Acids and Bases

What are acids? What properties do you associate with acids? What properties do you associate with bases? Acids: SINDY, formulae i.e. contain (ionisable) hydrogen;

Bronsted-Lowry Theory An acid is a proton (H+) donor e.g. in water, hydrochloric acid gives a proton to a water molecule, forming the hydronium ion. HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq) hydronium ion A base is a proton acceptor e.g. ammonia accepts an H+ ion from hydrogen chloride NH3 + HCl 

What do the bases need in order to be able to accept a proton? A lone pair of electrons to form a covalent bond Lone pair e-s

Conjugate pairs When an acid loses a proton, it does so reversibly. This means that the resulting species can accept a proton and is thus a base. A base formed from an acid is known as the conjugate base. Similarly, the acid formed when a base reversibly gains a proton is a conjugate acid. Note: an acid can only act as an acid in solution if a base is present to accept a proton – the reactions of acids are acid-base reactions. Similarly the reactions of bases are acid-base reactions.

HCl + H2O  H3O+ + Cl- H2O + NH3  NH4+ + OH- base 2 acid 2 acid 1 base 1 base 2 acid 2 H2O + NH3  NH4+ + OH- acid 1 base 1 Note – water can act as an acid or a base – it is amphoteric.

Acid-Base pairs Acid Conjugate base Base Conjugate acid HCl H2SO4 H2O CH3COO- NH3 Base Conjugate acid OH- H2O NH3 C2H5NH3+ H2SO4

Practice! Identify the conjugate acid-base pairs in the following: CH3COOH + OH-  CH3COO- + H2O HSO4- + NH4+  H2SO4 + NH3 C2H5NH2 + HCl  C2H5NH3+ + Cl- H2SO4 + NH3  HSO4- + NH4+

Note: when an acid loses an H+ ion, it does so reversibly to produce its conjugate base. The species resulting from a base gaining an H+ ion is called the conjugate acid of that base.

Strong or weak? A strong acid or base dissociates completely in solution e.g. HCl + H2O  H3O+ + Cl- for a strong acid the position of equilibrium lies so far to the right that 100% dissociation is assumed i.e. all the molecular HCl, once dissolved in water dissociates (ionises) to yield ions. Strong bases dissolve completely in water and are fully ionised or completely dissociated to form hydroxide ions. e.g. HCl + H2O → H3O+ + Cl-

E.g. hydrochloric acid

Note: hydrochloric acid is called a monobasic acid since i.e. they give only hydrogen ion per formula unit whereas sulphuric acid is a dibasic acid because i.e. it gives two hydrogen ions per formula unit. Strong acids include Strong bases include hydrochloric acid oxides or hydroxides of nitric acid metals, particularly those sulphuric acid (during 1st of group I ionisation only) 1 mole of acid reacts with 1 mol of base 1 mol acid reacts with 2 mols base

Weak acids and bases involve partial dissociation in solution – the interaction with water can be shown: CH3COOH(aq) + H2O(l)  CH3COO-(aq) + H3O+(aq) NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) In these equilibria, the equilibrium position lies more to the left i.e. in a solution of ethanoic acid, for example, there are more undissociated ethanoic acid molecules than there are ethanoate and hydronium ions. Note: generally, a strong acid gives rise to a weak conjugate base whereas a weak acid gives rise to a strong conjugate base. Similarly for bases.

Strong acids and bases are much better conductors of electricity than weak acids or bases – why? Weak acids include Weak bases include Ethanoic acid ammonia Phosphoric (V) acid amines Ethanedioic acid Methanoic acid Carbonic acid What do we use to express how acidic or alkaline a solution is?