Lecture 4: Aqueous solution chemistry
Lecture 4 Topics
Brown, chapter 4
1. Solutes & solvents
4.1
• Electrolytes & non-electrolytes
• Dissociation
2. Solution concentration & stoichiometry
• Molarity & interconversion
• Dilution
4.5 – 4.6
Types of aqueous chemial reactions
3. Precipitation reactions
• Complete ionic equations
4. Neutralization reactions
• Acids & bases
• Neutralization reactions
• Non-hydroxide bases produce gases
• Titration
4.2
4.3
4.6
• Summary of complete ionic equations
5. Reduction & oxidation reactions
• Oxidation numbers
• Oxidation of metals by acids & salts
• Activity series
4.4
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
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.
Oxidation: loss of electrons
Reduction: gain of electrons
Students often get oxidation & reduction confused. How can
you remember which is which?
OIL RIG -> ‘oxidation involves loss; reduction involves gain’
2Na0 + 2H2O --> 2Na+1OH-1 + H20
e- lost
e- gained
Where do you commonly see redox? Corrosion of metals is oxidation.
How do you tell which reactant was reduced & which was oxidized?
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.
p.135-6
Redox reactions happen at electrodes
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.
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 charge
CO3-2 - O = (3x-2) = -6; so C has an oxidation number of +4
What is the oxidation number of S in each example:
Start with atoms whose
H = (2x1) = +2 | S = -2
H2S
oxidation no. are known &
S8
elemental, so = zero
then assign S to balance.
SCl2
Cl = (2x-1) = -2 | S = +2
Na2SO3
Na = (2x1) = +2 | O = (3x-2) = -6 | so S must = +4
-2
SO4
O = (4x-2) = -8 | so S must be +6
p.137-8
Let’s have a look at metals & acids:
 e- transferred from Mg to H+1,
oxidizing the Mg from metal to ion, and
reducing H+1 to H2 gas
 Mg ribbon being oxidized by HCl
p.138-9
Oxidation of metals by acids & salts
This is classical corrosion! These are displacement reactions:
A + BX  AX + B A is the metal corroded; B is H or a cation; X is the anion.
• A displaces B for the company of X
• In practice it’s like an exchange rxn with fewer partners.
Oxidation by acid: Zn(s) + 2 HBr(aq)  ZnBr2(aq) + H2(g)
Per atom oxidation #: 0
+1/-1
+2/-1
0
Zn oxidized
H reduced
Oxidation by salt:
Oxidation #:
Mn(s) + Pb(NO3)2(aq)  Mn(NO3)2(aq) + Pb(s)
0
Mn oxidized
+2/-1
+2/-1
0
Pb reduced
Generally, the metal that begins in elemental form is oxidized, so
either the ___H___ or the __cation__ is reduced.
Complete & net ionic equations?
Mn(s) + Pb(NO3)2(aq)  Mn(NO3)2(aq) + Pb(s)
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
Activity Series: Prediction of Redox
Specifically, this table allows us to predict whether a metal
will be oxidized by a specific salt or by acid.
Any elemental metal (in lefthand 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 yes
b) Ni(s) + H+1
yes
+1
c) Cu + H
NO
acid
Which are oxidized by
Pb(NO3)2? Zn, Cu, Fe
In lab 3 why Cu wire could
be used to ‘hold’ Mg
^
^
Why are precious
metal considered
precious?
p.140-2
Activity Series: Prediction of Redox
Can you reduce metals?
Yep, it’s like running an oxidation reaction backwards, and “plates” metals.
Here Ag is reduced & plates on; Cu is oxidized & “dissolves”.
p.140-2