topic 9

Question 1

oxidation is the —– of oxygen
reduction is the —– of oxygen

Answer 1

oxidation is gain of oxygen
reduction is loss of oxygen

Question 2

oxidation is the —– of hydrogen
reduction if the —— of hydrogen

Answer 2

oxidation is loss of hydrogen
reduction is gain of hydrogen

Question 3

oxidation is the —– of electrons
reduction is the ——- of electrons

Answer 3

oxidation is the loss of electrons
reduction is the gain of electrons

Question 4

what happens to oxidising/reducing agents in redox reactions?

Answer 4

the oxidising agent is reduced (gains electrons), the reducing agent is oxidised (loses electrons)

Question 5

group 1 metals always have an oxidation number of ?

Answer 5

+1

Question 6

group 2 metals always have an oxidation number of ?

Answer 6

+2

Question 7

aluminium always has an oxidation number of ?

Answer 7

+3

Question 8

fluorine always has an oxidation number of ?

Answer 8

-1

Question 9

oxygen is always …

Answer 9

-2 except when in peroxides, where it is -1

Question 10

KMnO4 is potassium manganate (VII)- what does the (VII) refer to?

Answer 10

the oxidation number on the manganese ion

Question 11

define an oxidation number

Answer 11

reflects the no of electrons the atom uses in bonding to another element

Question 12

how can you tell if an element has been oxidised or reduced by its oxidation numbers?

Answer 12

oxidised if its oxidation number increases
reduced if its oxidation number decreases

Question 13

hydrogen is always…

Answer 13

+1 except when in metal hydrides, where it is -1

Question 14

what does the activity series do?

Answer 14

it ranks metals according to the ease with which they undergo oxidation

Question 15

describe how you would write a redox reaction equation

Answer 15

1. write down word equation as a symbol equation and balance it
2. work out the oxidation numbers of the elements that change
3. work out the increase and decreases in oxidation number and balance them
4. add H2O and H+ as needed to balance O and H

Question 16

state and explain 2 different redox titrations

Answer 16

1. acidified manganate (VII) ions and iron (II) ions;
   MnO4– (aq) + 8H+ (aq) + 5Fe2+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
   - This reaction needs no indicator as the manganate (VII) is a strong purple colour which disappears at the end point, so the titration is self-indicating
2. iodine and thiosulfate ions:
   2S2O32– (aq) + I2 (aq) → 2I–(aq) + S4O62– (aq)
   - The light brown/yellow colour of the iodine turns paler as it is converted to colourless iodide ions
   - When the solution is a straw colour, starch is added to clarify the end point
   - The solution turns blue/black until all the iodine reacts, at which point the colour disappears.

Question 17

give the 2 equations for concentration in parts per million

Answer 17

mass of component in solution/total mass of solution x10^6

or

mass of solute in mg/volume of solution in dm3

Question 18

in the case of solubility of oxygen in water, we calculate the amount dissolved in …

Answer 18

1dm3

Question 19

high concentration of dissolved oxygen =

Answer 19

low level of pollution

Question 20

what does BOD stand for and what does it measure?

Answer 20

biochemical oxygen demand- the amount of oxygen used (for bacteria) to decompose the organic matter in a sample of water over a specified time period, usually 5 days, at a specified temperature- in ppm

Question 21

give 3 sources of organic matter in a body of water

Answer 21

• untreated sewage
• brewery waste
• abattoirs

Question 22

describe the Winkler Method

Answer 22

1. a precipitate of manganese (II) hydroxide is made
   Mn2+ (aq) + 2OH- (aq) -> Mn(OH)2 (s)
2. this precipitate will react with any oxygen present in the water sample to form a brown precipitate of MnO(OH)2
   2Mn(OH)2 (s) + O2(g) -> 2MnO(OH)2 (s)
3. The brown precipitate is react with an excess of iodide ions, creating iodine
   MnO(OH)2 (s) + 4H+ (aq) + 2I- (aq) -> Mn2+ + I2 + 3H2O
4. The amount of iodine formed is determined by titrating the sample with sodium thiosulphate, Na2S2O3 (redox titration)
   I2 + 2S2O32- -> 2I- + S4O62-

Question 23

give the Winkler method ratios

Answer 23

4:2:2:1
S2O32-: I2: MnO(OH)2: O2

Question 24

what takes place in electrochemical cells?

