### Chemical Reactions and Reaction Stoichiometry

```Chemical Reactions and Reaction
Stoichiometry
Chemistry, The Central Science, 13th edition
Theodore L. Brown, H. Eugene LeMay, Jr., Bruce E. Bursten, Murphy, Woodward, and Stoltzfus.
3.1 Chemical Equations
Chemical Equations
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Chemical equations represent chemical reactions
•
•
Ex. 2 H2 + O2  2 H2O
The “+” is read as reacts with while the “” reads as yields, produces,
makes, forms, etc.
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Subscripts are used to tell how many of each element is in the molecule
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Coefficients are used to tell how many molecules/moles are needed for the
reaction
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The reactants are on the right side of the equation
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The products are on the left side of the equation
Balancing Equations
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To construct a balanced chemical equation:
1.
Write the reactants on the left
2.
The products on the right
3.
Balance the equation using the smallest possible coefficients
NEVER change the subscripts of a molecular formula (or ionic formula)
•
Subscripts are important to tell the reader what the chemical is
Example
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Methane reacts with oxygen to form carbon dioxide and water
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CH4 + O2  H2O + CO2 (unbalanced)
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Decide where you want to begin balancing and where you should balance last
•
I suggest balancing diatomic elements last as a rule
CH4 + 2 O2  2 H2O + CO2 (balanced)
Indicating the State of Reactant and
Products

Symbols indicating the physical state of each reactant and product are often
shown in the chemical equations.

The symbols are


Gas (g)

Liquid (l)
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Solid (s)
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Aqueous (aq)
Sometimes symbols that represent what helps the reaction along is written
above the arrow

Δ is for heat

λ is for light
3.2 Simple Patterns of Chemical
Reactivity
Simple Patterns of Chemical Reactivity
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
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Combination Reactions
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Two or more substances react to form one product

A + B  AB
Decomposition Reaction

A single reactant breaks apart to form two of more substances

AB  A + B
Usually forms carbon dioxide and the metal oxide
Bicarbonate decomposition


ABC  AB + C etc…
carbonate decomposition


or
Forms carbonate, water, and carbon dioxide
Hydrate Decomposition

Usually the loss of water by heating
Hydrates…
Combustion Reactions

Combustion Reactions

Rapid reactions that produce a flame

Most reactions observed involve oxygen from air

The products are typically carbon dioxide and water
Single or Double Replacement


Single Replacement

A + BC  AC + B

AB + C  AC + B
Double Replacement


AB + CD  AD + CB
Special types of Reactions

Acid/base, acid/carbonate, acid/metal,
3.3 Formula Weights
Formula Weights

The formula weight (FW) of a substance is the sum of the atomic weights (AW)
of the atoms in the chemical formula of the substance
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
FW = Σ (type of element)(subscript of element)
If the chemical formula is that of a molecule, the formula weight is also
called the molecular weight (MW)
Percentage Composition from Chemical
Formulas

The percent composition or elemental composition comes straight from the
chemical formula/molecular formula


The calculation depends on the formula weight of the substance, the atomic
weight of the element of interest, and the number of atoms of that element in the
chemical formula
Practice

What is the percent composition of carbon in C12H22o11
3.4 Avogadro’s Number and the Mole

The mole is the amount of matter that contains as many objects as the
number of atoms in exactly 12g of isotopically pure 12C


Scientists call this Avogadro’s number, N
Practice

Determine which of the following samples contains the fewest sodium atoms.
A) 1 mol sodium oxide B) 45 g sodium fluoride C) 50 g sodium chloride D) 1 mole
sodium nitrate

Calculate the number of H atoms in 0.350 mol of C6H12O6

How many moles of ions are found in a solution of sodium phosphate
Molar Mass

The molar mass of one mole of any element is the average mass of one 6.02 x
1023 atoms of the naturally occurring mixture
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
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Units: g/mol
Calculate the molar mass of the following:

Sodium chloride
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Ethane
A sample of an ionic compound containing iron and chlorine is analyzed and
found to have a molar mass of 126.8 g/mol.

What is the charge of the iron?
Interconverting Masses and Moles

How many moles of sodium bicarbonate are in 508g of sodium bicarbonate?

