Period 3 Elements - Reactions with Oxygen and Water — AQA A-level Chemistry

SPEC 3.2.4 - PROPERTIES OF PERIOD 3 ELEMENTS

Period 3 Reactions: With Water & Oxygen

How Period 3 elements react with oxygen and water, and the patterns across the period

Overview of Period 3 Elements

Period 3 contains eight elements: sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), and argon (Ar). The most obvious physical trend is from metals on the left to non-metals on the right.

Sodium, magnesium, and aluminium are metallic — they are shiny (when freshly exposed to air), conduct electricity, and react with dilute acids to give hydrogen and salts. Silicon is a semi-metal (metalloid) — it conducts electricity to some extent, making it useful in semiconductor devices. Phosphorus, sulfur, and chlorine are typical non-metals with low melting and boiling points. Argon is a noble gas, existing as separate atoms and chemically unreactive.

Redox Reactions

The reactions of Period 3 elements with oxygen and water are all redox reactions. Every element starts with an oxidation state of zero, and after reacting, ends up with a positive or negative oxidation state. The element is oxidised (loses electrons) while oxygen or hydrogen in water is reduced (gains electrons).

Reactions with Oxygen

All elements in Period 3 (except argon) react with oxygen to form oxides. Their oxides can all be prepared by direct reaction (burning) of the element with oxygen, and the reactions are exothermic. In their highest oxides, each element adopts an oxidation state equal to its group number — with sulfur as the exception.

The oxidation state of the Period 3 element in its oxide generally increases as we move from left to right across the period. This is a key trend to remember for exam questions.

ElementOxideOx. StateFlame ColourReaction

Na

Na_{2} O

+1YellowVigorous

Mg

MgO

+2Bright whiteVigorous

Al

Al_{2} O_{3}

+3Brilliant whiteSlow (oxide layer)

Si

SiO_{2}

+4WhiteSlow (strong heating)

P

P_{4} O_{10}

+5White smokeSpontaneous (white P)

S

SO_{2}

/

SO_{3}

+4 / +6BlueBurns steadily

Storage Conditions

Sodium is stored under oil to prevent contact with air and moisture. White phosphorus is stored under water because it spontaneously ignites in air. These storage requirements are frequently examined.

Equations: Metals with Oxygen

Sodium burns brightly in air with a characteristic yellow flame to form white sodium oxide:

0 0 +1 -2

2Na_{(s)} + \frac{1}{2} O_{2 (g)} \to Na_{2} O_{(s)}

Magnesium ribbon burns in air with a brilliant white flame. The white powder produced is magnesium oxide. If burning magnesium is lowered into a gas jar of pure oxygen, the flame is even more intense:

0 0 +2 -2

2Mg_{(s)} + O_{2 (g)} \to 2MgO_{(s)}

Aluminium powder burns brightly when heated and lowered into a gas jar of oxygen, giving aluminium oxide — a white powder. However, aluminium is always coated with a strongly bonded surface layer of Al₂O₃ (approximately 2-3 nm thick) which protects it from further reaction. This process is called passivation. Even if the surface is scratched, the exposed aluminium reacts instantly with oxygen in the air and reseals itself.

0 0 +3 -2

4Al_{(s)} + 3O_{2 (g)} \to 2Al_{2} O_{3 (s)}

Why This Matters

Because of passivation, aluminium appears unreactive despite being high in the reactivity series. This is why aluminium is used for aircraft bodies, window frames, and cooking utensils — it doesn't corrode in normal conditions.

