Elements From the Sea

Question 1

[ES1] How is atom economy calculated?

Answer 1

atom economy = (molar mass of desired product / molar mass of all products) x 100

Question 2

[ES1] What are the benefits of a high atom economy?

Answer 2

• Less waste is produced from the reaction, which makes it cheaper.
• The reaction requires less natural resources so it is more sustainable.

Question 3

[ES2] What is electrolysis?

Answer 3

Electrolysis is the breaking down of a substance using electrical energy.

Question 4

[ES2] How is electrolysis done in the lab?

Answer 4

Using the following setup:

Two non-reactive electrodes are connected to the power supply and places in the solution, the power is turned on and electrolysis starts.

Question 5

[ES2] How does electrolysis work?

Answer 5

The cations (negative ions) are attracted to the cathode and are oxidised.

The anions (positive ions) are attracted to the anode and reduced.

The compound is therefore broken up as the two ions do not need each other to be stable.

Question 6

[ES2] What is a half equation?

Answer 6

A half equation shows the reaction that happens at the one electrode in an electrolysis reaction.

(e.g. 2Cl⁻₍ₐᵩ₎ ⇒ Cl₂ ₍ᶢ₎ + 2e⁻ )

Question 7

[ES2] What are the equations for the electrolysis of NaCl solution?

Answer 7

Half equations:
2Cl⁻₍ₐᵩ₎ ⇒ Cl₂ ₍ᶢ₎ + 2e⁻
2H⁺₍ₐᵩ₎ + 2e⁻ ⇒ H₂₍ᶢ₎

2Cl⁻₍ₐᵩ₎ + 2H⁺₍ₐᵩ₎ ⇒ Cl₂₍ᶢ₎ + H₂₍ᶢ₎

Question 8

[ES2] What is an oxidising agent?

Answer 8

A substance that removes electrons from other substances by accepting electrons and getting reduces.

Question 9

[ES2] What is a reducing agent?

Answer 9

A substance that donates electrons to other substances by getting oxidised.

Question 10

[ES2] What does the Roman Numeral indicate in copper(I) sulfide?

Answer 10

The oxidation number of copper is +1.

We can, therefore, infer that the formula for the compound is Cu₂S.

Question 11

[ES2] What does the Roman Numeral indicate in sodium chlorate(I)?

Answer 11

The oxidation number of chlorine is +1 in the compound.

The oxidation number of oxygen and sodium do not change very often so we can assume that they are -2 and +1 respectively.

We can, therefore, infer that the formula of the compound is NaClO.

Question 12

[ES3] What does chlorine look like under standard conditions?

Answer 12

Chlorine is a pale green gas under standard conditions.

Question 13

[ES3] What does bromine look like under standard conditions?

Answer 13

Bromine is a red-brown liquid under standard conditions.

Question 14

[ES3] What does iodine look like under standard conditions?

Answer 14

Iodine is a violet solid under standard conditions.

Question 15

[ES3] Why does the reactivity of halogens decrease down the group?

Answer 15

As we go down group 7 the number of electrons shells increase, this increases the shielding from the nucleus as well as the atomic radius.

Both of these factors decrease the nuclear attraction of outer electrons, making it harder for the halogen to gain electrons.

Question 16

[ES3] Why does the volatility of halogens decrease down the group?

Answer 16

Halogens only have intermolecular induced di-di (london) forces between their molecules.

As we move down the group the number of electrons in the halogen molecule increases.

Thus the di-di forces between the molecules become stronger, taking more energy to break.

Question 17

[ES3] Why are halogens more soluble in cyclohexane than water?

Answer 17

Halogen molester are non-polar, so they do not solve very well in polar solvent like water, but they will dissolve readily in non-polar solvents like cyclohexanes.

Question 18

[ES3] What are the results of the halogens’ reaction with Ag⁺ ions?

Answer 18

Chlorine: white precipitate.

Bromine: creme precipitate.

Iodine: yellow precipitate.

Question 19

[ES3] What are the solubilities of silver halide precipitates in ammonia?

Answer 19

Silver chloride: soluble in dilute ammonia.

Silver bromide: soluble only in concentrated ammonia.

Silver iodide: insluble in dilute and concentraed ammonia.

Question 20

[ES3] How do halogens react with halide ions?

Answer 20

Less reactive halide ions are displaced.

Chlorine will displace bromine and iodide ions.
Bromine will only display iodide ions.
And iodine will not displace halide ions,

(e.g. 2Br⁻₍ₐᵩ₎ + Cl₂₍ᶢ₎ ⇒ Br₂ ₍l₎ + 2Cl⁻₍ₐᵩ₎ )

Question 21

[ES3] How are hydrogen halides prepared?

