17.2 - Preparation & Properties of Gases

Choosing a way to collect a gas

• Two properties of a gas decide how it's collected:
  1. Whether it dissolves in water
  2. If it does dissolve, whether it's denser or less dense than air
• Ask about solubility first:
  • A gas that does not dissolve in water (or only slightly) is collected over water, regardless of density.
  • A gas that does dissolve in water is collected by displacement of air, where density decides the method.

A gas with a low solubility in water - Collection over water

• A gas jar or measuring cylinder is filled with water and inverted in a trough of water over a beehive shelf.
• The delivery tube runs into the trough and under the shelf, so the gas bubbles up through the shelf into the mouth of the jar.
• As gas bubbles into the jar, it displaces the water downwards.

A soluble gas that is denser than air

• The gas jar stands upright with the delivery tube reaching near the bottom.
• The gas sinks to the bottom and pushes the less dense air up and out.
• This is called downward delivery (or upward displacement of air).
• Used for sulfur dioxide and hydrogen chloride.

A soluble gas that is less dense than air

• The gas jar is held upside down with the delivery tube pointing up into it.
• The gas rises to the top and pushes the denser air down and out of the mouth.
• This is called upward delivery (or downward displacement of air).
• Used for ammonia.

Hydrogen

• Colourless, odourless gas.
• Insoluble in water and less dense than air.
• Diatomic, so its molecules are H₂.

Preparing hydrogen

Made by reacting a metal with dilute hydrochloric acid. Zinc is normally used:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Magnesium works too and gives the same gas:

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Magnesium reacts more vigorously than zinc and is harder to control, so zinc is the better choice for a steady preparation. Hydrogen is insoluble in water, so it's collected over water.

What hydrogen is used for

• Meteorological (weather) balloons - It's less dense than air, so the balloon rises.
• Hardening oils - Liquid oils are turned into solid fats used in margarines and spreads.
• Clean fuel - Burning it gives only water, which is not a pollutant:
  • 2H₂(g) + O₂(g) → 2H₂O(l)

Oxygen

• Colourless, odourless gas.
• Only slightly soluble in water and slightly denser than air.
• Diatomic, so its molecules are O₂.

Preparing oxygen

• Made by the catalytic decomposition of hydrogen peroxide.
• Manganese(IV) oxide (manganese dioxide) is added as a catalyst to speed up the reaction:
  • 2H₂O₂(aq) → 2H₂O(l) + O₂(g)

Oxygen is collected over water because of its low solubility in water.

What oxygen is used for

• Medicine - To supply patients who can't take in enough oxygen through normal breathing.
• Welding - Burned with a fuel gas to produce a flame hot enough to melt metal.

Carbon dioxide

• Colourless, odourless gas.
• Low solubility in water and denser than air.

Preparing carbon dioxide

Made by reacting calcium carbonate (marble chips) with dilute hydrochloric acid:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

Because its solubility is low, carbon dioxide is collected over water.

Carbon dioxide and limewater

Limewater is a solution of calcium hydroxide. Bubbling carbon dioxide through it turns it milky because insoluble calcium carbonate forms as a white precipitate:

CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

With excess carbon dioxide, the milkiness clears. The calcium carbonate reacts further with water and excess gas to form soluble calcium hydrogencarbonate:

CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

What carbon dioxide is used for

• Carbonated drinks - Dissolved under pressure; when opened, the gas escapes and the drink fizzes.
• Fire extinguishers:
  • It does not support combustion.
  • It's denser than air, so it smothers the fire and keeps oxygen away.

Nitrogen

• Colourless, odourless, and tasteless gas.
• Insoluble in water.
• Diatomic, so its molecules are N₂.

Why nitrogen is so unreactive

• The two nitrogen atoms in an N₂ molecule are held together by a strong triple covalent bond (three shared pairs of electrons).
• A large amount of energy is required to break this bond, so nitrogen is very unreactive.

What nitrogen is used for

• Coolant - Liquid nitrogen is extremely cold and chills substances rapidly.
• Food packaging - Provides an unreactive atmosphere that stops food from spoiling.