Transcript 8.2 Chemical Equations law of conservation of mass reactants products diatomic molecule coefficient Big Idea Ch8 1.

8.2 Chemical Equations
law of conservation of mass
reactants
products
diatomic molecule
coefficient
Big Idea Ch8
1. In Chemical changes (reactions) atoms
are rearranged, never made or destroyed
2. Mass stays the same in chemical
changes (reactions)
3. Chemical reactions release energy or
absorb energy to break and make
chemical bonds
8.2 Chemical Equations
Is matter conserved in
chemical reactions?
• Chemical equations show that in chemical
reactions, atoms rearrange, but no atoms
are gained or lost.
8.2 Chemical Equations
Conservation of Mass
• The law of conservation of mass The
mass before a chemical reaction is equal to
the mass after the reaction.
8.2 Chemical Equations
Chemical Equation
• Reactants react to produce Products
• Reactants react to produce Products
• Reactants react to produce Products
• Reactants react to produce Products
• Reactants react to produce Products
Quick check
Paper reacts with oxygen to produce carbon dioxide
and ashes
WHAT ARE THE REACTANTS?
WHAT ARE THE PRODUCTS?
Is the mass of the paper and oxygen the same as
the carbon dioxide, water, and ashes?
8.2 Chemical Equations
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.
Reactants
=
Products
8.2 Chemical Equations
Balancing Chemical Equations
• “Tin + oxygen → tin oxide” reads as “tin
plus oxygen produces tin oxide.”
To save space let’s use the chemical
symbols
Sn + O2
REACTANTS
SnO2
PRODUCTS
Balancing Equations
8.2 Chemical Equations
• Use the symbols and formulas instead
of words.
Elements, Compounds,
and Molecules (cont.)
8.2 Chemical Equations
8.2 Chemical Equations
Elements, Compounds,
and Molecules (cont.)
• Some elements are diatomic, that is, they
contain two atoms.
8.2 Chemical Equations
Equations for Common
Chemical Reactions
Reaction of methane:
CH4 + O2 → CO2 + H2O balance hydrogen
CH4 + O2 → CO2 + 2H2O balance oxygen
CH4 + 2O2 → CO2 + 2H2O
8.2 Chemical Equations
Equations for Common
Chemical Reactions (cont.)
Baking soda and vinegar:
NaHCO3 + HC2H3O2 → CO2 + H2O + NaC2H3O2
8.2 Chemical Equations
Using Parentheses with Formulas
• When counting atoms inside parentheses,
the subscript multiplies all atoms inside.
Ca + H2O → Ca(OH)2 + H2
Ca + 2H2O → Ca(OH)2 + H2
8.2 Chemical Equations
Which is a diatomic molecule?
A O2
B H2O
C CH4
D N
8.2 Chemical Equations
What is potassium bromide (KBr)?
A an element
B a mixture
C a compound
D a diatomic molecule
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
8.3 Energy and Chemical Change
law of conservation of energy
exothermic process
endothermic process
8.3 Energy and Chemical Change
Conservation of Energy
• The law of conservation of energy states
that energy cannot be created or destroyed,
simply changed from one form to another.
8.3 Energy and Chemical Change
• Where is the energy from sunlight?
8.3 Energy and Chemical Change
Net Energy
During a chemical reaction, reactant bonds
are broken and product bonds are formed:
– Breaking bonds releases energy.
– Forming bonds absorbs energy.
– The total of energy in and energy out
makes the entire reactions release or
absorb energy
8.3 Energy and Chemical Change
Net Release of Energy
(cont.)
• An exothermic process releases energy.
• The products in an exothermic reaction
have less energy than the reactants.
8.3 Energy and Chemical Change
Net Absorbtion of Energy
(cont.)
• An endothermic process absorbs energy.
• The products in an endothermic reaction
have a higher energy than the reactants.
8.3 Energy and Chemical Change
Chemical Bonds
• Molecules have chemical energy stored in
their bonds.
• Energy is stored in the food that you eat
and transferred to your cells.
• Where is the energy from sunlight?
• Which is endothermic or exothermic?
8.3 Energy and Chemical Change
Products have a higher energy level
than the reactants in an endothermic
reaction.
A False
B True
8.3 Energy and Chemical Change
____ bonds ____ energy.
A Breaking; releases
B Breaking; requires
C Forming; requires
D Ionic; release
8.3 Energy and Chemical Change
The law of ____ states that energy is
neither created nor destroyed in a
chemical reaction.
A conservation of mass
B definite proportions
C conservation of matter
D conservation of energy
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Which is a chemical change?
A copper turning green from
exposure to air
B drying clothes
C ice melting
D none of the above
Which is a chemical property?
A boils at 100°C
B has a volume of 500 ml
C does not rust
D has a luster
Which does NOT describe an
exothermic reaction?
A There is a net release of energy.
B The reactants have a higher energy
than the products .
C The reactants are lower in energy
than the products.
D The products are less stable than the
reactants.
What is the coefficient of H2O?
Ca + 2H2O → Ca(OH)2 + H2
A 1
B 0
C 4
D 2
How many (OH) molecules are in
the products?
Ca + 2H2O → Ca(OH)2 + H2
A 1
B 2
C 3
D 4
SCI 5.b
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
SCI 5.c
Energy is ____ in exothermic
reactions.
A absorbed
B created
C released
D stored
SCI 3.b
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
SCI X.X
Which of the following is a
physical change?
A hydrogen burning
B iron rusting
C copper turning green when exposed
to air
D none of the above
SCI 3.f
Symbols represent ____ and ____
represent molecules.
A elements; formulas
B formulas; equations
C molecules; formulas
D elements; equations
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