Overview
This lecture covers the properties, laboratory preparation, testing, and chemical reactions of HCl gas and HCl acid, including key experiments and common exam questions.
Properties of HCl Gas
- HCl gas is colorless, pungent-smelling, and acidic in nature.
- It does not change the color of dry litmus paper but turns moist litmus red.
- HCl gas is highly soluble in water.
- High solubility demonstrated by the Fountain Experiment.
Laboratory Preparation of HCl Gas
- HCl gas is prepared by reacting sodium chloride (NaCl) with concentrated sulfuric acid (H2SO4) at temperatures below 200°C.
- Reaction: NaCl + H2SO4 → NaHSO4 + HCl (below 200°C).
- At temperatures above 200°C, sodium sulfate (Na2SO4) forms, which creates a hard crust that blocks further reaction, risks breaking glass apparatus, and wastes fuel.
- Sulfuric acid is preferred over nitric acid because it is non-volatile and prevents contamination of HCl gas.
Drying HCl Gas
- HCl gas is dried by passing it through concentrated H2SO4, which acts as a drying agent.
- Calcium oxide (quick lime) is not used as a drying agent because it reacts with HCl gas.
Fountain Experiment
- Demonstrates HCl's high solubility by creating a vacuum when water enters a vessel filled with HCl gas, causing atmospheric pressure to push blue litmus solution upward, turning it red.
HCl Acid Preparation and Properties
- HCl acid is made by dissolving HCl gas in water.
- Due to high solubility, rapid dissolution may cause back-suction of water, damaging the setup.
- Back suction is prevented using an inverted funnel arrangement which provides a larger surface area and slows water entry.
- HCl acid turns blue litmus paper red, methyl orange pink, and keeps phenolphthalein colorless.
Chemical Reactions of HCl Acid
- With metals above hydrogen: produces metal chloride and hydrogen gas (e.g., Zn + 2HCl → ZnCl2 + H2).
- With bases: forms salt and water (e.g., NaOH + HCl → NaCl + H2O).
- With carbonates: forms salt, water, and CO2 (e.g., Na2CO3 + 2HCl → 2NaCl + H2O + CO2).
- With sulfites: forms salt, water, and SO2 gas.
- With sulfides: forms salt and hydrogen sulfide (H2S) gas.
- With thiosulfates: forms salt, water, SO2 gas, and yellow sulfur precipitate.
- With nitrates (e.g. lead nitrate, silver nitrate): forms white precipitate of lead chloride or silver chloride.
Testing for HCl Gas
- HCl gas is identified by the formation of dense white fumes (ammonium chloride) when brought into contact with ammonia.
Key Terms & Definitions
- HCl Gas — Gaseous hydrogen chloride, colorless, highly soluble, acidic, pungent.
- HCl Acid — Aqueous solution of hydrogen chloride, strong acid properties.
- Drying Agent — Substance that removes water vapor from a gas (e.g., concentrated H2SO4).
- Dehydrating Agent — Substance that removes chemically bound water (e.g., from compounds).
- Back Suction — Ingress of water into the gas generator due to vacuum from rapid gas dissolution.
- Fountain Experiment — Demonstrates high HCl solubility and acidic nature.
- Inverted Funnel Arrangement — Prevents back suction during HCl acid preparation.
Action Items / Next Steps
- Review the differences between drying and dehydrating agents.
- Memorize the key reactions of HCl acid with metals, bases, carbonates, sulfites, sulfides, thiosulfates, and nitrates.
- Understand and be able to describe the Fountain Experiment and inverted funnel setup.
- Prepare answers to common board questions regarding HCl lab preparation and properties.