Download Unit 10: Formulas, Equations, and Amounts

Unit 10: Formulas, Equations, and Amounts
What’s the point?
We need a way to clearly and unambiguously describe what chemical compounds are and
how they transform.
Chemical compounds
A compound is a combination of two or more elements with their atoms in a specific
ratio. Compounds may consist of molecules, which are electrically neutral sets of atoms
bonded together, or of ions, which are atoms or groups of atoms with positive or negative
electric charges. A molecular or ionic formula identifies the atomic composition of a
compound.
The number of atoms of each element in a molecule or ion is denoted by a subscript after
the element symbol. The subscript is omitted if just one atom is present. For example,
H2S is a molecule comprising two atoms of hydrogen and one of sulfur. The electric
charge of an ion is indicated by a superscript following the ionic formula. Individual
ionic charges are usually not shown when ions combine to a neutral compound. For
example, two sodium ions Na+ and one sulfide ion S–2 combine to form the compound
Na2S.
Depiction
The structure of a compound can be described in a variety of ways. A molecular or ionic
formula tells how many atoms of each kind of element are in a molecular or formula unit.
A structural formula shows how individual atoms are bonded to each other. A dot
structure shows how the outer, or valence, electrons of each atom configure to join the
atoms. Depictions that show how the atoms arrange in three dimensions include balland-stick models, tube structures, and space-filling models. Physical models and
computer renderings can show the shapes in three dimensions.
Names of ions
Ions made of single atoms with one or more electrons added or removed are named from
the name of the element. Positive ions are just given the name of the element, such as
sodium Na+ or calcium Ca+2. If the atom can make ions with different charges, the
charge is given by roman numerals in parentheses, such as chromium(III) for Cr+3 and
chromium(VI) for Cr+6. Atomic ions with negative charges are given the “-ide” suffix,
such as chloride Cl– and sulfide S–2. Polyatomic negative ions are often given “-ite” or
“-ate” suffixes, and positive ions often “-ium.”
Ionic compounds
The formula unit of an ionic compound consists of the smallest number of each ion
sufficient to make the combination electrically neutral. For example, one Ca+2 and one
O–2 balance charges to be Al2O3.
Ionic compound names
Ionic compounds are named with the name(s) of the positive ions followed by the names
of the negative ions, such as sodium bromide NaBr. The number of each ion is not
indicated in the name unless it would otherwise be ambiguous. Thus Na2SO4 is sodium
sulfate and NaHSO4 sodium hydrogen sulfate, but NaH2PO4 is sodium dihydrogen
phosphate and Na2PO4 is disodium hydrogen phosphate.
Chemical reaction equations
Chemical transformations (reactions) are shown as reactants products. A quantitative
description of a reaction shows the number of each species of reactant consumed and the
number of each species of product formed. These numbers are shown as coefficients
before each species, for example the combination of two molecules of hydrogen gas H2
with one molecule of oxygen gas O2 to produce two molecules of water H2O is shown as
2 H2 + O2 2 H2O.
Individual atoms do not change in chemical reactions; they merely exchange partners. A
reaction equation is balanced if there are exactly as many atoms of each element in the
reactants as in the products. The reaction equation 2 H2 + O2 2 H2O is balanced: the
reactants and products each contain four atoms of hydrogen and two atoms of oxygen.
Chemical amounts
Because meaningful quantities of material contain enormous numbers of atoms, it is
convenient for us to use the unit mole, which is 6.022 1023 of something. The number
6.022 1023 is Avogadro’s number, sometimes abbreviated NA, the number of atoms in
exactly one gram of 12C. An element’s atomic mass listed in the periodic table is the
mass in grams of one mole of atoms of that element. So, a mole of zinc has a mass of
65.37 g, and a 100-g sample of pure zinc contains 1.53 moles of zinc atoms, or 1.53 mol
Zn for short.
The mass of a mole of a compound can be found from its formula and form the atomic
masses of the elements it contains. The molar mass of the compound is the total molar
mass of all its constituent atoms. For example, the molar mass of carbon dioxide CO2,
composed of one atom of carbon (atomic mass 12.011) and two atoms of oxygen (atomic
mass 15.999) is 12.011 + 2(15.999) = 44.009.
Equivalent amounts
The coefficients in a reaction equation tell the smallest whole-number amounts of
molecules or formula units of reactants consumed and products created in the reaction. A
given reaction always proceeds in the same proportions. The proportions of moles of he
substances are the same as the proportions between the reaction coefficients. For
instance, in the reaction CH4 + 4 Cl2 CCl4 + 4 HCl, four moles of Cl2 are consumed,
one mole of CCl4 is produced, and four moles of HCl are produced for every mole of CH4
consumed. If a different amount of CH4 reacts, correspondingly different amounts of the
other substances will be consumed or created. Equivalent amounts can be found from the
reaction coefficients; here, one mol CH4 is equivalent to 4 mol Cl2, 1 mol CCl4, and 4
mol HCl.
Limiting reagent
When reactants are not present in equivalent amounts, only the one with the fewest
equivalents reacts fully. Only equivalent amounts of the other reactants are consumed
and equivalent amounts of the products are created. A simple example is the reaction
NaOH + HCl NaCl + H2O; if 3 mol NaOH and 4 mol HCl are combined, only 3 mol
of each will react and NaOH is the limiting reagent.
A more complicated example is 4 Al + 3 O2 2Al2O3. If 1 mol each of Al and O2 are
present, only 3/4 mol of O2 would be needed to react with all the Al but 4/3 mol Al would
be needed to react with all the O2. So, there is more than enough O2 for all the Al, but not
enough Al for the O2. The result is that all the limiting reagent Al will react but only 3/4
of the O2 will react. 1/2 mol of Al2O3 will be produced and 1/4 mol unreacted O2 will
remain.