CHAPTER 4 – CHEMICAL EQUATIONS AND STOICHIOMETRY
A chemical reaction is depicted using a chemical equation. In a chemical
equation, the formulas for the ___________ are written to the left of the arrow
and the formulas to the ____________ are written to the right. Their physical
states are also indicated as shown below.
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
A chemical equation should always be balanced. The relative amounts of the
reactants and products are shown by numbers, the ______________, before the
formulas.
2 AgNO3(aq) + K2CrO4(aq) → Ag2CrO4(s) + 2 KNO3(aq)
P4(s) + O2(g) → P4O10(s)
Balancing a chemical equation:
Step 1: Write the correct formulae for the reactants and products (make sure the
formula for each molecule is correct or else the equation is meaningless)
Step 2: Balance one element at a time.
Step 3: Balance polyatomic ions as a group.
Step 4: Balance large molecules before small molecules.
Step 5: Balance oxygen last (unless there is another element in more molecules;
if so, balance that element last).
Step 6: Verify that the number of atoms of each element is balanced
Step 7: Ensure that the coefficients are the smallest possible whole numbers
(“simplest whole number ratio”).
Step 8: Add states of matter.
e.g. Write a balance equation for the combustion of a former gasoline additive,
tetraethyllead, Pb(C2H5)4 to form lead oxide, water, and carbon dioxide.
Chemical equations can be added and subtracted.
H2SO4 + NaOH → NaHSO4 + H2O
NaHSO4 + NaOH → Na2SO4 + H2O
Chemical equations can be multiplied or divided by any number.
2 × [AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)]
Stoichiometry
The balanced equation for a reaction tells us the correct mole ratios of reactants
and products. This is called stoichiometry. A mole ratio based on the
coefficients for the two chemicals in the balanced equation is called a
_______________________.
Mass is always conserved in chemical reactions. The “Law of conservation of
mass” states that _________________________________________________
__________________________________________.
e.g. What mass of oxygen is required to completely combust 454 g of propane,
C3H8? What are the masses of CO2 and H2O produced? (Remember that
stoichiometric calculations always use moles.)
Limiting reactant (or limiting reagent)
In laboratory experiments, reactions are often carried out with an excess of one
reactant over that required by stoichiometry. This is done to ensure that one of
the reactants in the reaction is consumed completely. A limiting reactant is the
one whose amount determines or limits the amounts of product formed (the
other reactant(s) is present in excess).
e.g. combustion of gasoline (C8H18) – write the balanced reaction and indicate
the limiting reactant.
e.g. In our body glucose (C6H12O6) is oxidized to carbon dioxide and water to
provide energy. If you oxidize 18.00 g of glucose with 4.00 g of oxygen
gas
(a) how much carbon dioxide is produced?
(b) which is the limiting reactant?
Percent Yield
The maximum quantity (calculated) of product that can be obtained from a
chemical reaction is called the _________________ yield. In reality, the
quantity of the material that is actually obtained in a laboratory or a chemical
plant is often less and is called the __________ yield.
Percent yield, which specifies how much of the theoretical yield was obtained,
is defined as
Percent yield =
actual yield
theoretical yield
× 100%
e.g. Perchloric acid (aq) reacts with tetraphosphorus decaoxide (s) to form
phosphoric acid (aq) and dichlorine heptaoxide (g). 4.00 g of
tetraphosphorus decaoxide is treated with excess perchloric acid.
Calculate the mass of dichlorine heptaoxide formed if the percent yield
is 80.00%.
Quantitative Chemical Analysis
Chemical analyses can be carried out to identify and quantify an unknown
substance in a given sample.
Titrations are examples of quantitative chemical analysis. In the lab we
determined the oxalic acid content of an impure acid sample. The impure
sample was titrated with a known concentration of NaOH and by equating the
mole ratio we were able to calculate the mass of pure oxalic acid and thereby
calculate the oxalic acid content of the impure acid sample.
e.g. 1.056 g of impure acid was titrated with 0.5005 M NaOH. 25.65 mL of the
NaOH was required to attain the end-point. Calculate the percent oxalic
acid in the given sample.
A material of unknown composition can be converted to one or more
substances of known composition whose amount can be related to the amount
of the original, unknown sample.
e.g. The most important ore mineral of silver is argentite (Ag2S, silver sulfide).
To analyze a mineral sample for the quantity of silver, the ore is digested
with nitri acid to form AgNO3.
Ag2S(s) + 4HNO3(aq) → 2AgNO3(aq) + S(s) + 2NO2(g) + 2H2O(l)
The aqueous solution of silver nitrate is then treated with potassium
chromate to form a silver chromate precipitate.
2 AgNO3(aq) + K2CrO4(aq) → Ag2CrO4(s) + 2 KNO3(aq)
What is the mass percent of silver in the mineral sample if a 0.5694 g of
argentite produced a 0.6876 g of Ag2CrO4 precipitate.
Finding an empirical formula by chemical analysis
e.g. A 6.00 g of unknown organic compound containing C, H and O produces
13.18 g of carbon dioxide and 7.19 g of water upon complete combustion.
What is the empirical formula of the organic compound (CxHyOz)?
Important Concepts from Chapter 4
• balancing chemical equations
• calculations
o stoichiometry
o limiting reactants
o percent yield
o quantitative analysis