Download COUNTING ATOMS

Chemical
Reactions
Chemical Reactions
• Combination of two or more elements
bonded together to make a new substance.
• Example:
– Na + Cl = NaCl
Real-Life Examples
Evidence of CR
• Production of gas
• Change of color
• Change in energy
(heat/light)
• Odor (gas formed/can’t see)
• Precipitate is formed
– Substance forms on top of a
solution.
Examples of CR
• Fire/explosions
• Fireworks
• Making salt
• Stomach digesting food
CHEMICAL
EQUATIONS
CHEMICAL EQUATIONS
• A Chemical Equation is a representation of
a chemical reaction expressed as a formula.
• An Example of a Chemical Equation is
when Carbon and Oxygen react to produce
Carbon Dioxide.
• Carbon + Oxygen = Carbon Dioxide
• C + O2  CO2
CHEMICAL EQUATIONS
• The substances that change in a reaction are
called the reactants:
• C + O2  CO2
• The new substances that are formed as a
result of the reaction are called the products:
• C + O2  CO2
• The arrow represents the energy needed to
complete the reaction.
CHEMICAL EQUATIONS
• Most equations have subscripts.
• Subscripts are numbers that appear after
elements in a chemical reactions that
represents the number of each atom present.
• Example:
• H2 + O2  H20
• N2 + H2  NH3
CHEMICAL EQUATIONS
• Some equations have a coefficient.
• Coefficients are numbers that appear before
elements in a chemical equation that change
the number of reactants or products.
• Example:
• H2 + O2  H20
• The correct way to write this equation is:
• 2H2 + O2  2H2O
• The coefficients change the number of
hydrogen and water molecules present.
1. The combination of two or more
elements bonded together to make
a new substance is called a
A. Chemical Reaction
B. Coefficient
C. Subscript
D. Element
2. Which of the following is not
considered evidence of a chemical
reaction?
A. Formation of a precipitate
B. Production of a gas
C. Change in color
D. Cutting a piece of paper in half
3. A chemical equation is
A. Numbers that appear after elements in a
chemical reaction that represent the
number of each atom present.
B. A representation of a chemical reaction
expressed as a formula.
C. Substances that change in a reaction.
D. The new substances that are formed as a
result of the reaction.
4. In a chemical equation, the
coefficients are
A. Carbon and Oxygen
B. The new substances that are formed as a
result of the reaction.
C. Numbers that appear before elements in a
chemical equation that change the number
of reactants or products.
D. Substances that change in a chemical
reaction.
5. In the equation C + O2 CO2,
the reactants would be
A. C + O2
B. The arrow
C. CO2
D. The plus sign
6. In the equation C + O2 CO2,
the products would be
A. C + O2
B. The arrow
C. CO2
D. The plus sign
COUNTING
ATOMS
RULES FOR COUNTING
ATOMS
1. SUBSCRIPTS only refer to the
atom that they are BEHIND. For
example…
H2S
There are TWO atoms of
HYDROGEN and only ONE atom
of SULFUR.
COEFFICIENTS
2. COEFFICIENTS apply to the entire
compound. You MULTIPLY the
coefficients and SUBSCRIPTS.
2 H2S
ATOMS OF HYDROGEN: 4
ATOMS OF SULFUR: 2
IF THERE ISN’T A SUBSCRIPT BEHIND AN
ELEMENT, ASSUME THERE IS ONLY ONE ATOM OF
THAT ELEMENT!
PARENTHESES
3. If elements or compounds are inside of
PARENTHESES, then the SUBSCRIPT
behind the parentheses applies to
everything inside.
Ba(OH)2
ATOMS OF BARIUM:
1
ATOMS OF OXYGEN:
2
ATOMS OF HYDROGEN: 2
LET’S PRACTICE!
MgCl2
Atoms of Magnesium: 1
Atoms of Chlorine: 2
Al2S3
Atoms of Aluminum: 2
Atoms of Sulfur: 3
PRACTICE
H2SO4
Atoms of Hydrogen: 2
Atoms of Sulfur: 1
Atoms of Oxygen: 4
CH3OH
Atoms of Carbon: 1
Atoms of Hydrogen: 4
Atoms of Oxygen: 1
THIS COULD BE A LITTLE
TRICKY…
Ca3(PO4)2
Atoms of Calcium: 3
Atoms of Phosphorus: 2
Atoms of Oxygen: 8
Al2(SO4)3
Atoms of Aluminum: 2
Atoms of Sulfur: 3
Atoms of Oxygen: 12
What about this BAD BOY???
2Ca3(PO4)2
Atoms of Calcium: 6
Atoms of Phosphorus: 4
Atoms of Oxygen: 16