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CHEMICAL
REACTIONS
SIGNS OF A
CHEMICAL REACTION
Evolution of heat and light
Formation of a gas
Formation of a precipitate
Color change
SYMBOLS IN CHEMICAL
REACTIONS
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Many symbols are used when writing a chemical reaction
A BALANCING ACT
LAW OF CONSERVATION
OF MASS
• mass is neither created nor destroyed in a
chemical reaction
• total mass stays the same
• atoms can only rearrange
4H
36 g
2O
4H
2O
4g
32 g
CHEMICAL REACTION
VOCABULARY
Chemical Reaction: One or more substances, the reactants change
into one or more new substances, the products
Reactant: A substance present at the start of a reaction
Product: A substance produced in a chemical reaction
Chemical Equation: An expression representing a chemical reaction;
the formulas of the reactants (on the left) are connected by an arrow
with the formulas for the products (on the right).
Example:
Products
6
Reactants
PRACTICE:
Write the equation for…
• Copper can be produced by heating copper (II)
sulfide with oxygen gas. Sulfur dioxide is also
produced.
PRACTICE:
Write the equation for…
• Water is formed by the explosive reaction
between hydrogen and oxygen
ANSWERS
CuS + O2 → Cu + SO2
2 H2 + O2 → 2 H2O
MEANING OF CHEMICAL FORMULA
Chemical
Symbol
Meaning
Composition
H2O
One molecule
of water:
Two H atoms and one O atom
2 H2O
Two molecules
of water:
Four H atoms and two O atoms
One molecule
of hydrogen
peroxide:
Two H atoms and two O atoms
H 2 O2
MEANING OF CHEMICAL FORMULA
subscript is written after the
• When a __________
chemical symbol for an element it
# of that element
indicates the___________________.
• When a coefficient is written in front of an
atom or molecule it indicates the
number
____________
of that atom or molecule
that is present.
• If there are subscripts in that
atom/molecule they need to be
multiplied
____________
by the coefficient.
UNBALANCED AND BALANCED EQUATIONS
H
Cl
Cl
Cl
H
H
H
Cl
Cl
H
Cl
Cl
H
H
H2 + Cl2  HCl (unbalanced)
reactants
H
Cl
Timberlake, Chemistry 7th Edition, page 167
H2 + Cl2  2 HCl (balanced)
reactants
products
2
1
2
1
H
Cl
2
2
products
2
2
HOW DO WE GO FROM UNBALANCED
TO BALANCED EQUATIONS?
MAY NOT
• You ____________
change the formula of
your atom/ion/molecule!
• This means you may not change the
___________
!
subscript
• Instead, you may put a coefficient
__________
of your atom/ion/molecule to
in front
balance the number of atoms after you
have ___________
multiplied it by the subscript
BALANCING STEPS
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients as needed to make #s equal on
both sides of the equation.
Coefficient  subscript = # of atoms
4. Reduce coefficients to lowest possible ratio, if
necessary.
5. Double check atom balance!!!
HELPFUL TIPS
• Balance one element at a time.
• Update ALL atom counts after adding a
coefficient.
• If an element appears more than once per
side, balance it last.
• Balance polyatomic ions as single units.
•“1 SO4” instead of “1 S” and “4 O”
BALANCING
EXAMPLES
P
+
O2
→
P4O10
H2
+
O2
→
H 2O
AgNO3 + CuSO4 Ag2SO4 + Cu(NO3)2
BALANCING EXAMPLE
Aluminum and copper(II)chloride react to
form copper and aluminum chloride.
2 Al + 3CuCl2  3 Cu + 2AlCl3
2 1
Al
Al
1 2
3 1
Cu
Cu
1 3
6 2
Cl
Cl
3 6
BALANCING EXAMPLE
Aluminum and copper(II)chloride react to form copper and aluminum chloride.
Step 1. Determine the Correct Formula
Aluminum = Al
Copper(II)chloride = CuCl2
Copper = Cu
Aluminum chloride = AlCl3
Step 2. Write the skeleton equation
Al + CuCl2
Cu + AlCl3
Step 3. Determine the # of atoms in reactants and products
Reactants: 1 Al, 1 Cu, 2 Cl
Products: 1 Al, 1 Cu, 3 Cl
Step 4. Balance the elements one at a time by using coefficients.
Start with Cl since it is the only one that isn’t balanced. To make both sides match we use
coefficients to make both sides match. When we make that change it then effects the others so
we have to use coefficients to balance those as well.
2Al + 3CuCl2
3Cu + 2AlCl3
Step 5. Check each atom or polyatomic ion to be sure they are equal on both sides of
the equation.
Reactants: 2 Al, 3 Cu, 6 Cl
Products: 2 Al, 3 Cu, 6 Cl
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Step 6: Make sure all the coefficients are in the lowest possible ratio.