Chemical Reactions
Unit 8:
Chemical Reactions and Equations
• What are chemical reactions and how do
they occur?
• How are chemical reactions classified?
• How are products of chemical reactions
predicted?
• How are chemical reactions symbolically
represented?
• Is the Law of Conservation of Matter
obeyed when a chemical reaction takes
place?
• Chemical Reaction (Rxn)
– Change in identity of a substance(s)
• Reactants (Reagents)
– Substance(s) you start with
– Left Side
• Products
– Substance(s) you end up with
– Right Side
• Reactants turn into/create the products
Chemical Equations
• Chemical Equation
– Formula for a reaction
– Arrow separates the reactants from
products
Reactants  Products
– Substances separated with a + sign
• Read:
– Arrow as: “reacts to form” or “yields”
– + as “and”
Equation Symbols
• Symbols after formula indicate substance’s
state of matter:
• (s)
– Solid
• (l)
– Liquid
• (g)
– Gas
Equation Symbols
• (aq)
– Aqueous solution
• Dissolved in water
– 
• Solid is formed (precipitate)
– 
• Gas is produced as a product
– Gas is evolved
Equation Symbols
• Reversible reaction
• Heat is supplied to the reaction
Pt
 
• Catalyst (Can be an element or compound)
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Writing Equations
• Word Equation
– Uses words to illustrate substances
• Ex: Copper + chlorine  copper (II) chloride
• Ex: Copper reacts with chlorine to form copper
(II) chloride.
• Skeleton Equation
– Uses formulas
– Not necessarily balanced
• Ex: Cu + Cl2 CuCl2
Catalysts
• Catalyst
– Substance that
speeds up a reaction
– Not changed or used
up by the reaction
– Lowers Activation
Energy
• Energy needed to
start the reaction
• Enzymes
Reaction Energy
• Exothermic
– Give off heat
– Ex: Explosions
• Endothermic
– Absorb heat
– Feel cold
afterwards
– Ex: Ice packs from
the nurse
Balancing Equations
• Law of Conservation of Mass
– Atoms can’t be created or destroyed
– In rxn, all atoms we start with we must
end up with
• Balanced Equation
–Same number of each element on
both sides of the equation
– Biological catalysts
Balancing Equations
1. Write the equation with the correct formulas
for all reactants and products
2. Count the number of atoms of each element on
both sides
3. Balance the elements one at a time by adding
Coefficients
– Number in front of substance
4. Save H and O until LAST
5. Recount to make sure it is balanced
Balancing Don’ts
• Never change a subscript to balance an
equation
– If you change the formula, you change the
substance
– H2O is a different compound than H2O2
• Never put a coefficient in the middle of a
formula
– 2 NaCl is okay, Na2Cl is not
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Decomposition
Synthesis
• Synthesis Reaction
– 2+ substances combine to make one compound
– Examples:
• Ca + O2 CaO
• SO3 + H2O  H2SO4
– Predict the products if reactants are two elements
• Cross charges and write ionic formula
• Ex: Mg + N2  Mg3N2
Decomposition
• For Binary Compounds:
–Decompose into elements
–Examples:
electricity
2 H2O 

 2 H2 + O2
1. __
2 HgO   2 Hg + O2
2. __
• Decomposition
– One reactant breaks up into 2+
substances
– Energy is usually required
– Examples:
electricity Na + Cl
NaCl 
 
2

CaCO3   CaO + CO2
Decomposition
• If the compound has more than two
elements you will only be asked to
identify and balance them

• __ NiCO3 
 
  ___NiO + ___ CO2

  __KCl
2
2
3 O2
• __KClO
+__
3
Metals
Single Replacement
Activity Series
• Single Replacement
–One element replaces another
–Reactants are an element and a compound
–Products will be a different element and a
different compound
–Examples:
• K + NaCl  Na + KCl
• F2 + NaCl  NaF + Cl2
• Some metals are more
reactive than others
• Activity Series of
Metals
– Lists metals in order
of reactivity
– Higher on the list is
more active
– Used to determine if
rxn happens
Lithium
Decreasing Activity
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Chromium
Iron
Nickel
Tin
Lead
Hydrogen
Copper
Silver
Mercury
Platinum
Gold
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Activity Series
Single Replacement
– If free metal is higher than cation in
compound:
• Reaction occurs
• Free metal replaces cation
• Cation is neutral, free element as
product
– If free metal is lower than cation:
• No reaction
• Write “NR” or “No Reaction” after arrow
• Metal/Water Reactions
–Think of water as HOH
–Metals replace one of the H,
combine with hydroxide (OH-)
–H2 released
–Example:
• 2 Li + 2 H2O  2 LiOH + H2
Single Replacement
Single Replacement
• Activity Series of
Halogens
– Lower mass
halogen is more
active
• F replaces all, I
does not replace
– You can use list on
“Activity Series” or
periodic table
• Halogen Reactions
–Halogen that is kicked out is
diatomic element on other side
–Examples:
1. __ Br2 + __
2 NaI 2 NaBr + I2
2. __ Cl2 + __ CaF2 No Rxn
Double Replacement
• Double Replacement
– Two compounds switch partners
– Reactants must be two ionic compounds
or acids
– Usually in aqueous solution
– Ex: NaOH + AlCl3 
Double Replacement
•
Double Replacement
–
Will only happen if one of the products is:
• Insoluble in water and forms a precipitate
• A gas that bubbles out
• A covalent compound (usually water)
– NaOH + AlCl3 Al+3 OH- + Na+1 Cl-1
– 3 NaOH + AlCl3 Al(OH)3 + 3 NaCl
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Combustion
• Combustion
– Oxygen gas reacts with Hydrocarbon
• Compound composed of only C, H, and
maybe O
– Products will be CO2 and H2O
• Known as complete combustion
Combustion
Put a 2 here
If this is odd
CxHy + O2CO2 + H2O
Balance last
Balance first
• Balancing Tips:
– Balance H2O First
– If coefficient is odd
• Put a 2 in front of the hydrocarbon
– Balance O2 last
Net Ionic Equations
• Dissociation
– Separation into cations and anions
– Generally occurs in aqueous solution
Net Ionic Equations
• Complete Ionic Equation
– If aqueous, write compound as its ions
– If a solid, liquid, or gas, leave the formula as is
– Complete Equation:
HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
– Complete Ionic Equation:
H+(aq) + Cl-(aq) + Na+ (aq) + OH-(aq)  Na+(aq) + Cl-(aq) + H2O(l)
Net Ionic Equations
• Spectator Ions
– Appear on both sides of the equation
• Same state/charge on both sides
– Are present in solution, but do not affect
reaction
• Net Ionic Equation
– Shows what ions actually react
• Does not include spectator ions
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