chemical names and formulas
DESCRIPTION
Chemical Names and Formulas. Molar Mass. Molar mass relates moles to grams The molar mass is the mass in grams of 1 mole of an element or compound Use the mass numbers from the periodic table, instead of amu’s, the units are grams. Molar masses are computed in the typical way. - PowerPoint PPT PresentationTRANSCRIPT
Chemical Names and Formulas
Molar Mass Molar mass relates moles to grams The molar mass is the mass in grams of 1 mole of
an element or compound Use the mass numbers from the periodic table, instead of
amu’s, the units are grams
Molar masses are computed in the typical way
Percentage Composition
Percentage composition is the percentage by mass of each element in a compound
The chemical formula gives the ratio of atoms in a compound
The chemical formula can also give the ratio of ions to one another
Significance of a Chemical Formula
For molecules, the formula gives the number of atoms of each element present in the molecule
For ionic compounds, the formula gives the simplest ratio of cations (+) and anions (-) in the crystalline lattice Called a formula unit
The subscript refers to what is immediately in front of it
Monatomic Ions
By gaining or losing electrons, single atoms will become ions
Cations (+) will keep their atomic name Anions (-) will change their ending to
-ide Rather than gain or lose, some atoms
will share their electrons This is called a covalent bond
Binary Ionic Compounds
Made of two different elements The total number of positive and
negative charges must be equal The cation will be written first, the
anion second
Al2O3
Aluminum Oxide
Aluminum +3Oxide -2
Stock System of Nomentclature
Some elements form two or more cations
System uses Roman numerals to indicate an ion’s charge
Polyatomic Ions Atoms that are covalently bonded to
one another, but as a group have gained or lost electrons (forming a charge)
Many are oxyanions (contain oxygen) The most common one ends in –ate The one with less oxygen ends in –ite If there is one with less oxygen than –ite,
it is also given the prefix hypo- If there is more oxygen than –ate, it is
given the prefix per-
Naming Binary Molecular Compounds There are two naming systems
Stock system Prefixes
Covalent-Network Compounds
Some covalent compounds do not consist of individual molecules Each atom is joined to all of its neighbors
in a covalently bonded, 3-dimensional network
The subscripts in these formulas indicate the smallest whole-number ratio of the atoms in a compound
The naming scheme is the same as other molecular compounds (SiC, SiO2, Si3N4)
Silicon Carbide, Silicon Dioxide, Trisilicon tetranitride)
Acids and Salts
Most acids are either Binary or Oxyacids Binary acids consist of two elements
Usually hydrogen and one of the halogens Oxyacids contain H, O, and a third element
Usually a non-metal
An ionic compound composed of a cation and the anion from an acid is often referred to as a salt.
Oxidation Numbers
In order to indicate the general distribution of electrons among the bonded atoms in a molecular compound or a polyatomic ion, oxidation numbers, also called oxidation states, are assigned to the atoms composing the compound or ion.
p. 216
Saturated Hydrocarbons
Saturated Hydrocarbons are hydrocarbons in which each carbon atom in the molecule forms four single covalent bonds with other atoms
Hydrocarbons that contain only single bonds are alkanes
A homologous series is one in which adjacent members differ by a constant unit
For alkanes, the series formula is CnH2n+2
The systematic naming system for compounds was developed by IUPAC The basic part is the
name of the longest carbon chain
The number of carbon atoms is reflected in the prefix, while the suffix –ane indicates the molecule has all single bonds
The naming of branched-chain alkanes also follows a systematic method
The branches are called alkyl groups They are formed when a hydrogen is removed
from an alkane molecule Named by changing the suffix from –ane to –yl.
To name this molecule, locate the longest continuous carbon chain that contains the most straight chain branches Do not be tricked by the way it is drawn, the
longest chain may be bent
Name the parent hydrocarbon and add the suffix –ane
Next identify the alkyl groups Arrange the names in
alphabetical order in front of the name of the parent hydrocarbon
If there are multiple groups, use the prefix di, tri, etc.
Place the location numbers of each alkyl group in front of the name (lowest possible number)
Small alkanes have very weak London dispersion forces only (for IM attractions) and have low BP’s
Longer alkanes have greater London dispersion forces and have higher BP’s
Natural gas is a fossil fuel composed primarily of alkanes containing one to four carbon atoms
Petroleum is a complex mixture of different hydrocarbons that varies greatly in composition Anywhere from 1 to 50 carbon atoms Petroleum is separated into its different
portions through fractional distillation
When alkanes combust, they produce carbon dioxide, water, and heat The carbon dioxide is a
greenhouse gas When gasoline ignites before
the flame front reaches it, there is a decrease in the amount of power (engine knock) Octane rating is a measure of the
anti-knocking properties of gs
Unsaturated Hydrocarbons Unsaturated hydrocarbons are hydrocarbons
in which not all carbon atoms have four single covalent bonds
Alkenes are hydrocarbons that contain double covalent bonds An alkene with one double bond has two less
hydrogens that the corresponding alkane CnH2n
The rules for naming alkenes are similar those for naming alkanes The parent hydrocarbon is the longest
one that contains the double bond The suffix –ene is added to the prefix The number are established so that the
first carbon in the double bond has the lowest number (this number is placed at the beginning of the name)
If there is more than one double bond, the suffix is modified to: 2=diene, 3=triene, etc.
Ethene is the smallest alkene Ethene is the hydrocarbon
produced in the greatest quantity in the US Used in the synthesis of plastic
and alchohol Can cause the ripening of fruit
Hydrocarbons with triple covalent bonds are alkynes
The general formula is CnH2n-2 The naming rules are similar to the naming
rules for alkenes except the suffix –yne is used
The smallest alkyne is ethyne (acetylene) that is used for welding
Effusion and Diffusion
Diffusion is the constant motion of gas molecules that causes them to spread out in their container
Effusion is the process whereby the molecules of a confined gas pass through a tiny opening of their container
The rates of effusion and diffusion depend on the relative velocities of gas molecules The velocity of a
gas varies inversely with its mass (lighter molecules move faster than heavier molecules at the same temperature)
212
KE mv
2 21 12 2A A B BM v M v
We know that
and since the KE for all gases isthe same at a given temperaturethen for gases A & B:
The M represents the molar mass
2 21 12 2A A B BM v M v
Can be multiplied by 2 to give 2 2A A B BM v M v
And then rearranged to give
2
2A B
AB
v MMv
orBA
B A
Mvv M
This leads to Graham’s law of effusion which states that the rates of effusion of gases at the same temperature and pressure are inversely proportional to the square roots of their molar masses
The law only provides the relative rates of effusion based on the relative velocities of the molecules
__
B
A
Meffusion rateAeffusion rateB M
Density varies directly with molar mass so density can be substitutedfor M
__
B
A
densityeffusion rateAeffusion rateB density
This is used to separate U-238 and U-235by allowing gaseous uranium to pass through porous membranes (over timeand many membranes, the U-235 becomes concentrated)