DESCRIBING CHEMICAL
REACTIONS
Reactants: Zn + I2
Product: Zn I2
Chemical Reactions
In the course of a chemical reaction, the
reacting substances are converted to new
substances.
Indications of a Chemical Reaction:
Changes of Matter
• Physical Changes:
□ Changes in matter that do not change the
composition of a substance.
• Changes of state, temperature, volume, etc.
• Chemical Changes:
□ Changes that result in new substances.
• Combustion, oxidation, decomposition, etc.
1.Color change
• The visible light absortivity of product molecules is different
that that of reactant molecules.
2.Production of a gas
• Bubbling of product out of reactant mixture.
3.Change in Temperature
• Energy is absorbed or released from reactant mixture.
4.Formation of a Precipitate
• A solid is formed and falls out of the reactant mixture.
5.Production of Light
• Electromagnetic radiation emanates from the reactant
mixture.
Chemical Reactions are Represented by:
1. Word Equations
Word equations describe a chemical
reaction by listing the names of reactant
and product molecules on opposite sides
of an arrow representing the direction of
the reaction
Water → Hydrogen + Oxygen
When writing word equations, list the
reactant particles on the left side of the
arrow separated by “+” signs, and product
particles on the right side of the arrow, also
separated by “+” signs.
Write the word equation for the
reaction represented below:
Reactants: Zn + I2
Product: Zn I2
Word equations are not very
descriptive and are only used to explain
what reactants and products are involved in
the reaction.
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Anatomy of a Chemical Equation
2. Formula Equations
Chemical equations use symbols,
subscripts and coefficients to describe
the identities and relative amounts of
substances undergoing chemical
reactions.
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (g)
2 H2O (l) → 2 H2(g) + O2(g)
Coefficient describing
the number of
reactant particle
reacting
Subscript – part
of chemical
formula of the
reactant
Subscript symbol
denoting the
reactant is in the
liquid phase
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (g)
Products appear on the
right side of the equation.
Coefficients are inserted to
balance the equation.
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (g)
The states of the reactants and products
are written in parentheses to the right of
each compound.
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
Reactants appear on the
left side of the equation.
Subscripts and Coefficients Give
Different Information
CO2 (g) + 2 H2O (g)
• Subscripts tell the number of atoms of
each element in a molecule
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Subscripts and Coefficients Give
Different Information
Formula equations are preferred to word
equations because they:
• Use abbreviated chemical formulas rather
than the full names
• Include the physical states of reactants and
products
• Use symbols that describe the reaction
environment
• Subscripts tell the number of atoms of
each element in a molecule
• Coefficients tell the number of
molecules
• Use coefficients to describe the number of
particles actually undergoing the reaction
When writing formula equations:
When writing formula equations:
1.The correct chemical formula must be
used for all reactants and products.
2.The physical states must be given for
each reactant and product
Using the rules of nomenclature, each
reactant and product must be defined as
an atom, metallic element, alloy,
molecular element, molecule or ionic
compound and represented accordingly.
Physical States at Room Temperature
• Elements can exist metallic solids,
molecules, and individual atoms.
Physical States at Room Temperature Con’t.
•Molecules can be (s), (l) or (g); you will
just have to learn their natures.
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Physical States at Room Temperature Con’t.
NaCl Crystal
Lattice
• All natural ionic
compounds are
solids
• However, ionic compounds can
sometimes dissolve into solution.
Many reactions
involve ionic compounds
dissolved in water called
- aqueous solutions,
represented by (aq).
When writing formula equations:
3.The correct
symbols
must be
included to
describe the
environment.
When writing formula equations:
4.The law of conservation of mass must be
satisfied
•
In any physical or chemical
change, the mass of the
reactants must equal the total
mass of the products.
Or, mass is conserved
•
Coefficients are added to
chemical equations to satisfy
the Law of Conservation of
Mass.
Lavoisier, 1788
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