Electrode potentials

Question 1

Explain the function of a salt bridge

Answer 1

The ions in the ionic substance in the salt bridge move through the salt bridge

To maintain charge balance / complete the circuit

Question 2

The aluminium used as the electrode is rubbed with sandpaper prior to use, suggest a reason for this

Answer 2

to remove the oxide layer on the aluminium

Question 3

A simple salt bridge can be prepared by dipping a piece of filter paper into potassium carbonate solution. explain why such a salt bridge would not be suitable for use in this cell

Answer 3

the carbonate ion would react with the acid in the SHe between the carbonate and Al3+
so the cells emf would be altered conc of h+ and al3+ change

Question 4

give the meaning of the term reducing agent and oxidising agent

Answer 4

electron donor, loses electrons basically

electron acceptor

Question 5

suggest why potassium chloride would not be a suitable salt for the salt bridge in the cell, with copper as the second electrode

Answer 5

chloride ions would react with the copper ions, cu2+ or (cucl4)2- ions formed.

Question 6

in the external circuit of the cell, the electrons flow through the ammeter from the right to the left, suggest why the electrons move in this direction

Answer 6

the cu2+ ions in the left hand electrode are more concentrated so the reaction of cu2+ will occur at the left hand electrode.

Question 7

Explain why the current in the external circuit of the cell falls to 0 after the cell has operated for some time

Answer 7

eventually, the copper ions in each electrode will be at the same concentration

Question 8

suggest why the recharging of a lithium cell may lead to the release of co2 into the atmosphere

Answer 8

electricity for recharging the cell may have come from power stations burning fossil fuels.

Question 9

solar cells generate an electric current from sunlight. these cells are often used to provide electrical energy for illuminated road signs. explain why rechargeable cells are connected to these solar cells

Answer 9

solar cells do not supply electrical energy all the time

they can store electrical energy for use when solar cells arent working.

Question 10

when asked for conditions of electrode potentials reacs

Answer 10

always 298k and 100kpa

Question 11

use the fact that ag reaction= 0.44v and fe as -1.22 and explain why the redox reactions that occur when iron powder is added to an excess of aqueuous silver nitrate

Answer 11

E0 Ag> E0 Fe

So ag will oxidise iron into iron 2+ and then further oxidise that into iron 3+

Question 12

deduce one essenetial property of the non reactive porous seperator labelled in the diagram

Answer 12

allows ions to pass through

Question 13

suggest the function of the carbon rod in the cell

Answer 13

allows electrons to flow

Question 14

the zinc electrode acts as a container for the cell and is protected from external damage. suggest why a cell often leaks after being used for a long time

Answer 14

the zinc has been used up.

Question 15

one environmental advantage of a rechargeable cell compared to a non

Answer 15

metal compounds re used

Question 16

suggest why ethanol can be considered to be a carbon neutral fuel

Answer 16

co2 released by combustion is equal to the co2 taken up by photosynthesis.

Question 17

give one reason why the emf of the lead acid cell changes to stationary after several hours

Answer 17

the reagents are used up

Question 18

what type of cell has a graph where the conc of reagents are constant and the voltage remains constant and why

Answer 18

fuel cell
because
reagents or fuel is supplied continuously so the concs of reagents remain constant

Question 19

WHEN COMBINING OVERALL HALF EQUATIONS

Answer 19

make sure you add everything even the h’s just noe electrons if they cancel

and make sure you flip the oxidation one bc their always written in the reduced form

Question 20

when identifying an oxidising or reducing agent

Answer 20

remember a good oxidising agent is a good reducing agent which means its the most positive one. the acc thing is on the left bc reducing agents on right.

if there is a mole ratio before it eg 2 nioh2 dont put that in as the oxidisng agent its just nioh2 also if oh is - o is 2- that means the ni will be 3+

Question 21

use data from the table to explain why dilute hcl cannot be used to acidify potassium managanate in sol

Answer 21

e 0 for mno4 is more positive than that for cl2

manganate would oxidise the cl-

Question 22

give two reasons why it is essential to avoid this reaction in a titration between potassium manganate 7 and iron 3 ions

Answer 22

cant see end point due to the brown colour

larger titre than expected

Question 23

potassium manganate is an oxidising agent suggest one reason why a 0.0200moldm-3 solution of potassium manganate does not need to be kept away from flammable material

Answer 23

because the solution is very dilute

Question 24

Why is the potential exactly 0.00 V under these conditions?