Answer 21

X = halogen

X⁻ + H₃PO₄ ⇒ HX + H₂PO₄⁻

Question 22

[ES3] How does HBr react with H₂SO₄?

Answer 22

2HBr + H₂SO₄ → SO₂ + Br₂ + 2H₂O

Question 23

[ES3] Why is HF a weaker acid than HCl?

Answer 23

The F⁻ ion forms ionic bonds with the H₃O⁺ ions in solution

Which means there is an equilibrium between the bound and unbound forms of the acid.

H₃O⁺F⁻ ⇌ H₃O⁺ + F⁻

Question 24

[ES5] What are some uses of chlorine?

Answer 24

• Sterilisation of water.

- Bleaching for use in the paper or textile industry.

Question 25

[ES5] What are some risks associated with the storage and transport of chlorine?

Answer 25

• Chlorine is extremely toxic and can cause irreversible lung damage and eye damage upon exposure.
• Some chlorine compounds, such as those made when exposed to water, and carcinogenic.

Question 26

[ES4] When is a system in dynamic equilibrium?

Answer 26

• When the rate of forwarding reaction is equal to the rate of reverse reaction in a reversible reaction.
• When the concentration of reactants and products remain constant.

Question 27

[ES4] What is K𝒸

Answer 27

• An equilibrium constant provides information about the position of equilibrium.
• The magnitude of the constant indicated whether there are more reactants or more products in an equilibrium system.

Question 28

[ES4] What is K𝒸 for the following reaction?

aA + bB ⇌ cC + dD

Answer 28

K𝒸 = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ

Question 29

[ES4] What happens to the equilibrium position if the temperature is increased?

Answer 29

The endothermic reaction is favoured, increasing the equilibrium concentration of the products of that reaction.

Question 30

[ES4] What happens to the equilibrium position if the temperature is decreased?

Answer 30

The exothermic reaction is favoured, increasing the equilibrium concentration of the products of that reaction.

Question 31

[ES4] What happens to the equilibrium position if the pressure is increased?

Answer 31

The side of the equation with the least moles of gas is favoured, so the equilibrium position moves to that side.

Question 32

[ES4] What happens to the equilibrium position if the pressure is decreased?

Answer 32

The side of the equation with the most moles of gas is favoured, so the equilibrium position moves to that side.

Question 33

What is Le Chatelier’s principle?

Answer 33

For any change that is made to the conditions of a reversible reaction, the equilibrium position will shift to counteract the change.

Question 34

[ES2] What are the rules for working out oxidation states?

Answer 34

• Oxygen: -2 (unless with F or a peroxide ion)
• Group 1: +1
• Group 2: +2
• Aluminium: +3
• Group 7: -1 (unless with oxygen or a more reactive halogen)
• Fluorine: -1
• Hydrogen: +1 (unless when metal hydride)
• Ions: must total their charge
• compound: must total 0
• Elements: 0

Question 35

[ES2] What are oxidation states used for?

Answer 35

• Naming inorganic compounds.
• Deciding what has been Oxidised and reduced.
• Deciding oxidising and reducing agents in a redox reaction.
• Balancing redox equations.

Question 36

[ES3] What happens at the cathode if the metal in the salt comes from Group 1 or 2 or is aluminium?

Answer 36

Hydrogen made at cathode.

2H₂O(l) + 2e⁻ → 2OH⁻(aq) + H₂(g)

Question 37

[ES3] What happens at the cathode if a salt has a metal (that is not Group 1 or 2 or aluminium)?

Answer 37

Metal made at cathode.

Cu²⁺(aq) + 2e⁻ → Cu(s)

Question 38

[ES3] What happens at the cathode if acid is electrolysed?

Answer 38

Hydrogen made at cathode.

2H⁺(aq) + 2e⁻ → H₂(g)

Question 39

[ES3] What happens at the anode if an unreactive electrode electrolyses a salt that is halide?

Answer 39

Halogen made at anode.

2Cl⁻(aq) → Cl₂(g) + 2e⁻

Question 40

[ES3] What happens at the anode if an unreactive electrode electrolyses a salt that is sulfate or nitrate?

Answer 40

Oxygen made at anode.

2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻

Question 41

[ES3] What happens at the anode if an unreactive electrode electrolyses a hydroxide?

Answer 41

Oxygen made at anode.

4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻

Question 42

[ES3] What happens at the anode if a reactive anode is used?

Answer 42

Cu(s) → Cu²⁺(aq) + 2e⁻

Question 43

[ES5] What are some benefits of chlorine?

Answer 43

• Used in water treatment, were added to the water to kill bacteria and other pathogens.
• Household bleach product to kill bacteria on surfaces and clothing. This is done by bleach, which is an oxidising agent, by breaking bonds in coloured chemicals to form colourless products.