Transcript Balancing Chemical Equations and Stoichiometry

Chemistry 100 Chapter 3
Balancing Chemical
Equations and Stoichiometry
The Chemical Reaction
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What happens in a chemical reaction?
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Example
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2 H2 + O2  2H2O
Starting materials (reactants) are
converted into different chemical
substance(s) (the product(s)).
Described by ‘standard shorthand’ chemical equation
Balancing Equations
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Balanced equations have the same
number of atoms of a given element on
the LHS and the RHS
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Law of conservation of mass.
All the reactants and products must be
identified!
Chemical equations report the results of
experimentation!
A General Method of Balancing
Chemical Equations.
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Write the basic, or ‘skeletal’ equation,
showing the formula of all reactants and
products.
Balance the equation according to the
law of conservation of mass.
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Balance by adjusting the coefficients in
front of the chemical formulas, never by
adjusting the subscripts.
Best to start with an atoms that appears
only once on the left and right hand side of
an equation.
A Balanced Chemical
Equation
Chemical Reaction Types
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Combustion reaction  can be written for
any compound containing C, H, O or C, H, S
etc. O2 (g) is present as a reactant and is
usually in excess.
We can also have combination and
decomposition reactions
Combination reaction  two or more
substances combine to form one product
2Mg(s) + O2(g)  2MgO(s)
Reactions Types (II)
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Decomposition reaction  one
substance breaks down to form two or
more different substances
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MgCO3(s)  MgO(s) + CO2(g)
NH4Cl(s)  NH3(g) + HCl(g)
Decomposition of sodium azide, NaN3
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2NaN3(s)  2Na (s) + 3N2(g)
Molecular Masses
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How do we use the atomic masses of
elements to determine the molecular
masses of molecules?
We simply add the masses of the
constituent elements in the molecule.
The Mole and Avogadro’s
Number
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NOTE: 1 amu = 1.6605 x 10-24 g; this
is a very small mass!!
It is not a very convenient unit of
measurement on the laboratory scale.
In chemistry, we use a special unit (the
mole) when dealing with atoms,
molecules, and ions.
The Definition of a Mole
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1 mole (SI definition)
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the amount of substance that contains as
many elementary particles (atoms,
molecules, ions) as there are atoms in 12
grams (exactly) of carbon-12.
This number is called Avogadro's
number (it is an experimentally
determined quantity).
The Molar Mass
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1 mole = 6.022 x 1023 particles.
1 mole of carbon-12 has a mass of
12.00000 .... g exactly!
The molar mass of any element in
g/mole is the same numerically as its
atomic mass in amu’s!
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The molar mass of a compound is the
mass (in grams) of 1 mole of a
compound.
Numbers in periodic table
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e.g., Cl = 35.453 g/mole
= 35.453 amu/atom
Converting Between Moles
and Molecules
Masses of Anions and Cations
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Electron mass is small
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mass of Na+  mass of Na
mass of Cl-  mass of Cl
mass of O2-  mass of O atom!!
Percent Composition
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Percent composition is the percent by
mass of each element in a compound.
Q: If we have the percent composition
of the compound, can we calculate its
empirical formula?
A Schematic for Doing Empirical
Formula Calculations
Chemical Analysis
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Use a variety of techniques to obtain
the identity and the % by mass of each
element in a sample.
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Combustion analysis  a sample
containing C, H, or C,H, and O is
combusted (burned).
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(C,H,O)  CO2 + H2O
all the C in original compound gets converted
to CO2
all the H in original compound is converted to
H2O
Combustion Analysis
Quantitative Information from
Chemical Reactions
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Example
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A+B C+D
Normally, we start out with certain
quantities of reactants.
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How much product can we expect?
How much reactant would we need to
obtain a specified amount of product?
The Mole Method
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Stoichiometric coefficients in the
balanced chemical equation represent
the number of moles of reactants and
products.
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certain mass of reactants  ? mass of
products
initial volume of reactants  ? Volume
of products
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Example
N2 (g) + 3 H2 (g)  2 NH3 (g)
1 mole N2  3 moles H2  2 moles NH3
the  symbol means stoichiometrically
equivalent
A Schematic of the Mole
Method
Limiting Reagent
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Chemical equations  give the
molecular or molar ratios of reactants
needed and the products obtained.
N2 (g) + 3 H2 (g)  2 NH3 (g)
 We assumed 100% complete conversion
of of the N2 (g) and H2 (g) to NH3 (g).
 Normally, the reactants will react in the
proper ratio until one of them is
consumed completely.
The Definition of the Limiting
Reagent
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The amount of product formed is limited
by the reactant that is completely
consumed  the limiting reagent.
The amount of product obtained
assuming complete consumption of the
limiting reagent  theoretical yield.
The reactant that is left over  the
excess reagent.
Limiting Reagents
The Limiting Reagent
Schematic
Mass of A
and B
Obtain Theor. Yield
Use Molar
Masses of A, B
Moles of A, B
Use amount of
Limiting Reagent
Determine Limiting
Reagent
Use Coefficients from balanced
Equation