1. Unit 7 Chemical Equations

2. Chemical Equations
Reactants – the substances that exist before a chemical change (or reaction) takes place.
Products – the new substance(s) that are formed during the chemical changes.
CHEMICAL EQUATION indicates the reactants and products of a reaction.
Objectives:
To identify seven elements that occur naturally as diatomic molecules, H2, N2, O2, F2, Cl2, Br2, I2. [HOBrFINCL twins or BrINClHOF brothers]
To write a chemical equation from the description of a chemical reaction.
REACTANTS  PRODUCTS

3. 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
Objective:
To state four observations that are evidence for a chemical reaction.
Photo of precipitate:

4. 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)

5. Additional 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
And or added to
(s)
(l)
(aq)
(g) +

6. Additional Symbols Used in Chemical Equations
Alternative to (g), but used only to indicate a gaseous product
or 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
heat
2 atm
pressure
0 oC
MnO2

7. Chemical Equations
aluminum oxide
Shows the kind of reactants and products, their relative amounts, and the physical states of compounds.
4 Al(s) O2(g) Al2O3(s)
A chemical equation is an expression that gives the identities and quantities of the substances in a chemical reaction
Chemical formulas and other symbols are used to indicate the starting material(s) or reactant(s), which are written on the left side of the equation, and the final compound(s) or product(s), which are written on the right side. An arrow, read as yields or reacts to form, points from the reactants to the products.
Abbreviations are added in parentheses as subscripts to indicate the physical state of each species:—(s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution.
A balanced chemical equation is when both the numbers of each type of atom and the total charge are the same on both sides. A chemical reaction represents a change in the distribution of atoms but not in the number of atoms.
The letters (s), (g), and (l) are the
physical states of compounds.

8. The numbers in the front are called
Chemical Equations
aluminum oxide
The numbers in the front are called
coefficients.
4 Al(s) O2(g) Al2O3(s)
A chemical equation is an expression that gives the identities and quantities of the substances in a chemical reaction
Chemical formulas and other symbols are used to indicate the starting material(s) or reactant(s), which are written on the left side of the equation, and the final compound(s) or product(s), which are written on the right side. An arrow, read as yields or reacts to form, points from the reactants to the products.
Abbreviations are added in parentheses as subscripts to indicate the physical state of each species:—(s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution.
A balanced chemical equation is when both the numbers of each type of atom and the total charge are the same on both sides. A chemical reaction represents a change in the distribution of atoms but not in the number of atoms.
First: the coefficients give the number of molecules (or atoms) involved in the reaction. In the example reaction, four molecules of aluminum react with three molecules of oxygen and produce two molecules of aluminum oxide.
Second: the coefficients give the number of moles of each substance involved in the reaction. In the example reaction, four moles of aluminum react with three moles of oxygen and produce two moles of aluminum oxide.

9. Chemical Equations The Law of Conservation of Mass
Because the same atoms are present
in a reaction at the beginning (reactants)
and at the end (products), the amount
of matter in a system does not change.
Law of Conservation of Matter means that nothing is “lost” or “stolen”. In the real world retail theft costs business lots (much of the theft is actually employee theft)!
The Law of Conservation of Mass
states: Matter can neither be created or destroyed
20%
100%
Chemical
Factory
100%
80%
Kotz web

10. Chemical Equations
Because of the principle of the conservation of mass, all equations must be balanced.
There must be the same number of atoms and the same kind of atoms on both sides of equation. Reactants = Products
Lavoisier, 1788
What a ladies man!

11. Visualizing a Chemical Reaction2
Na Cl NaCl 2
___ mole Na 10 10
___ mole Cl2 5 5
___ mole NaCl 10 10 ?

12. Balancing Chemical Equations
Balanced Equation – the number of each atom of each element must be the same on each side (reactant and product) of the reactions.
Objective:
To write balanced chemical equations by inspection.
Balanced chemical equation
– Provides qualitative information about the identities and physical states of the reactants and products
– Provides quantitative information because it tells the relative amounts of reactants and products consumed or produced in the reaction
– The number of atoms, molecules, or formula units of a reactant or product in a balanced chemical equation is the coefficient of that species
– Mole ratio of two substances in a chemical reaction is the ratio of their coefficients in the balanced chemical equation
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

13. Balancing Chemical Equations
Counting atoms - to be able to write a balanced equation you must be able to count the number of atoms of each element in a substance correctly
Example: 6 H2O
Coefficient
Subscript
If no subscript is present, it is understood to be a “1”.
The coefficient tells how many representative particles there are in a substance. If no coefficient is present, it is understood to be “1”.

14. Balancing Chemical Equations
Coefficient
Example: 6 H2O
Subscript
subscript ______ H x coefficient _____ = ______
subscript ______ O x coefficient _____ = ______
______ Total atoms 2 6 12 1 6 6 18

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

16. Rules for Balancing Chemical Equations
Count the number of each element on each side of the equation
Balance each element one at a time
Determine the coefficients that make the equation balance.
Balance polyatomic ions, then metals, then hydrogen, then oxygen LAST
Reduce if necessary

17. Balancing Chemical Equations
Watch youtube video lessons for balancing equations by clicking the links below.
Watch youtube video lessons for completing worksheets.
Review: Counting Atoms in Compounds
Review: Forming and Naming Compounds
Which Equations are Balanced?
Balancing Equations
Word Equations

18. Don’t forget the Diatomic Elements
Writing Word Equations
Don’t forget the Diatomic Elements
H2, N2, O2, F2, Cl2, Br2, and I2

19. Writing Word Equations
See any diatomic??? Cl Br Mg H Ca S
 Cl2 Diatomic
 Br2 Diatomic
 Mg
 H2 Diatomic
 Ca S

20. Writing Word Equations
Write a balanced equation for the reaction between chlorine
and sodium bromide to produce bromine and sodium chloride.
1) Look up the oxidation number for each element in a compound.
chlorine sodium bromide  bromine + sodium chloride
No charge
Na Br 1-
No charge
Na Cl 1-
2) Write the correct formulas for all reactants and products by swapping and dropping. Don’t forget diatomic elements!
Cl NaBr  Br NaCl
3) Determine the coefficients that make the equation balance.
1 Cl NaBr  Br NaCl

21. Writing Word Equations
Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate.
1) Look up the oxidation number for each element in a compound.
aluminum sulfate + calcium chloride  calcium sulfate
+ aluminum chloride
Al (So4) 2-
Ca Cl 1-
Ca (So4) 2-
Al Cl 1-
2) Write the correct formulas for all reactants and products by swapping and dropping.
Al2(SO4) CaCl  CaSO AlCl3
3) Determine the coefficients that make the equation balance.
1 Al2(SO4) CaCl  CaSO AlCl3