Transcript Writing And Balancing Equations

Writing And Balancing
Equations
Chemistry
Dr. May
The Road To Writing Sentences
 Know
the alphabet
 Know vowels and consonants
 Know how to spell words
 Know the meaning of words
 Know sentence structure
 Know capitalization and punctuation
The Road To Writing Equations
 Know
the atomic symbols
 Know oxidation numbers of elements
 Know the charge on polyatomic ions
 Be able to create the correct formulas
 Identify the reactants and products
 Balance the equation
The Language of Equations
Ca(OH)2 + H2SO4  CaSO4 + 2 H2O

The equation above is like a sentence

The compounds Ca(OH)2, H2SO4, CaSO4, and H2O
are like correctly spelled and chosen words

The elements Ca, O, H, and S are like the letters of
the alphabet
Equations in English
Ca(OH)2 + H2SO4  CaSO4 + 2 H2O
 One
molecule of calcium hydroxide plus
one molecule of sulfuric acid yields one
molecule of calcium sulfate plus two
molecules of water.
Equations
Zn

+
2 HCl
 ZnCl2 +
H2 
On the left, Zn and HCl are reactants
 On the right, ZnCl2 and H2 are products
 The arrow () means “yields”
 The up arrow () indicated a gas product
 The 2 in front of HCl is a coefficient
 The small 2 to the right of H is a subscript
“Skeleton” Equation

Identify the reaction
 Zinc
reacts with hydrochloric acid to give
zinc chloride and hydrogen gas

Determine the correct formulas and write the
“skeleton” (unbalanced) equation

Zn
+
HCl
 ZnCl2 +
H2 
Coefficients

A coefficient placed before a formula
multiplies every atom in that formula by that
number
2 H2SO4

2 H2SO4 means 2 molecules of sulfuric acid
but also

4 H atoms, 2 S atoms, and 8 O atoms
Conservation Of Mass
 A balanced
equation contains the same
number of each kind of atom on each
side of the equation
 The
balanced equation obeys the
Law of Conservation of Mass
Balance The Equation
Zn
+
HCl
 ZnCl2 +
H2 

Count atoms of each element on each side

Balance each element by placing whole numbers
(coefficients) in front of each formula as needed
Zn
+ 2 HCl
 ZnCl2 +
H2 
Write Equation
 Calcium
hydroxide reacts with sulfuric
acid to give calcium sulfate and water
Calcium = Ca+2
Hydroxide = OH1
Hydrogen = H+1
Sulfate = SO42
Ca(OH)2 + H2SO4  CaSO4 + H2O
Balance Equation
Ca(OH)2 + H2SO4  CaSO4 + HOH
Count atoms on both sides
1Ca, 2OH, 2H, 1SO4
1Ca, 1OH, 1H, 1SO4
Balance the water
Ca(OH)2 + H2SO4  CaSO4 + 2 HOH
Write Equation
 Aluminum
hydroxide plus sulfuric acid
gives aluminum sulfate plus water
Aluminum = Al+3
Hydrogen = H+1
Hydroxide = OH1
Sulfate = SO42
Al(OH)3 + H2SO4  Al2(SO4)3 + H2O
Balance The Equation
Al(OH)3 + H2SO4 
Al2(SO4)3 +
1 Al, 3 OH, 2 H, 1 SO4
2 Al, 3 SO4, 1 H, 1 OH
2 Al(OH)3 + H2SO4  Al2(SO4)3 +
HOH
HOH
2 Al(OH)3 + 3 H2SO4 
Al2(SO4)3 +
2 Al(OH)3 + 2 H2SO4 
Al2(SO4)3 + 6 HOH
HOH
Write The Equation
 Propane
burns to produce carbon
dioxide and water
C 3H 8
+
O2

CO2
+
H2O
The above skeleton equation must be balanced
Balance The Equation
C3H8

+
O2

CO2
+
First balance the carbons
C3H8 + O2  3 CO2
H2O
+
H2O
+
+
4 H2O
4 = 10

Next balance the hydrogens
C3H8 + O2  3 CO2
6
 Finally balance the oxygens
C3H8
+
5 O2
2 x 5 = 10

3 CO2
+
4 H2O
Chemical Equations

Chemical equations are the short hand
description of a chemical reaction

It tells us:





The reactants and products
The formulas of the reactants and products
The number of molecules of reactants and products
The number of atoms of each element
The number of moles of each substance
The End
 This
presentation was created for the
benefit of our students by the Science
Department at Howard High School of
Technology
 Please
send suggestions and
comments to [email protected]