Download Conserving Matter - Hobbs High School

Conserving Matter
Keeping Track of Atoms
Law of Conservation of Matter
• In a chemical reaction, substances are
changed to new forms, not created or
destroyed.
• The Law of Conservation of Matter has
been determined by careful observation:
In a chemical reaction, matter is
neither created nor destroyed.
Balanced Chemical Equations
• Remember that we learned to take ATOM
INVENTORIES of both the reactants and
the products in a chemical reaction.
• Because of the Law of Conservation of
Matter, we learned that the number of
atoms of an element in the reactants is
equal to the number of atoms of the same
element in the products.
• In a balanced chemical reaction, there are
EQUAL numbers of each kind of atoms on
BOTH sides of the equation.
Balancing Equations
• Is this equation balanced?
C3H8 + O2 → CO2 + H2O
Take an atom inventory:
Reactant side
Product side
___ carbon atoms
___ hydrogen atoms
___ oxygen atoms
___ carbon atoms
___ hydrogen atoms
___ oxygen atoms
• Is this equation balanced?
4Ag + 4 H2S + O2 → 2 Ag2S + 4H2O
Take an atom inventory:
Reactant side
Product side
___ silver atoms
___ hydrogen atoms
___ sulfur atoms
___ oxygen atoms
___ silver atoms
___ hydrogen atoms
___ sulfur atoms
___ oxygen atoms
What is a mole?
• Well, yes, but
we’re not
discussing biology
or dermatology
now.
• We want the
CHEMIST’S mole.
So far we have discussed chemical
equations in terms of atoms and
molecules.
2 CuO(s) + C(s) → 2 Cu(s) + CO2(g)
But for a metal-processing plant making
copper metal, they need to know how
much carbon is needed to react with a
large amount of copper (II) oxide ore.
The Mole
So chemists devised a counting unit called
the MOLE. You use counting units all the
time—dozen, pair—to mean a specific
number.
A mole is 6.02 x 1023 units of a substance.
That’s 602 000 000 000 000 000 000 000
water molecules or
602 000 000 000 000 000 000 000 atoms of
copper.
This number is commonly known as
Avogadro’s number.
Molar Mass
• The MOLAR MASS of a substance is the
mass of one mole (6.02 x 1023 units) of
that substance.
• The mass in grams of one mole of a
substance is equal to the atomic weight of
that substance.
What is the molar mass of carbon?
What is the atomic weight of carbon?
So, the molar mass is 12.01 grams.
1 mol C = 12.01 g C
Molar Masses of Compounds
• What is the molar mass of O2?
What is the atomic weight of oxygen?
16.00 g.
The subscript tells us we have two
moles of oxygen, so the molar mass is
2 mol O x 16.00 g O = 32.00 g O
1 mol O
so 1 mol O2 = 32.00 g O2
What is the molar mass of carbon dioxide?
1 mol C = 12.01 g C
1 mol O = 16.00 g O
1 mol C x 12.01 g C = 12.01 g C
1 mol C
2 mol O x 16.00 g O = 32.00 g O
1 mol O
Molar mass of CO2
= (12.01 g + 32.00g) = 44.01 g CO2