Download 7.2 Writing Chemical Equations

Chapter 7
Chemical Reactions
Introduction

Chemical changes always involve the formation
of new substances with properties that are
different from the original substances.

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Chemical equations to describe these chemical
reactions
To conform to the law of conservation of
mass, equations must be balanced
Introduction

There are five general types of chemical
reactions

Understanding these types of chemical reactions
allow you to predict what will happen when
substances undergo a chemical change.
7.1 Chemical Reactions

Chemical reactions are constantly occurring;
these reactions both occur in the world around
you and within you.

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Examples: digestion, photosynthesis, rust, batteries,
cooking.
All chemical reactions, both simple and
complex, involve changing substances.
7.1 Chemical Reactions

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In chemical reactions, one or more substances,
the reactants, change into one or more new
substances, the products.
When writing chemical reactions, the reactants
are separated from the products with an arrow.

The arrow means “yields” or “reacts to product”
Reactants → Products
7.1 Chemical Reactions
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John Dalton explained the way in which
substances change in his atomic theory.
In a chemical reaction the ways in which the
atoms are joined together are changed.
As reactants are changed into products, bonds that
hold atoms together are broken and new bonds are
formed.
 The atoms themselves are neither created nor
destroyed (the law of conservation of mass)

7.1 Chemical Reactions

Chemical reactions can be described in different
ways: descriptions, word equations, skeleton
equations, and balanced equations.
In writing chemical equations, reactants are written
to the left of the arrow and the products are written
to the right.
 Separate reactants and products are separated by
plus signs.

7.1 Chemical Reactions

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Description: “Iron reacts with oxygen to
produce iron (III) oxide (rust)”
Word Equation:
iron + oxygen → iron (III) oxide
Description: “Methane is burned in oxygen to
produce carbon dioxide and water”
Word Equation:
methane + oxygen → carbon dioxide + water
7.2 Writing Chemical Equations
Word equations can be difficult to work with, so
chemical formulas are often used to communicate
more effectively.
Word Equation:
iron + oxygen → iron (III) oxide
Skeleton Equation:
Fe(s) + O2(g) → Fe2O3(s)

7.2 Writing Chemical Equations


A skeleton equation is a chemical equation
that does not indicate the relative amounts
of reactants and products
A catalyst is a substance that speeds up a
reaction without being changed.

Because it is not a reactant or a product, the formula
for a catalyst is written above the arrow.
7.2 Writing Chemical Equations
Symbol
+
→
=
→
←
(s)
(l)
(aq)
(g)
∆
heat
→
→
Pt
→
Explanation
Used to separate two reactants or two products
“Yields,” separates reactants from products
An alternative to →
Used in place of → for reversible reactions
Reactant or product in the solid state; placed after the formula
Reactant or product in the liquid state; placed after the formula
An aqueous solution; the substance is dissolved in water
Reactant or product in the gaseous state; placed after the formula
Indicates that heat is supplied to the reaction
A formula above the arrow represents a catalyst (in this example,
platinum)
7.2 Writing Chemical Equations
Description: “Manganese (IV) oxide catalyzes the
decomposition of an aqueous solution of
hydrogen peroxide into water and molecular
oxygen.”
Word Equation:
manganese (IV) oxide
hydrogen peroxide
→
water + oxygen
Skeleton Equation:
MnO
H2O2(aq) → H2O(l) + O2(g)
2