3.2.4 Properties of Period 3 elements and their oxides

Describe the reaction of magnesium with oxygen and write the balanced symbol equation.

2Mg(s) + O₂(g) → 2MgO(S) Magnesium burns in air with a bright white flame. The white powder produced is magnesium oxide. If burning magnesium is lowered into pure oxygen the flame is even more intense.

Write the balanced symbol equation for the reaction between sodium and water.

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Describe the reaction of sodium with oxygen and write a balanced symbol equation.

2Na(s) + ½O₂(g) → Na₂O(S) Burns brightly in air (with a yellow flame) forming white sodium oxide.

Write an equation for the reaction of an excess of magnesium oxide with phosphoric acid.

3MgO + 2H₃PO₄ → Mg₃(PO₄)₂ + 3H₂O

Write the overall balanced symbol equation for the reaction between phosphoric acid and sodium hydroxide.

3NaOH(aq) + H₃PO₄(aq) → Na₃PO₄(aq) + 3H₂O(l)

With hot, concentrated sodium hydroxide, sodium aluminate is formed. The aluminium oxide acts as an acid. Write a balanced symbol and ionic equation for this reaction.

Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq) Al₂O₃(s) + OH⁻ → 2AlO²⁻ + H+

Write a balanced symbol equation and an ionic equation for Al₂O₃ acting as a base with HCl.

Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l) Al₂O₃(s) + H⁺ → 2Al³⁺ + H₂O

State the element in period 3 that has the highest first ionisation energy. Explain your answer.

Argon Largest number of protons Same Shielding

Explain in terms of structure and bonding why P₄O₁₀ and SO₃ have low melting points.

Covalent bonding. Have relatively low mps as they form simple molecular structures. The molecules are attracted to each other by weak intermolecular forces (dipole-dipole and van der Waals), take little energy to break.

What happens if you add sodium hydroxide to an aqueous solution of sulphur dioxide?

First sodium hydrogensulphate (IV) is formed: SO₂(aq) + NaOH(aq) → NaHSO₃(aq) Followed by sodium sulfate (IV): NaHSO₃(aq) + NaOH(aq) → Na₂SO₃(aq) + H₂O(l)

Na₂O, MgO, Al₂O₃ (some covalent character) all have ionic bonding. Explain in terms of structure why they have high melting points.

Giant Ionic Lattices Strong forces of electrostatic attraction between 1⁺,2⁺,3⁺ and O²⁻ take lots of energy to break and melt the compound. MgO has a high mp than Na₂O because Mg forms 2⁺ ions, attracting O²⁻ ions more strongly than the Na⁺. Al₂O₃ has a lower mp because the ³⁺ ions distort the oxygen's electron cloud making the bonds partially covalent.

Draw the displayed formula of the undissociated acid formed when sulfur dioxide reacts with water.

H₂O + SO₂ → H₂SO₃

H₃PO₄ (aq) ionises in stages. Write the balanced symbol equation for the first one.

H₃PO₄(aq) ⇌ H+(aq) + H₂PO₄⁻(aq)

Compare the melting point of an oxide of Lithium to an oxide of Sodium.

If Lithium were to react with oxygen, the oxide would have a greater melting point, as the lithium ion is smaller (fewer energy levels) and could attract the O2- ions more strongly.

SiO₂ has a high melting point of 1883 °C. Why is this in terms of structure and bonding?

It has covalent bonding. With a macromolecular structure (many strong covalent bonds to be broken).

Is Al₂O₃ melting point less than or greater than MgO?

Less than, as Al₂O₃ has some covalent bonding.

What is a lewis acid?

Lewis Acid → Accepts Electrons (H⁺ wants to accept an electron)

What is a lewis base?

Lewis Bases → Donate Electrons (OH- wants to donate an electron)

Describe the overall trend of the period 3 oxide solutions they form in water.

Metal oxides (on the left of the period) form alkaline solutions in water and non-metal oxides (on the right of the period) form acidic ones, whilst those in the middle don't react.