Answer 24

by definition

Question 25

In each case, state and explain if the value of the potential for this half equation would become more or less than 0.00
V, if it changes at all, if the change stated was made:
i) use hydrogen gas at 200 kPa
…………………………………………………………………………………………………………………………………………..

ii) use a piece of platinum with a greater surface area
…………………………………………………………………………………………………………………………………………..
…..
iii) use H2SO4(aq) with concentration 0.500 mol dm-3
…………………………………………………………………………………………………………………………………………
iv) use HCl(aq) with concentration 2.00 mol dm-3
…………………………………………………………………………………………………………………………………………..

v) use a temperature of 50°C

Answer 25

i) negative, equilibrium shifts left to side with less gas molecules to reduce pressure of H2
ii) no effect, allows for faster rate of transfer of electrons but has no effect on potential
iii) no effect, as concentration of H+ = 1.00 mol dm-3
iv) positive, equilibrium shifts right to lower concentration of H+
v) negative, equilibrium shifts left in endothermic direction to reduce temperature

Question 26

increasing the conc of the solutions used in the electrochem cell

Answer 26

makes the cells EMF more positive as fewer electrons are produced in the reaction.

Question 27

increasing the pressure of the cell

Answer 27

makes the cells emf more negative as more electrons are produced.

Question 28

when writing conventional cell representations

Answer 28

if theres like 2 cr3+ in the equation and 10h+ still add them but write them as cr3+ AND H+ dont put the numbers before them

Question 29

Pb2+(aq) + 2 e–
⇌ Pb(s) Eº = -0.13 V
b) If the same cell was set up under standard conditions except for the concentration of the Pb2+(aq) ions which would
be 0.500 mol dm-3
, how would the emf compare to part (a). Explain your answer

Answer 29

Eº (Pb2+/Pb) would become more negative as equilibrium shifts left to increases concentration of Pb2+ ions;
therefore emf would decrease and become less positive

Question 30

if electrode pot of Ag+/Ag is more than cu2+/cu then what happens what reaction occurs

Answer 30

ag+ oxidises copper so ag+ gains electorns from copper

Question 31

if electrode pot of mno4 > cl2 will it liberate cl2

Answer 31

yes as it will gain electrons from cl if electrode potential is higher

Question 32

What is the main advantage of a fuel cell over rechargeable and non-rechargeable cells.

Answer 32

does not go flat; does not need re-charging

Question 33

suggest one reason why water is not used between the reac of lithium and something

Answer 33

as lithium reacts with water to form lithium hydroxide

Question 34

requirement for a salt bridge

Answer 34

must not react with the electrolyte or ions in the solution

Question 35

when using electrode potential data to explain why smth oxidises something

Answer 35

say bc it has a MORE POS E0 value and refer to specific values.

Question 36

you are provided with a daniell cell that was set up using copper sulfate and it was allowed to produce elec till the conc of copper 2 ions had decreased to 0.50mol dm-3. you are also provided with a zinc electrode that you have to calculate the decrease of mass of. how would you carry out an experiment to confirm your answer of b for mass of zinc lost

Answer 36

Allow the cell to react until the conc of copper sulfate is 0.05moldm-3
then confirm this using a colorimeter
weigh the zinc electrode before and after the experiment to calculate the decrease in mass

Question 37

why kno3

Answer 37

inert
doesnt react with electrolyte or solution in elec
allows ions to move

Question 38

when writing a conventional cell notation with fe2(so4)3 and feso4 given

Answer 38

write them as Fe3+, Fe2+ higher charge first make sure you sep them into the ions

Question 39

2fe3+ +h2–> 2fe2+ +2h+

explain why the ammeter reading would fall to 0 after some time

Answer 39

bc the Fe3+ ions would be used up

Question 40

what if your writing a cell notation with two solid things like ag2o(s) and Ag (s)

Answer 40

on the side on the right to the salt bridge

put // Ag2O, Ag

Question 41

3 reasons why the redox reac between chlorine and water does not normally occur in the abscence of light

Answer 41

high ea
light provides the ea
light breaks the cl-cl bond

Question 42

use a half eq to show that h202–> 2h20 is a redox reac

Answer 42

oxygen is -1 in h2o2

changes to oxygen as -2 in h2o

Question 43

when your writing half equations

conventional representation of a glucose oxygen fuel cell

Answer 43

dont forget the electrons

include all the reactants and products

c/c6h12o6,H+/co2//o2/H+,H2o/Pt
reaction species that have been oxidised or are about to be reduced go next to the salt bridge. Oxidation states go next to the salt bridge. The reason why H+ ions aren’t next to the salt bridge is since they aren’t about to be reduced, whereas the O2 is about to be reduced

Question 44

There are two ways to use hydrogen as a fuel for cars. One way is in a fuel cell to
power an electric motor, the other is as a fuel in an internal combustion engine.
Suggest the major advantage of using the fuel cell.

Answer 44

a fuel cell converts more of the available energy from the combustion of hydrogen into kinetic energy of the car.
internal combustion engines waste more heat energy