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Chemical Reactions
Chapter 6
Chemical Arithmetic
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Balancing Equations
The mole
Gram - mole conversions
Mole - mole relationships in chemical
equations
• Mass relationships in chemical equations
• Per cent yeilds
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Chemical Equations
• Chemical Equation- A shorthand way of
representing a chemical reaction
Starting substances -------> Final substances
(Reactants)
(Products)
• Physical states of reactants and products may
indicated
–
–
–
–
Solid
Liquid
Gas
Aqueous solutions (aq)
(s)
(l)
(g)
dissolved in water
Properties of Chemical Equations
• Conservation of mass- matter is neither
created or destroyed in chemical reactions
• Chemical equations must be balanced
– # of atoms and kinds of atoms must be the same
on both sides of the equation (coefficients)
– Total charge on all species on the left must
equal the total charge of all species on the right
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Guidelines for Balancing Equations
• Subscripts in formulas may not be changed to
balance the equation
– Use coefficients in front of formula to balance equation
• Balance elements (or “chemical groups”) that
appear in only one substance on each side of the
equation first
• Go back and forth correcting with each round the
effect of your balancing of an element or group
• Fractions may be used to balance an equation
initially. However, all coefficients should be
integers at the end
Step by Step Procedure for
Balancing Chemical Equations
• 1) Write an unbalanced equation, using correct
formulas for all reactants and products
• 2) Add appropriate coefficients to balance the
numbers of atoms of each element
• 3) Check equation to ensure the #s and types of
atoms are the same on both sides of the
equation
• 4) Make sure the coefficients are reduced to their
lowest whole number values
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Some Equations to Balance
_H3PO4 + _NaOH --> _Na3PO4 + _H2O
_CH3CH3 + _O2 --> _CO2 + _H2O
_AgNO3 + _MgCl2 --> _AgCl + _Mg(NO3)2
_BaCl2 + _(NH4)2SO4 --> _BaSO4 + _NH4Cl
The Mole
• Formula Weight (FW) - Sum of the atomic
weights of all atoms in a formula
– Formula unit- all of the atoms in a formula
– If the substance is composed of molecules, the formula
weight is also called the molecular weight
• Mole (mol) - The amount of a substance equal to
the formula weight of the substance in grams
– Contains Avogadro’s number (6.02 x 1023) of formula
units
– The formula weight in grams is often called the molar
mass
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Some Mole Problems
• Calculate the molecular weight of
Ibuprofen, C13H18O2.
• How many molecules of ascorbic acid
(vitamin C, C6H8O6) are in a 500 mg tablet?
• What is the mass in grams of 5.0 x 1020
molecules of aspirin (C9H8O4)?
Molar Mass as a Conversion Factor
• Using the molar mass as a conversion factor allows us to
convert moles to grams and grams to moles
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Relationships Involving the Mole
Stoichiometry
• Stoichiometry - Quantitative relationships involving
chemical equations and formulas
• The meaning of a chemical equation
– The coefficients in a balanced equation indicate the
relative number of formula units of each substance that is
involved in the reaction
2 C8H18 + 25 O2 --> 16 CO2 + 18 H2O
When 2 molecules of octane are allowed to react with 25
molecules of oxygen, 16 molecules of carbon dioxide and
18 molecules of water are formed
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2 C8H18 + 25 O2 ----> 16 CO2 + 18 H2O
2 molecules +
25 molecules --> 16 molecules + 18 molecules
12(2)
"
12(25)
"
12(16)
"
12(18)
"
24
"
300
"
192
"
216
"
2 dozen "
25 dozen "
16 dozen "
18 dozen "
6.02x1023(2) "
6.02x1023(25) "
6.02x1023(16) "
6.02x1023(18) "
2 moles C8H18 + 25 moles O2
16 moles CO2 + 18 moles H2O
• The coefficients in a balanced equation indicate the relative number of
moles of each substance that is involved in the reaction.
Stoichiometry- continued
2 C8H18 + 25 O2 ----> 16 CO2 + 18 H2O
2 mol C8H18
=
16 mol CO2
2 mol C8H18
=
18 mol H2O
25 mol O2
=
16 mol CO2
2 mol C8H18
=
25 mol O2
16 mol CO2
etc.
