Sodium sulfide

Key takeaways

Sodium sulfide belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sodium sulfide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sodium sulfide from memory before moving on to harder problems.

Reference excerpt

Sodium sulfide is a chemical compound with the formula Na2S. More common is its hydrate Na2S·9H2O. Both the anhydrous and the hydrated salts are colorless solids, although technical grades of sodium sulfide are generally yellow to brick red owing to the presence of polysulfides. It is commonly supplied as a crystalline mass, in flake form, or as a fused solid. They are water-soluble, giving strongly alkaline solutions. When exposed to moisture, Na2S is immediately hydrolyzed to give sodium hydrosulfide. Sodium sulfide has an unpleasant rotten egg smell due to the formation of hydrogen sulfide by hydrolysis in moist air. Some commercial samples are described as Na2S·xH2O, where a weight percentage of Na2S is specified. Commonly available grades have around 60% Na2S by weight, which means that x is around 3. These grades of sodium sulfide are often marketed as "sodium sulfide flakes". These samples consist of NaSH, NaOH, and water.

Structure The structures of sodium sulfides have been determined by X-ray crystallography. The nonahydrate features S2- hydrogen-bonded to 12 water molecules. The pentahydrate consists of S2- centers bound to Na+ and encased by an array of hydrogen bonds. Anhydrous Na2S, which is rarely encountered, adopts the antifluorite structure, which means that the Na+ centers occupy sites of the fluoride in the CaF2 framework, and the larger S2− occupy the sites for Ca2+.

Production Industrially Na2S is produced by carbothermic reduction of sodium sulfate often using coal:

Na2SO4 + 2 C → Na2S + 2 CO2 In the laboratory, the salt can be prepared by reduction of sulfur with sodium in anhydrous ammonia, or by sodium in dry THF with a catalytic amount of naphthalene (forming sodium naphthalenide):

2 Na + S → Na2S

Reactions with inorganic reagents The sulfide ion in sulfide salts such as sodium sulfide can incorporate a proton into the salt by protonation:

S2− + H+ → SH− Because of this capture of the proton ( H+), sodium sulfide has basic character. Sodium sulfide is strongly basic, able to absorb two protons. Its conjugate acid is sodium hydrosulfide (SH−). An aqueous solution contains a significant portion of sulfide ions that are singly protonated.

S2− + H2O ↽ − ⇀ {\displaystyle {\ce {<=>>}}} SH− + OH− SH− + H2O ↽ − ⇀ {\displaystyle {\ce {<<=>}}} H2S + OH− Sodium sulfide is unstable in the presence of water due to the gradual loss of hydrogen sulfide into the atmosphere. When heated with oxygen and carbon dioxide, sodium sulfide can oxidize to sodium carbonate and sulfur dioxide:

2 Na2S + 3 O2 + 2 CO2 → 2 Na2CO3 + 2 SO2 Oxidation with hydrogen peroxide gives sodium sulfate:

Na2S + 4 H2O2 → 4 H2O + Na2SO4 Upon treatment with sulfur, sodium polysulfides are formed:

2 Na2S + S8 → 2 Na2S5

Pulp and paper industry In terms of its dominant use, sodium sulfide is primarily used in the kraft process in the pulp and paper industry. It aids in the delignification process, affording cellulose, which is the main component of paper. It is used in water treatment as an oxygen scavenger agent and also as a metals precipitant; in chemical photography for toning black and white photographs; in the textile industry as a bleaching agent, for desulfurising and as a dechlorinating agent; and in the leather trade for the sulfitisation of tanning extracts. It is used in chemical manufacturing as a sulfonation and sulfomethylation agent. It is used in the production of rubber chemicals, sulfur dyes and other chemical compounds. It is used in other applications including ore flotation, oil recovery, making dyes, and detergent. It is also used during leather processing, as an unhairing agent in the liming operation.

Reagent in organic chemistry

Installation of carbon-sulfur bonds Alkylation of sodium sulfide give thioethers:

Na2S + 2 RX → R2S + 2 NaX Even aryl halides participate in this reaction. By a broadly similar process sodium sulfide can react with alkenes in the thiol-ene reaction to give thioethers. Sodium sulfide can be used as nucleophile in Sandmeyer type reactions.

Reducing agent Aqueous solution of sodium sulfide will reduce nitro groups to amine. This conversion is applied to production of some azo dyes since other reducible groups, e.g. azo group, remain intact. The reduction of nitro aromatic compounds to amines using sodium sulfide is known as the Zinin reaction in honor of its discoverer. Hydrated sodium sulfide reduces 1,3-dinitrobenzene derivatives to the 3-nitroanilines.

Other reactions Sulfide has also been employed in photocatalytic applications.

Safety Consisting of the equivalent of sodium hydroxide, sodium sulfide is strongly alkaline and can cause chemical burns. It reacts rapidly with acids to produce hydrogen sulfide, a gas which is both highly toxic and potentially explosive. Sodium sulfide hydrolyses in water to form smaller amounts of hydrogen sulfide which also makes it very toxic to aquatic life.

References

Illustrations

Worked examples

Example 1 — a first encounter with Sodium sulfide

Start with the simplest possible case. Write down what Sodium sulfide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sodium sulfide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sodium sulfide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sodium sulfide

In research: Sodium sulfide appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sodium sulfide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sodium sulfide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fluorite crystal structure, Photographic chemicals, Sodium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Sodium sulfide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Sodium sulfide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sodium sulfide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sodium sulfide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sodium sulfide in simple terms?

Sodium sulfide is a chemical compound with the formula Na2S. More common is its hydrate Na2S·9H2O.

Why does Sodium sulfide matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sodium sulfide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sodium sulfide.