Physical Science: Ch 7 Notes

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Physical Science Ch 7 Notes
A. Chemical Reactions

• When a substance undergoes a chemical change, a
  chemical reaction has taken place.

Reactions are known to produce unexpected color
changes, a precipitate, gas, or a change in
temperature
Common chemical reactions rust forming,
digestion of food, bleaching hair, milk souring,
apples turning brown, photosynthesis, and the
combustion of anything!
Chemical equation a representation of a
chemical reaction in which the reactants and
products are expressed as formulas
A chemical equation is written as Reactants ?
Products
Reactants the substances that undergo change
Products the new substances formed as a result
of that change
Ex C O2 ? CO2 carbon reacts with oxygen gas
to form carbon dioxide
Ex H2O ? H2 O2 water decomposes into
hydrogen gas and oxygen gas
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Physical Science Ch 7 Notes
B. Balancing Chemical Equations This must be
done to follow the Law of Conservation of Mass
states that mass is neither created nor destroyed
Therefore, the products of a chemical reaction
are not created out of thin air and the atoms of
the reactants are not destroyed in the chemical
reaction!
To balance a chemical equation 1. Write the
reaction in symbol form 2. Draw a vertical line
down from the arrow in the equation and, on each
side, write the symbols of each of the atoms
present 3. Count and record the numbers of each
atom present on each side of the arrow 4. Use
coefficients to increase the number of atoms
until they are balanced (Do NOT change the
subscripts already present in the chemical
formulas!)
Coefficients the numbers that appear before the
chemical formulas in a balanced chemical equation
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Physical Science Ch 7 Notes
C. Classifying Reactions chemical reactions are
often classified by the type of reactant or the
number of reactants and products
Synthesis reaction a reaction in which two or
more substances react to form a single substance
General form A B ? AB
Example 2H2 O2 ? 2H2O
Decomposition reaction a reaction in which a
compound breaks down into two or more simpler
substances
General form AB ? A B
Example 2H2O ? 2H2 O2
Combustion reaction a reaction in which a
substance reacts rapidly with oxygen often
producing heat and light The products are ALWAYS
carbon dioxide and water
General form X O2 ? CO2 H2O
Example CH4 2O2 ? CO2 2H2O
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Physical Science Ch 7 Notes
Single Displacement a reaction in which one
element takes the place of another element in a
compound
General form A BC ? B AC
Example Mg HCl ? MgCl2 H2
Double Displacement a reaction in which two
different compounds exchange positive ions and
form two new compounds
General form AB CD ? AD CB
Example HCl PbO ? H2O PbCl2
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Physical Science Ch 7 Notes
D. Chemical Bonds and Energy
Chemical energy the energy stored in the
chemical bonds of a substance
Chemical reactions involve the breaking and
formation of chemical bonds
The BREAKING of chemical bonds REQUIRES energy
The FORMATION of chemical bonds RELEASES energy
During a chemical reaction, energy is either a
required reactant or is released as a product
Endothermic reaction reaction absorbs energy
from its surroundings as a reactant
Exothermic reaction reaction releases energy to
its surroundings as a product