Download Reactions and Equations

Reactions and Equations
Learning Objectives
• Accurately represent chemical reactions with
equations.
• Balance chemical equations.
Review
• What is a chemical change?
• How do you know that a chemical change has
taken place?
Review
• Remember that in a
chemical change, a
substance undergoes a
change in chemical
composition.
• A chemical change
always involves the
conversion of a material
into one or more new
substances.
Review
• Signs of a chemical
change
– Temperature change
– Release of energy in the
form of light or sound
– Gas formation
– Precipitate formation
– Color change
– Odor change
Review
Write formulas for the following:
1. Barium nitrate
2. Sulfurous acid
3. Copper (II) sulfide
4. Nitrogon trioxide
Chemical Reactions
• A chemical reaction is a process by which the
atoms of one or more substances are
rearranged to form different substances.
Chemical Equations
• A chemical equation is a statement that uses
chemical formulas to show the identities and
relative amounts of the substances involved in
a chemical reaction.
Chemical Reaction
• Chemical equations are used to represent chemical
reactions.
– Reactants are the starting substances
– Products are the substances that are formed
Chemical Reactions
• A subscripted number tells
you how many atoms of
an element there are
directly to the left of the
number.
– Ex: there are two atoms of
hydrogen in a water
molecule
Chemical Reactions
• A coefficient is the number
written in front of a
reactant or product.
– Whole numbers
– Not written if one
• A coefficient tells you how
many of that atom or
molecule there is. The
coefficient applies to all
atoms in the molecule.
– Ex: There are two water
molecules
– Ex: There are 4 hydrogen
atoms and 2 oxygen atoms
3
Chemical Reactions
• In equations, symbols
are used to show the
physical states of the
reactants and products.
–
–
–
–
Solid (s)
Liquid (l)
Gas (g)
Aqueous (aq)
• Aqueous means dissolved
in water.
Learning Check
Identify each of the
following:
• A
• B
• C
• D
• E
• F
Learning Check
• How many sodium
oxide molecules are
there?
• How many sodium
atoms are there on the
left side of the
equation?
• How many oxygen
atoms are there in the
water molecules on the
left side of the
equation?
Learning Check
• Write a chemical equation for the reaction in
which hydrogen gas reacts with oxygen to
yield water (remember that both hydrogen
and oxygen are diatomic molecules).
Balancing Chemical Equations
• The law of conservation of mass states that mass is
neither created nor destroyed during a chemical
reaction—it is conserved.
• In other words, the mass of the reactants equals the
mass of the products.
Chemical Equations
• To accurately represent
a chemical reaction by
an equation, the
equation must show
equal numbers of
atoms of each reactant
and each product on
both sides of the arrow.
• Such an equation is
called a balanced
chemical equation.
Balancing Chemical Equations
• To balance an equation, you must find the
correct coefficient for the chemical formula.
Step 1
Write the skeletal equation for the reaction.
Step 2
Count the atoms of the elements in the
reactants.
Step 3
Count the atoms of the elements in the
products.
Step 4
1. Change the coefficients to make the number
of atoms of each element equal on both
sides. DO NOT CHANGE COEFFICIENTS! Write
coefficients in lowest possible ratio.
Step 5
Check your work.
Diatomic Molecules
When writing out chemical equations, it is important to
know which elements form diatomic molecules.
There are seven elements that form diatomic molecules.
The seven diatomic elements are:
Hydrogen (H2)
Nitrogen (N2)
Oxygen (O2)
Fluorine (F2)
Bromine (Br2)
Iodine (I2)
Chlorine (Cl2)
How to Remember the Diatomic Elements
• The elements ending with "-gen" including halogens
form diatomic molecules. An easy-to-remember
mnemonic for the diatomic elements
is: Have No Fear Of Ice Cold Beverages
Example
Write a balanced chemical equation for the reaction
of carbon monoxide gas and oxygen gas that produces
carbon dioxide gas.
Example
Write the balanced chemical equation for the reaction
in which aqueous sodium hydroxide and aqueous
calcium bromide react to produce solid calcium
hydroxide and aqueous sodium bromide.
Practice
Write the balanced chemical equation for the reaction
in which, in water, iron (III) chloride reacts with
sodium hydroxide, producing solid iron (III) hydroxide
and sodium chloride.
Practice
Write the balanced chemical equation for the reaction
in which liquid carbon disulfide reacts with oxygen
gas, producing carbon dioxide gas and sulfur dioxide
gas.