Chemical Equations and
Reactions
Describing Chemical Reactions
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A process by which one or more
substances are changed into one or
more different substances
The original substances are
reactants, the resulting substances
are the products
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The mass of reactants and products
must be equal
Indications of a Chemical Reaction
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Certain easily observed changes
usually indicate that a chemical
change has occurred
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Evolution of heat and light
Production of a gas
Formation of a precipitate – a solid that
separates from solution
Color change
Characteristics of Chemical Equations
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The equation must represent known
facts
The equation must contain the
correct formulas for the reactants
and products
The law of conservation of mass
must be satisfied
Can be either a word equation or a
formula equation

There are several
symbols used in
chemical
equations
Significance of a Chemical Equation
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The coefficients of a chemical
reaction indicate relative, not
absolute, amounts of reactants and
products
The relative masses of the reactants
and products of a chemical reaction
can be determined from the
reaction’s coefficients

The reverse reaction for a chemical
equation has the same relative amounts
of substances as the forward reaction
Balancing Chemical Equations
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1. Identify the names of the
reactants and the products, and
write a word equation
2. Write a formula equation by
substituting correct formulas for the
names of the reactants and the
products
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3. Balance the formula equation
according to the law of conservation
of mass
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Balance atoms one at a time
Balance the atoms that are combined
and appear only once on each side
Balance polyatomics that appear on
both sides as single units
Balance H and O atoms last
4. Count atoms to be sure that the
equation is balanced
Types of Chemical Reactions
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Synthesis Reactions

A+X
AX
Reactions of Elements with Oxygen
and Sulfur
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One simple type of synthesis
reaction is the combination of an
element with oxygen to produce an
oxide of that element
2Mg(s) + O2(g)  2MgO(s)
Sulfur behaves in a similar way
8Ba(s) + S8(s)  8BaS(s)
Reactions of Metals with Halogens
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Most metals form ionic or covalent
compounds with the Group 17
elements
2Na(s) + Cl2(g)  2NaCl(s)
Synthesis Reactions with Oxides
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Oxides of active metals react with
water to produce metal hydroxides
CaO(s) + H2O(l)  Ca(OH)2(s)
Many oxides of nonmetals in the
upper right react with water to
produce oxyacids
SO2(g) + H2O(l)  H2SO3(aq)
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Decomposition Reactions
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AX
A+ X
A single compound undergoes a reaction that
produces two or more simpler substances (the
reverse of synthesis reactions)
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SingleReplacement
Reactions
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One element
replaces a similar
element in a
compound
A + BX
AX + B
Y + BX BY + X
Activity Series
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A single
replacement
reaction will occur
if the single
element is more
active that the one
in the compound
Double-Replacement Reactions
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The ions of two compounds
exchange places in an aqueous
solution to form two new
compounds
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AX + BY
AY + BX
One of the products is usually a
precipitate, an insoluble gas, or a
molecular compound (usually water)
Combustion Reactions
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A substance combines with oxygen,
releasing a large amount of energy in the
form of light and heat
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2H2 + O2
2H2O
C3H8 + 5O2
3CO2 + 4H2O
Energy & Enthalpy
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The heat of reaction is the quantity of
energy released or absorbed as heat
during a chemical reaction
Thermochemical equations includes
the quantity of energy released or
absorbed as heat during the chemical
reactions

2H2(g) + O2(g)  2H2O(g) + 483.6 kJ
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The quantity of energy released
corresponds to the molar quantities of
the equation
The enthalpy change is the amount of
energy absorbed or lost by a system
as heat during a process at constant
pressure
DH = Hproducts – Hreactants
A postive DH corresponds to an
endothermic reaction, A negative DH
is exothermic
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The coefficients refer to the number
of moles and can be written as
fractions
Physical states must be included in
these reactions
If the equation is multiplied by a
factor, the energy change is
multiplied by that same factor
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The heat of formation is the energy
released or absorbed as heat when
one mole of a compound is formed
by combination of its elements
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Given for the standard states of the
elements (state at STP)
DH  standard heat
0
DH  standard heat of formation
0
f
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Elements in their standard states are
defined as have a standard heat of
formation of 0
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a negative heat of formation means that
a compound is more stable than its
component elements
Postitive or only slightly negative heats
mean that it is more stable in its
elemental form and will tend to
decompose
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The heat of combustion is the energy
released as one mole of material is
combusted
Thermochemical equations can be
added together to give changes for
other reactions
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Hess’s law states that the overall
enthalpy change in a reaction is
equal to the sum of enthalpy
changes for the individual steps in
the process
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If a reaction is reversed the sign of is
also reversed
If the reaction is multiplied by a factor,
so is the DH
p.
520
The Reaction Process
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Reaction mechanisms
are the step-by-step
sequence of reactions
by which the overall
chemical change
occurs
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Species that appear in
some steps but not in
the net equation are
known as
intermediates
Homogeneous
reactions have
reactants and products
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Collision theory describes the conditions
required for the effective collision of
reactants to produce a product
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Must have a minimum amount of energy
Must have a correct orientation
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The energy
required for
reactants to
come
together and
form the
activated
complex is
know as the
activation
energy
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The activated complex is a
transitional structure that results
from an effective collision that
persists while old bonds are
breaking and new bonds are
forming
Reaction Rate
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The change in concentration of
reactants per unit time as a reaction
proceeds is called the reaction rate
Chemical kinetics is the study of
reaction rates and reaction
mechanisms
Reaction rate depends on both the
frequency of collision and collision
efficiency
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5 factors that influence reaction rate
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1. Nature of reactants
2. Surface area – the greater the
surface area, the faster the reaction
3. Temperature – higher T, more and
faster collisions, higher rate
4. Concentration – more reactants,
more collisions, faster rate
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5. Presence of catalysts – substances
that change the rate of a chemical
reaction without actually reacting
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Typically provides and alternative
pathway for the reaction that has a lower
activation energy
Homogeneous is in the same phase,
heterogeneous is in a different phase
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An equation that relates reaction
rate and concentrations of reactants
is called the rate law for the
reaction
For the reaction A + B  2C
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The forward rate law is R=k[A][B]
The reverse rate law is R=k[C]2
Note the concentration is raised to its
molar coefficient
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If a reaction occurs in a sequence of
steps, the rate law is determined by
the slowest step
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This is the rate determining step
Estimated Enthalpy from Bond
Energies
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Think of bond energy as the energy
required to pull a bond apart.
Keeping the sign convention in
mind, all bond energies are
positive!
In order to use bond energies to
calculate heat of reaction (enthalpy
of reaction), you substitute bond
energies into this equation:
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You need to be very careful when
applying this formula because the
order of terms is the opposite of the
order used in the heats of
summation calculations.
The molar enthalpy of combustion of ethanol
based on bond energies is -1398 kJ/mol. The
accepted value is -1368 kJ/mol. There is a
difference due to the use of average bond
energies.
Ex: Combustion of methane