Chemical Equations
loosely based on Chap 4 Sec 4
of Jespersen 6th edition
Dr. C. Yau
Spring 2014
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Writing Balanced Chemical Equations
Read the question carefully and figure out
which is/are the reactants and which is/are
the products. Include physical states as
much as possible.
Balance the equation so that there are the
same number of atoms of each element on
the right side as on the left side by
changing ONLY the coefficients of the
equation.
(Remember conservation of matter: No
atoms are created nor destroyed in a
chemical reaction.)
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Example 4.14 p.129
Sodium hydroxide and phosphoric acid,
H3PO4, react as aqueous solutions to give
sodium phosphate and water. The
sodium phosphate remains in solution.
Write the balanced equation for this
reaction.
Do Practice Exercises 24 & 25 p.130
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Example
Write the balanced equation for the
combustion of decene, C10H20 (l).
Note: Combustion (burning) always means
a reaction of oxygen gas.
Combustion of hydrocarbons (compound
with only H and C) with excess oxygen
gives CO2 and H2O. REMEMBER THIS!
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Example
Write the balanced equation for the
combustion of decane, C10H22 (l).
Practice at home: (l) = liquid
Write the balanced equations for the combustion
of a) nonene, C9H18 (l) b) combustion of nonane,
C9H20 (l) and c) nonanone C9H18O (l).
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Working with Chemical Equations
Be able to translate equations into words,
both at the particulate level and at the
macroscopic level.
Ca (s) + 2H2O (l)
Ca(OH)2 (aq) + H2 (g)
particulate level:
One Ca atom reacted with 2 molecules of
H2O to form one f.u. of Ca(OH)2 and one
molecule of H2.
macroscopic level:
One mol Ca atoms reacted with 2 mol of
H2O molecules to form one mol Ca(OH)2
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and one mol H2 gas.
Example 4.15 p.131
How many moles of sodium phosphate can
be made from 0.240 mol of sodium
hydroxide by the following unbalanced
reaction?
NaOH(aq) + H3PO4 (aq)
Na3PO4 (aq) + H2O (l)
Useful tips: The coefficients of a balanced equation gives
us conversion factors for moles to moles.
Useful tips: When working with more than one substance
in dim.analysis, you MUST GO THRU MOLES!
Do Practice Exercises 26 & 27 p. 132
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What does “go through moles” mean?
It means somewhere in your dim. analysis you will
be using a conversion of...
 # mol A 


#
mol
B


Remember it is NOT...
#gA


#
g
B


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Example 4.16 p. 133
Portland cement is a mixture of the oxides of
calcium, aluminum and silicon. The raw
material for its calcium oxide is calcium
carbonate, which occurs as the chief
component of a natural rock, limestone. when
calcium carbonate is strongly heated it
decomposes. One product, carbon dioxide, is
driven off to leave the desired calcium oxide as
the only other product.
A chemistry student is to prepare 1.50 x 102 g of
calcium oxide in order to test a particular
"recipe" for Portland cement. How many grams
of calcium carbonate should be used,
assuming that ALL will be converted?
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Example 4.17 p.134
The thermite reaction is one of the most spectacular
reactions of aluminum with iron(III) oxide by which
metallic iron and aluminum oxide are made. So much
heat is generated that the iron forms in the liquid state.
The heat from this reaction was used in welding train
tracks.
A certain welding operation requires at 86.0 g of iron each
time a weld is made. What is the minimum mass, in
grams, of iron(III) oxide that must be used for each weld?
Also calculate how many grams of aluminum are needed.
Do Practice Exercises 28 & 29 p. 135
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The Thermite Reaction
Fe2O3 (s) + 2Al (s)  2Fe (s) + Al2O3 (s) + HEAT
The reaction requires ignition at a very high
temperature.
One method of ignition is to light a piece of Mg.
Another method of ignition is to add conc
sulfuric acid to a mixture of KClO3 and sugar.
For those who took CHEM 108 on the Catonsville
campus may remember the precautions when
you worked with potassium chlorate in one of
your experiments!
http://www.youtube.com/watch?v=AlGFCWBeyDk&NR=1
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