Acids and Bases Part 3

Strong and Weak Acids and Bases Consider the acid dissociation reaction: HA   H + + A - If this equilibrium lies to the right, it means that the acid has dissociated fully and is said to be a strong acid. In this case it will exist virtually entirely as ions in solution. For example: HCl   H + + Cl - In this case the reverse reaction can be considered to be negligible so it is usually written HCl  H + + Cl - Strong Acids completely dissociate (split up) into ions in aqueous solution examples HCl  H + (aq) + Cl¯ (aq) MONOPROTIC (1 replaceable H + ) HNO 3  H + (aq) + NO 3 ¯ (aq) H 2 SO 4  2H + (aq) + SO 4 2- (aq) DIPROTIC (2 replaceable H + )

Strong and Weak Acids and Bases If the equilibrium lies to the left, it means that the acid has dissociated only partially and is said to be a weak acid. Here it will exist almost entirely in the undissociated form. Ex: CH3COOH  H + + CH3COO - The strength of an acid is a measure of how readily it dissociates in aqueous solution. This is an inherent property of a particular acid dependent on its bonding.

Weak acids partially dissociate into ions in aqueous solution. Example: ethanoic acid CH 3 COOH (aq)   CH 3 COO¯ (aq) + H + (aq) When a weak acid dissolves in water an equilibrium is set up: HA (aq) + H 2 O (l)   A¯ (aq) + H 3 O + (aq) To make calculations easier the dissociation can be written like this: HA (aq)   A¯ (aq) + H + (aq) The weaker the acid: the less it dissociates and the more the equilibrium lies to the left. The relative strengths of acids can be expressed as K a or pK a values The dissociation constant for the weak acid HA is K a K a = [H + (aq)] [A¯(aq)] mol dm -3 [HA(aq)] Strong and Weak Acids and Bases

Be careful not to confuse two different pairs of opposites. Strong and weak acids or bases refer to their extent of dissociation; concentrated and dilute refer to the amount of water added to the solution. Similar to acids, bases can be described as strong or weak on the basis of the extent of their dissociation. For example, the strong base NaOH dissociates fully, so its equilibrium lies to the right, producing a high concentration of ions. NaOH  Na + + OH -

Strong and Weak Acids and Bases On the other hand, a weak base such as NH 3 ionizes only partially so its equilibrium lies to the left and the concentration of ions will be low. NH 3 + H 2 O  NH OH - Weak acids and bases are much more common than strong acids and bases. The following acids are examples of strong acids: 1)HCl, hydrochloric acid 2) HNO 3, nitric acid 3) H 2 SO 4, sulfuric acid

Strong and Weak Acids and Bases The following bases are strong bases: 1) LiOH, lithium hydroxide 2) NaOH, sodium hydroxide 3) KOH, potassium hydroxide 4) Ba(OH) 2, barium hydroxide Strong acids and bases dissociate almost completely in solution; weak acids and bases dissociate only partially in solution. Distinguishing between strong and weak acids and bases: Owing to their greater dissociation in solution, strong acids and strong bases will contain a higher concentration of ions than weak acids and weak bases.

Strong and Weak Acids and Bases Three properties that can help distinguish between weak and strong acids and bases: 1) Electrical conductivity: Strong acids and strong bases will show higher conductivity than weak acids and bases. This can be measured using a conductivity meter. 2) Rate of reaction: Rates of reactions will be faster in strong acids. This is not an easy way to distinguish between strong and weak acids and bases. 3) pH: The pH scale can be used to directly compare the strengths of acids (providing they are of equal molar concentrations) It is a scale in which the higher the H + concentration, the lower the pH value. Universal indicator or a pH meter can be used to measure pH.