Lesson Notes By Weeks and Term v4 - SHS 3

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Performance objectives

• Identify and describe the properties of key gases.
• Design and perform experiments to prepare these gases.
• Test for the presence of hydrogen, ammonia, and carbon (IV) oxide.
• Explain the real-life significance of these gases.

Lesson summary

Welcome, future scientists and engineers! Today, we delve into the fascinating world of gases. Gases are all around us, from the air we breathe to the bubbles in our favourite soft drinks ('minerals'). In Ghana, we use LPG (a mixture of gases) for cooking in our homes, and farmers rely on fertilisers made from ammonia gas to grow our food like maize and cocoa. Understanding how to prepare and identify common gases in the laboratory is a fundamental skill for any student of chemistry. This lesson will equip you with the knowledge and a-thinking process to safely design experiments to produce and test for hydrogen, ammonia, and carbon (IV) oxide.

Lesson notes

This section covers the preparation, properties, collection, and testing of three important laboratory gases. A key principle we will use repeatedly is: The method used to collect a gas depends on its physical properties. Density: Is the gas less dense or denser than air? If less dense (lighter) than air, we use downward displacement of air (upward delivery). If denser (heavier) than air, we use upward displacement of air (downward delivery). Solubility: Does the gas dissolve in water? If insoluble or sparingly soluble, it can be collected by the displacement of water. If very soluble, it *cannot* be collected over water.

Let's explore each gas in detail. A. Hydrogen Gas (H₂) Principle of Preparation: Hydrogen gas is typically prepared in the laboratory by the reaction of a moderately reactive metal (above hydrogen in the reactivity series) with a dilute acid. The metal displaces hydrogen from the acid. General Equation: `Metal + Dilute Acid → Salt + Hydrogen Gas` Reactants and Equation: Common Reactants: Granulated Zinc (Zn) and dilute Tetraoxosulphate(VI) acid (H₂SO₄) or Hydrochloric acid (HCl). Reasoning: Zinc is chosen because it is reactive enough to produce hydrogen at a steady rate but not too reactive (like sodium) to be explosive. Balanced Chemical Equation: `Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)` *(Zinc + Dilute Tetraoxosulphate(VI) acid → Zinc tetraoxosulphate(VI) + Hydrogen)* Experimental Setup and Procedure: Apparatus: Flat-bottom flask, thistle funnel, delivery tube, gas jars, trough of water (or beehive shelf). Procedure: Place some granulated zinc in the flat-bottom flask. Set up the apparatus as shown in diagrams (thistle funnel dipping below the acid level to prevent gas from escaping). Add the dilute H₂SO₄ through the thistle funnel to react with the zinc. Effervescence (fizzing) occurs immediately, and hydrogen gas is produced. Allow the first few bubbles to escape as they are mixed with air from the flask. Collection of Hydrogen Gas: Properties for Collection: Hydrogen is the lightest known gas (much less dense than air). It is insoluble in water. Methods: Displacement of Water: The gas is bubbled through water in an inverted gas jar filled with water. The gas displaces the water. This method gives a pure sample of the gas. Downward Displacement of Air (Upward Delivery): An inverted gas jar is used, and the delivery tube is placed at the top of the jar. The lighter hydrogen fills the jar from the top downwards, pushing the heavier air out. Confirmatory Test for Hydrogen: Test: Insert a lit splint into a test tube or gas jar containing hydrogen. Observation: The gas extinguishes the flame with a characteristic 'pop' sound. Chemical Reaction: `2H₂(g) + O₂(g) → 2H₂O(l)` (a mini-explosion)

