electrochemical & electrolytic cells

Question 1

dichromate reduction half equation

Answer 1

Cr2O7(2-) + 14H(+) +6e –> 2Cr(3+) +7H2O

Question 2

permanganate reduction half equation

Answer 2

MnO4(-) + 8H(+) + 5e –> Mn(2+) + 4H2O

Question 3

direct redox reaction

Answer 3

reactants mixed together in a single beaker: both oxidation and reduction occur in the same vessel (usually results in release of thermal energy)

Question 4

indirect redox reaction

Answer 4

two reactants in different containers and ox and red occur simultaneously

Question 5

explain that there is no spontaneous reaction between a reductant and an oxidant

Answer 5

(reductant) is a weaker reductant than the conjugate reductant of (oxidant) and hence no spontaneous reaction will be observed
(oxidant) is a weaker oxidant than the conjugate oxidant of (reductant) and hence no spontaneous reaction will be observed

Question 6

oxidation

Answer 6

loss of e-
always at anode
reductant undergoes oxidation
increase in oxidation number

Question 7

reduction

Answer 7

gain of e-
always at cathode
oxidant undergoes reduction
decrease in oxidation number

Question 8

rules for determining oxidation number

Answer 8

1. atom in elemental form =0
2. simple ion = charge of ion
3. Hydrogen is +1 unless in metal hydrides (eg NaH or CaH2) and it is -1
4. oxygen is usually -2 except when in peroxide compounds (eg. H2O2, baO2) = -1 and when bound to flourine = +2
5. neutral compound must add to 0
6. polyatomic ion must equal charge of that ion
7. in covalent compounds (without oxygen/hydrogen)more electronegative element has neg ox number. this is equal to charge of that ion

Question 9

galvanic cell

Answer 9

electrochemical cell in which chemical energy from spontaneous redox reactions is converted into electrical energy

Question 10

anode

Answer 10

negatively charged electrode in a galvanic cell where oxidation occurs

Question 11

cathode

Answer 11

positively charged electrode in a galvanic cell where reduction occurs

Question 12

salt bridge

Answer 12

connection which allows the flow of ions between two half cells to maintain electrical neutrality of the cell

Question 13

standard half cell

Answer 13

cell consisting of both members of the conjugate redox pair and an electrode at standard conditions

Question 14

electrode

Answer 14

electrical conductor that is either unreactive or involved in a reaction

Question 15

standard electrode potential

Answer 15

potential of the half reaction (reduction), given in volts, relative to the standard hydrogen electrode

Question 16

structural ft of galvanic cells

Answer 16

two half cells (allows chem energy -> electrical energy)
salt bridge prevents polarisation and completes circuit
each half cell contains conjugate redox pairs
electrons flow through external circuit

Question 17

electric potential calculation

Answer 17

E overall = E(oxidant) - E(reductant)

E overall = E(cathode) - E(anode)

Question 18

if sodium chloride forms in a reaction it is a…

Answer 18

a precipitate

Question 19

explain why iron is protected from rusting when blocks of Mg or Zn are attached to it

Answer 19

Because Zn(s) and Mg(s) will preferentially oxidise when exposed to oxygen because they are stronger reductants than Fe(s), then electrons transfer from the Mg and Zn and protect the Fe from degradation. (anodic protection)

Question 20

fuel cell

Answer 20

a special type of galvanic cell where there is a continuous supply of reactants allowing for the continuous conversion of chemical energy to electrical energy

Question 21

general features of a fuel cell

Answer 21

O2 inlet at cathode (+)
fuel inlet at anode (-)
porous electrodes for large SA:V and faster rate of reaction
net reaction is a combustion reaction
electrolyte can be alkaline or acidic

Question 22

adv of fuel cells

Answer 22

high energy conversion efficiency
low chemical pollution (H2 based fuels particularly)
fuel flexibility
quiet
continuous supply of electricity

Question 23

disadvantage of fuel cells

Answer 23

Distribution storage and transport of hydrogen can be difficult (highly flammable and needs high pressure)
Manufacturing process and materials can be expensive
need reliable and continual supply of fuel

Question 24

primary cells

Answer 24

non-rechargeable electrochemical cells in which a chemical reaction generates electrical energy

Question 25

secondary cells

Answer 25

rechargeable electrochemical cells in which a chemical reaction generates electrical energy

Question 26

recharge

Answer 26

conversion of electrical energy into chemical energy through redox reactions with the aid of an external power source

Question 27

discharge

Answer 27

conversion of chemical energy to electrical energy as a result of spontaneous redox reactions

Question 28

batteries

Answer 28

devices consisting of cells that convert chemical energy to electrical energy

Question 29

factors influencing cell selection

Answer 29

initial & operating cost
size, shape & weight
voltage provided
shelf life

Question 30

for recharge to occur…

Answer 30

products of the discharge reactions must stay in contact with the electrodes

Question 31

polarity of electrodes when recharging

Answer 31

anode = postive
cathode = negative