redox + electrode potentials

Question 1

what are the half equations for MnO4- and Fe2+

Answer 1

MnO4- + 8H+ + 5e-&raquo_space; Mn2+ + 4H2O
- Mn is reduced

Fe2+&raquo_space; Fe3+ + e-
- Fe is oxidised

Question 2

what is the full equation for the redox titration reaction between MnO4- and Fe2+

Answer 2

MnO4- + 5Fe2+ + 8H+&raquo_space; 5Fe3+ + Mn2+ + 4H2O

Question 3

what colour change is seen in redox titrations involving manganate (VII)

Answer 3

a natural colour change of pink&raquo_space; colourless is seen
- this titration is self indicating, as MnO4- = pink and everything on the RHS is colourless
when the solution turns permanently pink/colourless this indicates there is no more Fe2+/MnO4- to react with

Question 4

what are the half equations for iodine I2 and thiosulfate S2O2 2-

Answer 4

I2 + 2e-&raquo_space; 2I-
- iodine is reduced

2S2O3 2-&raquo_space; S4O6 2- + 2e-
- thiosulfate is oxidised

Question 5

what is the full equation for the redox titration reaction between iodine I2 and thiosulfate S2O2 2-

Answer 5

2S2O3 2- + I2&raquo_space; 2I- + S4O6 2-

Question 6

what indicator is used in an iodine/thiosulfate redox titration + why

Answer 6

iodine is naturally brown-yellow, as the titration occurs this colour fades to yellow then to colourless - as the end point is approached (solution is the colour of hay) starch is added, turning the solution blue-black, which makes the end point more easily observable
at the end point the blue-black colour will have disappeared and the solution will be colourless

Question 7

oxidising agent definition

Answer 7

a substance that accepts electrons from another species in a redox reaction, becoming reduced itself and oxidising the donor species

Question 8

reducing agent definition

Answer 8

a substance that donate electrons to another species in a redox reaction, being oxidised itself and reducing the receiving species

Question 9

oxidised definition

Answer 9

the loss of electrons

Question 10

reduced definition

Answer 10

the gain of electrons

Question 11

how do you find the correct number and placement of electrons when constructing half equations

Answer 11

use oxidation numbers, moles and ion charges to find the total for both sides of the equation, then add electrons on either side to balance them
e.g. +8-1 on LHS and +2 on RHS
= +7 on LHS and +2 on RHS
difference is 5 so place 5e- on LHS to balance 2 sides (as +7-5=+2)

Question 12

what is a trick for finding the number of water molecules made in a half equation

Answer 12

same as the number of oxygens available, and then add missing hydrogens as H+ ions - should be double the oxygens

Question 13

what is a source of H+ ions

Answer 13

acid

Question 14

standard electrode potential definition

Answer 14

the tendency to be reduced and gain electrons, relative to a hydrogen H2(g) half cell

Question 15

how can you tell which half cell will be reduced and which one will be oxidised

Answer 15

the half cell with the more + electrode potential will be reduced

Question 16

what 5 features need to be included in a diagram of 2 half cells + extra features if it is the diagram for the determination of standard electrode potential vs H2 half cell

Answer 16

• the 2 half equations happening at each electrode
• labels for the 1 moldm-3 solutions and solid material of the electrodes
• the voltmeter
• the salt bridge
• an indication of which is the positive and negative half cell
• the standard conditions of 298K + 100kPa
• the specific apparatus of a hydrogen half cell

Question 17

what is the standard electrode potential of a hydrogen half cell

Answer 17

0V

Question 18

how do you calculate whole cell electrode potentials
(E cell)

Answer 18

E anode - E cathode

Question 19

what are the names of positive and negative electrodes

Answer 19

+ = cathode
- = anode

Question 20

what is the purpose of a salt bridge

Answer 20

allows ions from electrolyte to travel between the 2 solutions, acting as charge carriers, completing the circuit and allowing half cells to remain electrically neutral

the salt bridge contains salt solution e.g. KNO3

Question 21

at which electrodes do reduction + oxidation take place

Answer 21

reduction occurs at the cathode (red cat)
oxidation occurs at the anode

Question 22

what are 3 limitations of predicting reactions using E values

Answer 22

• reaction rate - predictions for feasibility ΔG give no indication of Ea or reaction rate
• concentrations - E is measured with 1 moldm-3 solutions, if using different concs E will be different
• conditions - if standard conditions aren’t used then E will be different

Question 23

3 types of cells

Answer 23

• primary cells
• secondary cells
• fuel cells

Question 24

outline the function + uses of a primary cell

Answer 24

• they are non rechargeable/single use
• electrical energy is produced by oxidation and reduction at electrodes, these reactions cannot be reversed
• often used for low current, long storage devices e.g. wall clocks

Question 25

outline the function + uses of a secondary cell

Answer 25

• rechargeable
• cell reaction that produces electrical energy can be reversed by recharging, so chemicals are regenerated
• used for car batteries, cylindrical batteries, modern appliances

Question 26

outline the function + uses of a fuel cell

Answer 26

• do not need to be recharged
• electrical energy is generated from the reaction of a fuel with O2
• as long as fuel and O2 are available fuel cells can operate continually

Question 27

electrochemical cells in phones are based on Li+ ion transport - what are 2 risks of these if phone is damaged

Answer 27

• risk of fire
• Li+ ions are toxic

Question 28

2 reasons why methanol is a good fuel source

Answer 28

• it is a liquid so easy to transport, store and use
• doesn’t require extreme pressures or temps to store

Question 29

3 reasons why hydrogen is not often used as a fuel source

Answer 29

• as a gas it is difficult to store + transport – requires high pressures + low temps
• it is highly flammable
• not as energy efficient as other fuel sources