Transcript The Nature of Chemical Reactions

Chapter 8:
Chemical Reactions
Mrs. Wanstreet
The Nature of Chemical Reactions
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Chemical reactions:
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Occur when substances undergo chemical changes
to form new substances
Have reactants and products
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Reactants: substance that undergoes a chemical
change
Products: substance that is the result of a
chemical change
Energy and Reactions
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Energy must be added to break bonds
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There are several types of energy:
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What bonds?
Light
Thermal (heat)
Kinetic (motion)
Energy is transferred to the compound to
break bonds and rearrange the molecules
Forming New Chemical Bonds
Releases Energy!
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The formation of carbon dioxide and water
from isooctane and oxygen produces the
energy used to power engines!
As the bonds of isooctane break down,
energy is released as the new compounds
are formed
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The energy is in the form of heat
This is a Combustion Reaction!
Energy is Conserved in
Chemical Reactions
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Energy released by a reaction is stored as
chemical energy
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This is stored in the reactants
Chemical energy: energy stored within
atoms and molecules that can be released
when they react
Types of Reactions
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There are seven types of reactions to know:
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Exothermic
Endothermic
Synthesis
Decomposition
Combustion
Single-Displacement
Double-Displacement
Exothermic Reaction
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Exothermic: energy
transferred from the
reactants to the
surroundings
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Heat and light are the most
common forms of energy
given off
Examples:
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Explosions
Bioluminescence
Endothermic Reactions
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Endothermic: energy
transferred to the
reactants from the
surroundings
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Usually in the form of
heat
The result is cold on
the outside of the
beaker
Synthesis and Decomposition
Reactions
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Synthesis Reaction: two or more substances
combine to form a new substance
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A + B  AB
2H2 + O2  2H2O
Decomposition Reaction: one substance breaks
down, or decomposes, into two or more simpler
substances
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AB  A + B
2H2O  2H2 + O2
Single and Double Displacement
Reactions
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Single-Displacement Reaction: occurs when
one element replaces another in a compound
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A + BC  AC + B
Zn + Cu(NO3)2  Zn(NO3)2 + Cu
Double-Displacement Reaction: occurs when
elements switch partners
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AB + CD  AD + CB
AgNO3 + NaCl  AgCl + NaNO3
Precipitates
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A precipitate is usually formed in a doubledisplacement reaction
Precipitate: an insoluble compound formed
during a reaction
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It comes out of solution as a solid!
Precipitation Reactions:
Precipitate of Sulfur
White
substance is
the silver
chloride
precipitate
Combustion Reactions
Combustion of Methanol
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Not all combustion reactions release CO2
and water (Ex.: the combustion of
magnesium metal)
Reduction-Oxidation Reactions
“Redox” Reactions
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A reaction that occurs when electrons are
transferred from one reactant to another
Rusting is an example of a redox reaction
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Iron turns to Fe2O3 (rust) when exposed to the
environment (O2)
Each iron atom (Fe) loses 3e- and each oxygen
atom (O) gains 2e-
Oxidation of Iron
Balancing Chemical Equations
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The Law of Conservation of Mass: the
mass of what you start with has to be equal
to the mass of what you end with
This is why equations must be balanced!!!
A balanced equation says the number of
atoms you started with has to be equal to
the number of atoms you end with
Describing a Chemical Reaction
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Pb(NO3)2 + 2KI  PbI2 + 2KNO3
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How many potassium iodides are in the reactants side?
How many nitrate molecules are in the product side?
The reactant side?
Pb(NO3)2 + 2KI  PbI2 + 2KNO3
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How would you say it in words?
Lead (II) nitrate reacts with 2 Potassium iodides to
produce Lead (II) iodide and 2 Potassium nitrates
Balancing Equations
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A chemical equation is the written format of a
reaction
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All chemical equations MUST BE BALANCED!!!
Balanced equation has the same number of atoms
of each element on both sides of the equation
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YOU CAN ONLY ADD COEFFICIENTS TO
BALANCE! You CANNOT change subscripts!!!
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Remember coefficients are the big numbers, subscripts are
the little numbers below the element
Balancing Equations: Example 1
Is the equation balanced? Count the number of
EACH TYPE OF ELEMENT ON BOTH SIDES OF THE
EUQATION
Balancing Rules:
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Step 1: describe the reaction in words
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Step 2: write a chemical equation for the reaction
using chemical formulas
Step 3: Check the equation to see if there are
balanced atoms on both sides
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Make sure the reactants are on the left and the products are
on the right
Draw a line between the reactants and products, write out
each element, count the number of atoms of each element
on both sides
Step 4: If it isn’t balanced then choose coefficients
(big numbers) that balance the equation
Balancing Equations Example 2
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Silver Nitrate reacts with Sodium Chloride
to produce Silver Chloride and Sodium
Nitrate. Write the equation!!!
AgNO3 + NaCl  AgCl + NaNO3
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Check to see if the equation is balanced?!
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YES!!!!!
Try These:
Balance these equations:
 NaBr + Cl2  NaCl + Br2
 2NaBr + Cl2  2NaCl + Br2
 Al(OH)3 + CO2  Al(HCO3)3
 Al(OH)3 + 3CO2  Al(HCO3)3
 Mg + O2  MgO
 2Mg + O2  2MgO
More Practice!
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2NO2  N2O4
Already Balanced!
NaHCO3  H2O + CO2 + Na2CO3
2NaHCO3  H2O + CO2 + Na2CO3
NaCl + H2SO4  Na2SO4 + HCl
2NaCl + H2SO4  Na2SO4 + 2HCl