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Redox is the transfer of electrons
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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.
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?
p.135-6
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.
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)
p.140-2
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
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