Answer 24

chemical energy - electrical energy conversions

Question 25

state and describe the two types of electrochemical cell

Answer 25

1. voltaic (galvanic) cells- convert chemical energy to electrical energy; convert energy from spontaneous, exothermic chemical processes to electrical energy
2. electrolytic cells- convert electrical energy to chemical energy, bringing about non-spontaneous processes

Question 26

define an electrode

Answer 26

a conductor of electricity used to make contact with a non-metallic part of a circuit, such as the solution in a cell

Question 27

similarity between voltaic and electrolytic cells

Answer 27

oxidation always takes place at the anode
reduction always takes place at the cathode

Question 28

difference between voltaic and electrolytic cells

Answer 28

voltaic cell:
- cathode is positive electrode
- anode is negative electrode

electrolytic cell (CNAP):
- cathode is negative electrode
- anode is positive electrode

Question 29

a voltaic cell consists of

Answer 29

two half-cells

Question 30

define electrodes and half cells

Answer 30

electrodes: the two metal strips
half cells: beaker containing strip of metal atoms in equilibrium with aqueous solution of its ions

Question 31

Construct and annotate of both types of electrochemical cells.

Answer 31

tb

Question 32

Explain how a redox reaction is used to produce electricity in a voltaic
cell

Answer 32

1. Half-Cell Reactions:
   Each half-cell involves a redox reaction, consisting of an oxidation half-reaction and a reduction half-reaction.
   - Oxidation Half-Reaction: In one half-cell, a substance undergoes oxidation, losing electrons.
   - Reduction Half-Reaction: In the other half-cell, a different substance undergoes reduction, gaining the electrons lost in the oxidation half-reaction.
2. Electron Flow:
   Electrons flow from the substance undergoing oxidation to the substance undergoing reduction
   The electrons move through an external circuit, creating an electric current.
3. Ion Migration:
   Ions are involved in the redox reactions, and they migrate between the half-cells through a salt bridge or some other ionic conductor.
   The salt bridge maintains charge neutrality and prevents the buildup of excess charge in the half-cells.
   4.Cell Potential (Voltage):
   The difference in potential energy between the two half-cells is the cell potential or voltage of the voltaic cell.
   The cell potential is a measure of the cell’s ability to do work and is related to the standard reduction potentials of the half-reactions.
4. Electricity Generation:
   The flow of electrons through the external circuit generates electric power that can be used to do work or power electronic devices.

Question 33

explain how current is conducted in an electrolytic cell

Answer 33

1. Setup:
   An electrolytic cell consists of two electrodes (usually metal or graphite rods) immersed in an electrolyte solution.
   The electrolyte is a substance that can undergo electrolysis – a chemical process where an electric current is used to drive a non-spontaneous redox reaction.
2. Electrode Reactions:
   The electrodes are connected to the external power supply. The electrode connected to the positive terminal of the power supply is the anode, and the one connected to the negative terminal is the cathode.
   At the anode, oxidation occurs. Anions (negatively charged ions) move toward the anode and release electrons. These electrons flow through the external circuit toward the cathode.
   At the cathode, reduction occurs. Cations (positively charged ions) move toward the cathode and gain electrons. These electrons are supplied by the external circuit.
3. Ion Migration:
   The movement of ions in the electrolyte is crucial for the continuity of the process. In contrast to a salt bridge in a voltaic cell, in an electrolytic cell, ions move freely in the electrolyte solution.
   For example, if the electrolyte is a molten salt or a solution, positive ions move toward the cathode, and negative ions move toward the anode.
4. Redox Reaction:
   The redox reaction at the electrodes is non-spontaneous and requires an external energy source to drive it.
5. Overall Electrolysis Reaction:
   The overall electrolysis reaction is the sum of the oxidation and reduction half-reactions.
6. Current Flow:
   Electrons flow through the external circuit from the anode to the cathode, driven by the external power supply.
   Ions move through the electrolyte solution to maintain charge neutrality and complete the circuit.
7. Electrolysis Products:
   As a result of the electrolysis process, chemical changes occur at the electrodes, and new substances are formed.