Redox

Question 1

Define oxidation number

Answer 1

The charge assigned to an element which represents the number of electrons lost or gained by an atom of the element (the charge it would have if the electrons in each bond belonged to the more electronegative element)

Question 2

What is an electrochemical cell?

Answer 2

A cell comprising of two half cells. Electron transfer occurs.

Question 3

What is the purpose of a salt bridge?

Answer 3

1. Completes the circuit - movement of ions (K+ to cathode, NO3- to anode)
2. Maintains the potential difference

Question 4

What is a standard electrode potential and what is meant by standard conditions?

Answer 4

The potential difference measured between a half cell and a standard hydrogen electrode.
Conditions: 298K, 1 atm and 1 moldm-3

Question 5

What is a reference cell used for?

Answer 5

A SHE is used to measure the electrode potential because a half cell will not conduct alone.
A platinum electrode is used - it is inert to H+ ions, porous (increases s.a) and adsorbs hydrogen gas on its surface.

Question 6

How do you calculate the Ecell for a reaction?

Answer 6

Ecell = E+ve (right) - E-ve (left)

Question 7

What is the relationship between Ecell, delta S total and the equilibrium constant?

Answer 7

Ecell is directly proportional to delta S total and lnK

Question 8

How can you tell if a reaction is feasible from its Ecell value?

Answer 8

If Ecell is positive (>0), the reaction is feasible.

If Ecell is > +0.3V the reaction will go to completion.

Question 9

Why might a reaction not be feasible?

Answer 9

1. If the conditions are not standard - temp or conc may affect position of equilibrium.
2. If the kinetics are not favorable - Ea is too high so rate of reaction is too slow.

Question 10

How can hydrogen fuel be made? (What can it be sourced from?)

Answer 10

1. Natural Gas: CH4 + 2H2O -> CO2 + 4H2

2. Electrolysis of water

Question 11

State the advantages and disadvantages of hydrogen as a fuel?

Answer 11

Advantages: Less pollution and CO2; 100% atom economy (only water produced); more efficient than direct combustion; no nitrogen oxides
Disadvantages: Difficult to store and transport; expensive to produce and liquefy; limited lifespan (constant replacement); safety issues (flammable)

Question 12

What are the sources of ethanol and methanol fuels?

Answer 12

1. Biomass - agricultural waste
2. Non-renewable fossil fuels
3. Chemical recycling of CO2

Question 13

State the advantages and disadvantages of alcohol fuel cells

Answer 13

Advantages: renewable and sustainable biomass; easier to store than hydrogen; already existing infrastructure; higher energy density than hydrogen
Disadvantages: CO2 released (greenhouse gas); production uses land for food crops; higher food prices and starvation; highly flammable and poisonous

Question 14

How does a dichromate breathalyser work?

Answer 14

Ethanol from breath reduces dichromate ions (and are oxidised to ethanoic acid); showing orange to green colour change.
3C2H5OH + 2Cr2O7 2- + 16H+ -> 3CH3COOH + 4Cr2+ +11H2O

Question 15

How does an ethanol fuel cell breathalyser work?

Answer 15

A fixed volume of breath is fed into the anode. The alcohol content is proportional to the current. The current causes the needle in the meter to move, then return to rest by an operator.

Question 16

How does an infrared breathalyser work?

Answer 16

IR absorption is passed through captured breath to detect the absorbance by the compound (ethanol). A line in fingerprint region for ethanol is used.

Question 17

Give an advantage and disadvantage of:

a) Dichromate breathalyser
b) Fuel cell breathalyser
c) Infrared breathalyser

Answer 17

a) adv: portable, disadv: unreliable, chromium species are carcinogenic, extent of going green is judgmental,
b) adv: used as preliminary breath testing, disav: cannot be used in court (too many sources of error!)
c) adv: can be used in court, disav: not portable

Question 18

In the analysis of iron tablets:

a) Why can potassium permanganate not be used in titrations with chloride ions? Why is it unstable?
b) Why is so much sulfuric acid used in the experiment?
c) Why is an indicator not needed?

Answer 18

a) 1. KMnO4 is not a primary standard - it is easily reduced by any other reagent so it’s concentration changes Solid MnO2 always forms so it must be precipitated out of solution.
2. It oxidizes Cl- ions, making the titration inaccurate.
b) 1. To suppress the hydrolysis of Fe2+ ions which would take place
2. To supply H+ ions for reaction
c) Titration is self-indicating

Question 19

What is the equation for the reaction of thiosulfate ions with iodine?

i) Why is KIO3 added to excess KI used?
ii) What would be the colour change without starch
iii) Why is sodium thiosulfate used?

Answer 19

2S2O3 2- + I2 (aq) -> S4O62- + 2I-

i) Iodine does not dissolve well in water
ii) Pale yellow to colourless
iii) It is a strong reducing agent that is used to help measure the time required for a set amount of reaction (prevents colour from appearing until enough I-)

Question 20

What is percentage error and how do you calculate the percentage error of a burette?

Answer 20

Error due to uncertainty in measurements (e.g. thermometer, iodine evaporating, starch added too late)
% error = (+/-)error / titre value x 100