Download Introduction to Chemical Equations

9/26/2016
Recall: Indications of a Chemical Reaction
1.
2.
3.
4.
Heat/light given off or heat absorbed
Production of a gas
Formation of a new solid (precipitate)
A new color appears
Any of these indicate that a new substance is formed
• Reactants – the substances that exist before a
• To communicate that a chemical reaction has occurred, we
use
Chemical Equations
A + B → AB
chemical change (or reaction) takes place. (left side)
• Products – the new substance(s) that are formed during
the chemical changes. (right side)
REACTANTS  PRODUCTS
Recall Meaning of Chemical Formula
Chemical
Symbol
H2O
Meaning
One molecule
of water:
Composition
Two H atoms and one O atom
What is wrong with this Equation…?
A + B + C → AB
The element “C” disappears on the right side. We cannot destroy
matter!
A + B → BC
The element “C” appears unexpectedly on the right side. We
cannot create matter!
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
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9/26/2016
Unbalanced and Balanced Equations
H
Matter cannot be created or destroyed
Cl
Therefore we have to account for each
substance in a chemical equation by
making sure the left side equals the right
side.
WE MUST BALANCE THE EQUATION
CH4 + 2O2  CO2 + 2H2O
Cl
Cl
H
Cl
H
Cl
H
H
Cl
Cl
H
H
H2 + Cl2  HCl (unbalanced)
reactants
H2 + Cl2  2 HCl (balanced)
reactants
products
products
H
2
1
H
2
2
Cl
2
1
Cl
2
2
We use co-efficients to balance:
Unbalanced (Skeleton Equation):
Mg + O2 → MgO
Balanced Equation:
2Mg + O2 → 2MgO
Reactants
1 C atom
4 H atoms
4 O atoms
Products
1 C atom
4 H atoms
4 O atoms
Fe + O2 → Fe2O3
NEVER
Change the subscripts of a compound to balance!!!
CaO → Ca + O2
2CaO → 2Ca + O2
•
Take your inventory. Balance starting with the compound
with the most atoms but not oxygen
4Fe + 3O2 → 2Fe2O3
Final Check…can you reduce the
Coefficients?....No…Good!
• H2 + Cl2 → HCl
• H2 + Cl2 → 2HCl
• Al + HCl → AlCl3 + H2
• 2Al + 6HCl → 2AlCl3 + 3H2
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9/26/2016
Translating Words into Balanced
Chemical Equations
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
Reactants
+
2.
Write the correct formulas for all reactants and products.
3.
Determine the coefficients that make the equation balance.
Products
Showing Phases in
Chemical Equations
H2O(s)
1) Write a word equation for the reaction.
chlorine + sodium bromide  bromine + sodium chloride
2) Write the correct formulas for all reactants and products.
+
Write a word equation for the reaction.
water
Write a balanced equation for the reaction between chlorine
and sodium bromide to produce bromine and sodium chloride.
Cl2
1.
NaBr

Br2
+
NaCl
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
3) Determine the coefficients that make the equation balance.
Cl2
+
2NaBr

Br2
+
2NaCl
shape, and it shows little response to gravity. Cl 2(g)
• Aqueous Phase – this is a substance dissolved in water
to form a solution. HCl(aq)
You Try…
Create the word equations for the following
situations
HOMEWORK
• Solid sodium and gaseous chlorine combine to form solid sodium
chloride:
• Read pages 225 to 226
• Do questions #1 - 4 (on page 227)
Word Equation:
• Sodium(s) + Chlorine(g)  Sodium chloride (s)
• Read pages 230 to 232
• Do questions #2, 3, and 6
Skeleton Equation (Unbalanced):
• Na(s) + Cl2(g)  NaCl (s)
• Read pages 233 to 235
• Do questions #2-8
Balanced Equation:
• 2Na(s) + Cl2(g)  2NaCl (s)
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