Writing Chemical Equations

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Chemical Equations

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Chemical Equations

Chemical equations are concise representations of

chemical reactions.

+
+

⇒

CH4(g) + 2O2(g)

CO2(g) + 2H2O(g)

The formulas of the reactants (on the left) are connected by an
arrow with the formulas of the products (on the right).

To write a word equation, write the names of the reactants to the
left of the arrow separated by plus signs.

Write the names of the products to the right of the arrow, also
separated by plus signs.

Reactant + Reactant Product + Product

Chemical Equations

Symbols Used in Chemical Equations

Symbol

Explanation

(s)

(l)

(g)

(aq)

Pt

heat

∆

"yields" ­ separates reactants from products

yield sign for a reversible reaction

designates a reactant or product in solid
state; placed after the formula

reactant or product in liquid state

reactant or product in aqueous state

heat is supplied to the reaction

formula above or below yield sign indicates
use of a catalyst, in this case, platinum (Pt).

reactant or product in gaseous state

Word Equations

When ignited, methanegas reacts withoxygen gas

to producecarbon dioxide and steam.

CH4 gas

O2 gas

CO2 gas

H2O gas

+
+

⇒

+

+

This "skeleton" equation is not balanced.

+

+

Word Equations to Chemical Equations

Solid potassium chlorate decomposes in air to produce solid
potassium chloride and oxygen gas.

The word equation is:

potassium chlorate(s) potassium chloride(s) + oxygen(g)

The unbalanced "skeleton" equation is:

KClO3(s) KCl(s) + O2(g)

Skeleton Equations

A skeleton equation is a chemical equation that does not indicate
the relative amounts of the reactants and products.

Write the formulas of the reactants to the left of the yields sign
(arrow) and the formulas of the products to the right.

Here is the equation for rusting:

Metallic Iron reacts with oxygen in the air to produce iron (III)

oxide (rust).

Iron(s) + Oxygen(g)

iron(III) oxide(s)

Fe(s) + O2(g)

Fe2O3(s)

Write the word equation, then the skeleton equation

Aluminum sulfate reacts with calcium chloride to
produce calcium sulfate and aluminum chloride

Word Equations to Chemical Equations

Answer on next slide

Write the word equation, then the skeleton equation

Aluminum sulfate reacts with calcium chloride to
produce calcium sulfate and aluminum chloride

Word Equations to Chemical Equations

Aluminum sulfate + calcium chloride

calcium sulfate + aluminum chloride

Al2(SO4)3 + CaCl2 Ca(SO4) + AlCl3

Law of Conservation of Mass

“We may lay it down as an incontestable axiom
that, in all the operations of art and nature,
nothing is created; an equal amount of matter
exists both before and after the experiment.
Upon this principle, the whole art of performing
chemical experiments depends.”

­­Antoine Lavoisier, 1789