Topic 7

Question 1

What is the oxidation number of elements?

Answer 1

Zero

Question 2

What is the oxidation number of simple ions?

Answer 2

Charge

Question 3

What is oxidation?

Answer 3

Increase in oxidation number, loss of electrons (electrons on RHS)

Question 4

What is reduction?

Answer 4

Decrease in oxidation number, gain of electrons (electrons on LHS)

Question 5

Redox meaning

Answer 5

Stands for reduction and oxidation. In a redox reaction there are two species, one is oxidised and one is reduced

Question 6

Oxidation in terms of oxygen and hydrogen

Answer 6

Gain of oxygen, loss of hydrogen

Question 7

reduction in terms of oxygen and hydrogen

Answer 7

Loss of oxygen, gain of hydrogen

Question 8

What is an oxidant (oxidising agent)

Answer 8

• Oxidises the other species
• Gets reduced
• Gains electrons, oxidation number decreases

Question 9

What is a reductant (reducing agent)

Answer 9

• Reduces the other species
• Gets oxidised
• Loses electrons, oxidation number increases

Question 10

What are reactants and species?

Answer 10

Reactants are the full substance not ionic species eg ZnSO4 is the substance that acts as an oxidant but Zn2+ is the species that is reduced

Question 11

What colour is iodide ion and chloride ion?

Answer 11

Colourless, so is bromide

Question 12

What is electrolysis?

Answer 12

The breakdown of an ionic compound in molten or aqueous solution by the passage of the electric current

Question 13

What compounds can confuct electricity?

Answer 13

• Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
• Ionic compounds in the solid state cannot conduct electricity either since they haveno free ionsthat can move and carry the charge

Question 14

What is the current for in electrolysis?

Answer 14

Provides voltage that forces electrons to be gained and lost when it wouldn’t otherwise be possible

Question 15

Why does electrolysis occur?

Answer 15

The charged ions in the electrolyte allows electrons to flow.
- The electrons move through the external circuit towards the cathode
- These electrons are gained by the cations and transferred from the cathode. Reduction occurs at the cathode
- The electrons are lost from the anion and transferred from the anode. Oxidation occurs at the anode

Question 16

What is an electrode?

Answer 16

Electrodes are the rods that carry the electric current to and from the electrolyte. These rods are usually inert, as in they do not react- usually graphite or platinum. Anode is positive electrode and cathode is negative electrode

Question 17

What is electrolyte?

Answer 17

the compound that conducts electricity when molten and breaks down during electrolysis

Question 18

Why do electrodes need to be inert?

Answer 18

need to be inert such as graphite or platinum so that they don’t participate in a side reaction with the electrolyte

Question 19

What happens during electrolysis?

Answer 19

• During electrolysis, current needs to flow around the circuit. In order for this to occur, charge must be transferred around the circuit (current is a measure of the rate of flow of charge) by charge carriers.
• The power supply provides thecathodewith a supply ofelectrons, causing it to become negatively charged.
• Positive ions(cations) in the electrolyte move towards the cathode where theygain electrons to become atoms (in a positive ion, there are more protons than electrons, so in order to become an atom, they need to gain those extra electrons to become neutral). Soreduction occurs at the cathode.
• Negative ions(anions) in the electrolyte move towards the anode where theylose electrons to become atoms (in a negative ion, there are more electrons than protons, so in order to become an atom, they need to lose those extra electrons to become neutral). Sooxidation occurs at the anode.
• Theelectronsmove from the anode back towards the power supply which then goes back to the cathode creating a circuit.

Question 20

What are the three types of electrolyte solutions?

Answer 20

1. Molten (liquid, no water)
2. Dilute aqueous solution (lots of water)
3. Concentrated aqueous solution (less water)

Question 21

What happens in the electrolysis of a molten electrolyte?

Answer 21

• A molten ionic compound consists of only the two elements joined by ionic bonding
• During electrolysis, they always produce the corresponding element. Thepositiveion will migrate towards thecathodeand thenegativeion will migrate towards theanode, therefore, thecathodeproduct will always be themetal,and the product formed at theanodewill always be thenon-metal

Question 22

Electrolysis of molten lead(II) bromide

Answer 22

Cathode (reduction) reaction: Pb2+(aq) + 2e-→ Pb(s)
Anode (oxidation) reaction: 2Br-(aq) → Br2(aq)+ 2e-
Observations: There is bubbling at the anode as brownbromine gas is given off. At the cathode agrey lead metalis deposited on the surface of the electrode.

Question 23

What happens in the electrolysis of a dilute aqueous solution?

Answer 23

• In dilute aqueous electrolytes there are positive and negative ions from the salt and a large number of H+ and OH- ions
• At the anode (oxidation) negative non-metal ions and OH- ions from water are attracted. Only O2 forms (This is because OH- is the least reactive of the negative ions)
• At the cathode (reduction) positive metal ions and H+ ions from water are attracted. Either metal metal or H2 is formed depending on if the metal is more reactive than hydrogen (if metal is more reactive hydrogen gas forms, if metal is less reactive metal forms (ex: copper, silver))

Question 24

Most to least electronegative

Answer 24

F-
SO42-
NO3-
Cl-
Br-
I-
OH-

Question 25

Most to least electropositive

Answer 25

K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe2+ Sn2+ Pb2+ H+ Cu2+ Ag+

Question 26

What happens in the electrolysis of a concentrated aqueous solution?

