Download Fall.2008.Week9.Lesson.1 - reich

Chemistry SM-1131
Week 9 Lesson 1
Dr. Jesse Reich
Assistant Professor of Chemistry
Massachusetts Maritime Academy
Fall 2008
Class Today
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Tests
Chemical Reactions
Types of Chemical Reactions
Balancing Chemical Reactions
Solubility
Precipitation Reactions
Tests
• There are still people needing to take tests.
• They will hopefully be handed back on Friday
• The range was huge. A handful of you really
aced that last test. There was a small section
of Bs. There was a large section of 60-79, and
then there were a handful of 10-30.
• We’ll go over it soon.
• WIKI EXTENSION UNTIL NEXT WEDNESDAY
Chemical Reactions
• A chemical reaction is a rearrangement of
atoms.
• Reactants  Products
• Once a reaction has happened chemists often
want to collect 1 of the products. Often times
we’ll use solubility to help us… we’ll see how
that works in a sec.
Evidence of a Chemical
Reaction
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Heat and Light
Formation of solids
Gas emitting
Changing colors
Change in temperature
Some quick vocab
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(g) means the substance is a gas
(l) means the substance is a liquid
(s) means the substance is a solid
(aq) means the substance is aqueous
Aqueous means dissolved in water, which
does not necessarily mean the compound was
a liquid. Ethanol and sugar both become
aqueous, but only one of them was a solid at
room temperature.
Rules about Chemical
Equations
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Remember that law of conservation of mass?
Matter can neither be created nor destroyed.
We have to follow that!
So, whatever mass we started with on the
reactants side we’re going to have to have on
the right side.
• We do this by making sure reactions are
balanced. Mean we have the same number
and types of atoms on both sides of the .
Counting molecules and
atoms
• 2H2O
• 2 moles of water. To count atoms we’re going
to use mutiplication. 2x2H= 4H, 2x1O= 2O
• 4Mg(NO3)2
• 4x 1Mg= 1Mg, 4x2N = 8N, 4x6O= 24O
• The 2 and the 4 are called coefficients, just
like 5X in math where the 5 is the coefficient.
Types of Chemical
Reactions
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There are 5 main types of reactions
Combination aka synthesis
Decomposition
Combustion
Single displacement
Double displacement
(Acid Base, gas evolution, precipitation,
oxidation and reduction aka redox)
Combination Reactions
• 2 things come together to make 1 thing.
• Carbon and Hydrogen react to form the
compound methane.
• C + H2  CH4
• This is called a skeleton equation since it’s not
balanced. There is 1 C on the left, and 1C on
the right, but there are 2H on the left and 4H
on the right.
Balanced Chemical
Reaction
• C + 2H2  CH4
• Note, when I’m talking about “things” I mean
unique formulas. I recognize to balance this
reaction it requires 3 moles or 3 molecules on
the left, BUT, I’m only speaking about the
“types of molecules,” and I’m not invoking the
coefficients.
Combination Reaction 2
• Mg and oxygen react at high temperature
what is formed?
• Mg + O2  MgO
• O2 is diatomic when written into reactions
(HINClBrOF)
• (How’d I know MgO, well 4 steps: symbols,
charges, switcheroo, reduce)
• To balance it…
Balancing Chemical
Reactions
• ___ Mg + ___ O2  ___MgO
• 1- for all ionic compounds correctly establish
their formula
• 2-List all elements in the rxn under the arrow
• 3-Count the number of atoms of each type on
both sides of the equation
• 4-Starting with metals change the coefficients
until both sides are balanced. DO NOT
CHANGE CHEMICAL FORMULAS.
Balancing Chemical
Reactions
• ___ Mg + ___ O2  ___MgO
___Mg___
___O___
• ___ Mg + ___ O2  ___MgO
1 Mg 1
2 O 1 NOT BALANCED
• ___ Mg + ___ O2  _2_MgO
1 Mg 2
2 O 2 NOT BALANCED
• _2_ Mg + ___ O2  _2_MgO
1 Mg 2
2 O 2 BALANCED
More examples
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N2 + 3H2  2NH3
2Al + 3F2  2AlF3
P4 + 5O2  P4O10
SO3 + H2O  H2SO4
Decomposition
• When things decompose they break down.
This reaction is where 1 molecule breaks
down into several molecules.
• Iron(III) chloride decomposes at high
temperature into it’s elements.