How many moles of water are in 1.00 L of water, whose density is 99.3 g/mL?
Calculating Numbers of Molecules and
Atoms from Mass

How many glucose molecules are in 5.23 grams of glucose? (C6H12O6)

How many oxygen atoms are in the sample?

How many chlorine atoms are in 12.2g of carbon tetrachloride?
3.5 Empirical Formulas from Analysis
Empirical Formulas from Analyses

The ratio of the numbers of moles of all elements in a compound gives the
subscripts in the compound’s empirical formula
Practice Problems

Ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and the rest is oxygen
by mass. What is the empirical formula of ascorbic acid

A 2.144g sample of phosgene, a compound used as a chemical warfare agent
during WWI, contains 0.260 g C, 0.347g O and the rest is chlorine. What is the
empirical formula?
Molecular Formulas from Empirical
Formulas

We can obtain the molecular formula for any compound by calculating the
empirical formula and knowing the molecular mass of the compound.

Whole-number multiple = (molecular weight)/(empirical formula weight)

Practice:

Mesitylene, a hydrocarbon found in crude oil, has an empirical formula of 3 carbons
and 4 hydogens and an experimentally determined weight of 121 amu. What is its
molecular formula?
Practice

Cyclohexane, a commonly used organic solvent, is 85.6% C and 14.4% H by
mass with a molar mass of 84.2 g/mol. What is its molecular formula?

Side note: Cyclo- anything means that the linear chain hydrocarbon is now
cyclic
Degree of Unsaturation

How unsaturated is cyclohexane?
Combustion Analysis
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Combustion Analysis

Commonly used to determine the composition of hydrocarbons

When a hydrocarbon is fully combusted, the mass of water and carbon dioxide
collected can be used directly to determine the amount of carbon and hydrogen in
the original compound.
Practice

Isopropyl alcohol, sold as rubbing alcohol, is composed of C, H, and O. Combustion
of 0.255 g of isopropyl alcohol produces 0.561g of carbon dioxide and 0.306g of
water. Determine the empirical formula of isopropyl alcohol

Now solve using empirical formula calculations
Practice

The compound dioxane, which is used as a solvent in various industrial
processes, is composed of C, H, and O atoms. Combustion of a 2.203g sample
of this compound produces 4.401g carbon dioxide and 1.802g water. A
separate experiment shows that it has a molar mass of 88.1 g/mol. What is
the correct formula?
3.6 Quantitative Information from
Balanced Equations
Quantitative Information from Balanced
Equations

Coefficients in a chemical equation represents the relative numbers of
molecules in a reaction.

The coefficients can be used to determine the amounts of grams, molecules, or
moles of any substance needed or made in a reaction.
Practice

Determine how many grams of water are produced in the oxidation of 1.00g
of glucose?

Sodium hydroxide reacts with carbon dioxide to form sodium carbonate and
water. How many grams of sodium carbonate can be formed from 2.40g of
sodium hydroxide?
3.7 Limiting Reactants
Limiting Reactants
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When any reaction occurs there are always left over reagents in the reaction.


Using stoichiometry, one can determine the amount of excess reactant and which
reactant stopped the overall reaction
Limiting reactant

It determines or limits the amount of product formed
Practice

The most important commercial process for converting nitrogen from the air
into nitrogen-containing compounds is based on the reaction nitrogen gas
reacting with hydrogen gas to form ammonia. How many moles of ammonia
can be produced from 3 moles of nitrogen and 6 moles of hydrogen?
Theoretical and Percent Yield
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
Theoretical yield

The quantity of product calculated to form when all of a limiting reactant is
consumed

Calculated from stoichiometry
Percent Yield

Calculates the amount of product retrieved from an experiment compared to the
theoretical yield one should have obtained
Practice

Adipic acid, H2C6H8O4, used to produce nylon, is made commercially by a
reaction between cyclohexane and oxygen. Liquid adipic acid is formed as
well as water.
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
Assume that 25.0g of cyclohexane is used with excess oxygen present. What is the
theoretical yield of adupic acid? If you obtain 33.5g of adipic acid, what is the
percent yield for the reaction?
If 3.00g of Ti metal is reacted with 6.00g chlorine gas, to form 7.7f of
titanium (IV) chloride in a combination reaction, what is the percent yield of
the product?
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