• Announcements• Acid-Base Reactions

Lecture 10 t – 2 class days until exam

Mixing calcium iodide with silver sulfate gives?

20%20%20%20%20% 1. CaSO4

2. AgI3. CaSO4 & AgI4. No reaction 5. None of the above

Announcements

• Sample exam up on chemboard

• CAPA tutor schedule change (Wed 7-9, Sun 6-8– Still SC3031– This Sunday (before exam) there will hopefully be 3

tutors

Acid-Base Reactions

• One of the most fundamental reactions• Neutralization• Often called proton transfer reactions

OH– + H3O+ → 2H2O

What is an acid?• Something that can donate H+ ions (protons)

HCl + H2O → H3O+ + Cl–

• Strong acids completely dissociate, creating hydroniumions by donating ALL hydrogens to water– HCl, H2SO4, HNO3, others (Table 4.2)

• Weak acids only partially dissociate– loosely, they donate protons to OH–, but not to water– CH3CO2H (acetic), H3PO4 (phosphoric)

H+ vs. H3O+

• Originally, Arrhenius had described an acid using H+

• We now know that H+ does not exist in solution• When produced it will immediately react with a water

molecule to form hydronium ion, H3O+

H+ (aq) + H2O (l) H3O+ (aq)

• So, in water acids produce H3O+ ions

What is a base?

• Something that can accept H+ ions• Hydroxide (OH–) is a great proton acceptor• Strong bases often generate OH– ions in water

– NaOH, KOH, etc.• Weak bases often accept protons directly from hydronium

ions, but not from water– NH3 + H3O+ → NH4

+ + H2O– NH3 + H2O → N.R.

Titration

• Titration: adding a base to an acid (or vice versa!) until it is neutralized, to determine how much acid is present.

• The unknown acid is placed in a beaker or flask.

• A buret allows us to measure the volume of the base, which is called the titrant.– The titrant must be of known concentration

• An indicator is used to tell us when the solution is exactly neutralized, its color depends on the hydronium ion concentration, [H3O+].– Can also use pH meter

More Titration

• At the stoichiometric point, just enough hydroxide ions have been added to react with every acidic proton present.– Also called the end point

• The indicator changes colors at the stoichiometric point.– The solution is neutral

We have 100.00 mL of unknown base. This is titrated with 0.5863 M HCl. The endpoint is reached when 47.39 mL of acid have been added.

What is the concentration of the base?

54321

25%25%25%25% 1. 0.1885 M

2. 0.2778 M3. 0.8083 M4. 1.237 M

Oxidation – Reduction Reactions

• More commonly known as Redox reactions• These are electron transfer reactions

– Contrast with acid-base proton transfer• Our happy Magnesium reaction is a good example

Mg(s) + 2H3O+(aq) → Mg2+(aq) + H2(g) + H2O(l)

Electrons move from the Mg to the H3O+.

The charge on each changes.

Balancing Redox Reactions

• Easiest to separate reduction and oxidation steps– Each alone is called a half-reaction– As always, mass and charge must be conserved

Oxidation:Mg → Mg2+ + 2e–

Reduction:2H3O+ + 2e– → H2 + H2O

“LEO says GER”Loose electrons = oxidationGain electrons = reduction

“OIL RIG”Oxidation is loss

Reduction is gain

Mg + 2H3O+ + 2e– → Mg2+ + H2 + H2O + 2e–

Redox – Complex• Many details we will leave until next semester• We will cover relative ability of metals to oxidize/reduce

– Later we will quantify this with reduction potentials– For now, just use Table 4-3 (things like to flow CCW)

For Example

• Mg metal is mixed with strong acidMg is oxidized and H3O+ is reduced

Mg(s) + 2H3O+(aq) → Mg2+(aq) + H2(g) + H2O(l)

• Lead metal is mixed with zinc sulfateNo reaction

• Iron metal is mixed with copper nitrateFe is oxidized and Cu2+ is reduced

Fe(s) + Cu2+(aq) → Fe2+(aq) + Cu(s)or

2Fe(s) + 3Cu2+(aq) → 2Fe3+(aq) + 3Cu(s)

Aluminum perchlorate is mixed with nickel metal.

What happens?

54321

20%20%20%20%20% 1. Aluminum ions are reduced

2. Aluminum ions are oxidized3. Nickel is oxidized4. Both 1 and 25. None of the above

Today• Finish CAPA #6

By Friday• Master all the individual steps• Make plans for seminar

Rem

ember: Y

ou are done with the

homew

ork when you understand it!