Periodicity

Question 1

Which elements in period 3 react with cold water

Answer 1

Sodium, magnesium, chlorine

Question 2

Describe reaction of sodium with water

Answer 2

Vigorous. Sodium floats on surface of water and fizzes rapidly, melting because of the heat energy released by the reaction. Strongly alkaline solution of NaOH formed (pH 13-14). Redox reaction. Produces hydrogen gas.

Question 3

Describe the reaction of magnesium with water

Answer 3

Very slow at room temperature. Resulting solution less alkaline than one with sodium as magnesium hydroxide only sparingly soluble in water (pH about 10). With cold water produces Mg(OH)2. Reaction much faster with heated magnesium and steam and produces MgO. Both produce hydrogen gas. Both redox reactions.

Question 4

Reactions of period 3 elements with oxygen

Answer 4

Sodium burns brightly in air with yellow flame forming white Na2O. Magnesium burns in air with bright white flame forming white powder MgO. Aluminium powder burns brightly in air forming white Al2O3. Silicon forms SiO2 if heated strongly in O2. Red phosphorus must be heated to react, white spontaneously ignites in air and white smoke of P4O10 (and P2O2 if limited O2). Heated sulfur powder in glass jar of O2 burns with blue flame forming colourless SO2 gas (and some SO3).

Question 5

Why does aluminium oxide have some covalent character?

Answer 5

Aluminium forms very small ion with large positive charge so can approach closely to the oxygen ion and distort its electron cloud.

Question 6

What state are phosphorus oxides in at room temperature?

Answer 6

Solid but with low melting points

Question 7

Describe the structure of silicon dioxide

Answer 7

Giant covalent

Question 8

Reaction of oxides of sodium and magnesium with water

Answer 8

The sodium oxide reaction forms strongly alkaline solution of NaOH (pH 13-14). The magnesium oxide reaction forms Mg(OH)2 which is sparingly soluble (pH about 10).

Question 9

Reactions of oxides of aluminium and silicon with water.

Answer 9

They are both insoluble in water

Question 10

Reaction of phosphorus pentoxide with water

Answer 10

Forms phosphoric acid (H3PO4). Violent reaction. This ionises so the solution is fairly strongly acidic. pH 1-2

Question 11

Reactions of oxides of sulfur with water

Answer 11

Sulphur dioxide fairly soluble and reacts with 1 water to form H2SO3. This partially dissociates causing the acidity of the solution (pH 2-3). Sulfur trioxide reacts violently with water to produce H2SO4 which fully dissociates forming a strongly acidic solution (pH 0-1).

Question 12

Reactions of oxides of sodium and magnesium with acid

Answer 12

Both act as strong bases.
E.g- Na2O with H2SO4 makes Na2SO4 and water
MgO with HCl makes MgCl2 and water

Question 13

What does aluminium oxide react with?

Answer 13

Acids and alkalis. It is called an amphoteric oxide

Question 14

Reaction of aluminium oxide with HCl

Answer 14

Al2O3 and 6HCl goes to 2AlCl3 and 3H2O

Question 15

Reaction of aluminium oxide with sodium hydroxide

Answer 15

Sodium hydroxide is hot and concentrated.

Al2O3 and 2NaOH and water goes to 2NaAl(OH)4

Question 16

Reaction of silicon dioxide with bases

Answer 16

Acts as a weak acid (solid) and can for example react with hot concentrated sodium hydroxide to form Na2SiO3 and water.

Question 17

Reaction with phosphorus pentoxide with NaOH

Answer 17

It’s really the reaction between phosphoric acid and NaOH as that’s what is formed when it dissolves in water. H3PO4 has 3 OH groups and so 3 acidic H atoms so reacts in 3 stages.
H3PO4 + NaOH -> NaH2PO4 +H2O
NaH2PO4 + NaOH -> Na2HPO4 +H2O
Na2HPO4 + NaOH -> Na3PO4 +H2O
Overall is 3NaOH + H3PO4 -> NaPO4 + 3H2O

Question 18

Reaction of sulfur dioxide with sodium hydroxide

Answer 18

Aqueous solution. First forms NaHSO3.

Then this reacts with NaOH to form Na2SO3 and H20

Question 19

Order of melting points of highest oxides of period 3 elements

Answer 19

Mg, Al, Si, Na, P, S,