Chemical change & chemical reactions
• A process by which
one or more
substances is
transformed into
something
completely different
• Bonds are broken,
and / or new bonds
are formed
Link to beyond books
Hints that tell you a reaction has
happened
•
•
•
•
The absorption or release of heat.
The formation of a solid when two
liquids are poured together.
The creation of a change in color when
two colorless liquids are poured
together.
The formation
of
a
gas
within
a
liquid
More signs of chemical change
solution.
What does a chemical change look
like?
1.
2.
3.
4.
Energy and gas are given off
Energy is give off 2
Precipitate
Color change
Equation vocabulary
• Reactants: substances at the start of the
chemical reaction, placed on the left hand
side of an equation
• Products: Substances at the end of a
chemical reaction, placed on the right
hand side of an equation
• Yield side () used to separate reactants
and products
• H 2 + O 2 H 2O
Additional information
• State of matter:
– Gas (g)
– Liquid (l )
– Solid (s)
H2 (g) + O2 (g) H2O (l)
• Aqueous solution (aq)
– an aqueous solution means a solid dissolved in water
NaCl
(s)
(aq)
+AgNO3 (aq) NaNO3 (aq) +AgCl
Equation vocabulary
• Subscripts: Small numbers that are used to
identify the number of atoms in each of the
reactants ands products:
H 2 + O 2 H 2O
• Coefficients: Large numbers that indicate
how many molecules, atoms or formula units
are needed to balance the equation
2H2 + O2 2H2O
Catalyst
• A catalyst is a substance that causes a
reaction to occur at a faster rate.
• It is not used up in a reaction
• It does not change the amount of reactant
consumed or products formed
• It is written over the yield arrow
MnO2
H 2O 2

H2O + O2
Write “skeleton equations”
for the following changes
•
Sulfur burns in oxygen to form sulfur
dioxide
•
S (s) + O2 (g)  SO2 (g)
Remember, Skeleton equations simply
describe the “players”
•
Heating potasium chlorate in the
presence of the catalyst manganese
(IV) oxide produces oxygen gas.
Potassium chloride is left as a solid
MnO2
•
KClO3 (s)

KCl (s) + O2 (g)
Balanced Equation
Coefficients are used to identify the correct
number of atoms, molecules &/or formula units,
so that the conservation of matter is portrayed
C3H8 (g) + O2 (g) -----> 3CO2 (g) + 4H2O (g)
Note the use of coefficients in the
products
Rules for Balancing Equations
1.
2.
3.
4.
5.
Atoms cannot be created nor destroyed. An element
that appears in a reactant must appear in a product.
Never break up a polyatomic ion that appears
unchanged as a reactant and a product – treat it as a
single unit (a “super” atom)
If an element appears as a free element, such as C
(s) or O2 (g), as either a reactant or product,
balance that element last.
NEVER CHANGE SUBSCRIPTS!
Adjust atom numbers by the careful placement of
coefficients
Link to beyond books
Try some!!
• A solution of Silver nitrate is added to a
solution of hydrosulfuric acid. These react
to produce solid silver sulfide and aqueous
nitric acid
1. Write the reactants and products being
careful to follow the rules:
AgN03 (ag) + H2S (ag)  Ag2S (S) + HN03 (ag)
2. Make a list for the reactants and products:
Ag
N03
H
S
1
1
2
1
Ag
N03
2
1
H
1
S
1
Add coefficients to make the lists
identical
2AgN03 (ag) + H2S (ag)  Ag2S (S) + 2HN03 (ag)
Ag
N03
H
S
2
2
2
1
Ag
N03
2
2
H
2
S
1
Remember
1. Never change a subscript !!!
2. Use coefficients at the beginning of a
formula to adjust numbers of atoms –
keeping in mind that adding a
coefficient changes all atoms in the
fomula!!
3. Keep polyatomic ions as single units
4. Do isolated atoms or diatomics last
Try another one !
• Iron (III) chloride is added to calcium
hydroxide. Iron (III) hydroxide is
produced, as well as calcium chloride
• FeCl3 + Ca(OH)2 Fe(OH)3 + CaCl2
• 2FeCl3 + 3Ca(OH)2 2Fe(OH)3 + 3CaCl2
Can we add equations to these?