Equations: Non-Metals with Oxygen

Silicon requires strong heating to react with oxygen because of the high activation energy needed to break the strong Si-Si covalent bonds in the element:

0 0 +4 -2

Si_{(s)} + O_{2 (g)} \to SiO_{2 (s)}

Phosphorus exists as two main allotropes. White phosphorus spontaneously ignites in air (hence stored under water), producing white smoke of P₄O₁₀. Red phosphorus must be heated first. In limited oxygen, phosphorus trioxide forms:

0 0 +5 -2

P_{4 (s)} + 5O_{2 (g)} \to P_{4} O_{10 (s)}

(excess O₂)

0 0 +3 -2

P_{4 (s)} + 3O_{2 (g)} \to P_{4} O_{6 (s)}

(limited O₂)

Sulfur powder burns with a blue flame to form sulfur dioxide:

0 0 +4 -2

S_{(s)} + O_{2 (g)} \to SO_{2 (g)}

Sulfur trioxide is formed by further oxidation of SO₂ — this is the key step in the Contact Process for manufacturing sulfuric acid:

+4 -2 0 +6 -2

2SO_{2 (g)} + O_{2 (g)} ⇌ 2SO_{3 (g)}

(450°C, V₂O₅ catalyst, 1-2 atm)

Worked Example01

A Period 3 element X forms an oxide with the formula X₂O₃. The element burns with a brilliant white flame but appears unreactive under normal conditions.

(a) Identify element X.

(b) Write the balanced equation for the reaction with oxygen.

(c) Explain why X appears unreactive despite burning vigorously when heated.

(a)

From the formula X₂O₃: oxygen is -2, so total negative charge = 3 × (-2) = -6. For neutrality, 2X = +6, so X = +3. The Period 3 element with +3 oxidation state in its oxide is aluminium (Al).

(b)

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

(c)

Aluminium is coated with a thin layer (~2-3 nm) of Al₂O₃ that forms immediately on exposure to air. This oxide layer is impermeable and strongly adherent (passivation). It prevents oxygen and water from reaching the metal beneath. Even if scratched, the exposed aluminium instantly reacts with air to reform the protective layer.

Key insight: The question uses the oxide formula to test whether you can work backwards to identify the element — a common exam technique.

Reactions with Water - Sodium

Sodium and magnesium are the only metal elements in Period 3 that react with cold water. (Chlorine is the only non-metal that reacts with water — via disproportionation.)

The reaction of sodium with water is vigorous — the sodium floats on the surface (density 0.97 g/cm³), melts into a silvery ball due to the exothermic heat released (m.p. 98°C), and fizzes rapidly as hydrogen is evolved. A strongly alkaline solution of sodium hydroxide forms with pH 13-14.

Equation with Oxidation States

0 +1 -2 +1 -2+1 0

2Na_{(s)} + 2H_{2} O_{(l)} \to 2NaOH_{(a q)} + H_{2 (g)}

In this redox reaction, sodium is oxidised (0 → +1) and hydrogen in water is reduced (+1 → 0). Sodium loses one electron per atom to form Na⁺ ions.

Observations to Learn

Floats (low density) → melts into ball (exothermic, low m.p.) → fizzes (H₂ evolved) → moves rapidly (propelled by gas) → gets smaller/disappears (NaOH dissolves) → may ignite with yellow flame (large pieces) → solution turns indicator purple/violet (pH 13-14).

Reactions with Water - Magnesium

The reaction of magnesium with cold water is very slow — only a few bubbles of hydrogen form after several days. The solution is less alkaline (pH ~10) because magnesium hydroxide is only sparingly soluble.

With Cold Water (Very Slow)

Mg_{(s)} + 2H_{2} O_{(l)} \to Mg(OH)_{2 (s)} + H_{2 (g)}

(Mg(OH)₂ is sparingly soluble — forms a white precipitate/suspension)

The reaction is much faster with steam, producing a violent reaction with a bright white flame. Steam provides greater energy to overcome the activation energy barrier.

With Steam (Vigorous)

Mg_{(s)} + H_{2} O_{(g)} \to MgO_{(s)} + H_{2 (g)}

Why Sodium is More Reactive

Sodium loses 1 electron (1st IE = 496 kJ mol⁻¹). Magnesium loses 2 electrons (1st IE + 2nd IE = 738 + 1451 = 2189 kJ mol⁻¹ total). The much higher total ionisation energy for Mg²⁺ formation means more energy is required, making magnesium less reactive. Additionally, the sparingly soluble Mg(OH)₂ forms a protective layer that slows further reaction.

