3.3 Chemical Equations and the Law of Conservation of Mass
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Loud explosions, flashes of colour, puffs of smoke—these are all results you might expect from chemical
reactions.
Many chemical reactions are explosive, colourful, and smoky.
But many more take place in the world around you without your even noticing them.
However, whether they are obvious or not, chemical reactions are vital to maintaining and sustaining
your health and the health of all life on Earth.
Chemical Reactions
• Chemical changes involve breaking old
chemical bonds and forming new chemical
bonds.
• During a chemical reaction, the atoms are
neither created nor destroyed.
• All that really happens to the atoms is that they are rearranged.
Question:
Look at Table 3.6 and try to identify
the products and reactants for each
chemical reaction.
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Chemical Reactions
• The old substances whose bonds are broken are called reactants.
• The new substances that are formed as a result of the reaction are called products (products are
produced).
• 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
• The products are listed on the right side.
Chemical Equations Have Four Parts:
1. chemical formulas
2. subscripts for 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".
Question:
Translate the following equation into a sentence.
CaCl2(aq) + Na2SO4(aq) → CaSO4(aq) + 2NaCl(aq)
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Word Equations
• A chemical reaction can be expressed in a word equation.
• The reactants are on the left side of the arrow, and the products are on the right side of the
arrow.
Question:
When do you use a plus sign in a word equation?
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Conservation of mass
• The work of Antoine Lavoisier is hailed as a turning point in the history of chemistry.
• One example of his work was his study of a red powder called cinnabar, which is now known as
mercury (II) oxide.
HgO(s) → 2Hg(l) + O2(g)
45.05 g
41.72 g 3.32 g
His Experiment
• He placed mercury (II) oxide powder in a test tube, sealed it, and then weighed it carefully.
• Then he heated it for 12 days and observed that the red powder gradually changed into a grey
liquid.
• He reweighed the sealed tube after the reaction was complete and observed that its mass had not
changed.
• When he was finished with his mass measurements, he
opened the tube and noticed a rapid release of a gas which
was later learned to be oxygen. The grey liquid was mercury
metal.
• From this we derive, “The Law of Conservation of Mass”.
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.
Balancing Chemical Equations
• The law of conservation of mass states that during a chemical change or reaction, the mass of the
reactants is equal to the mass of the products.
• 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.
Example 1:
• Consider this equation.
• If a chemical equation is not balanced, then
coefficients have to be placed in front of the chemical formulas in order to balance the equation.
Example 2:
• Consider the equation for the decomposition
of mercury(II) oxide.
• What do we do?
Tips for Writing and Balancing Chemical
Equations
• A systematic approach should be used to write
and balance chemical equations.
Tips for Writing and Balancing Chemical Equations
1. Balance equations by adjusting coefficients, never by changing chemical formulas.
2. Balance metals first.
3. Add coefficients to any elements last.
4. Balance hydrogen and oxygen last. They often appear in more than one reactant or more than
one product, so it is easier to balance them after the other elements are balanced.
5. If a polyatomic ion appears in both a reactant and a product, treat it as a single unit.
6. Once you think the chemical equation is balanced, do a final check by counting the atoms of
each element one more time.
7. If you go back and forth between two substances, using higher and higher coefficients, doublecheck each chemical formula. An error in a chemical formula might be preventing you from
balancing the chemical equation.
8. The following elements exist as diatomic molecules: hydrogen, H2(g); nitrogen, N2(g); oxygen,
O2(g); fluorine, F2(g); chlorine, Cl2(g); bromine, Br2 (ℓ); and iodine, I2(s). Always
write them as shown here in a chemical equation.
A diatomic molecule contains two atoms of
the same element.
Section 3.3 Review
• In a chemical reaction, a reactant or reactants undergo a chemical change, forming a different
substance or substances called products. Reactants and products may be elements or
chemical compounds.
• According to the law of conservation of mass, the overall mass of the reactants is always equal
to the overall mass of the products in a chemical reaction.
• Chemists use descriptions, word equations, and balanced chemical equations to represent
chemical reactions.
• In a balanced chemical equation, there is always the same number of atoms of each element
on each side of the equation, in accordance with the law of conservation of mass.