Bond StrengthIntroduction Again

All acids and bases do not ionize or dissociate to the same extent. This leads to the statement that acids and bases are not all of equal strength in producing H+ and OH- ions in solution. The terms "strong" and "weak" give an indication of the strength of an acid or base. The terms strong and weak describe the ability of acid and base solutions to conduct electricity. If the acid or base conducts electricity strongly, it is a strong acid or base. If the acid or base conducts electricity weakly, it is a weak acid or base.

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Demonstration of Acid and Base Conductivity

The instructor will test the conductivity of various solutions with a light bulb apparatus. The light bulb circuit is incomplete. If the circuit is completed by a solution containing a large number of ions, the light bulb will glow brightly indicating a strong ability to conduct electricity as shown for HCl. If the circuit is completed by a solution containing large numbers of molecules and either no ions or few ions, the solution does not conduct or conducts very weakly as shown for acetic acid.

*

Ka = /

where HA is the concentration of the acid at equilibrium, and A- is the concentration of its conjugate base at equilibrium and for bases the expression will be

ceB>

where B is the concentration of the base at equilibrium and HB+ is the concentration of its conjugate acid at equilibrium

The stronger an acid is, the lower the pH it will produce in solution. pH is calculated by taking the negative logarithm of the concentration of hydronium ions. For strong acids, you can calculate the pH by simply taking the negative logarithm of its molarity as it completely dissociates into its conjugate base and hydronium. The same goes for strong bases, except the negative logarithm gives you the pOH as opposed to the pH. For weak acids and bases, the higher the Ka or Kb, the more acidic or basic the solution. To find the pH for a weak acid or base, you must use the K equation and a RICE table to determine the pH.

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All acids have a conjugate base that forms when they react with water, and similarly, all bases have a conjugate acid that reacts when they form with water.1 You can judge the relative strength of a conjugate by the (K_a) or (K_b) value of the substance because (K_a imesK_b) is equal to the ionization constant of water, Kw which is equal to (1 imes 10^-14) at room temperature. The higher the Ka, the stronger the acid is, and the weaker its conjugate base is. Similarly, the higher the Kb, the stronger the substance is as a base, and the more weakly acidic its conjugate acid is.1


Calculation of Ka

For an acid that reacts with water in the reaction

>

where each bracketed term represents the concentration of that substance in solution.

Relation of Kw, Kb, Ka

Partial List of Strong Acids: Hydrochlroic acid (HCl), Nitric Acid (HNO3), Perchloric Acid (HClO4), Sulfuric Acid (H2SO4)

Partial List of Strong Bases: Sodium Hydroxide (NaOH), Barium Hydroxide (Ba(OH)2), Calcium Hydroxide (Ca(OH)2), Lithium Hydroxide (LiOH) (Hydroxides of Group I and II elements are generally strong bases)

Partial List of Weak Acids: Acetic Acid (CH3COOH), Carbonic Acid (H2CO3), Phosphoric Acid (H3PO4)

Partial List of Weak Bases: Ammonia (NH3), Calcium Carbonate (CaCO3), Sodium Acetate (NaCH3COO)