9.2 Periodicity of chemical properties of the elements in Period 3

Question 1

Reaction of Na with oxygen

Answer 1

• burns in air with an orange flame to form white Na2O solid

Question 2

Reaction of Mg with oxygen

Answer 2

• burns in air with an intense white flame to form white MgO solid

Question 3

Reaction of Al with oxygen

Answer 3

• a layer of white Al2O3 solid forms very rapidly on exposure to air

Question 4

Reaction of Si with oxygen

Answer 4

• reacts with oxygen on HEATING to form white SiO2 solid

Question 5

Reaction of P with oxygen

Answer 5

• burns with a white flame in a plentiful supply of air to form white P4O10 solid (phosphorus pentoxide)

Question 6

Reaction of S with oxygen

Answer 6

• burns with oxygen with a blue flame to form SO2, a dense, colourless gas with a choking pungent smell
• SO2 is further oxidised in the atmosphere to form SO3 (reversible reaction)

Question 7

Reaction of Na with chlorine

Answer 7

• reduces chlorine on HEATING with a bright orange flame to form white NaCl solid

Question 8

Reaction of Mg with chlorine

Answer 8

• reduces chlorine on HEATING with an intense white flame to form white MgCl2 solid

Question 9

Reaction of Al with chlorine

Answer 9

• reduces chlorine on HEATING to form white AlCl3 solid

- AlCl3 undergoes dimerisation to form Al2Cl6 solid (to achieve stable octet)

Question 10

Reaction of Si with chlorine

Answer 10

• reduces chlorine on HEATING to form colourless SiCl4 liquid

Question 11

Reaction of P with chlorine

Answer 11

• reduces chlorine on HEATING to form PCl5, an off-white solid

Question 12

Reaction of Na with water

Answer 12

• reacts with COLD WATER vigorously to colourless NaOH and hydrogen gas

Question 13

Reaction of Mg with water

Answer 13

1. reacts very slowly with COLD WATER to form a white precipitate of Mg(OH)2 and hydrogen gas
2. reacts vigorously with STEAM and burns with a white flame to form white MgO solid and hydrogen gas

Question 14

Reaction of Na2O (basic) with water

Answer 14

1. reacts with water to form a strongly alkaline solution, NaOH, pH=13
2. reacts with acids to form salt and water
3. NaOH also reacts with acidic gases to form salt and water

Question 15

Reaction of MgO (basic) with water

Answer 15

1. reacts with water to form a weakly alkaline solution, Mg(OH)2, pH=9
   * MgO does not react readily with water due to the high charge density of Mg2+
2. reacts with acids to form salt and water

Question 16

Reaction of Al2O3 (amphoteric) with water

Answer 16

1. INSOLUBLE in water
2. reacts with HOT acids to form salt and water
3. reacts with alkalis to form salt

Question 17

Reaction of SiO2 (acidic) with water

Answer 17

1. INSOLUBLE in water
2. reacts with HOT CONCENTRATED alkalis to form colourless silicate solution and water
3. reacts with bases to form salt

Question 18

Reaction of P4O10 (acidic) with water

Answer 18

1. reacts with water to form a solution of phosphoric acid, H3PO4, pH=2
2. reacts with bases to form salt and water

Question 19

Reaction of SO2 (acidic) with water

Answer 19

1. reacts with water to form sulfurous acid, H2SO3, pH=1

2. reacts with bases to form salt and water

Question 20

Reaction of SO3 (acidic) with water

Answer 20

1. reacts with water to form a strongly acidic solution, H2SO4 sulfuric acid, pH=1
2. reacts with bases to form salt and water

Question 21

Reaction of NaCl with water

Answer 21

• white solid dissolves in water to form a colourless neutral solution, pH=7

Question 22

Reaction of MgCl2 with water

Answer 22

• dissolves in water to form a slightly acidic solution, pH=6.5

Question 23

Reaction of AlCl3 with water

Answer 23

• hydrolyses in water to form an acidic solution, pH=3

- the complex ion [Al(H20)6] 3+ hydrolyses with water to produce H3O+ ions which make the solution acidic

Question 24

Reaction of SiCl4 with water

Answer 24

• hydrolyses in water to form a strongly acidic solution
• effervescence, white fumes of HCl gas are produced
• white SiO2 solid forms

Question 25

Reaction of PCl5 with water

Answer 25

• hydrolyses in water to form a strongly acidic solution, pH=1
• white fumes of HCl gas are produced