=
18 mol H2O
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Stoichiometry- continued
• 2 C8H18 + 25 O2
----> 16 CO2 + 18 H2O
Solving Mole/Gram Conversion
Problems
• 1) Write a balanced chemical equation
• 2) Choose appropriate molar masses and mole
ratios to convert known information into
required information
• 3) Set up factor-label expression and calculate
the answer
• 4) Estimate or check answer using a ballpark
solution
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Summary of Mole and Gram Conversions
Some Conversion Problems
• Hydrogen fluoride reacts with glass (silicon
dioxide) in the following manner:
4 HF (g) + SiO2 (s) --> SiF4 (g) + 2 H2O (l)
a) How many moles of HF will react
completely with 8.90 mol of SiO2?
b) What mass of water (in grams) will be
produced by the reaction of 23.0 g of SiO2?
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Percent Yields
• Not all reactants are converted to products
• Losses may occur due to side reactions and
handling
• Percent yield = actual yield
X 100%
theoretical yield
• Calculations are based the actual mass of
the product for the actual yield and on gram
to gram conversions for the theoretical yield
Classification of Chemical
Reactions
• Precipitation reactions in which an insoluble
precipitate forms upon mixing aqueous
reactants
• Acid-base neutralization reactions in which an
aqueous acid reacts with an aqueous base to
form an ionic compound (a salt) and water
• Oxidation-Reduction reactions in which one or
more electrons are transferred between
reactants
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Solubility Rules
• Compounds are generally soluble if they contain
Group 1 cations (Li+, Na+, K+, Rb+, Cs+) or
ammonium ions (NH4+)
• Compounds are generally soluble if they contain
halide (Cl-, Br-, I-), nitrate (NO3-), perchlorate
(ClO4-), or sulfate (SO42-) anions.
• Exceptions: Ag+, Hg22+, & Pb2+ with halides and
Ba2+, Hg22+ and Pb2+ with sulfates
Acid-Base Neutralization Reactions
• Neutralization- When equal mole amounts of an
acid and a base are mixed together, both acidic
and basic properties of each disappear. The
resulting solution is said to be neutral.
• Water and a salt are formed
HA(aq) + MOH(aq) ---> H2O(l) + MA(aq)
acid
base
water
salt
Note- salt contains an anion other than OH- ion
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Net Ionic Equations
• Ionic Equation- an equation in which all ions are
explicitly shown
• Spectator ions- an ion that remains unchanged
on both sides of the reaction equation
• Net Ionic Equation- an equation that excludes
the spectator ions and shows only those ions that
are involved in the reaction
Oxidation-Reduction Reactions
• Oxidation-Reduction (Redox) Reaction- A chemical
reaction in which the oxidation numbers of elements
change because of a loss or gain of electrons
• Oxidation Number- A number that indicates the charge
that an atom in a molecule or polyatomic ion would
have if all bonds were ionic.
– Fictitious- No actual charge of this magnitude actually exists when
covalent bonds are present
– Useful for classifying a type of reaction as an oxidation-reduction
reaction
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Guidelines for Assigning Oxidation Numbers
• Assign an oxidation number of zero to any free element,
e.g., Na, Mg, Cl2, S8, etc.
• Assign the most electronegative element in a compound
or polyatomic ion a negative oxidation number equal to
the charge that atom would have if it were a monatomic
anion
• Assign a group 1, 2 or 3 element in a compound or
polyatomic ion an oxidation number of +1, +2, and +3,
respectively
• Assign any other element an oxidation number such that
the sum of oxidation numbers equals zero for a neutral
compound or the overall charge for a polyatomic ion
Redox Terminology- Part 1
• Oxidation- the loss of electrons
• Reduction- the gain of electrons
- Mnemonic to remember: LEO goes GER
• The species that contains the element whose
oxidation number increased (more positive) is said
to be oxidized
• The species that contains the element whose
oxidation number decreased (more negative) is said
to be reduced
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Redox Terminology- Part 2
• Reducing Agent - species that:
– Loses electrons
– Undergoes oxidation
– Becomes more positive (or less negative)
• Oxidizing Agent - species that:
– Gains electrons
– Undergoes reduction
– Becomes more negative (or less positive)
Redox Examples
• What species on the left was oxidized? Reduced?
• Which is the reducing agent? Oxidizing agent?
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Another Redox Reaction
• What species on the left was oxidized? Reduced?
• Which is the reducing agent? Oxidizing agent?
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