Acids, Bases and Salts

ACIDS, BASES AND SALTS

Properties of Acids, Bases and Salts:

Acids (Latin word acidus, meaning sour like lemon)

• Acids taste sour.
• Their aqueous solutions conduct electric current (electrolyte solutions)
• Acids turn blue litmus to red.
• pH less than 7
• Examples of acids:

Monoprotic acids

HNO₃ – nitric acid

HCl – hydrochloric acid

CH₃COOH – acetic acid (ethanoic acid) => (Organic acids contain -COOH group)

Diprotic acid

H₂SO₄ – sulfuric acid

Triprotic acid

H₃PO₄ – phosphoric acid

• Acids react with bases and form salt and water. This reaction type is called neutralization reaction.

HCl + NaOH –> NaCl + H₂O

Bases (Alkali: Arabic word for the ashes that come from burning certain plants; water solutions feel slippery and taste bitter like soap)

• Their taste is bitter like shampoo.
• Their solutions with water conduct electric current (electrolyte solutions)
• Bases are slippery.
• Bases turn red litmus to blue.
• Bases turn phenolphthalein to purple
• pH greater than 7
• They react with acids and form salt and water. (Neutralization reactions)

The Arrhenius theory: Swedish scientist Svante Arrhenius (1859–1927) proposed a theory explaining the nature of acids and bases according to their structure and the ions produced when they dissolve in water.

• Acids are any substances that dissociate to produce hydrogen ions (H⁺) when dissolved in water.

HA_(aq) –> H⁺_(aq) + A^–_(aq)

Hydrochloric acid:        HCl_(aq) –> H⁺_(aq) + Cl^–_(aq)

Nitric acid:                    HNO_3(aq) –> H⁺_(aq) + NO₃^–_(aq)

Acetic acid:                   CH₃COOH_(aq) –>  H⁺_(aq) + CH₃COO^–_(aq)

• In fact the hydrogen ion (H⁺) will associate with a water molecule to form H₃O⁺. One way to write the equation for an acid ‘HA’ dissolving in water is:

HA + H₂O_(l) –> H₃O⁺_(aq) + A^–_(aq)

• Bases are any substances that dissociate to produce hydroxide ions (OH^–) when dissolved in water.

MOH_(aq) –> M⁺_(aq) + OH^–_(aq)

Sodium hydroxide:       NaOH_(aq) –> Na⁺_(aq) + OH^–_(aq)

Barium hydroxide:        Ba(OH)_{2(aq) –>} Ba²⁺_(aq) + 2OH^–_(aq)

Ammonia:                     NH_3(aq) + H₂O –> NH₄⁺_(aq) + OH^–_(aq)

Salts (Ionic compounds)

• Ionic compounds that can result from the neutralization reaction of an acid and a base.
• They are composed of cations (positively charged ions) and anions (negative ions) so that the product is electrically neutral (without a net charge).
• Ionic compounds are dissociated into ions when they are dissolved in water.

Sodium Chloride:              NaCl_(s) + water –> Na⁺_(aq) + Cl^–_(aq)

Magnesium bromide:       MgBr_2(s) + water –> Mg²⁺_(aq) + 2Br^–_(aq)

Potassium nitrate:             KNO_3(s) + water –> K⁺_(aq) + NO₃^–_(aq)

Sodium acetate:                 CH₃COONa_(s) + water –> CH₃COO^–_(aq) + Na⁺_(aq)

Exercise: Dissociation equations of acids

Exercise: Dissociation equations of bases

Acidic Oxides & Basic Oxides:

Nonmetal oxides: (=Acidic oxides if #Oxygen>#Nonmetal)

CO₂, NO₂, N₂O₃, N₂O₅, P₂O₅, SO₂, SO₃, etc. Acidic oxides form acids when they’re dissolved in water.

CO_2(g) + H₂O_(l) –> H₂CO_3(aq) (Carbonic acid)

SO_2(g) + H₂O_(l) –> H₂SO_3(aq) (Sulfurous acid)

SO_3(g) + H₂O_(l) –> H₂SO_4(aq) (Sulfuric acid)

N₂O_5(g) + H₂O_(l) –> 2HNO_3(aq) (Nitric acid)

Metal oxides: (=Basic oxides except amphoteric metal oxides)

Na₂O, K₂O, MgO, CaO, etc. Basic oxides form bases when they’re dissolved in water.

Na₂O_(s) + H₂O_(l) –> 2NaOH_(aq)

CaO_(s) + H₂O_(l) –> Ca(OH)_2(aq)

NEUTRALIZATION REACTIONS:

• Acids react with bases and form salt and water. This reaction type is called neutralization reaction and also double-displacement reactions.
• The number of H⁺ ions should be equal to the number of OH^– ions for neutralization to form water (H⁺OH^–).

HCl + KOH –> KCl + H₂O

H₂SO₄ + 2NaOH –> Na₂SO₄ + 2H₂O

Neutralization Reactions of Oxides:

Sample:

Na₂O_(aq) +   HCl_(aq) –>  NaCl_(aq) +  H₂O_(l)

Basic             Acid            Salt          Water