Unit 12: Laboratory Preparation and Collection of Gases
Mastering the methods for generating and collecting common non-metal gases in the lab.
12.36 General Principles of Gas Collection
Once a gas is produced in a reaction, it must be collected. The method of collection depends on two key physical properties of the gas: its solubility in water and its density relative to air.
- Collection over Water: This is the preferred method for gases that are insoluble or only sparingly soluble in water (e.g., H₂, O₂, N₂). The gas is bubbled through water into an inverted gas jar, displacing the water.
- Upward Delivery (Downward Displacement of Air): This method is used for gases that are less dense than air (molar mass < 29 g/mol) and are soluble in water. The gas is passed into an upright gas jar, and because it is lighter, it fills the jar from the top down, pushing the heavier air out. Example: Ammonia ($NH_3$, Molar Mass = 17 g/mol).
- Downward Delivery (Upward Displacement of Air): This method is used for gases that are denser than air (molar mass > 29 g/mol) and are soluble in water. The gas is passed into an upright gas jar. Being heavier, it sinks to the bottom and fills the jar upwards, pushing the lighter air out. Examples: $CO_2$ (44 g/mol), $HCl$ (36.5 g/mol), $Cl_2$ (71 g/mol).
Solved Examples:
-
Why can't ammonia ($NH_3$) be collected over water?
Solution: Ammonia is extremely soluble in water due to its ability to form hydrogen bonds. If bubbled through water, it would simply dissolve instead of collecting in the gas jar. -
Which method would you use to collect carbon dioxide ($CO_2$, Molar Mass = 44 g/mol)? Explain.
Solution: Carbon dioxide is denser than air and moderately soluble in water. The best method is downward delivery (upward displacement of air).
12.37 Laboratory Preparation of Hydrogen (H₂)
Reaction: Reaction of a reactive metal with a dilute acid.
$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$
Collection: Insoluble in water, so collected over water.
12.38 Laboratory Preparation of Oxygen (O₂)
Reaction: Catalytic decomposition of hydrogen peroxide.
$2H_2O_2(aq) \xrightarrow{MnO_2} 2H_2O(l) + O_2(g)$
Collection: Sparingly soluble in water, so collected over water.
12.39 Laboratory Preparation of Nitrogen (N₂)
Reaction: Heating an aqueous solution of ammonium chloride and sodium nitrite.
$NH_4Cl(aq) + NaNO_2(aq) \xrightarrow{\Delta} N_2(g) + 2H_2O(l) + NaCl(aq)$
Collection: Insoluble in water, so collected over water.
12.40 Laboratory Preparation of Chlorine (Cl₂)
Reaction: Oxidation of concentrated HCl with a strong oxidizing agent like $MnO_2$.
$MnO_2(s) + 4HCl(conc) \xrightarrow{\Delta} MnCl_2(aq) + Cl_2(g) + 2H_2O(l)$
Collection: Denser than air and reacts with water. Collected by downward delivery.
12.41 Laboratory Preparation of Hydrogen Chloride (HCl)
Reaction: Reaction of solid sodium chloride with concentrated sulfuric acid.
$NaCl(s) + H_2SO_4(conc) \rightarrow NaHSO_4(s) + HCl(g)$
Collection: Denser than air and very soluble in water. Collected by downward delivery.
12.42 Laboratory Preparation of Sulphur Dioxide (SO₂)
Reaction: Reaction of a sulfite with a dilute acid.
$Na_2SO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + SO_2(g) + H_2O(l)$
Collection: Denser than air and soluble in water. Collected by downward delivery.
12.43 Laboratory Preparation of Carbon Dioxide (CO₂)
Reaction: Reaction of a carbonate with a dilute acid.
$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + CO_2(g) + H_2O(l)$
Collection: Denser than air and moderately soluble in water. Collected by downward delivery.
12.44 Laboratory Preparation of Ammonia (NH₃)
Reaction: Heating an ammonium salt with a strong base.
$2NH_4Cl(s) + Ca(OH)_2(s) \xrightarrow{\Delta} CaCl_2(s) + 2NH_3(g) + 2H_2O(l)$
Collection: Less dense than air and very soluble in water. Collected by upward delivery. The gas is passed through a drying tower of calcium oxide (CaO) to remove water.
Solved Examples:
- Why is a drying agent used in the preparation of ammonia?
Solution: The reaction also produces water vapor. Passing the gas mixture through a drying agent like calcium oxide removes the water, yielding pure ammonia gas. - Could concentrated sulfuric acid be used to dry ammonia gas?
Solution: No. Ammonia is a base and sulfuric acid is an acid. They would react together ($2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4$) instead of simply drying the gas. - What is a common reactant for producing CO₂ in the lab?
Solution: Marble chips (calcium carbonate, $CaCO_3$). - What type of reaction is the laboratory preparation of hydrogen chloride?
Solution: It is an acid-base reaction where the less volatile acid (sulfuric acid) displaces the more volatile acid (hydrochloric acid) from its salt. - Name two gases that are collected by downward delivery.
Solution: Carbon dioxide ($CO_2$) and Chlorine ($Cl_2$). - Name a gas that is collected by upward delivery.
Solution: Ammonia ($NH_3$).