Acids and Bases

Acids

• [Def.] A molecule which when dissolved in water, dissociates to form hydrogen ions

• All acids are sour

• Dissociation of acid molecule --> Breakage of covalent bond, with both bonding electrons retained by other atom. Hydrogen atoms thus loses its only electron, resulting in hydrogen ion

• Can conduct electricity; Even though they have simple molecular structure, still can give mobile ions in solution as charge carriers

• Important acids: 

  • • Hydrochloric acid (strong); HCl

    • Nitric acid (strong); HNO3

    • Sulfuric acid (strong); H2SO4

    • Ethanoic acid (weak); CH3CO2H

    • Phosphoric acid (weak) H3PO4

Acid strength

• [def.] The extent of dissociation in water to form hydrogen ions

• Only described as strong or weak

Strong acids:

• An acid that dissociates completely in water to produce a high concentration of hydrogen ions

• All acid molecules dissociate to form hydrogen ions. No aicd molecules remaining

Weak acids:

• An acid that dissociates partially in water to produce a low concentration of hydrogen ions

• Only small fraction dissociates. Most of the acid remains as molecules

• Use reversible arrows when writing chemical equation; Denote that only small fraction of reaction proceeds to the right of the equation

Importance of water for acids:

• Aqeuous solutions are only acidic because of hydrogen ions. This happens only when acid molecules dissolve in water and dissociate to form hydrogen ions. Acid molecules are not acidic if pure or dissolved in organic solvents

Acid basicity:

• Maximum number of hydrogen ions formed per acid molecule in water

Properties of acids:

• Sour

• Blue --> Red (Litmus)

• Contain hydrogen ions and conduct electricity by way of mobile ions

• React with metals (above hydrogen in metal reactivity series), carbonates and bases

Acid reaction with metals:

• Acid + Metal --> Salt + Hydrogen

• Observations:

  • Effervescence of colourless, odourless gas which extinguished a lighted wooden splint with a 'pop' sound

  • Grey solid dissolves to form a colourless solution

  • Heat is liberated

• *Salts:

  • Salts of nitric acid are nitrates, etc.

  • Are the products of neutralisation of acid and base

  • Ionic compound with giant ionic structure

Acid reactions with carbonates:

• Acid + Carbonate --> Salt + Carbon dioxide + Water

• Observations:

  • Effervescence of colourless, odourless gas, which forms a white precipitate (of calcium carbonate) when bubbled into limewater (aqueous calcium hydroxide).

  • White solid dissolves to form a colourless solution.

Acid reaction with bases (Neutralisation):

• Acid reacts with bases (metal oxides and metal hydroxide) to produce salt and water

• Acid + Base --> Salt + Water

• Observations:

  • White solid dissolves to form a colourless solution

  • Heat is liberated

• *Bases that are soluble in water are alkalis. Alkalis dissociate to form hydroxide ions

Use of acids:

Types of pH indicators:

• *pH relies on the concentration of acid; Does not measure strength of acid

Rest of the content can be found in the notes