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Gas Preparation and Gas Tests

Key Concepts

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Preparation of, and Testing for, Some Common Gases

Gas Example of Preparation Test
Decomposition of hydrogen peroxide

H2O2 → H2O + ½O2(g)

Thermal decomposition of some metallic oxides

2PbO → 2Pb + O2(g)

Ignites a glowing splint since oxygen supports combustion
acid1 + metal → salt + hydrogen gas

2HCl + Zn → ZnCl2 + H2(g)

active metal + water → hydroxide + hydrogen gas

Ca(s) + 2H2O → Ca(OH)2 + H2(g)

"Pop Test" : a lit taper will produce an audible "pop" when placed in hydrogen gas
Carbon dioxide
acid + carbonate → salt + water + carbon dioxide

2HCl + CaCO3 → CaCl2 + H2O + CO2(g)

Thermal decomposition of some carbonates

CuCO3 → CuO + CO2(g)

When CO2(g) is bubbled through colourless limewater (Ca(OH)2(aq)) the limewater turns milky due to the formation of a calcium carbonate (CaCO3(s)) precipitate.

When CO2(g) is bubbled through Ba(OH)2(aq) a white precipitate of BaCO3(s) forms.

Nitrogen dioxide
Thermal decomposition of some nitrates

2Ni(NO3)2 → 2NiO + O2+ 4NO2(g)

2Pb(NO3)2 → 2PbO + O2+4NO2(g)

Less active metals + nitric acid → nitrate + water + nitrogen dioxide

Cu + 4HNO3 → Cu(NO3)2 + 2H2O + 2NO2(g)

Nitrogen dioxide is a red-brown gas which is soluble in water and has a pungent odour.
Sulfur dioxide
Combustion of sulfur

S(s) + O2 → SO2(g)

Sulfites + acid → water + sulfur dioxide

SO32- + 2H+ → H2O + SO2(g)

Hot concentrated sulfuric acid + copper

3H2SO4 + Cu(s) → Cu2+ + 2HSO4- + 2H2O + SO2(g)

SO2(g) is bubbled through hydrogen peroxide solution (H2O2) to produce sulfuric acid (H2SO4). The addition of barium chloride solution (BaCl2(aq)) to this solution results in a precipitate of barium sulfate (BaSO4(s)).

SO2(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
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.

MnO2(s) + 2Cl- + 4H+ → Mn2+ + 2H2O + Cl2(g)

Electrolysis of molten sodium chloride (NaCl)

2NaCl → 2Na(Hg) + Cl2(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.)

Cl2(g) is a pale yellow-green gas which bleaches coloured materials and has a characteristic choking odour.

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Worked Example


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?

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

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 NO2(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 CO2(g) would have been produced, but CO2(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.