B. Carbon (IV) Oxide Gas (CO₂) Principle of Preparation: Carbon (IV) oxide is prepared by the action of a dilute acid on a metallic trioxocarbonate(IV). General Equation: `Acid + Trioxocarbonate(IV) → Salt + Water + Carbon (IV) oxide` Reactants and Equation: Common Reactants: Marble chips (Calcium trioxocarbonate(IV), CaCO₃) and dilute Hydrochloric acid (HCl). Reasoning: This reaction is not too vigorous and proceeds at a manageable rate. Dilute H₂SO₄ is *not* preferred because it forms insoluble calcium tetraoxosulphate(VI) (CaSO₄), which coats the marble chips and stops the reaction. Balanced Chemical Equation: `CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)` *(Calcium trioxocarbonate(IV) + Hydrochloric acid → Calcium chloride + Water + Carbon (IV) oxide)* Experimental Setup and Procedure: The apparatus is identical to the one used for hydrogen preparation (flat-bottom flask, thistle funnel, etc.). Marble chips are placed in the flask, and dilute HCl is added through the thistle funnel. Collection of Carbon (IV) Oxide Gas: Properties for Collection: It is about 1.5 times denser than air. It is sparingly soluble in water. Methods: Upward Displacement of Air (Downward Delivery): This is the preferred method. A gas jar is placed upright, and the delivery tube goes to the bottom. The heavier CO₂ fills the jar from the bottom up, pushing the lighter air out. To check if the jar is full, a lit splint placed at the mouth of the jar will be extinguished. Displacement of Water: This method can be used but may result in some loss of gas due to its slight solubility. Confirmatory Test for Carbon (IV) Oxide: Test: Bubble the gas through limewater (a saturated solution of Calcium Hydroxide, Ca(OH)₂). Observation: The limewater turns milky or cloudy due to the formation of an insoluble precipitate of calcium trioxocarbonate(IV). Chemical Equation: `CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)` Effect of Excess CO₂: If bubbling is continued, the milkiness disappears as the insoluble CaCO₃ reacts to form the soluble calcium hydrogentrioxocarbonate(IV). `CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)`

C. Ammonia Gas (NH₃) Principle of Preparation: Ammonia is prepared in the laboratory by heating an ammonium salt with a strong, non-volatile base (alkali). General Equation: `Ammonium Salt + Base → Salt + Water + Ammonia Gas` Reactants and Equation: Common Reactants: Ammonium chloride (NH₄Cl) and Calcium hydroxide (Ca(OH)₂), which is slaked lime. Reasoning: Both reactants are solids, so they are ground together to increase the surface area for reaction. Calcium hydroxide is used because it is a cheap, readily available strong base that is not volatile. Balanced Chemical Equation: `2NH₄Cl(s) + Ca(OH)₂(s) → CaCl₂(s) + 2H₂O(l) + 2NH₃(g)` *(Ammonium chloride + Calcium hydroxide → Calcium chloride + Water + Ammonia)* Experimental Setup and Procedure: Apparatus: Round-bottom flask (for even heating), delivery tube, gas jar, drying tower. Procedure: The mixture of NH₄Cl and Ca(OH)₂ is placed in the round-bottom flask. The flask is tilted downwards slightly to prevent water that condenses on the cooler parts of the flask from running back and cracking the hot glass. The mixture is gently heated. Drying and Collection of Ammonia Gas: Drying: The gas produced is moist. It must be dried. Drying Agent: Quicklime (Calcium oxide, CaO) is used. Reasoning: Ammonia is a basic gas. Therefore, an acidic drying agent like concentrated H₂SO₄ or a neutral one that reacts with it like fused CaCl₂ cannot be used. `2NH₃(g) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)` (Reacts) `CaCl₂(s) + 8NH₃(g) → CaCl₂·8NH₃(s)` (Forms an addition compound) CaO is a basic oxide and does not react with ammonia. Properties for Collection: It is less dense than air. It is extremely soluble in water. This is why it cannot be collected over water. Method: Downward Displacement of Air (Upward Delivery): The gas jar is inverted. The less dense ammonia fills the jar from the top down. Confirmatory Test for Ammonia: Litmus Paper: Brings a piece of damp red litmus paper to the mouth of the gas jar. It will turn blue, confirming the gas is alkaline. Hydrogen Chloride Gas: Bring a glass rod dipped in concentrated HCl near the gas. Dense white fumes of ammonium chloride are formed. Chemical Equation: `NH₃(g) + HCl(g) → NH₄Cl(s)`

Evaluation guide

• Design and perform experiments to prepare and test for gases (hydrogen,