redox and electrode potentials

Question 1

what is an oxidation number?

Answer 1

a measure of the number of electrons that an atom uses to bond with atoms of another element

Question 2

what is reduction?

Answer 2

gain of electrons
decrease in oxidation number

Question 3

what is oxidation?

Answer 3

loss of electrons
increase in oxidation number

Question 4

what is an oxidising agent?

Answer 4

a reagent that oxidises (takes electrons from) another species
it is reduced itself

Question 5

what is a reducing agent?

Answer 5

a reagent that reduces (adds electrons to) another species
it is oxidised itself

Question 5

steps for constructing redox equation

Answer 5

1 - identify reactants and products
2 - balance the elements which are oxidised or reduced and identify their change in oxidation number
3 - balance the change in oxidation number: total increase = total decrease
4 - check if any other element needs balancing

Question 6

manganate (VII) titration colour change

Answer 6

purple MnO4- ions are reduced to very pale pink Mn2+ ions

Question 7

iodine-thiosulfate titration equation

Answer 7

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

Question 8

what indicator is used for the iodine-thiosulfate titration?

Answer 8

add starch when solution is orange-brown coloured
the solution turns blue-black
as the thiosulfate is added, the solution starts to turn to straw colour
when the blue-black colour disappears, the reaction is complete

Question 9

significance of standard electrode potential, Eθ, values

Answer 9

more positive Eθ value shows greater tendency to gain electrons and undergo reduction
more negative Eθ value shows greater tendency to lose electrons and undergo oxidation

Question 10

what direction are the equations in the electrochemical series read in?

Answer 10

equations with more positive Eθ values undergo reduction and are read in the forward direction

Question 11

what are standard conditions?

Answer 11

298K temperature
101kPa pressure
1.00moldm-3 concentration

Question 12

what metal is used as the electrode in aqueous solutions?

Answer 12

Pt
because it is inert and is an electrical conductor

Question 13

what is the standard electrode potential?

Answer 13

the e.m.f. of a half cell compared with a standard hydrogen half cell, measured at 298K, with solution
concentrations of 1mol dm-3 and a gas pressure of 101kPa

Question 14

set up of an electrochemical cell

Answer 14

the half cells must be connected by a salt bridge
the cell potential is measured in volts from a voltmeter

Question 15

how is standard cell potential calculated?

Answer 15

Eθcell = Eθreduction - Eθoxidation
value must always be positive

Question 16

types of storage cells

Answer 16

primary (non-rechargeable)
secondary (rechargeable)

Question 17

what is a fuel cell?

Answer 17

a fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage

Question 18

redox equation for the hydrogen-oxygen fuel cell

Answer 18

H2 (g) + 1⁄2 O2 (g) ->H2O (l)

Question 19

how are species classified in the electrochemical series?

Answer 19

species at the top of the series are the strongest oxidising agents