Secs 4.3-4.4 Continued
Acid-base (neutralization) reactions
This is an extremely important class of reaction:
Acid(aq) + base(aq)  salt (aq) + H2O(l)
E.g., write balanced net ionic equations for the reactions
between:
Aqueous calcium hydroxide and aq. nitric acid
Aqueous acetic acid and aqueous barium hydroxide
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2. Neutralization reactions with gas formation
In general,
strong acid + metal { carbonates, bicarbonates, sulfide,
sulfites }  salt + acid
Carbonates and bicarbonates will form H2CO3
Sulfides will form H2S
Sulfites will form H2SO3
The acids formed in solution decompose & give off gases, e.g.,
Na2CO3(aq) + HCl(aq)  NaCl(aq) + H2CO3(aq)
then.....
H2CO3(aq) H2O(l) + CO2(g)
Write the balanced net ionic equation for this reaction!
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Write balanced net ionic equations for:
ZnS(s) and HCl(aq)
Solid potassium sulfite and aqueous hydrochloric acid
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4.4 Oxidation – Reduction (redox) reactions
oxidation: loss of ereduction: gain of ee.g., Ca(s) + 2H+(aq)  Ca2+(aq) + H2(g)
Which species is oxidized? which is reduced?
Note that when something is oxidized, something
else is reduced!
Oxidation numbers: allow us to keep track of which
species is gaining/losing e- in redox reactions
An increase in oxidation number indicates that a
species is oxidized
A decrease in oxidation number means that a species
is reduced
Recall how to deduce charges in ionic compounds, e.g.,
BaCl2…….
We want a similar model for molecular compounds
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Rules for assigning oxidation numbers
Oxidation number of an element in elemental form = 0
E.g., H2(g), F2(g), Na(s), I2(s)
Oxidation number of a monoatomic ion = its charge
E.g., K+, O2-, Cl-
E.g., what are the oxidation numbers of Fe and O in
Fe2O3?
What about a binary molecular compound, e.g., CO2?
Nonmetals: generally have negative oxidation numbers
Oxygen: usually -2 in both ionic and molecular
compounds (-1 in peroxides)
Hydrogen: +1 when bonded to a nonmetal, -1 when
bonded to a metal
Fluorine: always -1 in all compounds
Other halogens: generally -1 except when with
oxygen, then positive (e.g. Cl in ClO4-)
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Sum of oxidation numbers=0 for a neutral compound, and
equals the overall charge for an ionic species
e.g., find the oxidation state of the underlined element in
P2O5; SnBr4; Na2SO3
How to use this?
E.g., Which element is oxidized and which is reduced in the
following reactions?
Cl2(aq) + 2NaI(aq)  I2(aq) + 2NaCl(aq)
PbS(s) + 4H2O2(aq)  PbSO4(aq) + 4H2O(l)
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Oxidation of metals by acids and salts
Many metals react with acids to form salts and hydrogen
gas:
Metal(s) + acid(aq)  salt(aq) + H2(g)
e.g., write a balanced equation for the reaction between
Fe(s) and sulfuric acid.
When a metal reacts with an acid, what is oxidized? what
is reduced?
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Metals can also be oxidized by aqueous solutions of
various metal salts…..
e.g., Write a balanced equation for the reaction between
Fe(s) and nickel (II) nitrate to form solid nickel and iron (II)
nitrate.
Can we predict whether a certain metal will be oxidized by
acids or salts of other metals?
We list the metals in order of decreasing ease of oxidation
The easier a metal is oxidized (gives up electrons), the
more active the metal
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This ordering is known as the activity series
The activity series is used to predict reactivity as follows:
Any elemental metal on the list can be oxidized by the ions
of elements below it
e.g., Will Cu react with Fe2+? What about Ag?
e.g., Will Cu react with an acid solution? What about Al?
Note that the group 1 and 2 elements are the most active –
why?
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Problems du Jour
Classify each of the following substances as a
nonelectrolyte or strong/weak electrolyte in H2O
HF
NH3
Cu(NO3)2
Ni(OH)2
C2H5OH
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Problems du Jour
Which element is oxidized and which is reduced in the
following reactions:
Ni(s) + Cl2(g)  NiCl2(aq)
Fe2O3(s) + 3CO(g)  2Fe(s) + 3CO2(g)
Write balanced molecular and net ionic equations for the
reactions of
Aluminum with formic acid, HCHO2
Chromium with hydrobromic acid
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Problems du Jour
Based on the activity series, what is the outcome of the
following reactions?
Al(s) + NiCl2(aq) 
Mn(s) + HBr(aq) 
Ag(s) + Pb(NO3)2(aq) 
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