CHEMICAL EQUATIONS
Chapter 9
CHEMICAL REACTIONS
A chemical reaction describes a change in
composition; the process by which one or more
substances are changed into one or more
different substances.
 Reactions involve rearrangements of atoms.
 Chemical reactions release or absorb energy.
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CHEMICAL EQUATION
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An expression showing the formulas and the
relative amounts of the reactants and products in
a chemical reaction. A chemical equation
describes the type and number of atoms that are
rearranged during a reaction.
TIPS FOR BALANCING EQUATION
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Balancing chemical equations literally means
counting the number of times atom symbols appear in
the reactants and products to make sure the counts
are the same on both sides. Conservation of mass is
linked to "conservation " of element symbols.
The law of conservation of mass is met when the
count of element symbols on reactants side is equal to
the count of element symbols on the product side.
balanced equation has equal counts (number) of
atoms of each element in both reactants and products.
Equations are balanced by adjusting the multipliers
(coefficients) in front of formula symbols so the counts
of atoms are the same in reactants and products. The
subscripts are not changed.
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Subscripts in chemical formulas are NEVER
changed in the balancing process. Changing the
subscripts changes the identity of compounds and
the sense of the equation.
CLASS PRACTICE
Balance the following equations:
 Page 316
 Q.1.
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a,b,c,d
QUANTITATIVE RELATIONSHIP
Equations reveal the number of formula
units involved. The coefficients from a balanced
equation can help you to predict how much of
each reactant is needed to make a certain
amount of product.
 C6H12O6(aq) +6O2(g) 6CO2(g) +6H2O(l) +energy
 In this equation we can express as moles, formula units
or molecules.
 Mass is conserved and atoms are conserved. Total
number of moles and molecules do not necessarily
remain the same on both sides of the chemical equation
since molecules are created and destroyed as atoms are
rearranged.
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Balanced equations show proportion
 From the last slide you have seen that 1 mole of
glucose would require 6 moles of oxygen and
would produce 6 moles of water and 6 moles of
carbon dioxide . We can calculate from here how
many moles of each product will be produced by
increasing the moles of glucose. Instead of
counting the reactants and products we multiply
by a number; example 3.
 It is basically the same equation except
proportionately larger.
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Relative amounts in equations can be
expressed in moles.
 The molar interpretation of the previous reaction
can be expressed as ratios of reactants and
products.
 1mol C6H12O6: 6 mol O₂ : 6mol CO₂ :6 mol H₂O
 For any pair of molecules or formula units, a mole ratio
can be determined by comparing the coefficients from
the balanced chemical equation. Usually expressed as
lowest whole number ratios.
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EXOTHERMIC AND ENDOTHERMIC
REACTIONS
The amount of energy released or absorbed in a
chemical reaction is expressed as ∆H, where ∆
stands for “change in”, and H represents
enthalpy, the total energy content of a system.
 ∆H values are negative because energy is
released when the stronger bonds of the products
are created.
 In endothermic reactions energy is added to the
system and ∆H values are positive because the
energy needed to break the bonds increases the
total energy of the system.
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An exothermic reaction releases energy. The energy
change that accompanies a reaction can be written in the
equation.
Here the 213 kcal are a product and appear on the right
hand side of the equation. The reaction produces or
releases energy so the sign for the 213 kcal is negative, (213 kcal). The reaction is exothermic.
The amount of energy change is proportional to the mass of
material consumed in the reaction. If two moles of methane
are burned the heat effect will be doubled. If one mol of
methane is burned this way 213 kcal will be lost to the
surroundings. When the amount of CH4 is increased to ten
mols then the reactants will release 2130 kcal.
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An endothermic reaction uses energy as a reactant.
The energy change that accompanies a reaction can
be written in the chemical equation. The reaction
requires the addition of energy to the reactants to
form the bonds in the products. In this reaction 43
kcal are needed to make the reaction occur. The sign
for the energy change is +. This is an endothermic
reaction. The surroundings must provide energy to
make this reaction happen.
We are fortunate that the oxygen and nitrogen
require energy to force them to react. Otherwise we
could have lost our atmosphere in a burst of flame
years ago.
HOMEWORK
Page 324
 Q.5 and q.6 all
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CLASSIFICATION OF CHEMICAL REACTIONS
There are several types of chemical reactions:
 Combustion: A combustion reaction is when
oxygen combines with another compound to form
water and carbon dioxide. These reactions are
exothermic, meaning they produce heat. An
example of this kind of reaction is the burning of
napthalene:
C10H8 + 12 O2 ---> 10 CO2 + 4 H2O
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Synthesis: A synthesis reaction is when two or
more simple compounds combine to form a more
complicated one. These reactions come in the
general form of:
 A + B ---> AB One example of a synthesis
reaction is the combination of iron and sulfur to
form iron (II) sulfide: 8 Fe + S8 ---> 8 FeS Below
a skinny bird(reactant) and a worm(reactant)
combine to form a fat bird.
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Decomposition reaction a more complex
substance breaks down into its more simple
parts. One reactant yields 2 or more products.
Basically, synthesis and decomposition reactions
are opposites. For example, water can be broken
down into hydrogen gas and oxygen gas. The
chemical equation for this decomposition reaction
looks like:
reactant -------> product + product
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SINGLE DISPLACEMENT :In a single
displacement reaction a single uncombined
element displaces another in a compound. Two
reactants yield two products. For example when
zinc combines with hydrochloric acid, the zinc
displaces hydrogen. The chemical equation for
this single displacement looks like:
reactant + reactant ---------> product + product
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DOUBLE DISPLACEMENT :In a double
displacement reaction parts of two compounds
switch places to form two new compounds. Two
reactants yield two products. For example when
silver nitrate combines with sodium chloride, two
new compounds--silver chloride and sodium
nitrate are formed because the sodium and silver
switched places. The chemical equation for this
double displacement reaction looks like:
reactant + reactant ---------> product + product
CLASS PRACTICE
Page 330
 5,6,7 do all
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HOMEWORK
Q.28,29 do all
 Q.32 ; a and b
 Q.43 ; a,b and d.
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