Major Types of Chemical Reactions:
1. Combination Reactions (synthesis/composition) – reactions in which two or more substances (elements or
compounds) combine to form a single product.
A + X  AX
Examples:
a) Reactions with oxygen to produce oxides.
2Mg(s) + O2(g)  2MgO(s)
element + element
2KCl(s) + O2(g)  2KClO3(s) compound + element
b) Reactions of active metal oxides with water to produce metal hydroxides.
CaO(s) + H2O(l)  Ca(OH)2(s)
compound + compound
2. Decomposition Reactions (separation) – the breakdown of a compound into two or more substances usually
with addition of energy (heat, electricity, etc.).
AX  A + X
Examples:
a) Decomposition:
2H2O(l)  2H2(g) + O2(g)
binary compounds into elements
2KClO3(s)  2KCl(s) + O2(g)
ternary compounds into compound + element
b) Decomposition of metal hydroxides into metal oxides and water.
Ca(OH)2(s)  CaO(s) + H2O(g)
ternary compounds into compounds
3. Single Replacement Reactions – reactions in which one element replaces another element in a compound as
determined by the activity series.
A + BX  AX + B A replaces B
cation replaces cation
Y + BX  BY + X Y replaces X
anion replaces anion
Examples:
a) Replacement of hydrogen in water by a metal (group 1 and 2) to produce a metal hydroxide
and hydrogen.
2Na(s) + 2H2O(l)  2NaOH(aq) + H2
b) Replacement of a metal in a compound by a more reactive metal.
2Al(s) + 3Fe(NO3)2(aq)  3Fe(s) + 2Al(NO3)3(aq)
c)
Replacement of halogens by another halogen higher up in the periodic table (F is the most
active).
Cl2(g) + 2KBr(aq)  2KCl(aq) + Br2(l)
F2(g) + 2NaI  2NaF(aq) + I2(s)
Cl2(g) + 2KF(aq)  2KCl(aq) + F2(l) no reaction
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4. Double Replacement Reactions (ionic reactions in aqueous solution) – the ions of two compounds exchange
places in an aqueous solution to form two new compounds. One compound is either a precipitate, an
insoluble gas, water, a weak electrolyte, or a covalent compound while the other compound is usually
soluble and remains dissolved in solution as determined by the table of solubilities.
AX + BY  AY + BX
A, X, B, and Y are ions and AY and BX are ionic or molecular compounds.
Pb(NO3)2(aq) + 2NaI(aq)  PbI2(s) + 2NaNO3(aq) precipitate
FeS(aq) + 2HCl(aq)  H2S(g) + FeCl2(aq) Gas
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) Water
Above reaction is neutralization: acid + base  salt + water.
When ionic compounds react in aqueous solutions, usually only one ion from each compound reacts forming a
precipitate, an insoluble gas, water, a weak electrolyte, or a covalent compound. The other ions are spectator
ions and do not react. The use of net ionic equations (ionic equations without the spectator ions) focuses
attention on the actual reactions.
Ex. #1
Molecular Equation:
NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)
Ionic equation:
Na+ + Cl- + Ag+ + NO3-  AgCl(s) + Na+ + NO3Net ionic equation: Cl- + Ag+  AgCl(s)
Ex #2
molecular equation:
Pb(NO3)2(aq) + 2NaI(aq)  PbI2(s) + 2NaNO3(aq)
Ionic equation:
Pb+2 + 2NO3- + 2Na+ + 2I-  PbI2(s) + 2Na+ + 2NO3Net ionic equation:
Pb+2 + 2I-  PbI2(s)
5. Combustion Reactions – a substance (usually hydrocarbons) combines with oxygen releasing a large
amount of energy (usually light and heat). Products of complete combustion are normally carbon dioxide
and water.
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
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6. Oxidation –Reduction (Redox) reactions: any chemical reaction in which elements undergo changes in
oxidation number. These reactions include combination, decomposition, single replacement, and
combustion reactions.
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
Zn0(s) + 2H+1Cl-1(aq)  Zn+2Cl2-1(aq) + H20(g)
a) A substance undergoes oxidation or is oxidized if the oxidation number becomes more positive.
Zn(s)  Zn2+ + 2eN-3  N-2 + eZn is oxidized (reducing agent i.e. donates electrons)
b) A substance undergoes reduction or is reduced if the oxidation number becomes more negative.
H+1 + e-  H
N-1 + 2e- N-3
H is reduced (oxidizing agent i.e. accepts electrons)
Half Reactions – used extensively with Redox reactions and in describing electrochemical processes. The halfreaction is a reduction half reaction if electrons are on the reactant side and an oxidation half reaction if
the electrons are on the product side.
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
Zn0(s) + 2H+1Cl-1(aq)  Zn+2Cl2-1(aq) + H20(g)
Zn(s)  Zn2+ +2e-
oxidation half reaction
2H+ + 2e-  H2(g)
reduction half-reaction
Half-reactions may be combined to make an oxidation reduction reaction as long as the electrons all cancel.
Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g)
7. Nuclear Reactions: Unlike chemical reactions in which only electrons are shared or exchanged, nuclear
reactions involve a change in the nucleus of the atom accompanied by the release of enormous amounts of
energy.
A. Types of Nuclear Reactions:
1. Fusion: the combination of small light mass nuclei to form a heavier more stable nucleus, often called
thermonuclear reactions.
2. Fission: process in which a very heavy nucleus (mass number  200) divides to form smaller nuclei
of intermediate mass.
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B. Radioactivity or Radioactive Decay: the spontaneous decomposition of a nucleus to form a slightly
lighter and more stable nucleus while emitting particles, electromagnetic radiation, or both.
C. Types of Radioactice Decay:
1. Alpha decay (): radioactive decay that emits helium-4 nuclei known as alpha particles
4
4
226
222
4
2He or 2.
88Ra 
86Rn + 2
2. Beta decay (): radioactive decay that emits high speed electrons called beta particles
0
0
131
131
0
n  p + -10
-1e or -1.
53I 
54Xe + -1
3. Positron decay: radioactive decay that emits a positron which is a particle having the same mass as an
electron but an opposite (positive) charge; 01e or 01.
11
11
0
p  n + +10
6C  5B + 1e
4. Gamma decay: radioactive decay that emits gamma rays (high energy photons with very short
wavelengths) 00 or . Gamma decay is not normally shown in nuclear reactions.
131
131
0
53I 
54Xe + -1 + 
D. Particles involved in nuclear reactions:
a) proton (11p or 11H)
b) neutron (01n)
c) alpha particle (24 or 24He ): a helium nucleus that has a charge of +2; common in very heavy
nuclei.
d) beta particle (-10 or -10e): an electron emitted from the nucleus and formed from the breakdown
of one neutron into a proton and an electron. n  p + -10
e) positron, anti-electron, (+10 or +10e): a particle with the same mass of an electron but having a
positive charge.
Other:
f) gamma ray (): high energy radiation emitted from a nucleus as it changes from an excited state to
the ground state.
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E. Half Life: the time it takes for one half of the mass of a radioactive substance to decay.
Mr = Mo/2n
n = t/t½
where:
Mr = remaining mass
Mo = original mass
n = number of half lives
t = elapsed time
t½ = half life
Isotope
polonium-218
astatine-218
uranium-238
iodine-131
half-life
3.0 min
1.6 sec.
4.46 x 109 years
8.1 days
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