Why Different Products?

Cold water → Mg(OH)₂. Steam → MgO. At high temperatures, any Mg(OH)₂ immediately dehydrates: Mg(OH)₂ → MgO + H₂O. This is why MgO is the final product with steam.

Worked Example02

A student places a small piece of an unknown Period 3 metal into cold water. The metal sinks to the bottom, reacts very slowly with only a few bubbles appearing over several minutes, and when tested, the solution has a pH of approximately 10.

(a) Identify the metal and explain your reasoning.

(b) Suggest how the student could make this reaction proceed more vigorously.

(c) Write the equation for the vigorous reaction and state the product.

(a)

The metal is magnesium. Evidence: (1) It sinks — Mg has density 1.74 g/cm³, greater than water (Na would float). (2) Very slow reaction with cold water — characteristic of Mg. (3) pH ~10 — Mg(OH)₂ is sparingly soluble, giving a weakly alkaline solution (Na would give pH 13-14).

(b)

Use steam instead of cold water. Steam provides greater energy to the reaction, overcoming the higher activation energy required.

(c)

Mg(s) + H₂O(g) → MgO(s) + H₂(g)

Product: Magnesium oxide (white solid) and hydrogen gas. Magnesium burns with a bright white flame.

Key insight: This tests your ability to use observations (density, reaction rate, pH) to identify an unknown metal — a common practical-based question style.

Other Elements with Water

Aluminium does not react with water under normal conditions due to its protective oxide layer. If this layer were removed (e.g., by amalgamation with mercury), aluminium would react vigorously.

Silicon, phosphorus, and sulfur do not react with water. These non-metallic elements lack the electropositive character needed to reduce water.

ElementReaction with Cold WaterProductspH

SodiumVigorousNaOH + H₂13-14

MagnesiumVery slowMg(OH)₂ + H₂~10

Magnesium + steamVigorousMgO + H₂—

AluminiumNo reaction— (oxide layer)—

Si, P, SNo reaction——

Trend Summary

Reactivity with water decreases across Period 3 metals (Na > Mg > Al). This correlates with increasing ionisation energy and the formation of protective oxide/hydroxide layers.

Chlorine with Water

Chlorine is the only non-metal in Period 3 that reacts with water. It undergoes a disproportionation reaction — a redox reaction where the same element is simultaneously oxidised and reduced.

Disproportionation

In disproportionation, a single species acts as both oxidising agent and reducing agent. Here, chlorine (oxidation state 0) is simultaneously oxidised to +1 (in HOCl) and reduced to -1 (in HCl).

Cl_{2 (g)} + H_{2} O_{(l)} ⇌ HCl_{(a q)} + HOCl_{(a q)}

The equilibrium lies to the left — only a small proportion of chlorine reacts. The solution ("chlorine water") contains Cl₂, HCl, and HOCl. HOCl is a powerful oxidising agent used as a disinfectant in swimming pools and drinking water.

Worked Example03

Chlorine gas is bubbled through water and then sodium hydroxide solution is added.

(a) Write the equation for chlorine reacting with water and identify the oxidation state changes.

(b) Explain what happens to the position of equilibrium when NaOH is added.

(c) If 0.10 mol of Cl₂ completely reacted with excess NaOH, calculate the moles of each chlorine-containing product formed.

(a)

Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HOCl(aq)

Cl in Cl₂: 0 → Cl in HCl: -1 (reduction, gain of electron)

Cl in Cl₂: 0 → Cl in HOCl: +1 (oxidation, loss of electron)

(b)

NaOH neutralises both HCl and HOCl (both are acids):

HCl + NaOH → NaCl + H₂O

HOCl + NaOH → NaOCl + H₂O

Removing products shifts equilibrium to the RIGHT (Le Chatelier's principle), so more Cl₂ reacts.

