2.Redox

Question 1

reduction half equation of acidified dicromate

Answer 1

14H⁺ +Cr₂O₇^2- +6e^- –> 2Cr³⁺ +7H₂O

Question 2

OILRIG

Answer 2

oxidation is loss (of e^-)

reduction is gain (of e^-)

Question 3

define disproportionation

Answer 3

The oxidation and reduction of the same element in a redox reaction.

Question 4

Define oxidizing agent

Answer 4

An oxidizing agent causes a different species to be oxidized
Common O.A , O₂ , acidified manganate, acidified dicromate, H₂O₂
In the reaction the O.A gets reduced.

Question 5

hints for completing redox equations

Answer 5

add H⁺ (acidic) , OH^- (alkaline) , water (as a product) to balance them

Question 6

set up for a metal/metal cell

Answer 6

Question 7

what does the salt bridge do?

Answer 7

connects the two solutions, allowing ions to be transfered.

Filter paper soacked in ionic compount that doesnt react with half cells

Question 8

whats the pd caused by

Answer 8

differing electrod potentials.

(different speed of electron exchange)

Question 9

Diagram of standard hydrogen half cell

Answer 9

Question 10

What way are the half equations written for cells?

Answer 10

as a reduction

Question 11

why is pt electrodes used

Answer 11

if the half cell has no solid electrode.

pt is inert and doesnt react

allows e- exchange via a platinum black surface

Question 12

Fe²⁺ /Fe³⁺ half cell diagram

Answer 12

Question 13

what is standard electrode potential?

Answer 13

standard electrode potential of a cell is the emf of a half cell compared with a standard hydrogen half cell. measures at 298K with 1 molar solutions and atm gasses.

Question 14

which half cell is chosen as the negative terminal

Answer 14

the one with a more negative electrode potential

as it shows it has a greater tendancy to lose electrons.

Question 15

which redox reaction (reduction) goes to the right

Answer 15

the more positive one (electrode potential)

Question 16

limitations of using standard electrode potentials

Answer 16

• the reaction might not happen under standard condiditons
• so the potentials will be different
• Reaction may not happen (if electrode potential diff is large its much more likely)

Question 17

3 kinds of cell/battery.

Answer 17

• Non-recharagable cells (reaction is irreversible)
• recharagable (reaction is reversable when charging)
• Fuel Cells (needs a supply of fuel and oxygen constantly)

Question 18

Reactions in an alkaline fuel cell

Answer 18

H₂+2OH^-<==>2H₂O+2e^- negative end

O₂+2H₂O+4e^-<==>4OH^-

Question 19

typical fuel cells?

Answer 19

hydrogen

hydrogen rich fuels eg methanol

Question 20

fuel cell setup

Answer 20

Question 21

equation for methanol to hydrogen gas for fuel cells.

Answer 21

CH₃OH+H₂O–>3H₂+CO₂

250-300 degrees

Question 22

advantages of using methanol as a fuel instea dof turning it to hydrogen gas

Answer 22

liquid fuel is easier to store

can be generated from biomass

Question 23

advantages of fuel cell veicles?

Answer 23

• less pollution and less CO₂ (far better then combustion)
• Greater efficency (petrol is 20% efficent, fuel cells 40-60%)

Question 24

methods to store hydrogen

Answer 24

• as a presurised liquid (needs to be 2 cold tho)
• adsorbed onto a solid surface surface
• Absorbed within some solids

Question 25

limitations of hydrogen fuel cells

Answer 25

• transport of H₂ is costly and needs infrastructure
• hard to store the fuel as a liquid
• adsorbers and absorbes of the gas have limited lifetime
• fuel cells need regular replacements
• fuel cells use toxic chemicals to produce.

Question 26

E_cell =

Answer 26

E_{+ve terminal} - E_{-ve terminal}