Chapter 2 Compounds and Chemical Reactions

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Chapter 2Compounds and Chemical Reactions

• Essentially all elements combine to form
  compounds
• Compounds are of two types
• Molecular, which involve shared electrons and
  consist of electrically neutral, discrete
  particles called molecules
• Ionic compounds, which involve electron transfer
  and charged particles called ions

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• Chemical formulas are collections of chemical
  symbols that are used to describe elements and
  compounds
• Free elements are not combined with other
  elements in a compound
• Examples Fe (iron), Na (sodium), and K
  (potassium)
• Many nonmetals occur as diatomic molecules

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• Chemical formulas specify the composition of a
  substance
• NaCl is composed of the elements sodium and
  chlorine in a one-to-one (atom) ratio
• Fe2O3 is composed of the elements iron and oxygen
  in a two-to-three ratio
• CO(NH2)2 expands to CON2H4, but there are good
  reasons to write some compounds with parentheses
• Hydrates are crystals that contain water
  molecules, for example plaster CaSO4 2H2O
• When all the water is removed (by heating), the
  solid that remains is said to be anhydrous
  (without water)

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• Chemical equations describe what happens in a
  chemical reactions
• Hydrogen and oxygen combine to form water
• Hydrogen and oxygen are called reactants
• Water is called the product
• Reactants are separated from products with ?
• 2 H2 O2 ? 2 H2O
• Note that the ? is like an equal sign because
  both sides of the equation have the same number
  of each type of atom

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• This can be represented as

Note Mass is conserved because the number of
atoms of each type remains the same on each side
of the arrow. (Both sides of the arrow show 4 H
and 2 O atoms.) This equation is said to be
balanced.
The 2 in front of formulas H2 and H2O are
called coefficients. They indicate the number of
molecules of each type and can change when
balancing a chemical equation. The 2 in the
formulas H2 and H2O indicate atom ratios for the
compound and must not change.
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• It is sometimes useful to include the physical
  state of reactants and products
• For solids use s, liquids use l, gases use g, and
  for aqueous solutions use aq.
• For example, the reaction between stomach acid
  (an aqueous solution of HCl) and sodium carbonate
  (an antacid) can be written

2 HCl(aq) CaCO3(s) ? CaCl2(aq) H2O(l)
CO2(g)
Note You can verify this equation is balanced by
checking for mass balance each side shows 1 Ca,
1 C, 2 Cl, 2 H, and 3 O.
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• Almost all chemical reactions either absorb or
  give off energy, often as heat or light
• Kinetic and potential energy are both important
  in chemistry
• Kinetic energy is the energy an object has when
  moving
• Potential energy is the energy an object has due
  to its position
• Potential energy is stored energy because it
  can be converted into kinetic energy

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• Energy must also be conserved
• The Law of Conservation of Energy
• Energy cannot be created or destroyed it can
  only be converted from one form to another
• Heat and temperature are related to kinetic
  energy
• The temperature of an object is proportional to
  its average kinetic energy (average speed of its
  atoms)
• Heat or thermal energy is transferred between
  objects with different temperatures
• Heat flow spontaneously from hot to cold objects

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• Chemical energy is a form of potential energy
• The analysis of temperature changes in chemical
  reactions can provide information about the
  potential energy changes that occur
• The kinetic molecular theory of matter provides
  more details about chemical energy changes and is
  discussed in Chapter 7
• Energy can also be transferred as light, which
  will be covered later in the book

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• As a general rule, molecular compounds are formed
  when nonmetallic elements combine
• Many molecular compounds contain hydrogen

Group
Noble Period IVA VA
VIA VIIA Gas 2
CH4 NH3 H2O HF
Ne 3 SiH4 PH3 H2S
HCl Ar 4 GeH4 AsH3
H2Se HBr Kr 5
SbH3 H2Te HI Xe
Note The number of hydrogens that combined with
the nonmetal equals the number of spaces to the
right we have to move to get to the noble gas
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• Organic chemistry is a major specialty that deals
  with compounds containing mostly carbon and
  hydrogen
• Hydrocarbons contain only hydrogen and carbon and
  are organic compounds
• Alkanes are the simplest hydrocarbons
• General formula is CnH2n2

