Study of Compounds — Ammonia — Question 51
Back to all questionsQuestion 3(2015)
A metal ‘X’ has valency 2 and a non-metal ‘Y’ has a valency 3. If ‘Y ’ is a diatomic gas, write an equation for the direct combination of X and Y to from a compound.
X (metal) has a valency 2, hence,
X - 2e- ⟶ X2+
Y (non metal) has a valency 3. hence,
Y + 3e- ⟶ X3-
Therefore, the two combine as :
3X + 2Y ⟶ X3Y2
Chapter Overview: Study of Compounds
This chapter covers the detailed study of important compounds: Hydrogen Chloride (HCl), Ammonia (NH3), Nitric Acid (HNO3), and Sulphuric Acid (H2SO4). For each compound, students must learn the laboratory preparation, properties (physical and chemical), tests, and uses. HCl is prepared by heating NaCl with concentrated H2SO4 and is a strong monobasic acid. Ammonia is prepared by heating ammonium salts with alkalis and is the only common alkaline gas. Nitric acid is prepared in the lab by heating KNO3 with concentrated H2SO4 and is a strong oxidising acid. Sulphuric acid is manufactured by the Contact Process and is called the "King of Chemicals" due to its extensive industrial applications. The chapter demands thorough knowledge of balanced chemical equations, laboratory setups with drying agents, and the distinction between dilute and concentrated acid reactions. This is one of the most important chapters with very high exam weightage.
Key Reactions
| Compound | Key Reaction |
|---|---|
| HCl (Lab prep) | NaCl + H2SO4 → NaHSO4 + HCl↑ (below 200°C) |
| NH3 (Lab prep) | 2NH4Cl + Ca(OH)2 → CaCl2 + 2H2O + 2NH3↑ |
| NH3 fountain expt | NH3 is highly soluble in water, creating a pressure difference (red fountain) |
| HNO3 (Lab prep) | KNO3 + H2SO4 → KHSO4 + HNO3↑ |
| HNO3 + Cu | 3Cu + 8HNO3(dil.) → 3Cu(NO3)2 + 2NO↑ + 4H2O |
| H2SO4 (Contact Process) | 2SO2 + O2 ⇌ 2SO3 (V2O5 catalyst, 450°C) |
| Oleum formation | SO3 + H2SO4 → H2S2O7 (oleum); then H2S2O7 + H2O → 2H2SO4 |
| Dehydration by H2SO4 | C12H22O11 → 12C + 11H2O (sugar turns black) |
Must-Know Concepts
- HCl is dried using conc. H2SO4 (not CaO or NaOH as they react with HCl)
- NH3 is dried using CaO (quicklime), NOT conc. H2SO4 (acid reacts with basic NH3)
- Aqua regia (3 HCl : 1 HNO3) dissolves gold and platinum
- Conc. HNO3 makes iron passive (forms a protective oxide layer)
- Conc. H2SO4 is a dehydrating agent, oxidising agent, and non-volatile acid
- Dilute H2SO4 gives H2 with metals; conc. H2SO4 gives SO2 (acts as oxidising agent)
- Brown ring test confirms presence of NO3− ions
Dilute vs Concentrated HNO3
| Property | Dilute HNO3 | Concentrated HNO3 |
|---|---|---|
| With Cu | Colourless NO gas | Brown NO2 gas |
| With S | No reaction | S + 6HNO3 → H2SO4 + 6NO2 + 2H2O |
| With Fe | Reacts to give Fe(NO3)2 | Renders iron passive |
Important Diagrams to Practice
- Laboratory preparation of HCl gas with drying and collection setup
- Laboratory preparation and fountain experiment of ammonia
- Contact Process flowchart for H2SO4 manufacture
- Laboratory preparation of HNO3 with all-glass apparatus
Common Mistakes
- Using wrong drying agents: NH3 cannot be dried by H2SO4 or P2O5 (they react with it)
- Confusing products of dilute vs concentrated acid reactions with metals
- Not specifying the catalyst (V2O5) and temperature (450°C) in the Contact Process
- Writing that conc. HNO3 gives H2 with metals (it does not; it is an oxidising acid)
- Forgetting that SO3 is dissolved in H2SO4 (not water) in the Contact Process
Scoring Tips
- Learn at least 3-4 reactions for each compound (lab prep, with metals, with non-metals, special properties)
- For diagram questions, label every part including drying agent, delivery tube, and collection method
- Clearly distinguish between reactions of dilute and concentrated forms
- Remember: HNO3 with all metals (except Au and Pt) gives NO3− salt, never H2
Frequently Asked Questions
Why is H2SO4 called the "King of Chemicals"?
Because of its extremely wide range of industrial applications: manufacturing fertilisers, dyes, detergents, explosives, petroleum refining, and in car batteries. A country's industrial development is often measured by its H2SO4 production.
Why is SO3 not dissolved directly in water?
Dissolving SO3 directly in water produces a fine mist of H2SO4 that is difficult to condense. Instead, SO3 is dissolved in concentrated H2SO4 to form oleum (H2S2O7), which is then diluted with water to get H2SO4.
What does the ammonia fountain experiment prove?
It demonstrates the extremely high solubility of ammonia in water. When the flask of NH3 is opened to water, ammonia dissolves rapidly creating a partial vacuum, which draws water up as a red fountain (red due to phenolphthalein indicator in alkaline NH4OH).