Chemistry · Chemistry of the elements · Grade 9

Alkali metals and soda

Why is sodium stored in paraffin oil, and why is soda made from table salt and limestone?

The situation

In a fume cupboard in the chemistry room a lump of sodium lies under kerosene. The lab technician cuts a piece the size of a grain of rice from it and drops it into a crystallising dish of water behind a safety screen: the piece melts into a bead and darts about, hissing, while the water turns pink from the phenolphthalein. At the Kungrad Soda Plant, meanwhile, a sodium compound — soda ash — is made from table salt and limestone at hundreds of thousands of tonnes a year. Why are the alkali metals so reactive, how can they be identified, and how is soda made?

Topic

Alkali metals and soda production. Kungrad Soda Plant: soda from table salt, limestone and ammonia (Solvay process); sodium darts across water, releasing hydrogen

Key ideas

Alkali metals
Flame colour
Solvay process

Formulas

2 Na + 2 H_{2} O \overset{}{\to} 2 NaOH + H_{2} ↑

An alkali metal decomposes water: an alkali (phenolphthalein turns pink) and hydrogen are formed.

V (H_{2}) = \frac{1}{2} \cdot \frac{m}{A_{r}} \cdot V_{m}

Two moles of metal give one mole of H₂; at STP $V_{m}$ = 22.4 L/mol.

NaCl + NH_{3} + CO_{2} + H_{2} O \overset{}{\to} NaHCO_{3} ↓ + NH_{4} Cl

The key stage of the Solvay process: sparingly soluble baking soda precipitates.

2 NaHCO_{3} \overset{}{\to} Na_{2} CO_{3} + CO_{2} ↑ + H_{2} O

Calcination: baking soda is heated and turns into soda ash, and the CO₂ is used again.

2 NaCl + CaCO_{3} \overset{}{\to} Na_{2} CO_{3} + CaCl_{2}

The overall equation of the Solvay process (the ammonia returns round the cycle).

Theory

The alkali metals: structure and reactivity

The main subgroup of group I: lithium, sodium, potassium, rubidium, caesium, francium. They have one electron in the outer shell ( $n s^{1}$ ) and an oxidation state of +1 in compounds; they give this electron up easily — they are the strongest reducing agents. From Li to Cs the atomic radius increases (128 → 244 pm) and the first ionisation energy decreases (5.39 → 3.89 eV) — reactivity increases and the melting point falls (180.5 → 28.4 °C). All are silvery (caesium is golden), soft (they can be cut with a knife) and light: lithium (0.534 g/cm³) is the lightest metal. They tarnish quickly in air: $4 Na + O_{2} \overset{}{\to} 2 Na_{2} O$ , and when they burn: $2 Na + O_{2} \overset{}{\to} Na_{2} O_{2}$ ; lithium also combines with nitrogen: $6 Li + N_{2} \overset{}{\to} 2 Li_{3} N$ . That is why Na and K are stored under kerosene, while lithium (lighter than kerosene — it floats up) is stored under petroleum jelly or in argon. They are obtained by electrolysis of the melt: $2 NaCl \overset{}{\to} 2 Na + Cl_{2} ↑$ . Their hydroxides are strong bases (alkalis), and the strength of the base increases from LiOH to CsOH.

Reaction with water

$2 M + 2 H_{2} O \overset{}{\to} 2 MOH + H_{2} ↑$ (M = Li, Na, K…). Lithium fizzes quietly in water — it melts only at 180.5 °C, the piece does not melt, and the reacting surface stays small. Sodium melts from the heat of the reaction (97.8 °C), turns into a shiny bead, and the stream of hydrogen sends it skating across the water; 1 mol of sodium releases ≈ 184 kJ of heat. Potassium (melts at 63.5 °C) catches fire at once — the hydrogen burns with a pale violet flame, and at the end it often spatters with a pop. Rubidium and caesium react explosively. The solution becomes alkaline — phenolphthalein turns pink. Calculation: 0.46 g Na = 20 mmol → 10 mmol H₂ = 224 mL (STP).

