1. Chemical Equations Depict the kind of reactants and products
aluminum oxide
Depict the kind of reactants and products
and their relative amounts in a reaction.
reactants
product
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.
The numbers in the front are called
stoichiometric coefficients.

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

3. Chemical Equations 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.
The Law of Conservation of Matter
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)!
20%
100%
Chemical
Factory
100%
80%
Kotz web

4. 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

5. 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.

6. 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

7. 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
CH4 + 2 O2
CO2 + 2
H2O

8. Unbalanced and Balanced EquationsH Cl Cl H Cl Cl H H
H2 + Cl2  HCl
(unbalanced)
H2 + Cl2  2 HCl
(balanced)
reactants products H Cl
reactants products H Cl 2 1 2 2 2 1 2 2

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

10. Visualizing a Chemical Reaction2
Na Cl NaCl 2

11. 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

12. 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?
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
Determine whether the following equation is balanced.
2 Na + H2O  2 NaOH + H2
2 Na H2O  2 NaOH + H2

13. 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.

14. Balancing Chemical Equations
Other examples
NO(g) + O2(g)  NO2(g)
is it balanced?
Is this balanced?
NO(g) + O(g)  NO2(g)
Is this OK?
Is this balanced?
NO(g) + ½ O2(g)  NO2(g)
Is this OK?

15. Balancing Chemical Equations
An important point to remember
2 NO(g) + O2(g)  2NO2(g)
The 2 to the left of NO(g) and NO2(g) refers to the number of molecules present in the balanced equation.
It is a “multiplier” for every atom in the molecule.
The subscript 2 in O2 (g) and NO2(g) refers to the number
of atoms of this type that are present in each molecules
(or ionic compound).

16. Guidelines for Balancing Chemical Equations
1) polyatomic ions first ?
2) even / odd
(make all even)
3) H2O Mg(OH)2 2
H-OH vs.
4) single elements last
Example: need 13 oxygen atoms 13 2
“ ”
Multiply by O2 = 13
Method for balancing chemical equations
1. Identify the most complex substance.
2. Beginning with that substance, choose an element that appears in only one reactant and one product. Adjust the coefficients to obtain the same number of atoms of this element on both sides.
3. Balance polyatomic ions (if present) as a unit.
4. Balance the remaining atoms, usually ending with the least- complex substance and using fractional coefficients if necessary. If a fractional coefficient is used, multiply both sides of the equation by the denominator to obtain whole numbers for the coefficients.
5. Count the numbers of atoms of each kind on both sides of the equation to be sure that the chemical equation is balanced. 13 2
3X O Y Z
3X O Y Z 13 2 2
6X O Y Z

17. (NH4)3PO4 + Mg(OH)2  Mg3(PO4)2 3 + 6 NH4OH? 2
(NH4)3PO Mg(OH)2  Mg3(PO4)2 3 + 6
NH4OH
ammonium phosphate
magnesium hydroxide
magnesium phosphate
ammonium hydroxide
NH41+
OH1-
Now you try… 2
AlCl Li2CO  Al2(CO3) LiCl 3 6

18. 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

19. 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

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

21. Reactants  Products + C(s) + O2(g) CO2(g) + H2(g) + O2(g) H2O (l) 2 2
carbon oxygen carbon dioxide
Reactants Product
1 carbon atom carbon atom
2 oxygen atoms oxygen atoms
catalyst – speeds up reaction + Pt
H2(g) O2(g) H2O (l)
hydrogen oxygen water 2 Pt 2
Reactants Product
4 hydrogen atoms hydrogen atoms
2 oxygen atoms oxygen atoms
Reactants Product
2 hydrogen atoms hydrogen atoms
2 oxygen atoms oxygen atoms
Reactants Product
2 hydrogen atoms hydrogen atoms
2 oxygen atoms oxygen atoms Un
balanced

22. Reactants  Products
catalyst – speeds up reaction 2 2
Unbalanced

23. Pb2+ Cl1- K1+ I1- Pb2+ Cl1- K1+ I1- Cl2 Pb I K KCl PbI2 + +
lead (II) chloride potassium iodide
potassium chloride +
lead (II) iodide
Pb2+
Cl1-
K1+
I1-
Pb2+
Cl1-
K1+
I1-
Cl2 Pb + I K
KCl +
PbI2
(aq)
(ppt)
A reaction that yields an insoluble product, a precipitate, when two solutions are mixed
Are a subclass of exchange reactions that occur between ionic compounds when one of the products is insoluble
Used to isolate metals that have been extracted from their ores and to recover precious metals for recycling
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

