Balancing Chemical Equations

Balancing Chemical

Equations

Chemical Equations

• A useful way of describing a change is to

state what is present before and after the
change.

• Suppose you went home, and mixed the

following items:

• Flour, Baking Powder, Eggs, Sugar, Salt,

Milk. What do you get?:

Multiple Choice

If you mix flour, baking powder, eggs, sugar, salt, & milk together what should you get?

• soup

• cake

• pizza

• T-Bone Steak

• So, a useful description of a chemical

reaction tells you the substances present
before and after the reaction.

Chemical Reaction

• A chemical reaction can be understood in

two ways:

• (1) We know a chemical reaction has

occurred when the physical and chemical
properties of the matter have changed.

• And

• (2) We know a chemical reaction has

occurred because the atoms have
"rearranged" themselves.

• Both of these perspectives are important

to keep in mind whenever dealing with
chemical reactions.

Multiple Select

What are the ways we know that a chemical reaction has taken place?

• when the physical and chemical properties of the matter have changed.

• because the atoms have "rearranged" themselves.

• because the substance has changed colors.

• because the substance has become hot.

Reactant🡪Products

• The substances that must come in contact

before a chemical reaction can occur are
known as "Reactants" and the substances
that are formed from the reaction are
known as "Products". The chemist
denotes a chemical reaction with a
reaction arrow

• “ 🡪” This means YEILDSor =

Multiple Select

List the reactants in the following equation.

H + O --> OH

• H

• O

• OH

Wood+Oxygen---->Water+Carbon dioxide+Ash

• Generally, the reactants and products are

compounds or elements but more
descriptive equations are possible e.g.,

Mass Conservation

• Dalton stated in his atomic theory that

atoms could neither be created or
destroyed.

• From a chemical perspective all mass is

contained by atoms.

• Therefore, if atoms cannot be created

or destroyed then neither can mass.

• This is known as the principle of

"Conservation of mass".

• With this principle, important information

about a chemical reaction can be
obtained.

Multiple Choice

What does the Conservation of mass state?

• mass can neither be created or destroyed

• atoms can neither be created or destroyed.

• energy can neither be created or destroyed.

• your grade can neither be created or destroyed.

Wood +Oxygen---->Water+Carbondioxide+Ash

• According to the law of conservation of

mass, a chemical equation must be
balanced

• This means the total number of atoms on

the reactant side must be equal to the total
number of atoms on the product side.

•
• What is the product of this equation?

Multiple Select

What are the product of the following equation?

Wood +Oxygen---->Water +Carbon dioxide +Ash

• water

• wood

• oxygen

• carbon dioxide

• ash

• A chemical reaction must obey the

overriding principle of conservation of
mass. It does so through a balancing of
the reactants and products.

• consider the combustion of magnesium

metal:

• We carefully measure 10g of the metal,

set it on fire in the presence of air ( oxygen in the air) combust it, and then carefully weight the ash.

• We find that the ash weighs 16.6g.

• Using the principle of conservation of

mass we conclude that 16.6g - 10g =

• 6.6g of oxygen reacted with the

magnesium.

• A similar calculation can be performed on

all chemical reactions.

Balancing Equations

• In order to show that mass is

conserved during a reaction, a
chemical equation must be balanced.

• N2H4 + O2 -----> N2 + 2H2O

• You can balance a chemical equation by

changing the Coefficients.

Coefficient

• Is a number that appears before a formula

in a chemical equation to show the relative
proportions of each reactant and product.

• Such as: 2H or 5NO
• The 2 and 5 shows how many there is of

the entire compound.

• How many Nitrogen's are there in 5NO?

Multiple Choice

How many Nitrogens are there in 5NO?

• 1

• 3

• 5

• 0

Multiple Choice

How many Oxygens are there in 5NO?

• 1

• 3

• 5

• 0

Subscript

• A subscript on the other hand tell how

many of that element is present.

• Such as: MgCl2

• The 2 tells you that there is 2 Cl ions.
• How many Magnesium ions are there?

Multiple Choice

How many Magnesium ions are there in MgCl₂?

• 1

• 2

• 3

• 4

Multiple Choice

How many Chlorine ions are there in MgCl₂?

• 1

• 2

• 3

• 4

Multiple Choice

In the formula 5H_2,

What number represents the Coefficient?

• 1

• 2

• 5

• 0

Multiple Choice

In the formula 5H_2,

What number represents the Subscript?

• 1

• 2

• 5

• 0

Multiple Choice

In the formula 5H_2,

What element is represented by the letter "H"??

• Helium

• Hydrogen

• Hydroxide

• hyposodium

Multiple Choice

In the formula 5H_2,

How many "H's" do you have?

• 1

• 2

• 5

• 0

• Ethylene, C2H4, burns in the presence of

oxygen to produce carbon dioxide and
water vapor.

"Write a Skeletal equation for this reaction."

A Skeletal equation is an unbalanced equation, just the compounds in the reactant and products format.

Lets Balance the following chemical

equations together.

• H2O2 -----> H2O + O2

• Mg + HCl -----> H2 + MgCl2

Counting with MOLES

• Because chemical reactions often

involve large numbers of small particles,
chemists use a counting unit called the
mole to measure amounts of a
substance.

• A mole (mol) is an amount of a

substance that contains approximately
6.02 × 1023 particles of that substance.

Molar Mass

• A dozen eggs has a different mass than a

dozen oranges.

• The mass of one mole of a substance is

called a molar mass.

• A mole of carbon has a different mass

than a mole of sulfur,

• For a compound, you can calculate the

molar mass by adding up the atomic
masses of its component atoms, and then
expressing this sum in grams.

• A carbon dioxide molecule is composed of

one carbon atom (___.0 amu)

• and two oxygen atoms (2 × ____.0 amu =
• ___.0 amu).
• So carbon dioxide has a molar mass of

___.0 grams.

Mole-Mass Conversions

• Lets say that you have a block of CO2 that

is 95g. How many moles of CO2 is that?

• The molar mass of CO2 is 44.0g
• Use a conversion factor
• 44.0g CO2 1 Mole CO2
1 Mole CO2 44.0g CO2

• 95g CO2 × 1 Mole CO2 ═ 2.16 Moles of CO 2
44.0g CO2

• 2.16 Moles of CO 2 × 44.0g CO2═95g CO2

1 Mole CO2

Chemical Calculations