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Physical Science, 8e
Chapter 10
Chemical Reactions
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Core Concept
Chemical symbols, formulas, and
equations can be used to concisely
represent elements, compounds, and
what happens in a chemical reaction.
–3 types of formulas
Chemical Formulas
1. Empirical formula
– Identifies elements
present in terms of
simplest whole
number ratios
– Examples: table
salt, NaCl; water,
H2O
2. Molecular formula
– Identifies actual
number of atoms in
a molecular
compound
– Example: water,
H2O; not table salt,
NaCl (ionic
compound)
Chemical Formulas, cont.
3. Structural formula
– Represents
arrangement of
atoms within a
molecule
– Related to 3-D
structure of molecule
Empirical or Molecular
Formula?
Empirical
• Ionic - lacking discrete
unit, or molecule
• Composed of both
metallic and nonmetallic
elements
• Electronegativity
difference > 1.7
Molecular
• Covalent compounds
• Usually nonmetals bound
to nonmetals
• Molecular and empirical
formulas can be different
– Glucose – molecular
C6H12O6 versus empirical
CH2O
Molecular and Formula Weights
• Formula weight
– Sum of atomic
weights of all atoms
in chemical formula
• Molecular weight
– Formula weight of a
molecular
substance
– Term often used for
nonmolecular
substances, as well
Percent Composition of
Compounds
• Finding the mass percentage of an individual
element from the formula weight
Percentage Composition
Example
Chemical Reactions
• Occur through formation and breaking
of chemical bonds between atoms
• Involve changes in matter, creation of
new materials and energy exchange
• Chemical equations - concise
representation of chemical reactions
Chemical Equations
•
•
•
•
Reactants - substances existing before reaction
Products - substances existing after reaction
Word representation not sufficiently precise
Chemical symbols and formulas needed for
quantitative purposes
Balancing Equations
• Law of conservation
of mass - atoms are
neither created nor
destroyed in
chemical reactions
• Change coefficients
in front of chemical
formulas, not
subscripts within
formulas, to balance
mass of reactants = mass of products
Meaning of Subscripts and
Coefficients with a Chemical Formula
Stepwise Balancing Procedure
Generalizing Equations
• Combustion reaction
– A hydrocarbon in the presence of oxygen
reacts to produce carbon dioxide and water.
CH4 + O2  CO2 + H2O
Alternative Classification
1. Combination reactions
2. Decomposition reactions
3. Replacement reactions

(1-3 = redox reaction subclasses)
4. Ion exchange reactions
Oxidation – Reduction Reaction
• Oxidation – loss
of electrons
• Reduction –
gain of electrons
Combination Reactions
• Synthesis reaction in which two or more
substances combine to form single
compound
• X + Y  XY
Decomposition
• A compound is broken down into
simpler substances
• XY  X + Y
Replacement Reactions
• An atom or polyatomic ion is replaced in a compound
by a different atom or polyatomic ion.
Replacement Reactions
Al(s) + CuCl2(aq) →AlCl3(aq) + Cu(s)
2Al(s) + 3CuCl2(aq) →2AlCl3(aq) + 3Cu(s)
Ion Exchange Reactions
• A reaction that takes
place when the ions
of one compound
interact with the ions
of another
compound forming
– A solid precipitate
– A gas
– Water
AX + BY  AY + BX
Ion Exchange Reactions
__Ca(OH)2(aq) + __Al2(SO4)3(aq)  __CaSO4(aq) + __Al(OH)3(s)
3 Ca(OH)2(aq) + Al2(SO4)3(aq)  3 CaSO4(aq) + 2 Al(OH)3(s)
Information - Chemical
Equations
• Atoms are
conserved
• Mass is conserved
• Law of combining
volumes (gases)
– Gases at the same
temperature and
pressure contain
equal numbers of
molecules
Units of Measurement Used
with Equations
• Atomic mass unit = 1/12
mass of carbon-12
• One mole of a
substance contains
Avogadro’s number
(6.02x1023) of the basic
chemical unit of that
substance (atoms,
molecules, ions, …)
• A mole of carbon-12
atoms is defined as
having a mass of
12.00g
Molar Weights
• Gram atomic weight mass in grams equal to
atomic weight
• Gram formula weight mass in grams equal to
formula weight
• Gram molecular weight mass in grams equal to
molecular weight
Quantitative Use of Equations
Possible interpretations:
1. Molecular ratios of
reactants and
products
2. Mole ratios of
reactants and
products
3. Mass ratios of
reactants and
products