1. Chemical Reactions

2. What is a chemical reaction?
• A chemical reaction
is the process by
which the atoms of
one or more
substances are
rearranged to form
different
substances.

3. Chemical Reaction Indicators
• Some ways to tell that a chemical
reaction have occurred include:
1. Color change – a change indicates the particles
have changed.
2. Heat content change – did the temperature go up or
down.
3. Gas is produced – if it bubbles (without being
heated) it’s a gas!
4. Precipitate forms – a precipitate is a solid produced
during a chemical reaction between solutions.

4. Chemical Equations
• Chemical equations are used to
represent chemical reactions.
Fe (s) + Cl2 (g)  FeCl3 (s)
Reactants Products

5. Symbols Used in Equations
+ Used to separate two reactants
or products
 “Yields” separates reactants
from products
(s) Identifies solid state
(l) Identifies liquid state
(g) Identifies gaseous state
(aq) Identifies aqueous state – a
substance dissolved in water

6. Balanced equations
• Chemical equations MUST be balanced
to show that the number of atoms in
the reactants is the same as the number
in the products.
What goes in MUST come out!!!

7. Rules for Balancing
• The only place you can change any
number is the coefficient.
• A coefficient is a number written in
front of a chemical formula.
• Don’t forget diatomic molecules.
• Use the smallest ratio of coefficients
possible.

8. How to Balance
• If you are starting with words, write
the equation using formulas.
Example: hydrogen and oxygen gases
react to form water. (Hint: diatomics!)
H2 + O2  H 2O

9. H2 + O2  H 2O
2 hydrogens + 2 oxygens  2 hydrogens 1 oxygen
Notice that there are two hydrogen
atoms on each side however there are
two oxygen atoms in the reactants but
only one in the products. To balance this
we must insert a coefficient.
H2 + O2  2 H 2O

10. While that evens the number of oxygen
atoms – there are now four hydrogen
atoms in the products. To balance the
hydrogen we go back to the reactants
and insert a coefficient.
2H2 + O2  2H2O
Now there are equal number of atoms of
hydrogen and oxygen on each side of
the equation – it is now balanced.

11. Types of Reactions
• There are five types of chemical
reactions: synthesis, decomposition,
single-replacement, double-replacement,
and combustion.
• Replacement reactions are sometimes
called displacement reactions.
• Synthesis reactions are also called
combination reactions.

12. Synthesis or Combination
• Synthesis is a reaction in which two or
more substances react to produce a
single product.
A + B  AB
Na + Cl2  2NaCl

13. Decomposition Reactions
• Decomposition reactions occur when a single
compound breaks down into two or more
simpler substances.
AB  A + B or ABC  A + BC
2HI  H2 + I2
2Mg(ClO3)2 2MgCl2 + 3O2

14. Single-Replacement Reactions
• A reaction in which the atoms of one
element replace the atoms of another
element in a compound.
A + BC  AC + B
Mg + Zn(NO3)2  Mg(NO3)2 + Zn

15. Double-Replacement Reactions
• A reaction involving the exchange of
positive ions between two ionic
compounds dissolved in water.
AB + CD  AD + CB
2NaOH + CuCl2  2NaCl + Cu(OH)2

16. Combustion Reactions
• In a combustion reaction, oxygen
combines with a substance and releases
energy in the form of heat and light.
CH4 + 2O2  CO2 + 2H2O
*All hydrocarbons contain carbon and
hydrogen and burn in oxygen to yield
the same products – CO2 and H2O

17. Heat Content Changes
• An exothermic reaction occurs when more
energy is released forming new bonds than is
required to break bonds in the initial
reactants. *These reactions tend to feel
warm.
• An endothermic reaction occurs when a
greater amount of energy is required to break
the existing bonds in the reactants than is
released when the new bonds form in the
products. *These reactions tend to feel cold.