Topic 9: Redox Processes

Question 1

oxidation

Answer 1

• loss of electrons
• increase in oxidation no
• gain of oxygen
• loss of hydrogen

Question 2

reduction

Answer 2

• gain of electrons
• decrease in oxidation no
• loss of oxygen
• gain of hydrogen

Question 3

oxidation state

Answer 3

apparent charge of an atom in a molecule or ion.

• used to measure electron control or possession relative to the atom in pure element

Question 4

relation between reducing power and reactivity in metals

Answer 4

more reactive metals are stronger reductants

Question 5

relation between oxidizing power and reactivity in non-metals

Answer 5

more reactive non-metals are stronger oxidizing agents

Question 6

redox titration

Answer 6

redox reaction between an oxidizing agent and reducing agent

Question 7

chemically speaking, what happens in a redox titration?

Answer 7

electrons are transferred from reductant to oxidant

Question 8

redox titrations: analysis of iron with manganate (VII)

Answer 8

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

• uses KMnO4 in acidic soln. as oxidant
• oxidises Fe (II) ions to Fe (III)
• MnO4- is reduced to Mn2+
• colour change: deep purple to colourless

Question 9

redox titrations: iodine-thiosulfate reaction

Answer 9

2I- (aq) + oxidant -> I2 (aq) + reduced product

• oxidant reacts with excess iodides to form iodine diatomic molecules
• oxidants can be KMnO4, KIO3, K2Cr2O7, NaOCl, etc.

2S2O3 2- (aq) + I2 (aq) -> 2I- (aq) + S4O6 2- (aq)

• the liberated iodine is then titrated with sodium thiosulfate (Na2S2O3)
• using starch as an indicator (NOT added at the start but during titration
• initially forms deep blue colour due to starch, but as I2 is reduced to I-, the blue colour disappears

Question 10

redox titrations: winkler method (what it is, its function, etc)

Answer 10

• calculates dissolved oxygen content of water
• used to measure degree of pollution
• as oxygen is used by bacteria in decomposition reactions

Question 11

biological oxygen demand (BOD)

Answer 11

• amount of oxygen used to decompose organic matter in a sample of water over a specified time period
• usually 5 days at a specified temp
• high BOD = high quantity of degradable organic waste = low level of dissolved oxygen

Question 12

redox titrations: winkler method (process)

Answer 12

1. Dissolved oxygen O2 (g) is fixed by the addition of a manganese (II) salt, such as MnSO4. This causes oxidation of Mn (II) to higher oxidation states:
   2Mn2+ (aq) + O2 (g) + 4OH- (aq) -> 2MnO2 (s) + 2H2O (l)
2. Acidified iodide ions (I-) are added and are oxidised by Mn (IV) to I2:
   MnO2 (s) + 2I- (aq) + 4H+ (aq) -> Mn2+ (aq) + I2 (aq) + 2H2O (l)
3. Iodine produced is titrated with sodium thiosulfate:
   2S2O3 2- (aq) + I2 (aq) -> 2I- (aq) + S4O6 2- (aq)

Question 13

redox titrations: winkler method (ratio of O2 : S2O3 2-)

Answer 13

1:4

Question 14

voltaic cells

Answer 14

• generates electricity from spontaneous redox reactions
• separates two half-reactions into half-cells, allowing e-s to flow between them through an external circuit

2 connected half-cells = 1 voltaic cell

Question 15

electrode potential

Answer 15

charge separation between the metal and its ions in solution

Question 16

half cell

Answer 16

• where a half-reaction occurs

- simplest one is made by putting a strip of metal into a solution of its ions

Question 17

relationship between cell equilibrium and reactivity (of metal)

Answer 17

• more reactive metals are stronger reducing agents
• they have a higher tendency to lose electrons
• the less reactive the metal, the more to the right the equilibrium position will be

