Lecture 4: Aqueous solution chemistry Lecture 4 Topics Brown, chapter 4 1. Solutes & solvents 4.1 Electrolytes & non-electrolytes Dissociation 2. Solution concentration & stoichiometry 4.5 – 4.6 Molarity & interconversion Dilution Types of aqueous chemial reactions 3. Precipitation reactions 4.2 Complete ionic equations 4. Neutralization reactions 4.3 Acids & bases Neutralization reactions Non-hydroxide bases produce gases Titration 4.6 Summary of complete ionic equations 5. Reduction & oxidation reactions4.4 Oxidation numbers Oxidation of metals by acids & salts Activity series

Reduction & oxidation occur simultaneously to produce cations & anions. It’s all about the transfer of electrons. Acids & salts oxidize metals. Oxidation numbers are your guide. The activity series predicts who oxidizes whom.

Redox is the transfer of electrons

2Na 0 + 2H2O --> 2Na +1 OH -1 + H2 0 Oxidation: Reduction : Students often get oxidation & reduction confused. How can you remember which is which? OIL RIG -> ‘oxidation involves loss; reduction involves gain’ e- gained e- lost Where do you commonly see redox? Corrosion of metals is oxidation. How do you tell which reactant was reduced & which was oxidized? p Oxidation & reduction reactions ‘Redox’ reactions are really about the movement of electrons between reactants to product slightly altered products. Reduction & oxidation must are inextricably linked; both must occur. Why? Redox (like all reactions) must obey the Law of Conservation of Mass. E- can’t just disappear - if one reactant loses them the other must gain them. loss of electrons gain of electrons Well, you can reason it through. If oxidization causes a reactant to lose electrons, the charge of that reactant will increase (become more positive) when it becomes a product (more protons than e-). If reduction occurs the reactant gains electrons has its charge becomes more negative.

Redox reactions happen at electrodes

What is the oxidation number of S in each example: H2S S8 SCl2 Na2SO3 SO4 -2 p Oxidation numbers: guides to redox What’s an oxidation number? It’s a number that you can assign to each atom in a reaction. You compare each atom’s oxidation number before and after the reaction to determine whether an atom was oxidized or reduced. Start with atoms whose oxidation no. are known & then assign S to balance. Assigning oxidation numbers: 1. Elemental atoms (uncharged single symbols) have oxidation numbers of zero. 2. Monoatomic ions take their charge as their oxidation number. 3. Non-metals usually have negative oxidation numbers: O = -2 (except for peroxides) H is +1 when with non-metals; but -1 when with metals (LiH, CaH2; hydrides) All in column 7A have oxidation numbers of Polyatomic ions - all internal oxidation numbers must sum to the ions overall charge CO O = (3x-2) = -6; so C has an oxidation number of +4 H = (2x1) = +2 | S = -2 elemental, so = zero Cl = (2x-1) = -2 | S = +2 Na = (2x1) = +2 | O = (3x-2) = -6 | so S must = +4 O = (4x-2) = -8 | so S must be +6

p Let’s have a look at metals & acids:  Mg ribbon being oxidized by HCl  e- transferred from Mg to H +1, oxidizing the Mg from metal to ion, and reducing H +1 to H2 gas

This is classical corrosion! These are displacement reactions: A + BX  AX + B Oxidation by acid:Zn(s) + HBr(aq)  Oxidation by salt:Mn(s) + Pb(NO3)2(aq)  Generally, the metal that begins in elemental form is oxidized, so either the ___ H ___ or the __cation__ is reduced. Per atom oxidation #:0 +1/-1 +2/-1 0 Zn oxidized H reduced Oxidation #: 0 +2/-1 +2/-1 0 Mn oxidized Pb reduced Complete & net ionic equations? Mn(s) + Pb(NO3)2(aq)  Mn(s) + Pb +2 (aq) + 2NO3 -1 (aq)  Mn +2 (aq) + 2NO3 -1 (aq) + Pb(s) So here spectator ions are in exactly the same form on both sides. p Oxidation of metals by acids & salts A displaces B for the company of X In practice it’s like an exchange rxn with fewer partners. A is the metal corroded; B is H or a cation; X is the anion. ZnBr2(aq) + H2(g) 2 Mn(NO3)2(aq) + Pb(s)

p In lab 3 - why Cu wire could be used to ‘hold’ Mg Specifically, this table allows us to predict whether a metal will be oxidized by a specific salt or by acid. acid Any elemental metal (in left- hand column) can be oxidized by any ion (right column) below it. H + is just another metal ion. Will these reactions occur? a) Cu(s) + Ag +1 b) Ni(s) + H +1 c) Cu + H +1 Which are oxidized by Pb(NO3)2? Zn, Cu, Fe Why are precious metal considered precious? Activity Series: Prediction of Redox yes yes NO ^

Can you reduce metals? Yep, it’s like running an oxidation reaction backwards, and “plates” metals. p Activity Series: Prediction of Redox Here Ag is reduced & plates on; Cu is oxidized & “dissolves”.