The Strong vs. The Weak The Jekyl and Hyde of Acids and Bases

Review- The pH scale 0714 AcidNeutralBase The pH scale is based on the number of H+ and OH- ions Which side has the most H + ? Which side has the most OH - ? And where are they just about equal? Most H + (little OH - ) Most OH - (little H + ) Equal H + and OH -

H + and OH - again Another way of showing the H + /OH - concentrations is this diagram: PH [H + ] 1o -1 1o -14 H+H+ OH Ph

General Properties- Ionics n Ionic compounds ionize in aqueous solution –Which means… they break into ions n Inorganic acids and bases are ionic –So, they ionize in water too NaCl(s)  Na + + Cl - HCl  H + + Cl -

The few, the strong... n What makes an acid a strong acid? A strong acid has a very low pH Which means it has a lot of free H + ions - In chemistry, we say that it “dissociates” a lot Strong acids are also the ones which can do massive damage to your skin, eyes...

And the weak ones n What makes an acid a weak acid? A weak acid has a pH which is fairly close to 7 Which means it doesn’t have many free H + ions - it only dissociates slightly

Strong and Weak Ionization n Strong Acids and Bases completely ionize n Weak ones only ionize a little- making an equilibrium with ions and the compound CH 3 COOH  CH 3 COO - + H + HCl  H + + Cl -

Which is Which? n Take a look at the chart to the left and pick out the acid which has the most free H+ ions n How did you know? Nitric It has very low pH So, it’s a strong acid

Strength and Bases n Bases are similar Strong bases have HIGH pH (lots of OH - ions and not a lot of H + ions) - They dissociate a lot - Also, just like strong acids, strong bases can really damage your skin or eyes... Weak bases have a pH close to 7 - They dissociate very little

Try it again n Which one is the weak base? Ammonia (pH closest to 7) So, ammonia would be the least dangerous because it has very few free OH - ions

Quick Test n What is the pH of a strong acid? n Very low, close to zero n What is the pH of a weak base? n Just higher than 7 n What does the word dissociate mean? n Ions split when dissolved in H 2 O n What types of acids and bases dissociate a lot? n Strong acids and bases; so they have lots of free ions

Give and Take n And, you know that acids start with H, so they n and bases 0714 Strong AcidWeak AcidWeak BaseStrong Base Neutral donate H + accept H + to make HOH donate H + ions accept H + ions

Fancy New science Word Of the day AMPHIPROTIC: Substance that can act as an acid (H + donor) or a base (H + acceptor) Example: H 2 O can become H 3 O + or OH -

Fancy new science Word of The day #2 Conjugate Pair: 2 Substances which differ by one H +. One acts as an acid (H + donor) and one as a base (H + acceptor). Example: HCl/Cl -

Summary 0714 AcidNeutralBase Strong Acid Dissociates well Lots of free H + ions Weak Acid Dissociates slightly Few free H + ions Weak Base Dissociates slightly Few free OH - ions Strong Base Dissociates well Lots of free OH - ions

The End