The alkali metals: sodium and its group: WASSCE Chemistry

Drop a small piece of sodium into water and it fizzes, melts into a silver ball and races across the surface; potassium bursts into a lilac flame. The alkali metals of group 1 are the most reactive metals of all. This lesson covers their properties and trends, their reactions, flame tests and the extraction and uses of sodium.

Learning objectives

By the end of this lesson you should be able to:

name the alkali metals and relate their properties to their electron configuration
describe their physical properties and explain how they are stored
describe and write equations for their reactions with water, oxygen and chlorine
state and explain the trends in reactivity and melting point down the group
describe flame tests for lithium, sodium and potassium
describe the extraction and uses of sodium

The alkali metals

Group 1: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr).

Element	Configuration	Ion
Lithium	2, 1	Li⁺
Sodium	2, 8, 1	Na⁺
Potassium	2, 8, 8, 1	K⁺

Each has one outer electron, which it loses easily to form a +1 ion with a noble-gas configuration. That is why they are so reactive. They never occur free in nature; they are found as compounds such as NaCl and KCl.

Physical properties

Property	Detail
Soft	Can be cut with a knife
Low density	Li, Na and K float on water
Low melting points	Na melts at 98 °C, K at 63 °C
Shiny when freshly cut	But tarnish quickly in air as an oxide layer forms
Good conductors	Of heat and electricity

Storage: under paraffin oil (kerosene), to keep them away from air and water, with which they react.

Chemical properties

With water

The metal reacts to form an alkaline hydroxide and hydrogen:

2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)}

\begin{gathered}2\text{Na(s)} + 2\text{H}_2\text{O(l)} \\[6pt] \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)}\end{gathered}

Metal	Observations
Lithium	Floats; fizzes steadily; does not melt
Sodium	Floats; melts into a silvery ball (the reaction is exothermic and Na has a low melting point); darts about propelled by hydrogen; fizzes; may burn with a yellow flame; solution turns red litmus blue
Potassium	Floats; reacts violently; the hydrogen ignites with a lilac flame; may spit

With oxygen

They tarnish rapidly in air and burn when heated:

4\text{Na(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Na}_2\text{O(s)}

\begin{gathered}4\text{Na(s)} + \text{O}_2\text{(g)} \\[6pt] \rightarrow 2\text{Na}_2\text{O(s)}\end{gathered}

Sodium burns with a yellow flame. In excess oxygen it forms the peroxide, Na₂O₂. The oxides dissolve in water to form alkalis: Na₂O + H₂O → 2NaOH.

With chlorine

Sodium burns in chlorine, forming white sodium chloride:

2\text{Na(s)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{NaCl(s)}

\begin{gathered}2\text{Na(s)} + \text{Cl}_2\text{(g)} \\[6pt] \rightarrow 2\text{NaCl(s)}\end{gathered}

Trends down the group

Property	Trend (Li → K → Cs)	Reason
Reactivity	Increases	The outer electron is further from the nucleus and more shielded, so it is lost more easily
Atomic radius	Increases	More shells
First ionisation energy	Decreases	Outer electron held less strongly
Melting point	Decreases	Larger atoms; weaker metallic bonding
Density	Generally increases

Flame tests

Dip a clean nichrome or platinum wire (cleaned in concentrated HCl) into the sample, then hold it in a non-luminous Bunsen flame.

Ion	Flame colour
Lithium, Li⁺	Crimson red
Sodium, Na⁺	Golden yellow (persistent)
Potassium, K⁺	Lilac (best seen through blue cobalt glass)
Calcium, Ca²⁺	Brick red
Copper, Cu²⁺	Blue-green

Extraction of sodium: the Downs cell

Sodium is too reactive to be extracted by carbon reduction. It is obtained by the electrolysis of molten sodium chloride.

Calcium chloride is added to lower the melting point from 801 °C to about 600 °C, saving energy.
Cathode (iron, −): Na⁺ + e⁻ → Na. The molten sodium floats up and is collected.
Anode (graphite, +): 2Cl⁻ → Cl₂ + 2e⁻. Chlorine is a valuable by-product.
A steel gauze keeps the sodium and chlorine apart, because they would otherwise react.

Uses of sodium

Sodium vapour street lamps, which give an intense yellow light
Coolant (liquid sodium) in some nuclear reactors, because it conducts heat very well
Making sodium compounds such as sodium peroxide and sodium cyanide
Extracting titanium from titanium(IV) chloride

In Sierra Leone

The orange-yellow street lights seen on some roads and junctions are sodium vapour lamps. Electricity excites sodium atoms, which give out the same golden-yellow light you see in a sodium flame test. The colour is a "fingerprint" of sodium.

Common mistake

Alkali metals become more reactive down the group: potassium is more reactive than sodium. Their melting points fall down the group: potassium melts at a lower temperature than sodium.

Key points

Group 1 metals have one outer electron and form M⁺ ions; they are stored under paraffin oil.
They are soft, low-density (float on water) and have low melting points.
With water: hydroxide + hydrogen (Na melts into a ball and darts; K gives a lilac flame). With oxygen: oxides; with chlorine: chlorides.
Reactivity increases and melting point decreases down the group.
Flame tests: Li crimson, Na yellow, K lilac.
Sodium is extracted by electrolysis of molten NaCl (with CaCl₂) in the Downs cell.

WASSCE examination focus

WAEC tests this

Expect to describe and explain the reaction of sodium with water (observations and equation), to explain the trend in reactivity down group 1, to give flame-test colours, and to describe the extraction of sodium, including why CaCl₂ is added.

Quick check

Why are alkali metals stored under paraffin oil?
Why does sodium melt into a ball when it reacts with water?
Explain why potassium is more reactive than sodium.
What is the flame colour of potassium ions?

Answers:

To keep them from reacting with air (oxygen and moisture) and water.
The reaction is exothermic and sodium has a low melting point.
Potassium's outer electron is further from the nucleus and more shielded, so it is lost more easily.
Lilac.

The alkali metals: sodium and its group