Electrode Potentials

Question 1

what happens when a rod of metal is dipped into a solution of its own ions

Answer 1

an equilibrium is set up between the solid metal and the aqueous metal ions

Question 2

what is the simplest salt bridge made of

Answer 2

filter paper soaked in saturated solution of KNO3 (potassium nitrate)

Question 3

why are salt bridges necessary

Answer 3

complete the circuit but avoid further metal/ion potentials as does not perform electrochemistry
allows ion movement to balance the charge
do not react with electrodes

Question 4

symbol for salt bridge in standard notation

Answer 4

II

Question 5

what type of species goes on the outside in standard cell notation

Answer 5

most reduced

Question 6

what does I indicate in standard cell notation

Answer 6

phase boundary

Question 7

what happens at left hand electrode

Answer 7

oxidation
half cell with most negative E value

Question 8

what happens at right hand electrode

Answer 8

reduction
half cell with less negative E value

Question 9

conditions for standard hydrogen electrode

Answer 9

298 K
100 kPa
[H+] = 1.00 moldm-3

Question 10

what is the standard hydrogen electrode used for

Answer 10

comparing other cells against
E of SHE is defined as 0, so all other E values are compared against it

Question 11

why might you use other standard electrodes occasionally

Answer 11

cheaper/easier/quicker to use and can provide just as good as reference
platinum is expensive

Question 12

if E value is more negative what does it mean in terms of oxidising/reducing power

Answer 12

better reducing agent

Question 13

if E value is more positive what does it mean in terms of oxidising/reducing power

Answer 13

better oxidising agent

Question 14

what factors change E value

Answer 14

concentration of ions
temperature

Question 15

what happens if you reduce the concentration of the ions in the left hand half cell

Answer 15

equilibrium moves to the left to oppose the change of removing ions; releases more electrons, the E of the left hand cell becomes more negative, so e.m.f. of the cell increases

Question 16

how do you calculate the emf of a cell from E values

Answer 16

Ecell = Eright - Eleft
aka less negative - more negative

Question 17

when would you use platinum electrode

Answer 17

when both the oxidised and reduced forms of the metal are in aqueous solution

Question 18

why is platinum chosen

Answer 18

inert
good conductor to complete circuit

Question 19

how would you predict if a reaction would occur

Answer 19

take 2 half equations
find species that is being reduced
calculate its E value minus E value of oxidised
if E value > 0 reaction will occur

Question 20

what was the first commercial cell made from (Daniell cell)

Answer 20

zinc / copper (II)

Question 21

what are zinc/carbon cells more commonly known as

Answer 21

disposable batteries

Question 22

what are two reactions that take place in zinc/carbon cells

Answer 22

Zn oxidised to Zn 2+
NH4 + reduced to NH3 at carbon electrode

Question 23

reactions that occur in a lead/acid battery

Answer 23

Pb + SO4 2- –> PbSO4 + 2e-
PbO2 + 4H+ + SO4 2- –> PbSO4 + 2H2O

Question 24

how are cells rechargeable

Answer 24

reactions are reversible and are reversed by running a higher voltage through the cell than the cells E value

Question 25

nickel/cadmium cells are rechargeable AA batteries etc. what reactions occur at electrodes

Answer 25

Cd(OH)2 (s) + 2e- --> Cd (s) + 2OH- NiO(OH) (s) + H2O + e- --> Ni(OH)2 (s) + OH-

Question 26

where are lithium-ion cells used

Answer 26

mobile phones laptops

Question 27

reactions occur in lithium-ion cells

Answer 27

Li+ + CoO2 + e- --> Li+[CoO2]- Li --> Li+ + e-

Question 28

what is a fuel cell

Answer 28

cell used to generate electric current does not require electrical recharging

Question 29

what are the reactions that take place at the two electrodes in an alkaline hydrogen fuel cell

Answer 29

2H2 + 4OH- --> 4H2O + 4e- O2 + 2H2 + 4e- --> 4OH-

Question 30

why is it better to use a fuel cell than to burn H2 in air

Answer 30

in combustion, sulfur containing compounds and nitrogen containing compounds are produced due to high temperatures and the S and N in air. These are bad for the environment. this does not occur in a fuel cell; only product is water. more efficient

Question 31

disadvantages of fuel cells

Answer 31

hydrogen is a flammable gas with a low boiling point --> hard and dangerous to store and transport --> expensive to buy fuel cells have a limited lifetime and use toxic chemicals in their manufacture

Question 32

how do you find the weakest reducing agent from a table of electrode potential data

Answer 32

most positive E value then it is the PRODUCT of the reduction equation i.e. imagine equation going from right to left

Question 33

what is the reason that some cells cannot be recharged

Answer 33

reaction of the cell is not reversible - a product is produced that either dissipates or cannot be converted back into their reactants

Question 34

why might the e.m.f. of a cell change after a period of time

Answer 34

concentrations of the ions change - the reagents are used up

Question 35

how can the e.m.f. of a cell be kept constant

Answer 35

reagents are supplied constantly, so the concentration of ions are constant, E remains constant