5.2.3 redox and electrode potentials

Question 1

Reduction half equation

Answer 1

Br2 (aq) + 2e- > +2 Br - (aq)

electrons on left

electron acceptor, oxidising agent

Question 2

Oxidation half equation

Answer 2

2I- (aq) > I2 (aq) + 2e-

electrons on right

electron donor, reducing agent

Question 3

Thiosulfate redox titration

Answer 3

Redox titration between I2 and S2O3^2-

Yellow/brown solution > colourless

Starch indicator added near the end > from blue/black > colourless

Question 4

Manganate redox titration

Answer 4

Redox titration between Fe2+ and MnO4 -

Purple > colourless

MnO4-(aq) + 8H+ (aq) + 5Fe2+ (aq) > Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)

Question 5

why does a voltage form

Answer 5

more negative electron has more of a tendency to oxidise and release electrons compared to less negative

e.g. Zn > Zn2+ + 2e-

more electrons build up on more negative electrode

potential difference created between 2 electrodes

negative strip is negative terminal and less positive is positive terminal

Question 6

why use a high resistance voltmeter

Answer 6

stop the current from flowing in the circuit

in this state, it is possible to measure the maximum possible potential difference (E)

reactions will not be occurring as the very high resistance voltmeter stops the current from flowing

Question 7

salt bridge

Answer 7

allows free moving ions to conduct the charge

usually potassium nitrate

wire not used as the metal wire would set up its own electrode system with the solutions

Question 8

what happens if current is allowed to flow

Answer 8

reactions will then occur separately at each electrode

voltage falls to zero as reactants are used up

Question 9

measuring electrode potential of a cell

Answer 9

connect to another half-cell of known potential and PD between the 2 half-cells are measured

can assign a relative potential to each electrode by linking it to a reference electrode (hydrogen electrode) which is given a potential of O V

Question 10

standard electrode potential

Answer 10

potential difference measured when an electrode system is connected to the hydrogen electrode system, and standard conditions (1M, 100kPa, 298K) apply

Question 11

more negative half cell

Answer 11

oxidises

go backwards

Zn (s) > Zn2+ (aq) + 2e-

Question 12

less negative half cell

Answer 12

reduces

go forwards

Fe2+ (aq) + 2e- > Fe (s)

Question 13

Ecell

Answer 13

= Ered - Eox

more neg - less neg

Question 14

effect of concentration on cell e.m.f

Answer 14

increasing concentration of reactants > increase in EMF > side with fewer moles

Question 15

Effect of temperature on cell e.m.f

Answer 15

most Ecells are exothermic so increasing would decrease EMF

if positive, reaction might occur however reaction will not occur or will occur so slowly that it effectively doesn’t happen

if the reaction has a high activation energy, the reaction will not occur

Question 16

uncatalysed reaction between 2 negative ions

Answer 16

repulsion between ions results in high activation energy

making a very slow reaction

Question 17

cells

Answer 17

electrochemical cells can be used as source of electrical energy

are non-rechargeable when the reactions that occur within them are non-reversible

Question 18

hydrogen fuel cell

Answer 18

2H2 + O2 > 2H2O

maintain a constant voltage over time as they have a constant supply of O2 and H2 so maintain constant conc of reactants

Question 19

adv of fuel cells

Answer 19

less pollution and less CO2

greater efficiency

Question 20

limit of fuel cells

Answer 20

limited lifetime (regular replacement and disposal) and high production costs

use of toxic chemicals in their production