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5. Chemical Reactions

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• A process in which one or more substances is
changed into one or more new substances.
• In a chemical reaction, the substances
present before the chemical change are called
reactants.
• The substances present after the change are
called products.
Chemical Reaction

3. © 2015 Pearson Education, Ltd.

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3- Replacement reaction
• is a reaction in which one element is substituted for
another element in a compound.
• A + BC → B + AC
• 2K (s) + 2HCl (aq) → 2KCl (aq) + H2 (g)
4 - Combustion reaction
A combustion reaction is a reaction in which a
substance reacts with oxygen gas, releasing energy
in the form of light and heat. Combustion reactions
must involve O2 as one reactant

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Chemical Reactions in Automobiles: Combustion
A combustion reaction: In an automobile engine, hydrocarbons
such as octane (C8H18) from gasoline combine with oxygen from
the air and react to form CO2 and H2O.
Combustion reactions are a subcategory of oxidation–reduction
reactions, in which electrons are transferred from one
substance to another.

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• In chemical reactions, however, the changes
in mass are so minute that they can be
ignored. So we can say mass is neither
created nor destroyed in a chemical
reaction.
• Suppose that we burn 58 g of butane in a lighter. It
will react with 208 g of oxygen to form 176 g of
carbon dioxide and 90 g of water.

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Chemical Equations
It is a way to represent the chemical reaction.
It shows us:
• The chemical symbols of reactants and
products
• The physical states of reactants and products–
(s), (l), (g), (aq.)
• Balanced equation (same number of atoms on
each side)
2H2 (g) + O2 (g) 2H2O (l)
Reactants
(starting materials)
Products
( materials formed) 7

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Balancing Chemical Equations
The number of atoms of each element must be the same on both
sides of the equation.
C2H6 + O2 CO2 + H2O
Reactants Products
2 C 1 C
6 H 2 H
2 O 3 O
C2H6 + O2 CO2 + H2O
Reactants Products
4 C 4 C
12 H 12 H
14 O 14 O
C2H6 + O2 CO2 + H2O
2
7/2 3
2 7 4 6

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Balance the following equations
9
2
2
2
2 2 2
2
2 3
2 2
3
2
2
2
2
2
2
8
8

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The quantitative study of reactants and
products in a chemical reaction
CH4 + O2  CO2 + H2O
Stoichiometry
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Mass and mole Relationships
in Chemical Reactions

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The Mole Method:
Stoichiometric coefficients in a chemical equation
can be interpreted as the number of moles of each
substance.
N2 (g) + 3 H2 (g)  2NH3 (g)
N2
1 mole
6.022 ×1023 molecules
H2
3 mole
3×6.022 ×1023 molecules
NH3
2 mole
2×6.022 ×1023 molecules
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N2 (g) + 3 H2 (g)  2NH3 (g)
Recall that the coefficient on N2 is 1 but is not
explicitly written in the reaction Coefficients:
N2 = 1
H2 = 3
NH3 = 2
Using the coefficients we can write mole ratios
Mole Ratios
Definition: mole ratio gives the relative amounts of
reactants and products

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13
N2 (g) + 3H2 (g)  2NH3 (g)

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MOLE to MOLE Stoichiometry
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N2 (g) + 3H2 (g)  2NH3 (g)

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Silicon tetrachloride (SiCl4) can be prepared by heating Si
in chlorine gas:
Si (s) + 2Cl2 (g) SiCl4 (l)
In one reaction, 0.5 mole of SiCl4 is produced. How many
moles of molecular chlorine were used in the reaction?
Cl2 : SiCl4
2 mol : 1 mol
?? : 0.5 mol
n of Cl2 used = (0.507 molSiCl4 ×( 2 molCl2 /1 molSiCl4)
= 1.0 mol of Cl2
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Example

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Convert 85.0 g to moles:
If 85.0 g of CH4 is consumed by a person over a certain period,
what is the mass of CO2 produced?
CH4 + 2O2  CO2 + 2H2O
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Example
𝑤𝑡 (𝑔) = n x Mw (g/mol)
n (CH4) = wt/Mw = (85.0/16.04) = 5.30 mol(CH4)
CH4 + 2O2  CO2 + 2H2O
1 mole of CH4 1 mole of CO2
5.30 moles ??
n (CO2) = 5.30 mol(CH4) × (1mol (CO2) /1mol (CH4) ) = 5.30
mol(CO2)
wt (CO2) = n × Mw = 5.30 mol(CO2)× (44 g (CO2) / 1mol (CO2) )
= 233.2 g (CO2) =2.33×102 g (CO2)
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