redox 2 p1

Question 1

what is oxidation?

Answer 1

loss of electrons

Question 2

what is reduction?

Answer 2

the gain of electrons

Question 3

what is oxidation in terms of oxidation number?

Answer 3

oxidation is an increase of oxidation number

Question 4

what is reduction in terms of oxidation number?

Answer 4

reduction is a decrease in oxidation number

Question 5

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

Answer 5

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

Question 6

what are the two methods to measure standard electrode potential (E^o)?

Answer 6

1. metals or non-metals in contact with their ions in aqueous solution
2. ions of the same element with different oxidation numbers

Question 7

write a half-equation for zinc (s) to zinc (II)

Answer 7

Zn(s) ⇌ Zn2+(aq) + 2e-

Question 8

what is the simplest salt bridge made of?

Answer 8

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

Question 9

why are salt bridges necessary?

Answer 9

they complete the circuit allowing a flow of charge through ion movement. they also contain inert ions so they don’t react with the electrodes

Question 10

what symbol is used to represent a salt bridge in standard notation?

Answer 10

||

Question 10

what type of species goes on the outside (furthest from the salt bridge) in standard cell notation?

Answer 10

the most reduced species

Question 11

what does | indicate in standard cell notation?

Answer 11

phase boundary (solid/liquid/gas)

Question 12

how would an aluminium/copper cell be represented using standard cell representation?

Answer 12

Al(s)|Al3+(aq)||Cu2+(aq)|Cu(s)

Question 13

what happens at the left hand electrode?

Answer 13

oxidation occurs
the left hand electrode is the half cell with the most negative Eo value

Question 14

what happens at the right hand electrode?

Answer 14

reduction occurs
the right hand electrode is the half cell with the most positive E^o value

Question 15

which side of the cell has the most negative Eo value and what happens to the metal with the most negative Eo value?

Answer 15

the left hand electrode is most negative
the metal will be oxidised

Question 16

what conditions are required to use the standard hydrogen electrode?

Answer 16

temp - 298K
pressure - 100kPa
[H+] = 1.00 mol dm-3

Question 17

what is the standard hydrogen electrode used for?

Answer 17

used as a standard for comparing other cells against
Eo of SHE is defined as 0 so all other Eo values are compared to it

Question 18

why might you occasionally use other standard electrodes?

Answer 18

they are cheaper, easier and quicker to use and can provide just as good a reference
platinum electrodes of the SHE are expensive

Question 19

if an Eo value is more negative, what does it mean in terms of oxidising/reducing power?

Answer 19

it means it is a better reducing agent (i.e. easier to oxidise)

Question 20

if an Eo value is more positive, what does it mean in terms of oxidising/reducing power?

Answer 20

it means it is a better oxidising agent (i.e. easier to reduce)

Question 21

what factors change Eo values/

Answer 21

concentration of ions
temperature

Question 22

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

Answer 22

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

Question 23

how do you calculate the emf of a cell from Eo values?

Answer 23

Ecell = Ered(right) - Eox(left)

Question 24

when would you use a Platinum electrode?

Answer 24

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

Question 25

why is Platinum used?

Answer 25

it is inert so does not take part in the electrochemistry it is a good conductor that completes the circuit

Question 26

how would you predict if a reaction would occur?

Answer 26

1. take the 2 half equations 2. identify the species that is being reduced (this effectively the right hand electrode) 3. calculate its Eo value minus the Eo value of the species that is being oxidised (effectively the left hand cell) 4. if Eo cell > 0 a reaction will occur

Question 27

what was the first commercial cell made from (Daniel cell)?

Answer 27

zinc/copper (II)

Question 28

what are zinc/carbon cells more commonly known as?

Answer 28

disposable batteries

Question 29

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

Answer 29

Zn is oxidised to Zn2+ NH4 + is reduced to NH3 at carbon electrode

Question 30

IGNORE what are the reactions that occur in a lead/acid battery (car batteries)?

Answer 30

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

Question 31

how are some cells able to be recharged?

Answer 31

if the cell reactions are reversible, they can be reversed by running a higher voltage through the cell than the cell's Eo

Question 32

IGNORE Nickel/cadmium cells are rechargeable AA batteries. What reactions occur at the electrodes?

Answer 32

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

Question 33

where are lithium ion cells commonly used?

Answer 33

mobile phones and laptops

Question 34

IGNORE what reactions occur on discharge in lithium ion cells?

Answer 34

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

Question 35

what is a fuel cell?

Answer 35

a cell that is used to generate electric current; it does not require electrical recharging

Question 36

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

Answer 36

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

Question 37

why is it better to use a fuel cell than to burn H2 in air, even though the same overall reaction occurs?

Answer 37

in combustion, sulfur 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; the only product is water. also more efficient

Question 38

disadvantages of fuel cells?

Answer 38

hydrogen is a flammable gas with a low b.p. meaning it is hard and dangerous to store and transport - also expensive to buy fuel cells have a limited lifetime and use toxic chemicals in their manufacture

Question 39

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

Answer 39

the weakest reducing agent will have the most positive E value the species is the product of the reduction equation i.e. when the equation goes from right to left

Question 40

why can some cells not be recharged?

Answer 40

when the reaction of the cell is not reversible the product produced either dissipates or cannot be converted back into the reactants

Question 41

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

Answer 41

the concentrations of the ions change as the reagents are used up

Question 42

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

Answer 42

if the reagents are supplied constantly, the concentrations of the ions will be constant meaning E remains constant