Describing Chemical Reactions
Balancing Equations
Describing Chemical Reactions
Objectives
 Explain all parts of a chemical equation
 Explain the principle of conservation of mass
 Explain what a balanced equation is
Describing Chemical Reactions
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What is a chemical equation?
A short, easy way to show a chemical
reaction using symbols
Reactants- substances you have at the
beginning
Products- new substances produced after a
reaction
Describing Chemical Reactions
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Reactant + Reactant  Product + Product
Reactants are written on left
An arrow means “yields” or “produces”
Products are written on right.
+ signs separate 2 or more reactants or
products
Describing Chemical Reactions
Limestone (CaCO3) heated
 CaCO3  CaO + CO2
 How many reactants?
 How many products?
 What does the arrow mean?
Describing Chemical Reactions
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What is the law of conservation of mass?
During a chemical reaction, matter cannot be
created nor destroyed
The principle of conservation of mass- in a chemical
reaction, total mass of the reactants must equal the
total mass of the products
If you combine 14.0 grams of Fe with 8.0 grams of S,
then heat it to form a reaction, how many grams of
the product (FeS) will you have?
22 grams
Fe + S  FeS
Describing Chemical Reactions
Open System
 Matter can enter from or escape to its
surroundings
 What is an example of this from our lab?
 The carbon dioxide leaving as gas bubbles.
Describing Chemical Reactions
Closed System
 Matter is not allowed to enter or leave
 How could we have made the lab a closed
system?
Describing Chemical Reactions
Balancing chemical equations
 A chemical equation must show the same number of
each type of atom on both sides of the equation.
 A Coefficient is a number written in front of a
chemical formula in an equation.
 2 H2O
 How many hydrogens?
 4
 How many oxygens?
 2
Describing Chemical Reactions
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H2 + O2  H2O
H2 + O2  2 H2O
2 H2 + O2  2 H2O
Describing Chemical Reactions
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Mg + O2  MgO
Mg + O2  2 MgO
2 Mg + O2 2 MgO
Describing Chemical Reactions
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Ca + O2  CaO
Ca + O2  2 CaO
2 Ca + O2  2 CaO
Describing Chemical Reactions
Classifying Chemical Reactions
 Synthesis- 2 or more substances combine
to make a more complex substance.
 Decomposition- Complex substances are
broken down into simpler products.