Topic 9: Redox Processes

Question 1

What is a redox reaction?

Answer 1

Involves reduction and oxidation.

Question 2

List three ways of describing redox reactions.

Answer 2

1) in terms of specific elements - hydrogen and oxygen
2) in terms of electron transfer
3) in terms of oxidation number

Question 3

What is an antioxidant?

Answer 3

Substance that delays the onset of oxidation or slows down the rate at which oxidation occurs.

Question 4

List three main types of reaction that occurs in chemistry.

Answer 4

1) acid-base reactions
2) precipitation reactions
3) redox reactions

Question 5

OILRIG

Answer 5

Oxidation Is Loss of electrons

Reduction Is Gain of electrons

Question 6

Describe why combustion reaction cannot be described as redox reaction in terms of electron transfer.

Answer 6

For example, complete combustion of solid carbon (eg in the form of coal) in oxygen yields carbon dioxide:
C(s) + O2 (g) → CO2 (g)
However, carbon dioxide is molecular, with covalent bonds, so no ionic bonds are formed. this combustion reaction as a redox process in terms of electron transfer as in theory no electrons are lost or gained and carbon dioxide is a neutral species.

It’s oxidation by adding of oxygen.

Question 7

Define oxidation state.

Answer 7

An apparent charge of an atoms in a free element, a molecule or an ion.

Question 8

Define oxidation and reduction in terms of oxidation states.

Answer 8

Oxidation describes a process in which the oxidation state increases and reduction describes a process in which the oxidation state decreases

Question 9

What is the oxidation state of hydrogen?

Answer 9

The oxidation state of hydrogen is +1 when hydrogen
is bonded to a non-metal, such as in HCl and HNO3.
However, when hydrogen is bonded to a metal, for example in a metal hydride such as NaH, the oxidation number of hydrogen is 1.

Question 10

What is the oxidation state of oxygen?

Answer 10

The oxidation state of oxygen is usually -2, such as in H2O and H2SO4 . The main exception is in a peroxide
(a species with an -O-O- linkage) ; here the oxidation state of oxygen is -1. A typical example of such a compound is H2O2, hydrogen peroxide

Question 11

What is the oxidation state of group 17 elements?

Answer 11

The oxidation state of fluorine is -1 in all its compounds, for example HF, OF2, and LiF.
For the other
group 17 halogen elements the oxidation state is usually -1 in binary compounds (HI, NaCl, KBr) but in combination with oxygen in oxoanions and oxoacids the oxidation state is positive (for example, in HClO4 chlorine has a +7 oxidation state) .

Question 12

What are the activity series?

Answer 12

The activity series ranks metals according to the ease with which they undergo oxidation

Question 13

List three formulae used in volumetric analysis (titration).

Answer 13

1) Amount of substance in mol = n
n = m/M

2) n = volume (in dm^3) x concentration (in mol dm^(-3)
(If in cm^3 then divide all by 1000)

3)1/Va(na) = 1/Vb(nb) and hence
1/Va(Va x ca) = 1/Vb(vb x cb)

Va = volume of reactant A (in dm^-3)
cv = concentration of reactant A (in mol dm^(-3))

Question 14

What is the formula to calculate concentrations in parts per million?

Answer 14

concentration in ppm = (mass of component in solution/total mass of solution) x 10^6

= mass of solute in mg/volume of solution in dm^3

Question 15

What is the Winkler method?

Answer 15

Technique based on redox reactions used to measure the amount of dissolved oxygen in water.

High [dissolved oxygen] = low level of pollution.

Question 16

Define BOD.

Answer 16

BOD - biochemical oxygen demand.

BOD is the amount of oxygen required to oxidize organic matter in a sample of water at a definite temperature over a period of 5 days.

Measured in ppm.

<1 = pure water
100-400 = untreated domestic sewage

Question 17

What is the substance produced from carbon in aerobic and anaerobic conditions?

Answer 17

Aerobic: CO2

Anaerobic: CH4

Question 18

What is the substance produced from hydrogen in aerobic and anaerobic conditions?

