Chemical Reactions
• A basic definition of a chem rxn is:
• A Rearrangement of Atoms
• Really - it is substance(s) changing into
different substances
Here is an Example
• Notice the motion
Evidence of Chemical Reactions
• Visual Clues
A. Color changes
B. Formation of a solid (precipitate)
C. Flame occurs
D. Bubbles are produced (gas)
• Temperature changes
A. Heat is produced (exothermic)
B. Heat is absorbed (endothermic)
An Example
• Eggs, flour, sugar, baking soda, and oil all in
various amounts
• combine together
• to form Cake!
A chemical reaction
• The reactants (eggs, flour, etc.) have
changed and are no longer eggs, flour,
etc.
• There is a new substances called a
product cake in this case.
How do we write these?
• We write these reactions with + signs and
 signs.
• Eggs + Flour + Baking Soda + Sugar  Cake
• Be Careful! The Eggs, flour, etc. are not
the same. They don’t exist anymore!!!!
Another Example
• Hydrogen gas combined with oxygen gas
will produce dihydrogen monoxide liquid
• H2 + O2  H2O
• More examples on Help Page
Why the H2 and O2?!?
• These are called Diatomic Molecules
• Whenever you see hydrogen all by itself, it
will be paired up!
• There are 7 of them (see Periodic Table)
• There are other examples of this:
• Phosphorus all by itself is P4
• Sulfur all by itself is S8
Phases
• In most cases, phases must also be
written. You should always include
symbols for reactions you observe in the
laboratory!
• Three common phases:
• Solid
ex:
Cu(s)
• Liquid
ex:
H2O(l)
• Gas
ex:
Cl2(g)
Weird Phases
•
•
•
•
Plasma – not common
Aqueous
NaCl(aq)
These are solutions
Made of water and something dissolved in
water
• Mr. Wikrent’s favorite solution is soda. It has
many healthy nutrients dissolved in the water of
each delicious can.
• What is the difference between Cu and Cu+2?
What does a solution look like?
• To the naked eye it looks like a liquid
• Upon closer inspection it is many
substances mixed together
See an example
See another example
Back to our example
• Hydrogen gas combined with Oxygen gas
will produce Dihydrogen Monoxide liquid
H 2 + O 2  H 2O
• Should be:
• H2(g) + O2(g)  H2O(l)
BUT, matter is always conserved,
atoms must be conserved!!!!!!!!
In order to make water, there needs to
be 2 hydrogen molecules for every 1
oxygen molecule.
2 H2(g) + O2(g)  2 H2O(l)
Let’s look back at the Help Pages
• Help Pages
WE MUST BALANCE!!!
To correctly write and balance an equation:
1. Formulas MUST be correct
2. There must be the same number of each
kind of atom on each side of the arrow.
This is done by using coefficients
(numbers in front of a formula).
3. Do not change formulas to balance atoms!!
4. Follow a trial and error process. Balance
each species one at a time. Be prepared
to erase!!
Balancing simple equations
K + Cl2  KCl
Fe + O2  Fe2O3
LiCl  Li + Cl2
NaN3  Na + N2
NiO + Al  Al2O3 + Ni
C4H8 + O2  CO2 + H2O
Ag2O  Ag + O2
B2O3 + Mg  B + MgO
KBr + Cl2  KCl + Br2
Balancing Reactions containing
polyatomic atoms
•
Example
Li3PO4 + MgCO3  Li2CO3 + Mg3(PO4)2
1. Keep the polyatomic ions together on both
sides
2. Balance the metal ions first, or if they are
balanced, the polyatomic ions first, metals
last
If your formulas are correct, the equation
should balance
Balance:
Cu + AgNO3  Ag + Cu(NO3)2
NaI + Pb(NO3)2  NaNO3 + PbI2
Ca(CH3COO)2+ K2CO3 CaCO3+ KCH3COO
Word equations (writing equations from
observations)
Water is decomposed by electricity to produce
hydrogen and oxygen gases through a process
called electrolysis.
When copper(II)chloride is dissolved in water and
reacts with aluminum metal, copper metal and an
aluminum chloride solution is produced.
Lead(II)nitrate solution reacts with sodium
sulfate solution to produce lead(II)sulfate
solid and sodium nitrate solution.
Aluminum iodide solution reacts with lithium
hydroxide solution to produce aluminum
hydroxide solid and lithium iodide solution.
tetracarbon decahydride (butane) gas reacts
with oxygen gas to produce carbon dioxide
gas and water vapor.
Ammonia gas reacts with oxygen gas to
produce nitrogen gas and liquid water.
Remember the following:
1. Diatomic elements
hydrogen gas, nitrogen gas, oxygen gas
AND all halogens in their elemental form
2. The formulas and charges of the
polyatomic ions
WOW
• The key is that Formulas MUST be correct
and there must be the same number of
each kind of atom on each side of the
arrow.
• Now go be reactive and produce some
good work.
• Assignment is: (depending on how well
you work)