Acid/Base Properties In the past, we have classified acids and bases according to their observed properties ACIDS BASES Sour tastebitter taste Watery feelslippery feel Reactive with metalscan be corrosive Water solublemay be soluble in H 2 O

Extending Acid/Base Theories Arrhenius Theory (Svante Arrhenius ) Proposed that when salts dissolve in water, they dissociate completely into ions Acids- dissolve in water to produce H + (aq) ions Eg. HCl, HNO 3, H 3 PO 4, H 2 SO 4, HF Bases- dissolve in water to produce OH - (aq) ions Eg. NaOH, Ca(OH) 2, Mg(OH) 2 Neutralization- acid + base  water + a salt HCl(aq) + NaOH(aq)  HOH(ℓ) + NaCl(aq)

Extending Acid/Base Concepts Bronsted-Lowry Theory (1923 Johannes Bronsted, Thomas Lowry) Independently noted that some acid-base neutralization reactions can occur without dissolving ions in water Eg. HCl(g) + NH 3 (g)  NH 4 Cl(s) They concluded that acid/base reactions simply involve the transfer of a hydrogen ion from one molecule to another Although Arrhenius theory is very successful and useful for explaining acid/base chemistry, it has limitations WHY is Na 2 CO 3 (aq) BASIC?!? HELP WANTED

Extending Acid/Base Concepts HCl  H + + Cl – NaOH  OH – + Na + HCl + NaOH  H 2 O + NaCl H+ acceptorH+ donor Produces OH – in solution Produces H + in solution Bronsted- Lowry Arrhenius (grade 11) BaseAcid According to Bronsted-Lowry definitions: ACIDS- are proton (H + ) donors BASES- are proton (H + ) acceptors Comparing theories

Extending Acid/Base Concepts Bronsted-Lowry theory could be used to explain neutralization reactions that could not be explained by Arrhenius theory. HCl is an acid because it donates H +, NH 3 accepts H + and therefore is the base (NH 4 + and Cl – then form an ionic compound) H H H H H H H HN N Cl So, Na 2 CO 3 is basic because… CO 3 2- (aq) + H 2 O  HCO 3 1- (aq) + OH 1- (aq)

Strong vs. Weak Electrolytes DEMO conductivity of acids and bases 1.0 mol/L HCl(aq) and 1.0 mol/L CH 3 COOH(aq) 1.0 mol/L NaOH(aq) and 1.0 mol/L NH 3 (aq) The conductivity of these solutions varies eventhough they are all of the same concentration Substances that dissolve in water and completely dissociate into ions are called strong electrolytes Acids/Bases that dissolve in water and completely dissociate into ions are called strong acids/bases

Strong vs. Weak Electrolytes Some strong acids and bases: HCl(aq) + H 2 O(ℓ)  H 3 O + (aq) + Cl 1- (aq) NaOH(s) + H 2 O(ℓ)  Na 1+ (aq) + OH 1- (aq) fully dissociated! Hydronium ion

Strong vs. Weak Electrolytes Some substances are highly water soluble, yet they do not fully dissociate into ions upon dissolution. These substances are weak electrolytes Acids and bases that do not fully dissociate into ions upon dissolving are weak acids/bases When weak acid/bases are dissolved in water, a chemical equilibrium is established in the solution

Strong vs. Weak Electrolytes Weak acid/base equilibria(CLICK HERE!)(CLICK HERE!) CH 3 COOH(aq) + H 2 O(ℓ)  H 3 O + (aq) + CH 3 COO 1- (aq) NH 3 (aq) + H 2 O(ℓ)  NH 4 + (aq) + OH 1- (aq) The reactant sides of these equilibria are favoured, so complete dissociation of the acid/base molecules does not occur.

Oxyacids Oxyacids are mineral acids which contain oxygen combined with another non-metal Eg. HNO 3 (aq), H 2 SO 4 (aq), H 3 PO 4 (aq) Monoproticdiprotoictriprotic The relative strength (ability to donate protons) of an oxyacid increases with the number of oxygen atoms that are not bonded to an hydrogen atom in the acid molecule. Eg. H 2 SO 4(aq) sulfuric acid is stronger than H 2 SO 3(aq)

Oxyacids The ionization of polyprotic acids can be expressed using more than one chemical equation. Eg. H 3 PO 4(aq) ionization: H 3 PO 4(aq) + H 2 O (ℓ)  H 3 O + (aq) + H 2 PO 4 1- (aq) H 2 PO 4 1- (aq) + H 2 O (ℓ)  H 3 O + (aq) + HPO 4 2- (aq) HPO 4 2- (aq) + H 2 O (ℓ)  H 3 O + (aq) + PO 4 3- (aq)

Conjugate Acid/Base Pairs In any equilibrium expression involving a weak acid/base more than one Bronsted/Lowry acid and base can be identified CH 3 COOH(aq) + H 2 O(ℓ)  H 3 O + (aq) + CH 3 COO 1- (aq) Conjugate acid/base pair Conjugate acid/base pair- two substances that differ from each other by just one proton (H + ). CH 3 COO 1- (aq) is the conjugate base of CH 3 COOH(aq) H 3 O + (aq) is the conjugate acid of H 2 O(ℓ)

Conjugate Acid/Base Pairs HCN(l) + H 2 O  CN – (aq) + H 3 O + (aq) Forward: HCN is acid, H 2 O is base Reverse: H 3 O + is acid, CN – is base HCN(l) + H 2 O  CN – (aq) + H 3 O + (aq) For more lessons, visit Relative strengths of conjugate acid/base pairs The conjugate base of a weak acid must be a strong base The conjugate base of a strong acid must be a weak base The conjugate acid of a weak base must be strong acid The conjugate acid of a strong base must be a weak acid Q. Explain whether you would expect a solution of NH 4 Cl (aq) to be acidic or basic.

Amphoterism Any substance that can behave either as an acid or a base is amphoteric (amphiprotic) Eg. Bicarbonate buffer in blood, HCO 3 1- (aq) HCO OH 1-  H 2 O + CO 3 2- (as an acid) HCO H 3 O +  H 2 CO 3 + H 2 O (as a base)

Homework p.532 #1, 2 p.549 #2, 3 Read p.532 to 549 for next class