Space-filling models Black atoms carbon White
atoms hydrogen
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• Other classes of hydrocarbons exist
• Different classes of organic compounds are
  derived from hydrocarbons by replacing hydrogen
• For example alcohols result when a H is replaced
  by OH in a hydrocarbon

Methanol (wood alcohol), CH3OH, is related to
methane, CH4, by replacing one H with OH.
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• Inorganic compounds are substances not considered
  to be derived from hydrocarbons
• The rules for naming, or nomenclature, of simple
  inorganic compound is covered now (organic
  nomenclature is covered later)
• Binary compounds are compounds comprised of two
  different elements
• The goal is to be able to convert between the
  chemical formula and the name

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• The first element in the formula is identified by
  its English name, the second by appending the
  suffix ide to its stem

Chemical Name as
Name as Symbol Stem First Element
Second Element O ox-
oxygen oxide S
sulf- sulfur sulfide
N nitr- nitrogen
nitride P phosph- phosphorus
phosphide F fluor-
fluorine fluoride Cl
chlor- chlorine chloride
Br brom- bromine
bromide I iod- iodine
iodide
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• The number of each type of atom is specified with
  Greek prefixes

Greek Prefixes mono- 1 (often omitted)
hexa- 6 di- 2
hepta- 7 tri-
3 octa- 8
tetra- 4
nona- 9 penta- 5
deca- 10
Examples PF5 phosphorus pentafluoride
HCl hydrogen chloride N2O5
dinitrogen tetraoxide
Note many compounds have common names, like
water for H2O.
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• The subscripts in the formula of an ionic
  compound always specify the smallest whole-number
  ratio of the ions because molecules dont exist
  in ionic compounds
• The smallest unit of a compound is called the
  formula unit
• Positively charged ions have more protons than
  electrons and are called cations
• Negatively charged ions have more electrons than
  protons and are called anions
• The formula unit of an ionic compound always
  contains both cations and anions

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• Ionic compounds are composed of charged particles
  (ions)
• Ions can be formed from the reaction of metal
  with a nonmetal
• The metals form cations and the nonmetals form
  anions

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• The charges on many representative elements can
  be predicted
• Metals form cations
• The positive charge on the cation is the same as
  the A group number of the metal
• Nonmetals form anions
• The negative charge on the anion is equal to the
  number of spaces to the right we have to move in
  the periodic table to get to a noble
• Ionic compounds must be electrically neutral

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• Rules for writing Formulas of Ionic Compounds
• 1) The positive ion is given first in the
  formula.
• 2) The subscripts in the formula must produce an
  electrically neutral formula unit.
• 3) The subscripts should be the set of smallest
  whole numbers possible.
• 4) The charges on the ions are not included in
  the finished formula of the substance.

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• Ions formed by transition metals (Group IIIB
  VIIIB) and post-transition metals

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• Some polyatomic ions (ions with two or more
  atoms)

See Table 2.5 for a more polyatomic ions
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• Naming ionic compounds
• The name of the cation is given first followed by
  the name of the anion
• Cations
• If the metal forms only one positive ion, the
  cation name is the English name for the metal
• If the metal forms more than one positive ion,
  the cation name is the English name followed,
  without a space, by the numerical value of the
  charge written as a Roman numeral in parentheses
  (this is for the Stock system)
• Anions
• For monoatomic anions, the name is created by
  adding the ide suffix to the stem name for the
  element.
• For polyatomic ions, use the names in Table 2.5

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• To name a compound, you can use this flowchart

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• Summary of Properties
• Hardness and brittleness
• Molecular compounds tend to be soft and easily
  crushed because the attractions between molecules
  are weak and molecules can slide past each other
• Ionic compounds are hard and brittle because of
  the strong attractions and repulsions between
  ions

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• Melting points
• To melt the particles in the solid must have
  sufficient kinetic energy to overcome the
  attractions between particles
• Molecular compounds tend to have weak attractions
  between particles and so tend to have low melting
  points
• Many molecular compounds are gases at room
  temperature
• Ionic compound tend to have strong attractions so
  they have high melting points
• Nearly all ionic compounds are solids at room
  temperature

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• Electrical conductivity requires the movement of
  electrical charge
• Ionic compounds
• Do not conduct electricity in the solid state
• Do conduct electricity in the liquid state
• The ions are free to move in the liquid state
• Molecular compounds
• Do not conduct electricity in the solid or liquid
  state
• Molecules are comprised of uncharged particles