1. Magnesium and carbon dioxide
2. Energy is give off 2
3. Nitrogen triiodide detonation
Combination reactions
• Two or more substances come together
to form a compound
• A + B  AB
• Mg + O2  MgO
»Link to video
There are 5 basic types of
reactions
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•
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Combination Reactions
Decomposition reactions
Single replacement reactions
Double replacement reactions
Combustion reactions
– It is important to be able to identify each of the
basic types
Combination
Reactions
2H2 + O2 => 2H2O
Special combination reactions
• A metal oxide and water give a base
• A base is a compound with hydroxide
• K2O + H2O  KOH
• CuO + H2O  ?
• Right !! Copper(II) hydroxide
Special combination reactions
• A nonmetal oxide and water give an acid
• An acid is a compound with hydrogen
• SO3 + H2O  H2 SO3 and/or H2 SO4
• NO2 + H2O  ?
• Right !! Nitric or nitrous acid
Important fact
• Combination reactions are responsible for
acid rain.
• Sulfuric acid, nitric acid and carbonic acid
are involved.
• What nonmetal oxides are involved?
• Where do they come from?
Decomposition reaction
• A substance breaks down forming two
or more substances:
• AB A + B
• NI3= N2 + I2
(link to movie)
MnO2
• H2O2  H2O + O2
(link to movie)
Single replacement reactions
• A + BC  AC + B
• Aluminum + Iron(II) oxide Aluminum
oxide + Iron
• 2Al(s) Fe2O3(s) + --> Al2O3(s) + 2Fe(s)
• Link to movie
Single replacement: will it or will
it not happen?
• For metals, check
the reactivity
series
• A more reactive
metal will “bump”
a less reactive
one
Zn + HCl  ZnCl2 + H2
link to animation
Single replacement:
will it or will it not
happen?
• Magnesium is placed in sulfuric acid
• Mg + H2SO4  MgSO4 + H2
• zinc is placed in phosphoric acid
• 3Zn + 2H3PO4  Zn3(PO4)2
+ 3H2
• Potassium is placed in contact
with aluminum chloride
• 3K+ AlCl33KCl + Al
Single replacement: will it or will
it not happen?
• For non metals – look to the periodic
table (as an activity series)
• F is more reactive than Cl
• F2 + HCl  HF + Cl2
• Cl2
But –
+ HF  no reaction
Write single replacement
equations
• Magnesium is added
to lead nitrate
• Iron is added to
aluminum sulfide
• Sodium is added to
calcium carbonate
Write single replacement
equations
• Cl2 + NaI => ?
• I2 + NaCl => ?
Group VIIA
F
Cl
Br
• K + NaCl => ?
I
At
Double Replacement
• Two compounds interact
• The cations trade places
• AB + CD  AD +CB
• Na3PO4 + FeCl3 FePO4 + NaCl
• animation
What is the precipitate
• Silver nitrate and sodium
hydroxide react:
»Movie
• AgNo3 + NaOH  NaNO3 + AgOH
Check the solubility
tables
AlCl3 + NaOH  Al(OH)3 (s) + NaCl
(aq)
•
How do we know that the aluminum
hydroxide is the precipitate? We look at a
solubility table
•
If the solubility table indicates “I”
(insoluble), expect the substance to be a
precipitate
» link
Combustion reactions
• Fuel + oxygen gives carbondioxide, water,
and energy
• C3H8 (g) + 5O2 (g) -----> 3CO2 (g) + 4H2O (g)
• Fuels have carbon, hydrogen, and
sometimes oxygen
While any combination with oxygen can
be called “combustion”, the term is
usually restricted to reactions with
hydrocarbons, that produce water,
carbon dioxide and energy
Fuels:
Organic molecules:
The alkanes CnH2n+2
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Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10
Pentane (five carbons) C5H12
Hexane (six carbons) C6H14
Heptane (seven carbons) C7H16
Octane (eight carbons) C8H18
Each carbon develops 4 bonds
methane
Ethane : C2H6
Propane: C3H8
Link to bondit
Alcohols
Methanol: CH3OH
Can you write balanced equations
for these hydrocarbons?
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Methane (one carbon)
Ethane (two carbons)
Propane (three carbons)
Butane (four carbons)
Pentane (five carbons)
Hexane (six carbons)
Heptane (seven carbons)
Octane (eight carbons)