5.2.3 Redox and electrode potentials

Question 1

Define oxidising agent

Answer 1

accepts electrons from species being oxidised and is reduced

Question 2

Define reducing agent

Answer 2

donates electrons to species being reduced and is oxidised

Question 3

What are the steps for constructing a redox equation?

Answer 3

1. Write half equations (work out oxidation numbers and electrons, ensure charges balance)
2. Then add H2O to balance oxygens
3. Add H+ to balance hydrogens
4. Multiply each half equation to ensure there are equal numbers of electrons in each one
5. Add equations together and cancel electrons

Question 4

Example: Construct redox equation for reaction where MnO4- is reduced to Mn2+ and C2O4 2- is oxidised to CO2.

Answer 4

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

C2O42- -> 2CO2 + 2e-

= 2MnO4- + 16H+ +5C2O4 2- -> 2Mn2+ 8H2O + 10CO2

Question 5

What can manganate titrations be used for? How do they work? What is the endpoint?

Goes from —– to —— if Mn2+ is in the burette

Goes from —- to —- if MnO4- is in the burette

Answer 5

• iron (II) ions
• ethanedioic acid (COOH)2
• end point.= permanent pink colour
• MnO4- (purple) turns into Mn2+ (colourless).
• SELF INDICATING
• colourless to PALE PINK

Question 6

What acid must be used for manganate titrations? WHy?

Answer 6

DILUTE SULFURIC ACID

• Insufficient volume or weak acid = MnO2 produced instead
• HCl = Cl2 produced and greater manganate volume needed
• HNO3 = oxidising agent (we need reducing agent) and smaller volume of manganate will be used

Question 7

What can iodine/thiosulfate titrations be used for? What colour changes occur?

Answer 7

• S2O3- oxidised
• I2 reduced
• can be used for ClO- or Cu2+ ions

aqueous iodine appears yellow brown
during titration, iodine reduced back to I- ions and brown colour fades gradually.

Question 8

What indicator can be used for iodine/thiosulfate titrations?

Answer 8

STARCH

as end point approached and a pale straw colour is seen, add it = deep blue/black
as more sodium thiosulfate added, the colour fades -> at endpoint = colourless.

Question 9

How can you prepare Cu2+ ions to then analyse in iodine/thiosulfate titrations?

Answer 9

• soluble salts: dissolve in water
• insoluble salts: react with acids
• alloys: reacted and dissolved in conc HNO3, then neutralisation

add to iodine - brown mixture

Question 10

What do electrochemical cells do?

Answer 10

convert chemical energy to electrical energy

based on redox reactions

Question 11

How can you create an overall current and avoid heat energy?

Answer 11

two half cells interact to create overall current (chemicals must never mix though, since flow wouldn’t be controlled and it’d be heat energy instead)

Question 12

What is a metal/metal ion half cell?

Answer 12

• piece of metal dipped into solution of metal ions
• reactive metals tend to form ions, more unreactive tend to have positive charge on the metal.

Question 13

What is an ion/ion half cell?

Answer 13

• solution containing ions of same element in diff oxidation states
• platinum inert electrode used

Question 14

What is used when you combine the two half cells?

Answer 14

• filter paper soaked in aq concentrated ionic compound in order to balance charges = SALT BRIDGE

Question 15

How else do you represent electrochemical cells?

Answer 15

R | O | | O | R

Question 16

Define standard electrode potential

Answer 16

e.m.f of a half cell compared with a hydrogen half cell, under standard conditions. Hydrogen half cell = 0V

Question 17

Any half cell with a negative electrode potential has a greater tendency to…

Any half cell with a positive electrode potential has a greater tendency to..

Answer 17

undergo OXIDATION e.g. magnesium

undergo REDUCTION e.g. silver

Question 18

How do you measure a standard electrode? How do you calculate the electrode potential of an electrochemical cell?

Answer 18

• set up the half cell against a standard hydrogen one and see voltage recorded
• E(positive) - E(negative/less positive)

Question 19

Student modifies experiment to determine a combined conc of iron(II) and iron (III) ions in river water. They add excess zinc to 250dcm^3 sample of river water and warm gently. Why? E of Zinc is -0.76, E of iron (III) ions is +0.77.

Answer 19

To ensure all the iron (III) ions in the riverwater are reduced to iron (II) ions. Zinc is the reducing agent since it has a more negative E than Fe3+.

Question 20

Student modifies experiment to determine a combined conc of iron(II) and iron (III) ions in river water. They remove the excess zinc they added in step 1 by filtration. Why? E of Zinc is -0.76, E of iron (III) ions is +0.77. MnO4- E is +1.51.

Answer 20

• If unfiltered, MnO4- would oxidise zinc, since system 1 is more negative than 3. We do not want that affecting our titration results ( we do not want zinc ions in the solution)

Question 21

What are the limitations of electrode potential predictions?

Answer 21

• Rate: high activation energy might mean reaction effectively doesn’t proceed due to very slow rate
• Concentration: if not 1M, this will affect eqm and the electrode potential
• Other factors: if other conditions aren’t standard, and electrode potentials only apply to aq eqm and so many reactions that cant take place in solutions cant have values.

Question 22

What are primary cells?

Answer 22

non rechargeable, single use

chemicals will be used up, voltage will fall, cell will be recycled or discarded.

low current, long storage devices e.g. smoke detectors

most alkaline based on Zn/MnO2 and KOH electrolyte

Question 23

What are secondary cells?

Answer 23

rechargeable.

e.g. lead-acid car batteries, NiMH in radios, lithium-ion polymer cells in laptops and cameras

Question 24

What are fuel cells?

Answer 24

energy from fuel and oxygen used to create voltage

fuel and o2 flows into cell and products flow out
dont have to be recharged

Question 25

Describe features of hydrogen fuel cells

Answer 25

no CO2 produced

alkali/acid electrolyte (but produce same cell voltages for either)

Question 26

State one important feature of a fuel cell that is different from a conventional storage cell

Answer 26

Fuel reacts with oxygen to give electrical energy