(c)

From the equation, 1 mol Cl₂ produces 1 mol HCl and 1 mol HOCl.

With NaOH: HCl → NaCl (0.10 mol) and HOCl → NaOCl (0.10 mol)

Answer: 0.10 mol NaCl and 0.10 mol NaOCl

Practice

Question 1: A student burns element Y in oxygen and observes a blue flame. The gaseous product has a choking smell and turns damp blue litmus paper red.

(a) Identify element Y and write the balanced equation for this reaction.

(b) State the oxidation state of Y in the product.

(c) Explain why a different oxide of Y requires a catalyst to form, and write its equation.

(a)

Element Y is sulfur (S).

Evidence: Blue flame + choking gas + acidic (turns litmus red) = SO₂

Equation: S(s) + O₂(g) → SO₂(g)

(b)

Oxidation state of S in SO₂: Oxygen is -2, so S must be +4 (since +4 + 2(-2) = 0).

(c)

SO₃ is formed by oxidation of SO₂ in the Contact Process, which requires a catalyst (V₂O₅), high temperature (~450°C), and moderate pressure (1-2 atm). The reaction is reversible and exothermic.

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) [Contact Process]

Practice

Question 2: Aluminium is used to make aircraft bodies despite being more reactive than iron. Explain why aluminium is suitable for this purpose, with reference to its reaction with oxygen and water. Include the term "passivation" in your answer.

1

Although aluminium is high in the reactivity series, it undergoes passivation — the rapid formation of a thin (2-3 nm) but impermeable layer of aluminium oxide (Al₂O₃) on its surface.

2

This oxide layer is strongly adherent and prevents oxygen and water from reaching the metal beneath. Unlike iron oxide (rust), which is porous and flakes off, Al₂O₃ protects the metal completely.

3

Even if the surface is scratched, the exposed aluminium immediately reacts with oxygen in the air to reform the protective layer, "self-healing" the damage.

4

Therefore, aluminium does not corrode under normal atmospheric conditions, making it ideal for aircraft where low density and corrosion resistance are essential.

Practice

Question 3: A chemist needs to distinguish between samples of sodium and magnesium using only water and universal indicator paper.

(a) Describe the experiment and the observations for each metal.

(b) State the equations for both reactions.

(c) Explain why the pH values are different in terms of solubility.

(a)

Add each metal to separate beakers of cold water. Test the solution with universal indicator.

Sodium: Floats, melts into ball, fizzes vigorously, moves rapidly, may ignite (yellow flame). Indicator turns purple/blue (pH 13-14).

Magnesium: Sinks, very slow reaction, few bubbles over time. Indicator turns blue/green (pH ~10).

(b)

Sodium: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Magnesium: Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)

(c)

Sodium hydroxide (NaOH) is highly soluble in water, so a high concentration of OH⁻ ions forms → pH 13-14.

Magnesium hydroxide (Mg(OH)₂) is only sparingly soluble, so fewer OH⁻ ions dissolve → pH ~10.

Summary

Reactions with Oxygen

ElementOxideOx. StateFlameNotes

NaNa₂O+1YellowStored under oil

MgMgO+2Bright white—

AlAl₂O₃+3Brilliant whitePassivation

SiSiO₂+4WhiteNeeds strong heating

PP₄O₁₀ / P₄O₆+5 / +3White smokeWhite P stored under water

SSO₂ / SO₃+4 / +6BlueSO₃ via Contact Process

Key Exam Points

Na + water: vigorous, pH 13-14, floats, melts, fizzes

Mg + cold water: slow, pH ~10 (sparingly soluble hydroxide)

Mg + steam: vigorous, white flame, produces MgO

Cl₂ + H₂O: disproportionation (0 → -1 and +1 simultaneously)

Al passivation: oxide layer self-heals if scratched

Oxidation state trend: increases across Period 3 (+1 to +6)