• FeCl3  Fe + Cl2
• Not balanced, again HINClBrOF
Decomposition
• ___ FeCl3  ___Fe + ___Cl2
___Fe___
___Cl___ (count by atoms, not
Cl2)
• ___ FeCl3  ___Fe + ___Cl2
_1_Fe_1_
_3_Cl_2_
Decomp. Cont.
• _2_ FeCl3  ___Fe + _3_Cl2
_2_Fe_1_
_6_Cl_6_
• _2_ FeCl3  _2_Fe + _3_Cl2
_2_Fe_2_
_6_Cl_6_
• What’s the LCD of 3 and 2? 6 so Cl on both
sides needs 6. So how many FeCl3? Cl2?
Electrolysis of water
• Electrolysis is when you put a current through water. Water
turns into it’s elements. Write and balance the reaction.
• ___H2O  ___H2 + ___O2
_2H 2
_1O_2
• _2_H2O  ___H2 + ___O2
_4H_2
_2O_2
• _2_H2O  _2_H2 + ___O2
_4H_4
_2O_2
More examples of
Decomposition
• CaCO3  CaO + CO2
• 2 HgO  2 Hg + O2
• 2 ClO3  2KCl + 3O2
Combustion Reactions
• Combustion means burning and fire. What
two things does fire require? O2 and
something to burn. We normally burn
hydrocarbons (Hydro=H, Carbon = C therefore
stuff made up of H and C).
• The products are always CO2 and H2O.
• Methane and Oxygen burn write the equation.
• ___CH4+ ___O2  ___ CO2 + ___H2O
Balance a combustion
• ___CH4+ ___O2  ___ CO2 + ___H2O
__C __
__H __
__O __ (it comes from 2 places)
Balance this reaction now
Balanced Reaction
• _1_CH4+ _2_O2  _1_ CO2 + _2_H2O
1_C 1_
4_H 4_
4_O 4_ (it comes from 2 places)
Harder example
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C2H6 + O2  CO2 + H2O
2 C 1_
6 H 2_
2 O 3_
C2H6 + 3.5 O2  2 CO2 + 3 H2O
2 C 2_
6 H 6_
7 O 7_(no such thing as 0.5O2)
Double all the
coefficients
• 2C2H6 + 7 O2  4 CO2 + 6 H2O
4 C 4_
12 H 12_
14 O 14_
All combustion reactions will be just like one of
those two reaction.
Displacements
• These are the 2 hardest to tell apart when
starting.
• Single displacements typically have 1 lone
element on both sides of the reaction
• Double displacements look like the biggest
reactions out there, and you’ll see that the
two metals switch places
Single Displacement
• Magnesium metal starts making hydrogen gas
when it’s dropped in aqueous hydrochloric
acid. What’s the full reaction?
___Mg + ___HCl  ___ H2+ __ ?
Single Displacement
• ___Mg + ___HCl  ___ H2+ __ MgCl?
• NO! 4 steps when writing ionic salts ALWAYS!
So, it’s MgCl2
• ___Mg + ___HCl  ___ H2+ __ MgCl2
1 Mg 1_
1 H 2_
1 Cl 2_
___Mg + _2_HCl  ___ H2+ __ MgCl2
Single Displacement
• ___Mg + _2_HCl  ___ H2+ __ MgCl2
• Notice: Mg is elemental on the left side, and
Hydrogen is elemental on the right side.
• The single lone element was displaced by a
different lone element.
More Examples
• 3AgCl + Al  AlCl3 +3 Ag
• 2Na + H2O  H2 + 2NaOH
• Zn3N2 + 3Mg  Mg3N2 + 3Zn
Double Displacement
el double
• Silver nitrate and sodium chloride react to
form silver chloride and sodium nitrate. Write
and balance the reaction.
• KEY POINT: FIGURE OUT THE FORMULA FOR
EACH INORGANIC PIECE AND DON’T MESS
WITH THE FORMULA FOR THE REST OF THE
TIME!
El Double
• AgNO3 + NaCl  AgCl + NaNO3
• It’s already balanced.
• The Ag and Na switched places.
• That’s why it’s a double displacement.
El Double 2
• Barium nitrate and potassium sulfate reaction
to form barium sulfate and potassium nitrate.
Write and balance the equation.
Wiki Delay
• Due Wednesday of Next week. NO further
extensions will be announced.
• Homework for chapter 7 is due Monday.