Section 6.1: Chemical Equations
Objectives: Relate chemical changes and
macroscopic properties, demonstrate how
chemical equations describe chemical
reactions, Illustrate how to balance chemical
reactions by changing coefficients
Identify the changes that take place when a
cake is made:


When a substance undergoes a chemical
change
After a chemical reacts, it no longer has the
same chemical identity


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A precipitate (solid) forms
Heat is produced or absorbed
Odor changes
Gas is given off
Color change and bubbles given off indicates
a good chance a chemical change has
occurred, but they are not proof positive.
growth of a tree
melting butter
the use of food in the body
CO2 escaping from a can of soda
separation of the components of crude oil through
distillation
◦ a lake freezing
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Chemical Reaction (RXN): a process by which
one or more substances are changed into one
or more different substances; represented by
an equation.
Chemical Equation (EQN): shows what change
takes place, and shows the relative amounts
of various elements/compounds that take
part in these changes.
REACTANT: Substance that undergoes a
reaction (change)
PRODUCT: New substance formed
Example:
Iron and oxygen (reactants) → iron (III) oxide
(Fe3+)
(O2)
(Fe2O3)
Reactions can involve single/ or many
reactants or products
Reactants are placed to the left of an arrow and
products are placed to the right
Plus signs are used to separate reactants and also
to separate products
Example:
hydrochloric acid + sodium hydroxide → water + sodium chloride
(reactants)
(products)
Word equations can be converted into chemical
equations
Chemical formulas replace the names of
compounds or elements
Symbols in parentheses are put after formulas
to indicate the state of the substance
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous
Example:
HCl(l) + NaOH(s) → H2O(l) + NaCl(aq)
Energy is often released or absorbed during
chemical reactions


If energy is absorbed → endothermic reaction
If energy is released → exothermic reaction
For reactions that absorb (need) energy, the
word energy is often written along with the
reactants in a chemical equation
Example: 2H2O(l) + energy → 2H2(g) + O2(g)
For reactions where energy is released, the
word energy is often written along with the
products in a chemical equation
Example:
CH4(g) + 2O2(g) → CO2(g) + 2H20(g) + energy
Note: The word energy is not always written. If
it is important to know if energy is absorbed
or released or in some cases where the
reaction would not occur without the addition
of energy
The same amount of matter is contained in the
products and reactants (matter is neither
created nor destroyed)
Mass of reactants = Mass or products
In chemical reactions, atoms do NOT change,
they rearrange!
The number and kinds of atoms present in the
reactants of the chemical reaction are the
same as those present in the products
Counting atoms: The subscript after the
symbol for an element represents how many
atoms of that element are present
For a chemical equation to accurately represent
a reaction, the same number of each kind of
atom must be on the left side of the arrow as
are on the right side → It is said to be
balanced
To represent more than one unit taking part or
being formed in a reaction, use a
COEFFICIENT: a number is placed in front to
indicate the number of units involved
To balance equations, coefficients can be
added or changed on the reactants and
products
Subscripts CAN’T be changed – changing
subscripts changes the identity of that subs
Analyze
2. Set Up
3. Solve
4. Check
1.
1. Magnesium metal and water combine to form
solid magnesium hydroxide and hydrogen gas
Analyze: (word equation)
magnesium + water → magnesium hydroxide + hydrogen
Set up: (chemical equation)
Mg(s) + H2O(l) → Mg(OH)2(s) + H2(g)
Solve: (count atoms and then balance)
Left: Mg = 1, H = 2, O = 1
Right: Mg = 1, H= 4, O = 2
Mg(s) + 2H2O(l) → Mg(OH)2(s) + H2(g)
Check: (count atoms)
Left: Mg = 1, H = 4, O = 2
Right: Mg = 1, H= 4, O = 2
The equation is properly balanced.
Hydrogen and oxygen combine to form
gaseous water and release energy.
 Word equation:
Hydrogen + oxygen -> water + energy

Chemical equation:
H2(g) + O2(g) -> H2O (g) + energy
Balance:
2H2(g) + O2(g) -> 2H2O (g) + energy
P. 199 (#1,3)
Write word equations and chemical equations
 1) Magnesium and water combine to form
solid magnesium hydroxide and hydrogen
gas.

3) When energy is added to solid
manganese(II) sulfate heptahydrate crystals,
they break down to form liquid water and
solid manganese(II) sulfate monohydrate
P. 199 (#2,4)
Write word equations and chemical equations
2) An aq. Solution of dihydrogen dioxide and
solid lead (II) sulfide combine to form solid
lead (II) sulfate and liquid water.
4) Solid potassium reacts with liquid water to
produce aq. Potassium hydroxide and
hydrogen gas