9.1 Acid

• Definition:

• Acid is a chemical that, when dissolved in water, decomposes into hydrogen ions (H+ or protons).

• Examples of concentrated acids:

• Hydrochloric acid (HCl), sulfuric acid (H2SO4).

• These acids decompose in aqueous solution completely:

• HCl (aq) → H+ (aq) + Cl- (aq)

• H2SO4 (aq) → 2H+ (aq) + SO4 2- (aq)

• Examples of diluted acids:

• Carbolic acid (H2CO3), acetic acid (CH3COOH).

• These acids decompose partially:

• H2CO3 (aq) → 2H+ (aq) + CO3 2- (aq)

• CH3COOH (aq) → H+ (aq) + CH3COO- (aq)

• Strong acids: Fully decomposed in water (indicated by a single arrow).

• Weak acids: Only partially decomposed in water (indicated by a double arrow).

• Example: At 25°C, only 4 out of 1000 CH3COOH molecules will decompose in water, leaving the remaining molecules intact.

• Concentrated and Diluted Acids:

• Concentrated acid: More acid than water.

• Dilute acid: More water than acid.

• Properties of Acids:

• Taste: Sour taste (e.g., tamarind contains tartaric acid, which is sour).

• Litmus Test: Acid turns blue litmus paper red.

• Reactions Involving Acids:

• Reaction with Metallic Oxides:

• Acid reacts with metal oxides to produce salt and water.

• Example: CaO + 2HCl → CaCl2 + H2O

• Reaction with More Reactive Metals:

• Acid reacts with reactive metals to produce salt and hydrogen gas.

• Example: 2Na + 2HCl → 2NaCl + H2

• Acids in Food:

• Everyday food contains acids like:

• Milk (lactic acid), soft drinks (carbonic acid), lemon/orange (citric acid), tamarind (tartaric acid), vinegar (ethanoic acid), tea (tannic acid).

• These acids aid in digestion and prevent diseases.

• Weak acids in food don’t harm health.

• Stomach Acid:

• The stomach generates hydrochloric acid (HCl) for digestion.

• Excessive secretion or an empty stomach may cause a condition known as peptic ulcer.

9.1.1 Demonstrating Properties of Dilute Acids through Experiments

1. Taste:

• Dilute acids taste sour (e.g., tartaric acid from tamarind).

• Warning: Never taste acids in the lab as they can corrode your tongue.

2. Corrosive:

• Acids are corrosive to metals.

3. Litmus Test:

• Blue litmus turns red in the presence of an acid.

• Example: HCl, H2SO4, HNO3, and even tamarind or pickles turn blue litmus paper red.

4. Reaction with Reactive Metals:

• Acids react with reactive metals (e.g., Mg) to produce salt and hydrogen gas.

• Example: Mg + H2SO4 → MgSO4 + H2

5. Reaction with Metallic Carbonates:

• Dilute acids react with metallic carbonates to produce salt, water, and carbon dioxide.

• Example: CaCO3 + 2HCl → CaCl2 + H2O + CO2

• Carbon dioxide gas turns lime water turbid:

• Ca(OH)2 + CO2 → CaCO3 + H2O

• Further CO2 turns the solution opaque due to calcium bicarbonate:

• CaCO3 + CO2 + H2O → Ca(HCO3)2

6. Reaction with Metallic Bicarbonates:

• Metallic hydrogen carbonates react with dilute acids to produce salt, water, and CO2.

• Example: Ca(HCO3)2 + 2HCl → CaCl2 + 2H2O + 2CO2

7. Reaction with Metal’s Hydroxides (Alkali):

• Acid reacts with metal hydroxides to produce salt and water (neutralization).

• Example: NaOH + HCl → NaCl + H2O

8. Reaction with Metal’s Oxides:

• Acid reacts with metal oxides to form salt and water.

• Example: CaO + 2HCl → CaCl2 + H2O

9.1.2 The Role of Water in Chemical Properties of Acids

• Dilution and Ionization:

• Acids dissolve in water to produce hydrogen ions (H+), which are responsible for acidic properties.

• Example:

• Oxalic acid lattice: No change in litmus paper until dissolved in water, releasing H+ ions.

• Citric acid, ethanoic acid, and carbonic acid undergo partial decomposition in water, releasing some H+ ions.

• Strong acids like HCl, H2SO4, and HNO3 completely ionize in water, producing H+ ions and showing acidic properties.

• Weak acids like citric acid, acetic acid, and carbonic acid partially ionize, producing fewer H+ ions.

9.1.3 Concentrated Acids

• Definition:

• Concentrated acids contain less water and more acid.

• Concentrated Acids in Use:

• Common concentrated acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).

• Safety Precautions:

• Always wear gloves, goggles, a mask, and an apron when handling concentrated acids to avoid injury.

• Concentrated Hydrochloric Acid:

• Formed by dissolving hydrogen chloride gas in water.

• When opened, it creates a fog and releases a pungent smell.

• Wear a mask and goggles when opening.

• Concentrated Nitric Acid:

• Formed by dissolving nitrogen dioxide (NO2) gas in water.

• Brown container to prevent light decomposition.

• Creates a fog and strong smell when opened.

• Concentrated Sulfuric Acid:

• Formed by dissolving sulfur dioxide (SO2) gas in water.

• Releases a pungent smell when opened.