Alevel Chemistry - Topic 3 + 4

Question 1

Define oxidation number

Answer 1

The total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom

Question 2

Oxidizing agent

Answer 2

A species that oxidizes another species by removing one or more electrons, gaining electrons and therefore itself becomes reduced

Question 3

Reducing agent

Answer 3

A species that reduces another species by adding one or more electrons, loosing electrons itself and becoming oxidized

Question 4

Redox reaction

Answer 4

A reaction that involves both reduction and oxidation

Question 5

disproportionation

Answer 5

A single reaction involving the simultaneous oxidation and reduction of the an element

Question 6

What does oxidation mean?

Answer 6

• Loss of electrons
• Addition of oxygen
• Removal of hydrogen

Question 7

What are the five oxidation number rules?

Answer 7

• The oxidation number of a combined element = 0
• Group 1 and group 2 metals always have the oxidation numbers +1 and +2 respectively
• Oxygen is always -2 except in peroxides
• Hydrogen is always +1 except in metal hydrides like “NaH”
• The more electronegative element always take the negative oxidation number

Question 8

Balance by oxidation: MnO4 (-) oxides Fe (II) to Fe (III) and is itself reduced to Mn (+2)

Answer 8

5Fe(+2) + MnO4(-) + 8H(+) -> Mn(+2) + 4H2O + 5Fe(+3)

Question 9

What’s the color for Bromine water

Answer 9

Orange

Question 10

What’s the color for Iodine water

Answer 10

Brown

Question 11

How do we test for Br2 and I2 molecules and what is the results?

Answer 11

*The addition of an organic solution such as cyclohexane and shake

Br - Yellow/orange layer (Organic layer)
I - Purple layer (Organic layer)

*Both solution are usually the top layer. This is because the organic layer in less dense than water. This molecules sit in the organic layer because the formation of London dispersion forces are favored over breaking hydrogen bonds in water

Question 12

Write the oxidation reaction of the metals:
Na + Cl

Answer 12

2Na + Cl2 -> 2NaCl

2Na -> 2Na(+) + 2e(-)
Cl2 + 2e(-) -> 2Cl(-)

Sodium: 0 -> +1 (Oxidized)
Chlorine: 0 -> -1 (Reduced)

Question 13

Write the oxidation reaction between:
Chlorine + Water

Answer 13

Cl2 + H20 = HCl + HClO

1/2Cl2 + e- -> Cl-
1/2Cl2 -> Cl(+) + e-

Question 14

Explain the use of chlorine in water treatment

Answer 14

Hypochlorous acid is formed when Cl2 reacts with water. This can then further spilt in the equilibrium equation:

HClO = H(+) + ClO(-)

Chlorate ion is a powerful disinfectant and used in pool and drinking water to break down the cell walls and enzymes of bacteria. The concentration is high enough to kill bacteria but not to hurt us humans

Question 15

The reaction of Chlorine with HOT Alkali

Answer 15

3Cl2 + 6NaOH -> 5NaCl + NaClO3 + 3H2O

Question 16

The reaction of chlorine with COLD alkali

Answer 16

Cl2 + 2NaOH -> NaCl + NaClO + H2O

Question 17

What’s the formula for bleach

Answer 17

NaClO

Question 18

What is the trend of Oxidising and reducing ability of Halogen ions

Answer 18

Down the group - More ionizing but less reducing

Question 19

Write the reaction for NaCl + sulphuric acid

Answer 19

NaCl + sulphuric acid -> HCL + NaHSO4

Question 20

Write the reaction for NaBr + sulphuric acid

Answer 20

NaBr + sulphuric acid -> HBr + NaHSO4
2HBr + sulphuric acid -> Br2 + SO2 + H2O

Question 21

What’s the reaction for NaI + sulphuric acid

Answer 21

NaI + sulphuric acid -> HI + NaHSO4
a) 2HI + sulphuric acid -> I2 + SO2 + H2O
b) 6HI + sulphuric acid -> 3I2 + S + 4H2O
c) 8HI + sulphuric acid -> 4I2 + H2S + 4H2O

Question 22

Observation of HCl, HBr, or HI

Answer 22

Misty fumes

Question 23

Observation of Br2 + I2

Answer 23

Br2 - Brown gas
I2 - Black solid or purple fumes

Question 24

Oberservation of SO2

Answer 24

Colorless gas with choking smell

Question 25

Observation for solid sulphur

Answer 25

Yellow solid

Question 26

Observation of H2S

Answer 26

Colorless gas with rotten egg smell

Question 27

Precipitates of Halogen with ammonia solution is added? (Dilute and concentrated)

Answer 27

+ Dissolves / - no change

           Dilute   /     Concentrated  Chlorine      +                     + Bromine      -                      + Iodine           -                      -

Question 28

What is the color of ammonia halides?

Answer 28

White

Question 29

What do hydogen halides (Gas and not acids which are aq) form when reacted with water.

Answer 29

e.g.

HCl + H2O -> H3O+ + Cl-

Question 30

How can we store toxic compounds?

Answer 30

In oil, like barium chloride to stop contact with oxygen and water

Question 31

The reaction of a Metal with Water?

Answer 31

Forms a hydroxide + hydrogen gas

Question 32

Magneisum cannot react with water. How can it react with H2O?

Answer 32

With steam, vigoursly

Question 33

What is the trend of solubility of hydroxides and sulphates of metals?

Answer 33

Hydroxides are more soluble down the group
Sulphates are more soluble up the group

Question 34

Explain the decompsition of carbonates?

Answer 34

Group 1 carbonates dont decompose except lithium.
Group 2 metals do through the reaction:
MCO3 -> MO + CO2
With increasing difficulty down the groups since charge density decreases as charge is the same, but size increases

Question 35

Decompisiton of Nitrates?

Answer 35

All group 1 Metals except Lithium decompose like this:
MNO3 -> MNO2 + O2

Group 2 metals and lithium decompose like this:
MNO3 -> MNO + NO2 + O2

Nitrogen dioxide is a toxic brown gas
Thermal stability increases down the group because they decompose with increasing difficulty

Question 36

Explain decompisiton products of NaNO3 and MgNO3?

Answer 36

Mg2+ is a smaller cation than Na+
Hence Mg has greater charge density
More polarising power, distorting the NO3- anion electron cloud greater
It is more polarised so can form…instead of….
Na+ is more thermally stable as a result

Question 37

Apperance of halogens?

Answer 37

F - Very yellow pale gas
Cl - yellow-green gas
Bromine - orange/brown liquid
Iodine - Shiny, violet-black crystals