Lesson: Chemical Equations

Lesson: Chemical Equations

Compounds

●Atoms of different elements can use ionic
or covalent bonds to form compounds.

○Compounds are new substances

○Compounds have properties distinct
from the elements they are made of.

●Each compound has a consistent and
specific way to form.

○The ratio of each element’s atoms is
always the same

○The shape and type of bond is always
the same.

Formulas

Because the ratio of each element is consistent in a
compound, we can represent them with a formula

●A chemical formula uses symbols to
show what elements it contains. The
ratio is shown through subscripts.

○Subscript = number of atoms of
the element it is behind

○No subscript = 1 atom

Formulas

Some compounds are made of just 2
elements. We call them binary
compounds.

Others like sodium sulfate pictured
here are made of more than 2
elements and are called tertiary
compounds.

No matter how many elements make
up the compound, the formula rules
are the same. The ratios are in
subscripts.
Ratio: 2 sodium atoms: 1 sulfur atom: 4 oxygen atoms

Formulas

Some formulas can look complex
and intimidating. They really
aren’t that difficult.

If you see part of a formula in
parentheses (), it is a section that
is tied together, in some ways
acting like a single atom.

Subscripts on these “grouped”
elements tell how many groups
are in the molecule. The
subscripts inside tell what the
“groups” are made of.

Multiple Choice

How many different elements are present in H₂COCH₂

• 5

• 4

• 3

• 2

Multiple Choice

H₂SO₄

• H = 2, S = 4, O = 4

• H = 2, S = 1, O = 4

• H = 8, S = 4, O = 4

• H = 2, SO = 4

Multiple Choice

A ______ is a number written in front of a chemical formula.

• Coefficient

• Subscript

• Exponent

• Coordinator

Multiple Choice

How many atoms are present if there is no subscript following the element symbol?

• 0

• 2

• only 1

• 1 million

Multiple Choice

Coefficients tell the number of __________ in a formula.

• Molecules

• Atoms

• Elements

• Oxygen

Chemical Reactions

A chemical reaction occurs when bonds are
formed or broken between different atoms,
creating a new substance.

●Elements combining to form a compound

●A compound separating into its elements

●Compounds separating then recombining to
form new compounds

Writing Equations

Chemical Reactions can be described

●Hydrogen and oxygen combine to form water

●Zinc and hydrochloric acid react to produce zinc chloride
and release hydrogen gas.

●Lead II nitrate solution mixed with aqueous potassium iodide
produces lead II iodide precipitate and potassium nitrate
solution.

Writing Equations

These are called word description or
written descriptions. Like all
reactions they begin with substances
and end with different substances.

●Hydrogen and oxygen combine
to form water

●Zinc and hydrochloric acid
react to produce zinc chloride
and release hydrogen gas.

●Lead II nitrate solution mixed
with aqueous potassium iodide
produces lead II iodide
precipitate and potassium
nitrate solution.

Substances you begin with = reactants
Substances you end with = products

Writing Equations

Each substance in an equation is called a “species”.
When using symbols, species are separated by a + sign

+Does not mean add in a chemical
equation. It just separates the
species.

Writing Equations

Reactants (reagents) and Products are separated by an
arrow. The arrow indicates the direction of the reaction.

The arrow is usually read as “yields” or
“produces”.

Multiple Select

__CH₄ + __O₂ --> __CO₂ + __H₂O

What are the reactants in this equation?

• CH₄

• O₂

• CO₂

• H₂O

Multiple Choice

When writing a chemical equation, reactants are on the _____________ while products are on the _________________.

• top, bottom

• right, left

• left, right

• bottom, top

Multiple Choice

In a chemical equation...

• a new substance is formed

• new atoms are formed

• old atoms disappear

• a equal sign is always used

Multiple Choice

Products are

• old substances

• new substances

• atoms that disappear

• atoms that do not exist

Chemical Reactions are
the result of chemical
bonds breaking or
forming.

This allows substances to
change into other
substances.

Chemical Reactions
require energy to begin.
Some use more energy
during the reaction
(endothermic), some
release more energy
during the reaction
(exothermic).

Chemical Equations describe Chemical
Reactions

The left side of a chemical
equation shows the
substances you begin with.
These are called
“reactants”.

The right side of a chemical
equation shows the
substances after the
reaction has occurred.
They are called “products”.

Chemical Equations

Chemical Equations provide
lots of information about a
chemical reaction using
symbols and formulas.

Some symbols are optional.
However, it must always
include the formulas of the
reactants and products,
and an arrow to show the
direction.

Different substances (also
called species) are
separated by “+” signs.

Chemical Equations

Chemical Equations

Chemical Equations must follow the law of conservation of
matter. In other words, the amount of material you begin
with must match the amount of material you end with.

This can be measured as individual atoms, moles of elements,
or mass in grams. Consider the following equation:

H2 + O2→ H2O

Coefficients and subscripts can stand for molecules and atoms AND
they can represent moles of those substances.

Chemical Equations and Conservation of
Mass

H2
O2

H2O

Chemical Equations

Chemical Equations must follow the law of conservation of
matter. In other words, the amount of material you begin
with must match the amount of material you end with.

This can be measured as individual atoms, moles of elements,
or mass in grams. Consider the following equation:

H2 + O2→ H2O

Coefficients and subscripts can stand for molecules and atoms AND
they can represent moles of those substances.

Chemical Equations and Conservation of Mass

H2
O2

H2O

Chemical Equations and Conservation of Mass

H2
O2

H2O

grams

grams

grams

Chemical Equations

Coefficients are placed in an equation to make it follow the law of
conservation of matter by “balancing” the reactants and products.

NEVER CHANGE SUBSCRIPTS

Remember, a coefficient applies to the entire species it is in front of.

Reactant atoms must equal product atoms.

If a polyatomic ion shows up on both sides, treat it like a separate element.

R

P

Al

O

R

P

Multiple Choice

What are the smaller numbers that you CANNOT change in a chemical equation? 

• Coefficient

• Products

• Reactants

• Subscript

Multiple Choice

What are the larger numbers that you CAN CHANGE in a chemical equation? 

• Coefficient 

• Products

• Reactants

• Subscripts

Multiple Choice

What law governs the balancing of chemical equations? 

• Law of Energy

• Law of Conservation of Matter/Mass

• Law of Gravity

• Law of Matter Movement 

Multiple Choice

How many HCl molecules do you need to balance this equation? 
Mg +  __HCl --->  MgCl₂ + H₂

• 1

• 2

• 3

• 4

Multiple Choice

Which number should go in the blank?
Al(NO₃)₃ + _  NaOH -> Al(OH)₃ + 3 NaNO₃

• 1

• 2

• 3

• 0

Multiple Choice

If 100 grams of vinegar and 5 grams of baking soda are poured in a container, a small amount of gas will be produced. What will the final mass of the products be if the gas is trapped in the container?

• 100 g

• 104 g

• 105 g

• 110 g