Chemical Reactions & the
Law of Conservation of
• Chemical bonds are the ‘glue’ that holds the atoms
of elements together in compounds.
• Chemical bonds form when the electrons in the
electron cloud around the atoms interact.
Specifically, it is the ‘valence electrons’ of atoms that
• We learned about two types of chemical bonds –
Ionic and Covalent bonds.
• A chemical reaction produces
new substances by changing the
way in which atoms are arranged.
• In a chemical reaction, bonds
between atoms are broken and
new bonds form between different
atoms. Bonds broken new
Reactants and Products
• Reactants are the substances present at the beginning of a chemical reaction.
• Products are the substances formed by a chemical reaction.
• So chemical reactions turn reactants into products by rearranging atoms.
• Reactants and products can be elements or compounds depending on the reaction taking
• Example – the burning of natural gas
• Methane (CH4) and oxygen (O2) are the reactants. Carbon dioxide (CO2) and water (H2O)
are the products formed by the reaction.
• Reactants – bonds broken Products- new bonds formed
CH4 + O2 CO2 + H2O
Does matter disappear in a chemical
• Atoms are not changed by chemical reactions, but
merely rearranged into different compounds.
• Atoms are neither created nor destroyed by chemical
• So if atoms are neither created nor destroyed, matter
is neither created nor destroyed; it merely changes
• At one time scientists thought that chemical reactions
could create or destroy matter.
Law of Conservation of Mass
• In the late 18th century, Antoine Lavoisier proved that matter
can never be created or destroyed in a chemical reaction.
• His research and conclusion is called the law of conservation
• It states – ‘in a chemical reaction atoms are neither created
or destroyed. All atoms present in the reactants are also
present in the products.
Chemical Reactions can be described
by chemical equations
• The law of conservation of mass states that in a chemical reaction the total mass of
reactants is equal to the total mass of products.
• For example, the mass of Na plus the mass of Cl that reacts equals the mass of the
product NaCl (sodium chloride).
• A chemical equation is the ‘shorthand’ way to represent how atoms are rearranged in
a chemical reaction.
• The atoms in the reactants are shown on the left side of the equation and the atoms in
the products are shown on the right side of the equation.
• Because atoms are rearranged and not created or destroyed, the number of atoms of
each different element must be the same on each side of the equation.
Types of Chemical Reactions
• Synthesis – a chemical reaction when two or more simple substances
combine to form a new complex substance.
Basic form : A + X AX
Example: Na +Cl2 NaCl
• Decomposition – a chemical reaction when a complex substance breaks
down into two or more simpler substance.
Basic form: AX A + X
Example: Al2O3 Al + O2
• Single Replacement – a chemical reaction when a single uncombined
element replaces an element that is part of a compound.
Basic form: A + BX AX + B or AX + Y AY + X
Ex: Zn + CuCl2 ZnCl2 + Cu HgO + Cl2 HgCl+O2
• Double Replacement – a chemical reaction when different atoms in two
different compounds replace each other.
Basic form: AX + BY AY + BX
Example: MgCO3 + HCl MgCl2 +H2CO3
Chemical Equations must be
Chemical Equations must be balanced
• The same number of atoms of each element must appear on
both sides of a chemical equation.
• However, simply writing down the chemical formulas of reactants
and products does not always result in equal number of atoms.
• Therefore, you have to balance the equation to make sure the
number of atoms are equal on each side of an equation - the law
of conservation of mass.
• Also, equations must be balanced in order to determine ‘how
much’ of each compound is required.
Usefulness of Conservation of Mass
and balancing equations
• A compound called sodium azide (NaN3).
• It is a solid and takes up a very small amount of space -
130 grams( the size of a large tablespoon).
• The reason can be illustrated by the balanced chemical
reaction for this reaction.
2NaN3 2Na + 3N2 (a decomp. Reaction)
• According to this balanced equation, three
molecules of N gas are formed for every two
molecules of sodium azide that reacts.
• In fact, 67 liters of N gas are produced for every 130
grams of sodium azide in the reaction.
• This is the compound used for air bags in
2 H2 + O2 2 H2O
Rules for Balancing Equations
• Never add or change subscripts. You can only add coefficients to balance an equation.
• Only add coefficients in front of the chemical formula for the compound ( they indicate
how many molecules take part in the reaction). Make sure all coefficients are reduced to
the lowest common denominator.
• Chemical formulas can have both coefficients and subscripts. In these cases, multiply the
two numbers together to find the total number of atoms involved in the reaction.
• For example, 3O2 means there are 6 atoms of oxygen; i.e. 3 molecules containing
2 atoms each.
• Never ‘break’ a chemical compound (chemical bond). i.e., never put a coefficient or
subscript within a compound – for example, if NaCl is the compound you cannot do
this Na3Cl or this Na3Cl
• Never add reactants and never add products.
• Your goal is to make sure the number of atoms for each element are equal on both
sides of an equation.
Example of a Balanced Equation
2HgO 2Hg + O2
Hg =2 Hg=2
O =2 O =2