas enters at the cathode, reacts with H+ ions and electrons to form water.
Overall: H2(g) + 1/2 O2 –> H2O(l)

Question 39

What is an alkali hydrogen fuel cell?

Answer 39

Oxygen gas enters at the cathode, reacts with water to form OH- ions, travel to the anode and react with hydrogen ions to form water.
Overall: 1/2O2 + H2 –> H2O