Notes

1
Notes Chemical Equations

• Chapter 8, Lesson 2

2
Conservation of Mass

• When the end of the last period bell rang,
  everyone moved to a new room, and ended up in
  different groups in different rooms. The number
  of students IN the whole school did not change,
  only their arrangement.
• The amount of matter in a chemical reaction does
  not change, so the total mass at the start MUST
  equal the total mass at the end.

3
Conservation of Mass

• Matter is ALWAYS conserved in chemical reactions.
• Chemical equations show that in chemical
  reactions, atoms rearrange, but no atoms are
  gained or lost.

4
Conservation of Mass

• Antoine Lavoisier invented a balance to make more
  precise measurements.
• He also determined that the mass before and after
  a chemical reaction was the same.

Mass of elements BEFORE
Mass of elements AFTER
5
Conservation of Mass

• The law of conservation of mass states that the
  total mass before a chemical reaction is equal to
  the total mass after the reaction.

6
Chemical Equations

• Reactants are the starting materials in a
  chemical reaction and are placed on the LEFT
  side.
• Products are the ending materials in a chemical
  reaction and are placed on the RIGHT side.

Reactant Reactant ? Product
H2 O2 ? H2O2
7
Chemical Equations

• Tin oxygen ? tin oxide reads as tin plus
  oxygen produces tin oxide.
• Word equations can be long and do not show that
  mass is conserved.

8
Chemical Equations

• Instead of writing long word equations,
  scientists use symbols and formulas.
• Symbols represent atoms.
• Formulas represent molecules.
• Molecules may be elements or compounds.

9
Chemical Equations

• A molecule is the combination of 2 or more atoms.
• Some molecules have atoms that are alike, called
  diatomic (O2).
• Most molecules are made of more than one type of
  atom (H2O) a compound.

10
Chemical Equations
11
Chemical Equations

• Chemical equations use the symbols and formulas
  instead of words.

12
Balancing a Chemical Equation

• Atoms are neither gained nor lost in a reaction,
  so both sides of the equation must have the same
  number of atoms.

13
Balancing a Chemical Equation

• A subscript tells how many atoms of an element
  are in one molecule.
• A coefficient tells how many atoms, molecules, or
  formula units are in a reaction.

subscript
2H2O
coefficient
14
Balancing a Chemical Equation

• (i.e.) Oxygen reacts with hydrogen to form
  water.
• H2 O2 ? H2O

Reactants Reactants Products Products
H O H O
2 2 2 1

• This is unbalanced, and we CANNOT change the
  subscripts!
• We add coefficients, a number in FRONT of a
  chemical formula that tells you how many
  molecules or atoms of each reactant product act
  in the reaction. We do not write 1.

15

• We add a coefficient of 2 in front of the
  product
• H2 O2 ? 2 H2O

Reactants Reactants Products Products
H O H O
2 2 4 2
16

• The oxygen atoms are now balanced, but the
  hydrogens are not anymore. Add a coefficient of
  2 in front of the H2
• 2 H2 O2 ? 2 H2O

Reactants Reactants Products Products
H O H O
4 2 4 2
17
Balancing a Chemical Equation
18
Writing Balanced Equations

• Step 1 Determine the correct symbols and formulas
  for reactants and products.
• Step 2 Write reactant symbols and formulas to the
  left of an arrow and product symbols and formulas
  to the right.
• Step 3 Count the number of each kind of atom on
  both sides.
• Step 4 Use coefficients to make the number of
  each kind of atom the same on both sides of the
  arrow.
• Step 5 Check to see that each kind of atom
  balances.

19
Writing Balanced Equations

• Reaction of methane

CH4 O2 ? CO2 H2O balance hydrogen CH4 O2 ?
CO2 2H2O balance oxygen CH4 2O2 ? CO2 2H2O
balanced equation
20
Writing Balanced Equations

• Baking soda and vinegar

NaHCO3 HC2H3O2 ? CO2 H2O NaC2H3O2
21
Writing Balanced Equations

• When counting atoms inside parentheses, the
  subscript multiplies all atoms inside.

Ca H2O ? Ca(OH)2 H2
Ca 2H2O ? Ca(OH)2 H2
22
8.2 Chemical Equations
Which is a diatomic molecule? A O2 B H2O
C CH4 D N
23
8.2 Chemical Equations
What is potassium bromide (KBr)? A an element B a
mixture C a compound D a diatomic molecule
24
8.2 Chemical Equations
In the following equation, which is a
reactant? Ca 2H2O ? Ca(OH)2 H2
A H2 B (OH)2 C Ca D none of the above
25
What is the coefficient of H2O? Ca 2H2O ?
Ca(OH)2 H2 A 1 B 0 C 4 D 2
26
How many (OH) molecules are in the products? Ca
2H2O ? Ca(OH)2 H2 A 1 B 2 C 3 D 4
27
In a chemical reaction, which law states the
products have the same mass as the
reactants? A Law of Conservation of Water B Law
of Conservation of Energy C Law of Conservation
of Mass D Law of Conservation of Particles
28
If an equation is balanced, the number of atoms
in the products is ____ the number of atoms in
the reactants. A greater than B less
than C equal to D none of the above
29
Symbols represent ____ and ____ represent
molecules. A elements formulas B formulas
equations C molecules formulas D elements
equations