Transcript Balancing Chemical Equations

Chapter 11
What is a chemical reaction?
Reactions involve rearrangements
of atoms.
• Reactants are converted into products.
• The law of conservation of mass states
that mass cannot be created nor
destroyed.
• This means that the number of atoms in
the product must be equal to the
original reactants.
• Atoms do not become other kinds of
atoms, nor do they appear or disappear.
Chemical reactions release or
absorb energy
• Endothermic reaction- reaction in
which energy is absorbed.
- Bond formation always requires energy.
• Exothermic reaction- reaction in which
energy is released.
– Bond breaking always releases energy.
• Particles must collide for a chemical
reaction to occur.
• Example, when a safety match is lit, the
reaction begins when the two substances
are brought together by striking the match
head across the striking surface.
• If this collision happens with enough
energy, the bonds in the reactants are
broken, allowing new bonds to form
between the atoms.
Balancing Equations
Balancing requires patience
• To satisfy the law of conservation of mass,
you should insert coefficients into the
chemical equation in order to balance
the number of elements.
• There must be equal numbers of atoms for
each element on each side of the equation
when an equation is balanced.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2
CO2 + H2O
Carbon atoms
2
1
No
Hydrogen atoms
6
2
No
Oxygen atoms
3
3
Yes
• To balance this equation, there are two
carbon atoms in the reactant column and
only one in the product column.
• To balance the number of carbon atoms
you need to double the number of carbon
dioxide molecules in the products.
CH3CH2OH + ? O2
2CO2
+
? H2O
The 2 is called a coefficient and indicates that
there are two carbon dioxide molecules.
Coefficients are normal whole numbers, when
a coefficient is equal to 1, the 1 is not written for
simplicity.
Coefficients are written in front of the formula
and multiply the entire formula.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2
2CO2 + H2O
Carbon atoms
2
2
Yes
Hydrogen atoms
6
2
No
Oxygen atoms
3
5
No
• The carbon atoms are now balanced by
the hydrogen atoms are not. In addition
the oxygen atoms have been thrown out of
balance.
• Let’s start with hydrogen, there are six in
the reactant column, so we must add three
to the product column.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2
2CO2 + 3H2O
Carbon atoms
2
2
Yes
Hydrogen atoms
6
6
Yes
Oxygen atoms
3
7
No
• The oxygen atoms are still unbalanced.
• Multiplying the number of oxygen
molecules by three results in an equal
number of oxygen atoms.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH + 3O2
2CO2 + 3H2O
Carbon atoms
2
2
Yes
Hydrogen atoms
6
6
Yes
Oxygen atoms
7
7
Yes
• Now you have written a balanced chemical
equation.
• Replacing the question marks in the
formula equation with the right coefficients
now give a correct chemical equation.
Tips for balancing equations
1. Save the elements that appear in several
reactants or products for last. (usually oxygen
and hydrogen )
2. Balance lone elements last.
3. Balance the equation from left to right.
4. For ionic equations, be sure that charges are
balanced.