24. Pb2+ Cl1- K1+ I1- Pb2+ Cl1- K1+ I1- Cl2 Pb I K KCl PbI2 + +
A reaction that yields an insoluble product, a precipitate, when two solutions are mixed
Are a subclass of exchange reactions that occur between ionic compounds when one of the products is insoluble
Used to isolate metals that have been extracted from their ores and to recover precious metals for recycling
lead (II) chloride potassium iodide
potassium chloride +
lead (II) iodide
Pb2+
Cl1-
K1+
I1-
Pb2+
Cl1-
K1+
I1-
Cl2 Pb + I K
KCl +
PbI2
(aq)
(ppt)
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

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

26. Proportional Relationships
Stoichiometry
mass relationships between substances in a chemical reaction
based on the mole ratio
Mole Ratio
indicated by coefficients in a balanced equation
A balanced chemical equation gives the identity of the reactants and products and the accurate number of molecules or moles of each that are consumed or produced.
Stoichiometry is a collective term for the quantitative relationships between the masses, numbers of moles, and numbers of particles (atoms, molecules, and ions) of the reactants and products in a balanced reaction.
A stoichiometric quantity is the amount of product or reactant specified by the coefficients in a balanced chemical equation.
2 Mg + O2  2 MgO
Courtesy Christy Johannesson

27. Stoichiometry Steps Core step in all stoichiometry problems!!
1. Write a balanced equation.
2. Identify known & unknown.
3. Line up conversion factors.
Mole ratio - moles  moles
Molar mass - moles  grams
Molarity - moles  liters soln
Molar volume - moles  liters gas
Mole ratio - moles  moles
Core step in all stoichiometry problems!!
4. Check answer.
Courtesy Christy Johannesson

28. Stoichiometry Problems
How many moles of KClO3 must decompose in order to produce 9 moles of oxygen gas?
2KClO3  2KCl O2
? mol
9 mol
9 mol O2
2 mol KClO3
3 mol O2
= 6 mol KClO3
Courtesy Christy Johannesson

29. (NH4)3PO4 + Mg(OH)2  Mg3(PO4)2 3 + 6 NH4OH? 2
(NH4)3PO Mg(OH)2  Mg3(PO4)2 3 + 6
NH4OH
ammonium phosphate
magnesium hydroxide
magnesium phosphate
ammonium hydroxide
NH41+
OH1-
Now you try… 2
AlCl Li2CO  Al2(CO3) LiCl 3 6

30. Al2(SO4)3(aq) + 6 NH4OH(aq)  2 Al(OH)3 + 3 (NH4)2SO4 (ppt) (aq)
Identify the spectator ions and write a net ionic equation when an aqueous solution of aluminum sulfate is mixed with aqueous ammonium hydroxide.
Al3+
SO42-
NH41+
OH1-
Al3+
OH1-
NH41+
SO42-
sulfate hydroxide 
aluminum
aluminum
ammonium
ammonium
aluminum hydroxide +
ammonium sulfate
Al2(SO4)3(aq) NH4OH(aq)  2 Al(OH) (NH4)2SO4
(ppt)
(aq)
2 Al3+(aq) SO42-(aq) NH41+(aq) OH1-(aq)  2 Al(OH)3(ppt) NH41+(aq) SO42-(aq)
“spectator ions”
2 Al3+(aq) OH1-(aq)  2 Al(OH)3(ppt)
Net Ionic Equation

31. Meaning of Coefficients
2 atoms Na
1 molecule Cl2
2 molecules NaCl
2 Na + Cl NaCl
2 g sodium +
1 g chlorine =
2 g sodium chloride
2 mol sodium
1 mol chlorine
2 mol sodium chloride
(2 mol Na) x (23 g/mol)
(1 mol Cl2) x (71 g/mol)
(2 mol NaCl) x (58.5 g/mol)
46 g
71 g
117 g
117 g

32. IONIC BONDING: Formation of Magnesium ChlorideCl Cl Mg
Mg2+ Cl Cl
Loses 2e- Each gains 1e One magnesium ion Two chloride ions
Mg Cl1-
[(2+) (1-) = 0]
MgCl magnesium chloride

33. IONIC BONDING: Formation of Magnesium ChlorideCl Cl
Mg2+ Mg
Mg2+ Cl Cl
Loses 2e- Each gains 1e One magnesium ion Two chloride ions
Mg Cl1-
[(2+) (1-) = 0]
MgCl magnesium chloride