Answer 26

- In a concentrated aqueous solution, there are large amounts of positive and negative ions from the salt as well as H+ and OH- from water - At anode (oxidation), negative non-metal ions and OH- are attracted. If halide ions are present halogen gas will form, otherwise O2 will form - At cathode (reduction), positive metal ions and H+ are attracted. Either metal or H2 will form depending on which one is less reactive

Question 27

Electrolysis of concentrated aqueous sodium chloride

Answer 27

- At the negative cathode: There is H+ and Na+. H+ is discharged because it is less reactive than sodium and it gains electrons to form hydrogen gas (a colourless gas that causes bubbling at the cathode). 2H+ + 2e- → H2 - At the positive anode: There is Cl- and OH-. Cl- is discharged because it is a halide. It loses electrons to form chlorine gas (Cl2). 2Cl- → Cl2 + 2e- (pale green-yelloe gas, bubbling) - Left over solution: The sodium ions (Na+) and hydroxide ions (OH-) are left and form the sodium hydroxide solution (NaOH)

Question 28

Electrolysis of dilute sulfuric acid

Answer 28

- At the cathode: There are two H+ ions meaning there is no choice and they both form hydrogen gas. 2H+ + 2e- → H2 - At the anode: There is OH- and (SO4)2-. OH- is discharged because it is a dilute solution and sulphate is never discharged. The hydroxide ion is attracted there and loses its electrons to form oxygen gas and water. 4(OH)- → 2H2O + O2 + 4e- - Bubbles of gas are seen at both electrodes

Question 29

Electrolysis of aqueous copper(II) sulfate using copper electrodes

Answer 29

- The cathode increases in mass while the anode decreases. This occurs as copper atoms are oxidised at the anode and form ions while copper ions are reduced at the cathode, forming copper atoms. The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode. Therefore, the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode - The copper atoms in the electrode on the anode lose electrons and become copper ions. These are sent over to the cathode where they are turned back into copper atoms. Therefore, the anode loses weight and the cathode gains weight.

Question 30

Equation for oxygen at the anode

Answer 30

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

Question 31

Equation for hydrogen at the cathode

Answer 31

2H+ + 2e- ->H2

Question 32

What is electroplating?

Answer 32

Electroplating uses electolysis to coat one metal on another or to purify an impure metal (a process where the surface of one metal is coated with a layer of a different metal.)

Question 33

Common metals used for electroplating

Answer 33

Copper Chromium Silver Tin

Question 34

How to do electroplating through electrolysis?

Answer 34

The anode is made from the pure metal you want to coat your object with. The cathode is the object to be electroplated. The electrolyte is an aqueous solution of an ionic compound of the plating metal. The atoms in the electrode on the anode lose electrons and become ions. These are sent over to the cathode where they are turned back into atoms and plate the cathode.

Question 35

Two reasons for electroplating

Answer 35

1. To provide a protective coating to prevent corrosion 2. To create a shiny appearance

Question 36

Halogens and their electronic configuration

Answer 36

All diatomic molecules Fluorine (2,7) Chlorine (2,8,7) Bromine (2,8,18,7) Iodine (2,8,18,18,7) Astatine (2,8,18,32,18,7)

Question 37

Trends in Group VII

Answer 37

- Density increases as you go down the group (states change) - Reactivity decreases as you go down the group - Colour becomes darker as you go down the group - Melting and boiling point increases as you go down the group

Question 38

Halogens colour and state

Answer 38

Fluorine (yellow, gas) Chlorine (pale yellow-green, gas) Bromine (red-brown liquid) Iodine (Purple-black solid) Astatine (grey-black, solid)

Question 39

What are halogen displacement reactions?

Answer 39

This occurs when a more reactive halogen replaces a less reactive halide in a compound. In a displacement reaction, the more reactive halogen atoms oxidise the less reactive halide ions, causing the halide ions to lose electrons and form halogen atoms Ex: Fluorine will displace chlorine, bromine, iodine and astatine

Question 40

What is a fuel cell?

Answer 40

an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode. H2 → 2H+ + 2e-, O2 + 4e- → 2(O)2-

Question 41

What does a hydrogen-oxygen fuel cell do?

Answer 41

Uses hydrogen and oxygen to produce electricity (a voltage) with water as the only chemical product

Question 42

Advantages of hydrogen-oxygen fuel cells

Answer 42

- Can convert most of the chemical energy to electrical energy (no waste heat) - Water is the only product, therefore no harmful emissions (more ecofriendly) - Can be a renewable source of energy if hydrogen comes from a renewable source - Releases more energy per kilogram compared to petrol or diesel - Quieter so less noise pollution - No power is lost in transmission as there is no moving parts

Question 43

Disadvantages of hydrogen-oxygen fuel cells

Answer 43

- Hydrogen is obtained from methane and carbon is rejected so it contributes to greenhouse effect - Hydrogen is more difficult and expensive to store compared to petrol as it is very flammable and easily explodes when under pressure - Each fuel cell only makes a small voltage. A large number of fuel cells must be linked to produce a large voltage or current which is expensive - Fuel cells are affected by low temperatures and become less efficient - Only small number of hydrogen filling stations available

Question 44

Explain why the carbon anodes need to be replaced periodically

Answer 44

burns in oxygen / reacts with oxygen / oxidised by oxygen / forms carbon dioxide / forms carbon monoxide