Write the balanced symbol equation for the reaction between magnesium and water and describe the reaction and products.

Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g) Magnesium's slow room temperature reaction only forms a few hydrogen bubbles after some days. Resultant solution is less alkaline than for sodium as magnesium hydroxide is sparingly soluble.

Write the balanced symbol equation for the reaction between magnesium and steam.

Mg(s) + H₂O(l) → MgO(aq) + H₂(g) The reaction is faster with heated magnesium and steam and gives magnesium oxide and hydrogen.

Magnesium oxide reacts with water to give magnesium hydroxide, which is sparingly soluble in water and produces a somewhat alkaline solution. Write the balanced symbol equation for this reaction and write its ph.

MgO(s) + H₂O(l) → Mg(OH)₂(s) ⇌ Mg2+(aq) + 2OH⁻(aq) ph of solution ~ 9

Sodium and magnesium oxides are both bases. Why does Na₂O react with O²⁻?

Na₂O reacts because O²⁻ accepts a proton forming OH- ions

Sodium oxides react with water to give sodium hydroxide - strongly alkaline solution. Write the balanced symbol equation for this reaction.

Na₂O(s) + H₂O(l) → 2Na⁺(aq) + 2OH⁻(aq)

Both MgO and Na2O react with acids, only producing salt and water. How would you name the salt and write it in an equation?

Often the Pt.2 of the acid forms Pt.2 of the salt, with the metal as Pt.1. The initial oxides are solid (s) while the salt is aqueous (aq) as it's still dissolved in the excess acid.

Explain why sulfur compounds found in crude oil should be removed from the fractions before they're used for central heating fuel.

On combustion, SO₂ produced, which causes acid rain.

Describe the reactions of phosphorous and sulphur oxides with water.

Phosphorous and sulphur oxides are covalent molecules and react with water to form acid solutions.

What is the balanced symbol equation for the reaction between phosphorous pentoxide and magnesium oxide.

P₄O₁₀ + 6MgO → 2Mg₃(PO₄)₂

Non-metals typically form acidic oxides. Phosphorous pentoxide reacts quite violently with water to produce an acidic solution of phosphoric (V) acid. This ionises, so the solution is acidic. Write the balanced symbol equation for this reaction.

P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)

Red and white phosphorous are allotropes of phosphorous (same element with differently arranged atoms). What is the difference between red and white phosphorous?

Red phosphorous must be heated before it'll react with oxygen. White phosphorous spontaneously ignites in air and the white smoke of phosphorous pentoxide is given off.

Describe the reaction between sulphur and oxygen and write a balanced symbol equation for it.

S(s) + O₂(g) → SO₂(g) When sulphur powder is heated and lowered into pure oxygen it burns with a blue flame to form the pungent gas sulphur dioxide (a little sulphur trioxide also forms).

Sulphur(IV) oxide is fairly soluble in water and reacts with it to give an acidic solution of sulphuric (IV) acid (sulphurous acid). This partially dissociates producing ions, which cause the acidity of the solution. Write the balanced symbol equations for this reaction.

SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq) H₂SO₃(aq) ⇌ H⁺(aq) + HSO₃⁻(aq)

Write an equation for the reaction of sulfur trioxide with potassium hydroxide solution.

SO₃ + KOH → KHSO₄

Sulphur trioxide reacts violently with water to produce sulfuric acid (sulfuric(VI) acid). Write the balanced symbol equation for this reaction.

SO₃(g) + H₂O(l) → H₂SO₄(aq) → H⁺(aq) + HSO₄⁻(aq)

Silicon will form an oxide if it is heated strongly in oxygen. Describe the resultant oxide and write out a balanced symbol equation for its formation.

Si(s) + O₂(g) → SiO₂(S) The resultant oxide is a non-conductive, insoluble macromolecule! This is an acidic oxide that can neutralise bases!

Silicon dioxide will react as a weak acid with strong bases, e.g. with hot concentrated sodium hydroxide a colourless solution of sodium silicate is formed. Write a balanced symbol equation for this reaction.

SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l)

What is the balanced symbol equation for the reaction between silicon dioxide and calcium oxide?

SiO₂(s) + CaO(l) → CaSiO₃(l)

Explain why, in terms of structure and bonding, silicon dioxide is insoluble in water.

Silicon dioxide: macromolecular, water won't affect this structure due to strong covalent bonds throughout the 3-D lattice formed by Si⁴⁺ cations and O²⁻ anions.

Sodium and magnesium oxides, are composed of ions. Explain how this affects their sodium oxides behaviour with water.

Sodium oxide contains the oxide ion, O²⁻, which are a very strong base (strongly attracts protons) and so readily reacts with water to produce hydroxide ions - strongly alkaline solution.

Why does sodium react more vigorously with water than magnesium?

Takes less energy to lose one electron than it does to lose two!

Despite aluminium oxide being an ionic compound, it is insoluble. Why is this?

The bonding is too strong for the ions to be separated, partly because of the additional covalent bonding it has (Al³⁺).

The reactions between period 3 elements and oxygen are redox reactions, explain why.

The oxidation state of the Period 3 element increases and that of the oxygen decreases (0 to ⁻² in each case). The oxidation number of the Period 3 element in the oxide increases as you move from left to right across the period.

Why are the reactions between water and magnesium or steam redox reactions?

The oxidation state of the metal increases and that of some of the hydrogen atoms decreases.

Magnesium oxide's reaction with water produces a less alkaline solution than sodium oxide as it's less soluble than sodium oxide. Why is this?

This is because: Mg²⁺ has a greater charge than Na⁺, attracting the delocalised sea of electrons more strongly!

Describe the reaction between sodium and water and state what is produced.

Vigorous - sodium floats on water's surface and fizzes rapidly, melting due to heat energy released by the reaction. Strongly alkaline sodium hydroxide is formed.

How would you show that the period 3 metal oxides are ionic compounds?

You could heat until molten and see if the substance conducts electricity.

Aluminium oxides react with acids and alkalis, its an...

amphoteric oxide.

All Period 3 elements (aside from argon) are relatively reactive. Their oxides can all be prepared by...

direct reaction of the element with oxygen. The reactions are exothermic.

What is the pH of sulfur trioxide?

0 - 1

What is the pH of phosphorous (V) pentoxide?

1 - 2

Phosphoric (V) acid has three OH- groups, each has an acidic hydrogen atom. So, it'll react with sodium hydroxide in three stages, as each hydrogen in turn reacts with a hydroxide ion and is replaced by a sodium ion. Write the balanced symbol equations for these stages.

1. H₃PO₄(aq) + NaOH(aq) → NaH₂PO₄(aq) + H₂O(l) 2. H₃PO₄(aq) + 2NaOH(aq) → Na₂HPO₄(aq) + 2H₂O(l) 3. H₃PO₄(aq) + 3NaOH(aq) → Na₃PO₄(aq) + 3H₂O(l)

What is the pH of sodium oxide?

14

What is the pH of sulfur dioxide?

2 - 3

Write out the balanced symbol equation and describe the reaction between aluminum and oxygen.

2Al(s) + 1½O₂(g) → Al₂O₃(S) Aluminium powder burns brightly in air. But when aluminium powder is heated and then lowered into pure oxygen, it burns brightly to give aluminium oxide - a white powder.

Write the balanced symbol equation for the reaction between Phosphorous and oxygen.

4P(s) + 5O₂(g) → P₄O₁₀(s)

What is the pH of magnesium oxide?

9

Why is aluminium a good metal to coat window panes with?

Aluminium is a reactive metal, but it is always coated with a strongly bonded surface layer of oxide - protection from further reaction with water as its insoluble. Even if the surface is scratched, the exposed aluminium reacts rapidly with the air and seals off the surface.

Which oxides of period 3 elements are insoluble?

Aluminium oxide and Silicon dioxide

What is the pH of aluminium trioxide and silicon dioxide?

pH 7, as they don't dissolve in water and so the pH is of the solvent.