Gas Preparation and Gas Tests

Key Concepts

Gases can be produced in the school laboratory when:
⚛ a compound is heated and undergoes thermal decomposition
⚛ electricity is used to decompose a compound (electrolytic decomposition)

⚛ a substance reacts with water or an acid in a displacement reaction
⚛ a substance undergoes combustion in air

The table below summarises tests for the following gases commonly prepared in the school laboratory:
⚛ oxygen gas, O_2(g)
⚛ hydrogen gas, H_2(g)
⚛ carbon dioxide gas, CO_2(g)
⚛ nitrogen dioxide gas, NO_2(g)
⚛ sulfur dioxide gas, SO_2(g)
⚛ hydrogen sulfide gas, H₂S_(g)
⚛ chlorine gas, Cl_2(g)

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Preparation of, and Testing for, Some Common Gases

Gas	Example of Preparation	Test
Oxygen (O₂)	Decomposition of hydrogen peroxide H₂O₂ → H₂O + ½O_2(g) Thermal decomposition of some metallic oxides 2PbO → 2Pb + O_2(g)	Ignites a glowing splint since oxygen supports combustion
Hydrogen (H₂)	acid¹ + metal → salt + hydrogen gas 2HCl + Zn → ZnCl₂ + H₂(g) active metal + water → hydroxide + hydrogen gas Ca_(s) + 2H₂O → Ca(OH)₂ + H_2(g)	"Pop Test" : a lit taper will produce an audible "pop" when placed in hydrogen gas
Carbon dioxide (CO₂)	acid + carbonate → salt + water + carbon dioxide 2HCl + CaCO₃ → CaCl₂ + H₂O + CO_2(g) Thermal decomposition of some carbonates CuCO₃ → CuO + CO_2(g)	When CO_2(g) is bubbled through colourless limewater (Ca(OH)_2(aq)) the limewater turns milky due to the formation of a calcium carbonate (CaCO_3(s)) precipitate. When CO_2(g) is bubbled through Ba(OH)_2(aq) a white precipitate of BaCO_3(s) forms.
Nitrogen dioxide (NO₂)	Thermal decomposition of some nitrates 2Ni(NO₃)₂ → 2NiO + O₂+ 4NO_2(g) 2Pb(NO₃)₂ → 2PbO + O₂+4NO_2(g) Less active metals + nitric acid → nitrate + water + nitrogen dioxide Cu + 4HNO₃ → Cu(NO₃)₂ + 2H₂O + 2NO_2(g)	Nitrogen dioxide is a red-brown gas which is soluble in water and has a pungent odour.
Sulfur dioxide (SO₂)	Combustion of sulfur S_(s) + O₂ → SO_2(g) Sulfites + acid → water + sulfur dioxide SO₃^2- + 2H⁺ → H₂O + SO_2(g) Hot concentrated sulfuric acid + copper 3H₂SO₄ + Cu_(s) → Cu²⁺ + 2HSO₄^- + 2H₂O + SO_2(g)	SO_2(g) is bubbled through hydrogen peroxide solution (H₂O₂) to produce sulfuric acid (H₂SO₄). The addition of barium chloride solution (BaCl_2(aq)) to this solution results in a precipitate of barium sulfate (BaSO_4(s)). SO_2(g) passed through a concentrated, acidified solution of potassium dichromate (K₂Cr₂O₇) causes the dichromate solution to change colour from orange to green due to the formation of chromium(III) ions (Cr³⁺).
Hydrogen sulfide (H₂S)	sulfide + acid → salt + hydrogen sulfide FeS_(s) + 2HCl → FeCl₂ + H₂S_(g)	H₂S is often referred to as "rotten egg gas" as it smells just like rotten eggs! H₂S_(g) is passed through a solution of zinc chloride (ZnCl₂) and a white precipitate of zinc sulfide forms (ZnS_(s)). Pass H₂S_(g) over a damp filter paper impregnated with lead(II) nitrate (Pb(NO₃)₂), the paper turns black as lead(II) sulfide (PbS_(s)) forms.
Chlorine (Cl₂)	MnO_2(s) + 2Cl^- + 4H⁺ → Mn²⁺ + 2H₂O + Cl_2(g) Electrolysis of molten sodium chloride (NaCl) 2NaCl → 2Na(Hg) + Cl_2(g) (Na(Hg) is a mercury amalgam of sodium metal since sodium is slightly soluble in mercury but doesn't react with mercury as it does with other solvents like water.)	Cl_2(g) is a pale yellow-green gas which bleaches coloured materials and has a characteristic choking odour.

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Worked Example

Question:

Jo the Geologist has given Chris the Chemist a sample of a lead ore to identify.
The ore is most likely to be an oxide, carbonate or nitrate.
Chris uses gentle heat to decompose the lead ore in a fume cupboard.
The gas given off is then tested and the results recorded as shown below:

Test Observation

appearance colourless gas

odour odourless gas

test with lit taper taper continues to burn

test with glowing taper taper re-ignites

Is Joe's lead ore an oxide, a carbonate or a nitrate?

Test	Observation
appearance	colourless gas
odour	odourless gas
test with lit taper	taper continues to burn
test with glowing taper	taper re-ignites

Worked Solution: (based on StoPGoPS approach to problem solving)

What have you been asked to do?

Identify the lead ore

What information have you been given in the question?

(i) Lead ore could be an oxide, carbonate or nitrate
(ii) results of the gas tests:

Test Observation

appearance colourless gas

odour odourless gas

test with lit taper taper continues to burn

test with glowing taper taper re-ignites

Test	Observation
appearance	colourless gas
odour	odourless gas
test with lit taper	taper continues to burn
test with glowing taper	taper re-ignites

What is the relationship between what you know and what you need to find out?
(a) The most likely thermal decomposition reactions are:

(i) decomposition of oxide to produce oxygen gas

(ii) decomposition of carbonate to produce carbon dioxide gas

(iii) decomposition of nitrate to produce nitrogen dioxide gas

(b) Test for each of these gases:

(i) oxygen gas: colourless, odourless, supports combustion

(ii) carbon dioxide gas: colourless, odourless, does not support combustion

(iii) nitrogen dioxide gas: red-brown gas with a pungent odour

Determine the identity of the gas and infer the identity of the ore

(a) Gas is oxygen since it is odourless, colourless and supports combustion
(the lit taper was not extinguished so the gas supports combustion, the glowing taper re-ignited since the presence of oxygen allows combustion to re-commence).
(b) Since the gas is oxygen, the lead ore is most likely to be an oxide since oxides decompose to produce oxygen gas.

Is your answer plausible?

Work backwards:
(i) If the ore had been a nitrate then pungent, red-brown NO_2(g) would have been produced. The gas produced was colourless and odourless so the ore is not a nitrate.
(ii) If the ore had been a carbonate then odourless, colourless CO_2(g) would have been produced, but CO_2(g) does not support combustion and would have extinguished the lit taper, and, would never have allowed a glowing taper to re-ignite. So the ore is not a carbonate.
(iii) Since the ore was either an oxide, carbonate or nitrate, and since we find that it could NOT be a carbonate or a nitrate, the ore must be an oxide.

State your solution to the problem.

The sample of ore is a lead oxide.

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1. "acid" refers to a non-oxidizing acid such as dilute hydrochloric acid or dilute sulfuric acid.