Answer 18

Aerobic: H20

Anaerobic: CH4, NH2, H2S and H20

Question 19

What is the substance produced from nitrogen in aerobic and anaerobic conditions?

Answer 19

Aerobic: NO3^(-)

Anaerobic: NH3, amines

Question 20

What is the substance produced from sulfur in aerobic and anaerobic conditions?

Answer 20

Aerobic: SO4^(2-)

Anaerobic: H2S

Question 21

What is the substance produced from phosphorous in aerobic and anaerobic conditions?

Answer 21

Aerobic: PO4^(3-)

Anaerobic: PH3

Question 22

Define energy.

Answer 22

The capacity to d work.

Unit: joule (J)

Question 23

What is the law of conservation of energy?

Answer 23

It states that energy cannot be created or destroyed, but is converted from one form to another.

Question 24

List two main types of electrochemical cells.

Answer 24

1) Voltaic (or galvanic) cell - converts chemical energy (from spontaneous exothermic processes) to electrical energy
2) Electrolytic cells - convert electrical energy to chemical energy bringing about a non-spontaneous process.

Question 25

Study tip: EE

Answer 25

Electrical cell - Electrical to chemical

Question 26

Study tip: CROA

Answer 26

Cathode - Reduction

Oxidation - Anode

Question 27

Study tip: CNAP

Answer 27

Cathode - Negative
Anode - Positive

this is for electrolytic cell, the opposite is true for the voltaic cell.

Question 28

List three types of electrodes used in voltaic cell.

Answer 28

1) metal/metal-ion electrode
2) metal ions in two different oxidation states
3) gas-ion electrode

Question 29

In a voltaic cell, how are the two electrons connected?

Answer 29

via liquid junction called a salt bridge, which contains a strong electrolyte.

Question 30

What are three main functions of a salt bridge?

Answer 30

1) allows for a physical separation of cathode and anode, and therefore the oxidation and reduction processes
2) provides electrical continuity
3) reduces the liquid junction potential - a voltage generated when two different solutions come into contact

Question 31

How can you determine which metal will be oxidized and which metal will be reduced in a voltaic cell?

Answer 31

By looking at the activity series, for the Daniell cell, zinc (anode) is higher up in the series than copper (cathode), so it’s more easily oxidized.

Question 32

Write down the cell diagram of the Daniell cell.

Answer 32

Zn(s) |Zn2+

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

Question 33

Define a fuel cell and give an example.

Answer 33

Fuel cell is a voltaic cell (therefore based on redox processes).
Common type is the hydrogen-oxygen fuel cell, which uses the reaction between hydrogen and oxygen.

Question 34

How much energy in fuel can a fuel cell convert into electrical energy?

Answer 34

about 70%

Question 35

Name two advantages of a hydrogen-oxygen fuel cell and three disadvantages.

Answer 35

1) Water is the only component so it’s non-polluting
2) does not need recharging

Disadvantages

1) they are expensive to produce.
2) they are prone to poisoning by impurities in the fuel, which reduces their lifetime or requires expensive purification of the fuel.
3) storage of hydrogen is a problem.

Question 36

What is electrolysis?

Answer 36

Process by which electrical energy is used to drive a non-spontaneous chemical reaction.

Question 37

Describe an electrolytic cell

Answer 37

Consists of a single container, two electrodes (cathode and anode), a solution (electrolyte) and a battery, which can be considered as an electron pump.

Question 38

What are the 5 steps in working method for the electrolysis of a molten salt?

Answer 38

Step 1: Identify all species present.

Step 2: Identify which species are attracted to the cathode (negative electrode) and which species are attracted to the anode (positive electrode) .

Step 3: Deduce the two half-equations taking place at the cathode and anode and the overall cell reaction.

Step 4: Draw and annotate the electrolytic cell and show the direction of the movement of electrons and the direction of ion flow.

Step 5: State what would be observed at each electrode.