Science 1206
Conservation of mass
&
Balancing equations
Chemical Changes
• A Chemical change
occurs when matter
changes into a new and
different form.
• For instance when an
electric current is passed
though liquid water, the
water changes into two
gasses which are highly
flammable; hydrogen and
oxygen.
Hindenburg Disaster
• During a chemical reaction, the atoms are
neither created nor destroyed. All that really
happens to the atoms is that they are rearranged.
Water
4 H’s and 2 O’s
Hydrogen
4 H’s
Oxygen
2 O’s
• The old substances whose bonds are broken are called reactants
Reactants
Products
• The new substances that are formed as a result of the reaction are
called products. (products are produced)
Energy Changes in Chemical
Reactions
1. ENERGY change
• the absorption or the release of
heat and/or light, sound,
electrical energy.
• exothermic reactions release
energy (exo = out)
• endothermic reactions
absorb heat (endo = in)
Reactions: Heat = energy
The brilliant white light
associated with some
fireworks is due to the
release of energy when
magnesium reacts with
oxygen.
Equations
Chemical Equations
Equations are a way of representing
chemical changes/reactions
Chemical changes can be communicated in sentence form
or as chemical equations.
– Word equations – use of words to DESCRIBE what happens
– Chemical (skeleton) equations – use to symbols to SHOW what happens
Chemical changes can be communicated in sentence
form or as chemical equations.
• In a chemical equation, the reactants are listed on the left
side of an arrow symbol, and the products are listed on
the right side.
A chemical Equation
Chemical equations have four parts:
1. Chemical formulas
2. Subscripts for the States of matter
•
•
•
•
(s) solid
(l) liquid
(g) gas
(aq) aqueous - dissolved in water
3. Numerical coefficients
• indicates how many atoms/molecules are involved
4. Reaction symbols
• the "+" sign on the reactants (left) side is read as "reacts with"
• the arrow (  ) is read as "to produce"
• the "+" sign on the products (right) side is read as "along with".
Parts of a Chemical Equation/Reaction
Numerical Coefficients
Subscripts
Equation Practice
Given the equation:
6CO2( g )  6 H 2O(l )  1C6 H12O6( s )  6O2( g )
Pick out
a. A subscript ______________
b. The symbol meaning “to produce” ______
c. A chemical formula ____________
d. The symbol meaning “combines with” _______
e. A coefficient ______________
f. A reactant _______________
g. The symbol meaning “along with” _______
Complete the following:
• Chemistry Worksheet 2-1
Chemical Equations can be translated into a sentence
Words to use for formulas when writing chemical equations as sentences
Formula
Ionic
Molecular
Metal Element
Nonmetal Element
Word
Formula Unit
Molecule
Atom
Molecule
Let’s Try an Example
• Two molecules of diesel fuel reacts with 49 molecules of
oxygen to produce 32 molecules of carbon dioxide along
with 34 molecules of water.
Writing equations as sentences - Practice
The Law of Conservation of Mass
• In a chemical reaction the mass of the reactants before a chemical
reaction equals the mass of the products after the reaction is complete.
mass of reactants = mass products
• In other words, the mass you end up with equals the mass you started
with.
Ex: Burning Paper is a reaction that obeys the Law of Conservation of Mass
+
20 grams
Oxygen

+
20 grams
Balancing Equations
Any representation of a chemical equation must reflect the
Law of Conservation of Mass
• In order to write a chemical equation that obeys the law of
conservation of mass, the number of atoms of each element
must be the same on both sides of the equation.
the left hand Side = the right hand side
• Ex:
Balancing Equations - Practice
Let’s have a look at this equation and try to balance it.
Note: To balance equations we write Coefficients in front of each
formula.
___ HgO (s)  ___ Hg (l) + ___ O2 (g)
Rules for Balancing Equations
1.
When you insert a coefficient, each subscript in the formula is
Multiplied by the coefficient.

2.
Ex:
2 x 2( H )  4 H ' s
2H 2O =
2 x1(O)  2O ' s
The number of atoms of each element MUST be the same on
the left and right sides.
3.
Practice, Practice, Practice
Let’s Try another
___ Fe (s) + ___ O2 (g)  ___ Fe2O3 (s)
• Write a balanced chemical equation for the reaction of
cobalt with oxygen to produce cobalt (III) oxide
1. ___Mg(s) + ___O2(g) → ___MgO(s)
2. ___Cu(s) + ___AgNO3(aq) → ___Ag(s) + ___Cu(NO3)2 (aq)
3. ___KI(aq) + ___Pb(OH)2 (aq) → ___PbI2(s) + ___KOH (aq)
4. ___P4 (s) + ___F2 (g) → ___PF3 (l)
5. ___Cr2O3 (s) → ___Cr(s) + ___O2 (g)
Practice
• Complete the following activities
– Chemistry Worksheet 2-2