Lecture 4: Aqueous solution chemistry Lecture 4 Topics Brown, chapter 41. Solutes & solvents 4.1

• Electrolytes & non-electrolytes• Dissociation

2. Solution concentration & stoichiometry 4.5 – 4.6• Molarity & interconversion• Dilution

Types of aqueous chemial reactions3. Precipitation reactions 4.2

• Complete ionic equations4. Neutralization reactions 4.3

• Acids & bases• Neutralization reactions• Non-hydroxide bases produce gases • Titration 4.6

• Summary of complete ionic equations

5. Reduction & oxidation reactions 4.4• Oxidation numbers• Oxidation of metals by acids & salts• Activity series

Reduction & oxidation occur simultaneouslyto 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

2Na0 + 2H2O --> 2Na+1OH-1 + H20

Oxidation: Reduction:

Students often get oxidation & reduction confused. How canyou remember which is which?OIL RIG -> ‘oxidation involves loss; reduction involves gain’

e- gainede- lost

Where do you commonly see redox? Corrosion of metals is oxidation.

How do you tell which reactant was reduced & which was oxidized?

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Oxidation & reduction reactions‘Redox’ reactions are really about the movement of electrons betweenreactants 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 electronsgain of electrons

Well, you can reason it through. If oxidization causes a reactant to lose electrons, thecharge 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.137-8

Oxidation numbers: guides to redoxWhat’s an oxidation number? It’s a number that you can assign to eachatom in a reaction. You compare each atom’s oxidation number beforeand 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 -1.

4. Polyatomic ions - all internal oxidation numbers must sum to the ions overall chargeCO3-2 - O = (3x-2) = -6; so C has an oxidation number of +4

H = (2x1) = +2 | S = -2elemental, so = zeroCl = (2x-1) = -2 | S = +2Na = (2x1) = +2 | O = (3x-2) = -6 | so S must = +4O = (4x-2) = -8 | so S must be +6

p.138-9

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, soeither 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.138-9

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)

Mn(NO3)2(aq) + Pb(s)

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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 oxidizedby 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 byPb(NO3)2? Zn, Cu, Fe

Why are preciousmetal consideredprecious?

Activity Series: Prediction of Redox

yesyesNO

^ ^

Can you reduce metals? Yep, it’s like running an oxidation reaction backwards, and “plates” metals.

p.140-2

Activity Series: Prediction of Redox

Here Ag is reduced & plates on; Cu is oxidized & “dissolves”.