Chapter 10.
Chemical Calculations II
Chemical Reactions
Stoichiometry is the calculation of the
quantities of reactants and products
in a chemical reaction.
It's very important that we can do this.
We can plan the amounts of ingredients needed to make a compound
such as a drug.
Conservation of Mass
Atomic theory states that atoms are
not created or destroyed in a chemical reaction, they just trade partners.
The Law of Conservation of Mass
states that mass is neither created
nor destroyed in a chemical reaction.
The reaction equation for a chemical
change uses these principles.
Conservation of Mass
CH4 + 2 O2

CO2
+
2 H2O
Each side of the equation has
1 carbon, 4 oxygens, 4 hydrogens
Conservation of Mass
CH4 + 2 O2

16.05 g CH4
+64.00 g O2
80.05 g reactants
CO2
+
2 H2O
44.01 g CO2
+36.04 g H2O
80.05 g products
Chemical Equations
A chemical equation is a representation of
a chemical reaction that uses chemical
symbols instead of words to describe the
changes that occur in the reaction.
It's a sentence that describes something.
It's a mathematical equation that describes
a quantitative relationship.
Chemical Equations
Parts of a chemical equation:
(a) Reactants are compounds that are present
at the start of a reaction. Their formulas are
shown on the left of the reaction arrow
(b) Products are compounds that result from
the reaction. Their formulas are shown on
the right of the reaction arrow.
(c) The reaction arrow is read as "to produce"
and can be treated mathematically like "=".
(d) Coefficients show the number of moles of
each compound in the reaction.
Chemical Equations
A balanced chemical equation has coefficients
that show the number of moles of reactants
and products, such that the same number of
each type of atoms appear on both sides of
the equation.
A skeleton equation shows reactants and
products, but no coefficients. It must be
balanced to be useful in chemical calculations.
Chemical Equations
Balancing chemical equations by inspection
involves some guesswork and testing.
Start with the most complex compounds, and
work toward the simpler compounds and
elements.
Work with coefficients. Do not change subscripts in chemical formulas!
Make sure the result shows the lowest wholenumber ratio of coefficients.
Chemical Equations
Examples:
Balance the following skeleton equations.
(a)
(b)
(c)
(d)
H2 + O2  H2O
Fe2O3 + CO  Fe + CO2
C3H8 + O2  CO2 + H2O
C4H10 + O2  CO2 + H2O
Competency II-1
Equations and Moles
3 H2 + N2  2 NH3
This balanced equation can be read at the
molecular level or the molar level.
The 3:1:2 ratio holds for molecules and moles,
and for any quantities of reactants.
9 moles of H2 will react with 3 moles of N2 to
produce 6 moles of NH3
Equations and Moles
3 H2 + N2  2 NH3
We can treat the mole ratios as conversion
factors and calculate relative moles of
reactants and products based on these.
3 mol H2
1 mol N2
2 mol NH3
3 mol H2
2 mol NH3
1 mol N2
Equations and Moles
3 H2 + N2  2 NH3
How many moles of NH3 are produced if 4.85
moles of H2 are reacted with excess N2?
And how realistic is that problem? We can't
usually measure things in moles. We
have to use mass and convert.
Reaction Calculations
Typical types of calculations that are done for
chemical reactions:
(a) I have X amount of a reactant. How
much product can I get?
(b) I want to make X amount of a product.
How much of each reactant do I need?
(c) When I make X amount of the desired
product, how much of the side products
will be produced?
Reaction Calculations
These are called theoretical yield calculations, and they allow us to determine how
much of a product is formed, or a reactant
is needed, based on given quantities.
Reaction Calculations
In a theoretical yield problem, look for
(a) "Wants" the quantity requested.
(b) "Gots" the quantity given. Enough
information is given to figure out
moles of "gots".
(c) "Wants/Gots Factor" the # of moles
of the requested quantity divided by
the # of moles of the given quantity.
The balanced reaction equation gives
these numbers.
Reaction Calculations
Steps:
(a) Calculate moles of Gots
(b) Multiply that by Wants/Gots factor
to get moles of Wants
(c) Multiply moles of Wants by its
molar mass to get mass of Wants
Reaction Calculations
Example:
Fe2O3 + 3 CO  2 Fe + 3 CO2
With 500 g of Fe2O3 and excess CO,
how much iron metal (Fe) can be
produced?
Reaction Calculations
Example:
Fe2O3 + 3 CO  2 Fe + 3 CO2
With 500 g of Fe2O3 and excess CO,
(a) How much CO will be consumed?
(b) How much CO2 will be produced?
Competency II-2
Limiting Reactants
We often don't use exactly the amounts
of reactants called for in the balanced
equation. One (usually the cheapest!) is present in excess.
The limiting reactant is the reactant
that is entirely consumed when the
reaction stops. Other reactants are
present in excess.
Limiting Reactants
If I have 55 nuts, 48 bolts, and 92 washers,
how many sets of 1 bolt, 1 nut, 2 washers
can I make? What part limits me?
What is left over?
Limiting Reactants
Chemistry is the same way.
CH4O + NaI  CH3I + NaOH
If one starts with 750 g of both reactants,
what is the theoretical yield of CH3I (the
maximum that can be obtained)? Which
reactant is limiting?
Work this as two theoretical yield problems,
the lower result is the theoretical yield and
the reactant that produces it is limiting.
Actual and Percent Yield
We rarely obtain the theoretical yield of a product. The actual yield is the mass of product
obtained in a reaction. It's usually lower than
the theoretical yield, sometimes much lower!
It's determined by experiment.
The percent yield is the ratio
actual yield x 100%
theoretical yield
Actual and Percent Yield
Why do we care?
Percent yield shows how efficient the
reaction is, and/or how well the scientist
or student did the reaction.
Also, if a reaction routinely gives 50%
yield, one had better start with twice the
reactant as needed for the calculated
theoretical yield!
Actual and Percent Yield
Example:
If my theoretical yield for CH3I was 710 g,
but I only isolated 650 g, what was my
% yield?
Competency II-3