Title: Lesson 4 Strong and Weak Acids and Bases Learning Objectives: Understand the difference between strong and weak acids and bases Complete an experiment to explore the difference in properties of strong/weak acids and bases

Refresh 100 cm3 of a NaOH solution of pH 12 is mixed with 900 cm3 of water. What is the pH of the resulting solution? 1 3 11 13

Strong acids example: Good proton donor. Dissociation reactions go to completion, their conjugate bases are not readily able to accept a proton. Weak acids example: Poor proton donors. Equilibria of dissociation reaction lies to the left, their conjugate bases are readily able to accept a proton. Conjugate base Cl- shows virtually no basic properties Conjugate base CH3COO- is a stronger base than Cl- Acid dissociation favours the production of weaker conjugate bases.

Bases follow a similar pattern… Conjugate acid Na+ shows virtually no acidic properties Strong base example: Good proton acceptor. Dissociation reaction goes to completion, their conjugate acids do not readily donate protons. Weak base example: Poor proton acceptor. Equilibria of dissociation reaction lies to the left, their conjugate acids readily donate protons. Conjugate acid NH4+ shows virtually no acidic properties Base dissociation favours the production of weaker conjugate acids.

Strong Bases (Grp 1 hydroxides & barium hydroxide) Strong Acids (only six; know 1st three for IB) Strong Bases (Grp 1 hydroxides & barium hydroxide) Weak Acids carboxylic and carbonic acids Weak Bases ammonia and amines H2SO4, sulfuric acid* LiOH, lithium hydroxide CH3COOH, ethanoic acid and other organic acids C2H5NH2, ethylamine and other amines HNO3, nitric acid NaOH, sodium hydroxide H2CO3, carbonic acid Note CO2(aq) = H2CO3(aq) NH3, ammonia Note NH3(aq) = NH4OH(aq) HCl, hydrochloric acid KOH, potassium hydroxide H3PO4, phosphoric acid   HI, hydroiodic acid Ba(OH)2, barium hydroxide HBr, hydrobromic acid HClO4, perchloric acid Try to remember the main strong acids and bases… Then you can probably deduce any others you come across will be weak…

Strong, Concentrated and Corrosive In everyday English, strong and concentrated are often used interchangeably. In chemistry, they have distinct meanings: strong: completely dissociated into ions concentrated: high number of moles of solute per liter (dm3) of solution corrosive: chemically reactive Similarly, weak and dilute also have very different chemical meanings: weak: only slightly dissociated into ions dilute: a low number of moles of solute per liter (dm3) of solution It is possible for an acid or base to be strong but in a dilute solution, or weak and present in a concentrated solution.

Strong, Concentrated and Corrosive Similarly, weak and dilute also have very different chemical meanings: weak: only slightly dissociated into ions dilute: a low number of moles of solute per liter (dm3) of solution

Strong and Weak Acids Strong Acids: HA(aq)  H+(aq) + A-(aq) … The acid fully dissociates into ions For example: HCl(aq)  H+(aq) + Cl-(aq) Includes: hydrochloric, sulfuric, phosphoric, nitric Strong acids have weak conjugate bases Weak Acids: HA(aq)  H+(aq) + A-(aq) … The acid only partially dissociates into ions For example: HF(aq)  H+(aq) + F-(aq) Includes: hydrofluoric, ethanoic, carbonic Weak acids have strong conjugate bases

Strong and Weak Bases Strong Bases: BOH(aq)  B+(aq) + OH-(aq) …..the base fully dissociates into ions For example: NaOH(aq)  Na+(aq) + OH-(aq) Includes: group (I) hydroxides, barium hydroxide Strong bases have weak conjugate acids Weak Bases: BOH(aq)  B+(aq) + OH-(aq) …..the base only partially dissociates into ions For example: NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) Includes: ammonia, amines Weak bases have strong conjugate acids

Simulation http://phet.colorado.edu/en/simulation/acid-base-solutions

Distinguishing between strong and weak acids and bases The equilibrium has a profound effect on the properties of the acid/base Compared with strong acids of the same concentration, weak acids will: Have lower electrical conductivity (less mobile ions in from the same moldm-3) React more slowly (reactions depend on the concentration of H+ ions) pH is higher (lower concentration of H+ ions) Change pH more slowly when diluted However, they neutralise the same volume of alkali Weak bases follow a similar pattern

Solutions

Experiment Complete the experiment here, investigating strong/weak acids and bases When you give explanations, focus on reasons in terms of the equilibrium rather than just ‘because it is strong/weak’

Key Points Strong acids/bases dissociate fully into ions Weak acids/bases only partially dissociate, forming an equilibrium The strong/weak character has a significant effect on the chemical properties