Important compounds of sodium and potassium

NaOH is caustic soda: it is made by electrolysis of a solution of table salt, and used in the soap, paper and aluminium industries. Na₂CO₃ is soda ash: glass, detergents; Na₂CO₃·10H₂O is washing soda; NaHCO₃ is baking soda: it releases CO₂ on heating or with an acid (dough rises). NaCl is table salt, the main raw material of the chemical industry. KCl is a potassium fertiliser: the Dehkanabad potash plant (since 2010) makes potassium chloride from the sylvinite of the Tyubegatan deposit. KNO₃ is saltpetre (fertiliser, gunpowder), and K₂CO₃ is potash (found in plant ash). Sodium compounds can be converted into one another: $Na \overset{}{\to} NaOH \overset{}{\to} Na_{2} CO_{3} \overset{}{\to} NaHCO_{3}$ .

Flame colour and practical 2

When volatile salts are heated in a flame, the electrons of the cation rise to a higher level and, on returning, emit light of a definite colour: Li⁺ — deep red (670.8 nm), Na⁺ — yellow (589 nm), K⁺ — pale violet (766–770 nm and 404 nm), Ca²⁺ — brick red, Sr²⁺ — crimson, Cu²⁺ — bluish green. The yellow of sodium is so strong that it is present as an impurity even in potassium salts, so the potassium flame is viewed through blue cobalt glass — it absorbs the yellow light. Practical 2: three test tubes contain NaCl, KCl and CaCl₂: Na⁺ and K⁺ are identified by the flame test, while Ca²⁺ gives a white precipitate with a solution of soda: $CaCl_{2} + Na_{2} CO_{3} \overset{}{\to} CaCO_{3} ↓ + 2 NaCl$ . Before every test the loop is washed in hydrochloric acid and heated until the flame is colourless.

Soda production (the Solvay process)

The ammonia process was introduced into industry in the 1860s by the Belgian Ernest Solvay. 1) A saturated solution of table salt (brine) is purified and saturated with ammonia. 2) CO₂ is passed into it in columns: $NaCl + NH_{3} + CO_{2} + H_{2} O \overset{}{\to} NaHCO_{3} ↓ + NH_{4} Cl$ — NaHCO₃ is the least soluble salt in cold solution (9.6 g per 100 g of water at 20 °C; NaCl — 35.9 g, NH₄Cl — 37.2 g), so it precipitates. 3) The filtered NaHCO₃ is calcined: $2 NaHCO_{3} \overset{}{\to} Na_{2} CO_{3} + CO_{2} ↑ + H_{2} O$ . 4) The CO₂ is obtained by burning limestone: $CaCO_{3} \overset{}{\to} CaO + CO_{2} ↑$ . 5) The ammonia is recovered: $2 NH_{4} Cl + Ca (OH)_{2} \overset{}{\to} CaCl_{2} + 2 NH_{3} ↑ + 2 H_{2} O$ . Overall: $2 NaCl + CaCO_{3} \overset{}{\to} Na_{2} CO_{3} + CaCl_{2}$ ; the waste is a CaCl₂ solution. About 70 % of the sodium ends up in the soda, so in practice ≈ 1.5–1.6 t of salt is used per 1 t of soda. The Kungrad Soda Plant (Karakalpakstan) works by this process: it started up in 2006, and since 2016 its design capacity has been 200 thousand t of soda ash a year; the salt comes from the Barsakelmes deposit and the limestone from the Jamansoy deposit.

Safety and the model

Safety: take alkali metals only with tweezers and never touch them with your hands; the piece must not be bigger than a grain of rice, and blot off excess kerosene with filter paper; drop the metal into water in a fume cupboard behind a safety screen, wearing goggles; the vessel must be large and the water not too little; destroy leftover potassium and sodium in alcohol, not in water. If NaOH solution gets into the eyes, rinse at once with plenty of water. In the flame test be careful with the hydrochloric acid used to wash the loop. Model: the reaction of a piece proceeds over its surface ( $m^{1 / 3}$ decreases linearly; the rate constants are schematic), and the motion on the water is random; the volume of hydrogen is reduced to STP; the pH in the crystallising dish is calculated from the NaOH concentration for an ideal solution (absorption of CO₂ is ignored); the flame colours are the typical appearance (the view through cobalt glass is schematic); the plant diagram is simplified and the flow figures are representative.

Did you know?

The Kungrad Soda Plant (Karakalpakstan) started up in 2006 — it is cited as the only enterprise in Central Asia that makes soda ash; after the expansion in 2016 its design capacity is 200 thousand t a year.
The yellow light of low-pressure sodium street lamps comes from the lines of sodium atoms at wavelengths of 589.0 and 589.6 nm — the very same yellow as in the flame test.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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