Question 18

voltaic cell connections between half-cells

Answer 18

• external electronic circuit

- salt bridge

Question 19

external electronic circuit

Answer 19

• connected to the metal electrode in each half-cell
• can have a voltmeter attached to record generated voltage
• electrons flow from anode to cathode through wire

Question 20

salt bridge

Answer 20

• glass tube/strip of absorptive paper containing an aq soln of ions
• movement of these ions neutralise build-up of charge and maintains potential difference
• anions move in salt bridge from cathode to anode, which opposes the flow of e-s in external circuit
• cations move in salt bridge from anode to cathode
• solution chosen is typically aq NaNO3 or KNO3

Question 21

what is affected by the difference in reducing strength between electrodes in a voltaic cell?

Answer 21

• direction of electron flow (e-s always flow to the electrode with less reducing power)
• voltage generated (the greater the reducing power difference, the greater the voltage generated)

Question 22

electromotive force

Answer 22

• potential difference
• generated due to e- flow between half-cells
• depends on difference of reducing power
• also called cell potential/electrode potential

Question 23

standard hydrogen electrode

Answer 23

reference standard for measuring reducing power of half-cells

Question 24

standard hydrogen electrode

Answer 24

• modified form of pH electrode
• platinum is used as the conducting metal in the electrode
• as platinum is inert and doesn’t ionise
• it also acts as a catalyst for proton reduction reaction
• the form of platinum used is platinized platinum (the surface of the metal is covered with very finely divided platinum, or platinum black)
• this causes the electrode reaction to occur rapidly
• due to the large surface area helping in the adsorption of hydrogen gas

Question 25

standard conditions for measurement of standard electrode potential

Answer 25

- concentration: 1 mol/dm3 - pressure: 100 kPa - only pure substances - temp: 298 K - platinum is used as electrode if the half-cell doesn't include a solid metal

Question 26

standard electrode potential

Answer 26

- denoted by Eϴ - potential of a redox system to lose/gain electrons when connected to the SHE (assigned a value of 0 volts) via a salt bridge and high-resistance voltmeter

Question 27

standard reduction potential

Answer 27

- half-cells exist as equilibria - they can occur as either oxidation or reduction - std. electrode potential is always given for the reduction reaction - so standard electrode potential values are sometimes also called standard reduction potentials - they don't depend on total no. of electrons so they're not affected by stoichiometry - the more positive the Eϴ value, the more readily it is reduced

Question 28

determining spontaneity using Eϴ (cell)

Answer 28

- if Eϴ (cell) = positive, the rxn is spontaneous and the reverse is non-spontaneous - a voltaic cell always runs in the direction that gives a positive Eϴ (cell) value

Question 29

electrolytic cell

Answer 29

- uses external electrical energy source to bring about a redox rxn that would otherwise be non-spontaneous - redox rxns occur at electrodes - oxidation at anode and reduction at cathode - e- flow from anode to cathode

Question 30

determining products in electrolytic cells

Answer 30

1. Identify all ions in electrolyte and determine which will migrate to which electrode 2. Write the half-equation for the rxn at each electrode 3. Balance e-s lost and gained and add the two half-equations 4. Consider what changes would be observed as a result of the redox processes (e.g. colour change, precipitation of solid, gas discharge, pH change...)

Question 31

electrolysis of molten salt

Answer 31

- only 1 ion migrates to each electrode | - as ionic compounds have v high m.pts, another compound might be added to lower m.pt

Question 32

electrolysis of aq. soln.s

Answer 32

- cathode: H2O can be reduced to H2 - anode: H2O can be oxidised to O2 - difficult to predict as there is more than 1 redox rxn possible for each electrode

Question 33

selective discharge

Answer 33

the discharge of an ion at the electrode in cases where there is more than 1 possible redox rxn

Question 34

how to predict the products of electrolysis of aq. soln.s

Answer 34

- Eϴ values of the ions - relative conc.s of ions in electrolyte - nature of electrode