Preparation and properties of gas
BY WHAT METHODS ARE GASES USUALLY COLLECTED IN THE LABORATORY?
The method of collection of a gas depends on its solubility in water and its density. Gases that are insoluble or slightly soluble in water can be collected over water. Gases that are denser than air are collected by downward delivery, while those that are less dense are collected by upward delivery.
HOW IS OXYGEN PREPARED IN THE LABORATORY?
Use the diagram below to answer questions on the preparation of oxygen in the laboratory. Identify substance Q and write an equation for the reaction with hydrogen peroxide.
Oxygen can be prepared in the laboratory from the decomposition of hydrogen peroxide. This process is slow, so a catalyst is usually added to speed up the decomposition process. The catalyst used is manganese (IV) oxide, which would be substance Q in the diagram. When the catalyst is added, the hydrogen peroxide breaks down to produce oxygen and water. The equation for the reaction is given below:
2H2O2 (l) 2H2O (l) + O2 (g) Remember: MnO2 (catalyst) would not appear in the equation as it is unchanged at the end of the reaction.
WHAT IS THE METHOD OF COLLECTION OF THE OXYGEN GAS AND WHY?
The oxygen produced is collected by the downward displacement of water in a gas tube, and it is wet. To dry the gas, a drying agent such as concentrated sulphuric acid can be used to pass the gas through.
CITE THE PROPERTIES OF OXYGEN GAS THAT RELATE TO ITS USE.
Oxygen gas is colourless, odourless, tasteless and slightly soluble in water. It is, therefore, essential for breathing, especially for aquatic animals. It supports combustion and is important for the burning of fuels. The test for oxygen is that it relights or rekindles a glowing splint.
HOW IS CARBON DIOXIDE PREPARED IN THE LAB? WRITE THE EQUATION FOR THE REACTION.
In the laboratory, carbon dioxide is produced from the action of an acid on a carbonate. Reacting calcium carbonate (marble chips) with hydrochloric acid produces calcium chloride, water and the gas carbon dioxide.
Equation: CaCO3 (s) + 2 HCl (aq) CaCl2(aq) + CO2 (g) + H2O (l)
BY WHAT METHOD IS THE CO2 GAS BEING COLLECTED IN THE DIAGRAM? HOW COULD THE GAS BE DRIED?
The wet gas is collected by the downward displacement of water. This is used for gases that are not very soluble in water. The gas could also be collected by downward delivery (displacement of air) as it is denser than air and so collects at the bottom.
In water, carbon dioxide produces a weakly acidic solution, carbonic acid. CO2 (g) + H2O (l) H2CO3 (aq). Since carbon dioxide is acidic, it can be dried by passing the wet gas through concentrated sulphuric acid.
WHAT ARE THE PROPERTIES OF CARBON DIOXIDE AND HOW CAN IT BE TESTED?
Carbon dioxide does not support combustion, so it is used in fire extinguishers. The gas, when cooled, forms dry ice (solid carbon dioxide) and is used as a refrigerant.
The test for carbon dioxide is that it turns calcium hydroxide (lime water) milky or cloudy. The cloudy appearance is due to the precipitation of calcium carbonate.
Ca(OH) (aq) + CO (g) CaCO (s) + H O (l) 2 2 3 2
HOW IS AMMONIA PREPARED IN THE LABORATORY?
Ammonia is produced by the action of an alkali on an ammonium salt. When ammonium chloride and calcium hydroxide are heated, ammonia, calcium chloride and water are produced. The equation is given below:
2NH4Cl (s) + Ca(OH)2 (s) 2NH3 (g) + CaCl2 (s) + 2H2O (g)
NOTE: To dry the gas, calcium oxide (a base) is used instead of concentrated sulphuric acid or calcium chloride. Why? Ammonia is an alkaline gas and would react with the sulphuric acid. 2NH3 (g) + H2SO4 (l) ? (NH4)2SO4 (aq)
Ammonia also reacts with calcium chloride. Anhydrous calcium chloride is often used as a drying agent with neutral gases. Why is ammonia collected by the upward delivery method?
Ammonia is very soluble in water and is also lighter than air. So, notice that it is collected by the method of upward delivery. Being alkaline, it will turn damp red litmus to blue. This is a test for the gas.
Ammonia dissolves in water to form a weak alkali, ammonium hydroxide. It is important in the formation of ammonium fertilizers and in the production of nitric acid. How could hydrogen be prepared and tested in the laboratory? Write the equation for the reaction in the diagram.
Hydrogen can be obtained by the action of dilute acid on a metal. In the diagram, zinc and dilute hydrochloric acid react to produce a salt and hydrogen gas. Equation: Zn (s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g) The hydrogen produced can be tested with a lighted splint, which burns with a 'pop' (sound).