VII. Chemical Reactions
A.
Writing and Balancing a Chemical Reaction
1. Visible evidence that a chemical reaction occurred
a.
Change in color
b.
Formation of a gas
c.
Formation of a precipitate
d.
Heat released or absorbed
2. Chemical reaction - symbolic way to express a chemical change
3. Components of a chemical reaction
a.
Compounds and/or elements written on the left = reactants
b.
Compounds and/or elements written on the right = products
c.
Arrow written between
d.
Sample reaction - combustion of methane (natural gas)
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
Note: other symbols (g), (l), (s) refer to state of matter (gas, liquid, solid)
Reactants = CH4 and O2; Products = CO2 and H2O
4. Balancing a chemical reaction
a.
Key rules
(1) Need correct chemical formulas
(2) Do not change a chemical formula
(3) Balance reaction only by changing coefficients in front of formulas
b.
Begin by counting atoms of each element
c.
Frequently best to start with formula having the greatest number of atoms
d.
Do one formula at a time
e.
Polyatomic ions can be treated as a group if the same on both sides
f.
Finish by checking all atoms
g.
Examples
(1) Fe(s) + O 2(g) → Fe2O3(s)
(a) Start with Fe2O3; can balance the Fe with a 2 on the left, but the O is
more of a problem
(b) Note that 2 an 3 have as s common denominator 6, so multiplying the
O2 by 3 and the Fe2O3 by 2 will balance the oxygen.
Fe(s) + 3 O2(g) → 2 Fe2O3(s)
(c) Balancing is completed by putting a 4 in front of the Fe
4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)
(2) Al(s) + CuSO4(aq) → Cu(s) + Al2(SO 4)3(aq) ((aq) means in water)
(a) Start with Al2(SO 4)3; put a 2 in front of the Al and a 3 in front of the
CuSO 4 (to account for 3 SO42– ions)
(b) 2 Al(s) + 3 CuSO4(aq) → Cu(s) + Al2(SO 4)3(aq)
(3) Complete the balancing by putting a 3 in front of the Cu
2 Al(s) + 3 CuSO 4(aq) → 3 Cu(s) + Al2(SO 4)3(aq)
(3) Other examples
(a) Al(s) + HCl(aq) → AlCl3(aq) + H 2(g)
(b) Fe2(SO 4)3(aq) + KOH → Fe(OH) 3(s) + K2SO 4(aq)
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B.
Types of Chemical Reactions
1.
Combination reactions - two pure substances combining to form one compound
Examples:
a.
Mg(g) + O 2(g) → MgO (s) (metal + nonmetal)
b.
C(s) + O 2(g) → CO2(g) (nonmetal + nonmetal)
c.
SO 3(g) + H 2O(l) → H2SO 4(aq) (nonmetal oxide + water gives acid, basis of acid
rain)
b.
Na2O(s) + H2O(l) → 2 NaOH (aq) (metal oxide + water gives hydroxide, base)
2.
Decomposition reactions - one compound breaks down into two or more pure substances
(Often requires heating)
Examples:
∆ BaO
BaCO3(s) →
(s) + CO2(g) (heating carbonate yields metal oxide + CO2)
a.
∆
= heating the sample
Demo - heating HgO
∆
b.
2 NaHCO 3(s) → Na2CO3(s) + CO2(g) + H 2O(l)
(heating bicarbonate yields carbonate + CO2 + water)
This reaction is the basis of the leavening action of baking soda and baking powder
3.
Single-replacement reactions - element plus compound yields new element and new
compound
Examples:
a.
Mg(s) + CuCl 2(aq) → Cu(s) + MgCl2(aq)
b.
2 Al(s) + 3 H2SO 4(aq) → 3 H2(g) + Al2(SO 4)3(aq)
4.
Double-replacement reactions - between two ionic compounds, positive ions trade places
Examples:
a.
Pb(NO 3)2(aq) + 2 KI(aq) → PbI2(s) + 2 NaNO3(aq)
Driving force for this reaction is the formation of the precipitate PbI2.
b.
H2SO 4(aq) + 2 NaOH(aq) → Na2SO 4(aq) + 2 H2O(l)
Driving force here is the formation of water; this is also a neutralization reaction of
the general form acid + base gives a salt + water
5.
Oxidation-reduction reactions
a.
Overall characteristic - reaction involves one or more elements losing electrons
(oxidation) and one or more elenments gaining electrons (reduction)
b.
Memory device: LEO goes GER
(1) LEO = Loss of Electrons is Oxidation
(2) GER = Gain of Electrons is Reduction
c.
One example is single-replacement reactions
(1) Mg(s) + CuCl 2(aq) → Cu(s) + MgCl2(aq)
(2) Ionic form of equation:
Mg(s) + Cu2+ (aq) + 2 Cl –(aq) → Cu(s) + Mg2+ (aq) + 2 Cl –(aq))
(3) Divide into two parts
(a) Mg(s) → Mg2+ (aq) + 2e – (oxidation, loss of 2 electrons)
(b) Cu2+ (aq) + 2e – → Cu(s) (reduction, gain of two electrons)
(4) In balanced oxidation-reduction reaction, number of electrons lost = number
of electrons gained
Demo
Demo
Demo
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d.
e.
Another example is combination reactions
(1) 2 Al(s) + 3Cl 2(g) → 2 AlCl3(s)
(2) Dividing into the two parts
(a) 2 Al(s) → 2 Al3+ + 6e – (oxidation, loss of 3 electrons per Al)
(b) 3 Cl2(g) + 6e – → 6 Cl– (reduction, gain of one electron per Cl)
Organic compounds (1) Difficult to identify electron change since one isn’t dealing with ionic
compounds
(2) Other ways to determine that oxidation and reduction have occurred
(a) Look for changes in oxygen
(i) Gain of oxygen = oxidation
(ii) Loss of oxygen = reduction
(iii) Example: combustion of methane (CH4)
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
Carbon compound contains more oxygen, carbon was oxidized
(b) Look for changes in hydrogen
(i) Gain of hydrogen = reduction
(ii) Loss of hydrogen = oxidation
(iii) Conversion of formaldehyde to methanol
CH2O(l) + H 2(g) → CH3OH (l)
The carbon compound contains more hydrogen, was reduced
(3) Summary of increasing oxidation states of carbon (starting with methane)
CH4; CH3OH; CH 2O; HCO 2H; CO2
methane; methanol; formaldehyde; formic acid; carbon dioxide
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