1. Chemical Equations & Reactions

2. Describing a Chemical Reaction
Indications of a Chemical Reaction
Evolution of heat, light, and/or sound
Production of a gas
Formation of a precipitate
Color change

3. Chemical Equations Depict the kind of reactants and products
and their relative amounts in a reaction.
4 Al(s) O2(g) Al2O3(s)
The numbers in the front are called
stoichiometric coefficients.
The letters (s), (g), and (l) are the
physical states of compounds.

4. Chemical Equations 4 Al(s) + 3 O2(g) 2 Al2O3(s) This equation means:
4 Al atoms O2 molecules yield 2 molecules of Al2O3 or
4 Al moles O2 moles yield 2 moles of Al2O3

5. Chemical Equations Because the same atoms are present
in a reaction at the beginning (reactants)
and at the end (reactants), the amount
of matter in a system does not change.
The Law of Conservation of Matter

6. Chemical Equations
Because of the principle of the conservation of matter,
An equation must be balanced.
It must have the same number of atoms
of the same kind on both sides.
Lavoisier, 1788

7. Signs of Chemical Reactions
There are five main signs that indicate a chemical reaction has taken place:
release
input
change in color
change in odor
production of new
gases or vapor
input or release
of energy
difficult to reverse

8. Characteristics of Chemical Equations
The equation must represent known facts.
The equation must contain the correct formulas for the reactants and products.
The law of conservation of mass must be satisfied.
Reactants→Products

9. Word Equations
A WORD EQUATION describes chemical change using the names of the reactants and products.
Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water.
methane oxygen carbon dioxide water
Reactant
Product

10. Meaning of Chemical Formula
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
H2O2 One molecule
of hydrogen
peroxide:
Two H atoms and two O atoms

11. Symbols Used in Chemical Equations
“Yields”; indicates result of reaction
Used to indicate a reversible reaction
A reactant or product in the solid state;
also used to indicate a precipitate
Alternative to (s), but used only to indicate a precipitate
A reactant or product in the liquid state
A reactant or product in an aqueous solution
(dissolved in water)
A reactant or product in the gaseous state
(s)
(l)
(aq)
(g)

12. Additional Symbols Used in Chemical Equations
Alternative to (g), but used only to indicate a gaseous product
Reactants are heated
Pressure at which reaction is carried out, in this case 2 atm
Pressure at which reaction is carried out exceeds normal
atmospheric pressure
Temperature at which reaction is carried out, in this case 0 oC
Formula of catalyst, in this case manganese (IV) oxide,
used to alter the rate of the reaction D
2 atm
pressure
0 oC
MnO2

13. Showing Phases in Chemical Equations
H2O(s) H2O(l) H2O(g)
Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s)
Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H2O(l)
Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl2(g)

14. Types of Chemical Reactions
Synthesis (Combination) reaction
A B  AB
Decomposition reaction
AB  A + B
ASingle-replacement reaction
A BC  AC + B
BDouble-replacement reaction
AB CD  AD + CB
Acid base reaction
HX BOH  BX + HOH
CH O2  CO2 + H2O
Combustion reaction (of a hydrocarbon)
Ause activity series to predict
Bdriving force…water, gas, or precipitate

15. Synthesis Reaction Direct combination reaction (Synthesis)
2 Na Cl  NaCl Na Cl
Na+
Cl - Cl
Cl -
Na+ Na
General form: A B  AB
element or element or compound
compound compound

16. Decomposition Reaction Decomposition Reaction
2 H2O
2 H2O + +
2 H2
2 H2 O2 O2 H H H H O O O O H H + + H H O O O O H H
General form: AB
General form: AB A A + + B B
compound
compound
two or more elements
or compounds
two or more elements
or compounds

17. Single and Double Replacement Reactions
Single-replacement reaction
Mg CuSO4  MgSO Cu
General form:
A BC  AC B
Double-replacement reaction
CaCO HCl  CaCl H2CO3
General form:
AB CD  AD CB

18. Combustion Reactions
A substance combines with oxygen, releasing a large
amount of energy in the form of light and heat.
  Reactive elements combine with oxygen
P4(s) + 5O2(g)  P4O10(s)
(This is also a synthesis reaction)
  The burning of natural gas, wood, gasoline
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)

19. Unbalanced and Balanced EquationsH Cl Cl H H Cl H Cl Cl Cl H Cl H H
H2 + Cl2  HCl (unbalanced)
H2 + Cl2  2 HCl (balanced)
reactants products
reactants products H H Cl Cl

20. Meaning of Chemical Formula
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
H2O2 One molecule
of hydrogen
peroxide:
Two H atoms and two O atoms

21. Balancing Chemical Equations
Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product
What is the relationship between conservation of mass and
the fact that a balanced equation will always have the same
number of atoms of each element on both sides of an equation?
Determine whether the following equation is balanced.
2 Na + H2O  2 NaOH + H2
2 Na H2O  2 NaOH + H2

22. Balancing Chemical Equations
Write a word equation for the reaction.
Write the correct formulas for all reactants and products.
Determine the coefficients that make the equation balance.

23. Write a balanced equation for the reaction between chlorine
and sodium bromide to produce bromine and sodium chloride.
1) Write a word equation for the reaction.
chlorine sodium bromide  bromine + sodium chloride
2) Write the correct formulas for all reactants and products.
Cl NaBr  Br NaCl
3) Determine the coefficients that make the equation balance.
Cl NaBr  Br NaCl

24. Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate.
1) Write a word equation for the reaction.
aluminum sulfate + calcium chloride  calcium sulfate + aluminum chloride
2) Write the correct formulas for all reactants and products.
Al2(SO4) CaCl  CaSO AlCl3
3) Determine the coefficients that make the equation balance.
Al2(SO4) CaCl  CaSO AlCl3

25. CH4 + 2 O2  CO2 + 2 H2O Reactants Products 1 C atom 1 C atom
4 H atoms H atoms
4 O atoms O atoms