Transcript Chemical Reactions - Treasure Mountain: Welcome to

CHEMICAL REACTIONS
Reactants: Zn + I2
Product: Zn I2
Introduction
– Chemical reactions occur when bonds
between the valence electrons of atoms
are formed or broken, these atoms then
reform bonds with different atoms in new
arrangements.
– Chemical reactions involve changes in
matter, making new materials with new
properties.
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• The charcoal used in a grill is basically carbon. The carbon
reacts with oxygen to yield carbon dioxide. The chemical
equation for this reaction, C + O2  CO2, contains the same
information as the English sentence but is shorter, easier &
shows the number of atoms.
Parts of a Chemical Reaction
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– Chemical equations show the conversion of
reactants (the molecules shown on the left of
the arrow) into products (the molecules shown
on the right of the arrow).
• A “+” sign separates molecules on the same
side
• The arrow is read as “yields”
• Example
C + O2  CO2
• This reads “carbon plus oxygen react to yield
carbon dioxide”
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Chemical Equations
– Know your terms!
Product
Reactants
4 Al +
Coefficient
3 O2 ---> 2 Al2O3
Subscripts
(Number of an
element’s
atoms in a
molecule)
Subscripts
(Number of an
element’s
(number of molecules)
atoms in a
Chemical Symbol molecule) Chemical Formula
(combination of element symbols
(abbreviation of element)
in a compound)
Chemical Equations
Because of the principle of the
conservation of matter,
an equation
must be
balanced.
It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
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Balancing Equations
– When balancing a chemical reaction you
may add coefficients in front of the
compounds to balance the reaction, but
you may
not
change the subscripts.
• Changing the subscripts changes the
compound.
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Subscripts vs. Coefficients
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• The subscripts
tell you how
many atoms of
a particular
element are in a
compound. The
coefficient tells
you about the
quantity, or
number, of
molecules of
the compound.
The Combustion of Methane
(natural gas: a fossil fuel)
__CH4 + __O2
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•
•
•
•
__CO2 + __H2O
Each person in a group of 4 should build one molecule listed in the above
equation. Use the color key to find the correct atoms. All of the holes represent
empty valence electrons in the atoms that need to be filled, so you may need to
use double bonds which are the longer more flexible connectors in the model
kits.
Put the reactants on one side of the table and the products on the other. Count
the atoms. Do the number of atoms in the reactants match the number of atoms
in the products?
Which molecules do you need more of to balance the equation? Remember you
can only change the number of molecules, not the subscripts!
Make the molecules and count the atoms again.
Write the balanced equation in your notebook.
The combustion of methane in air generates
CO2, which is a significant green house gas.
However, methane itself is 20 X more potent as
a greenhouse gas than carbon dioxide.
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The Synthesis of Water
__H2 + __O2
__H20
1. Construct the reactants and products with the model set.
2. Count the atoms on each side of the equation. What do you
need more of?
3. Make more molecules until the number of atoms is the same
on each side of the equation.
4. Write the formula for the balanced equation in your notebook.
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• This equation looks too simple to be interesting, but the
reaction is fast and explosively energetic. The ignition
temperature for the reaction is 580-590 OC. (youtube video)
Balancing Equations
2 H2 + ___ O2 --->
___
2 H 2O
___
What Happened to the Other Oxygen
Atom?????
This equation is not balanced!
Two hydrogen atoms from a hydrogen
molecule (H2) combines with one of the
oxygen atoms from an oxygen molecule (O2)
to form H2O. Then, the remaining oxygen
atom combines with two more hydrogen
atoms (from another H2 molecule) to make a
second H2O molecule.
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Bombardier Beetle spray!
(a decomposition reaction)
__H2O2
__H2O + __O2
1. Make the molecules of hydrogen peroxide, water and
oxygen.
2. Count the atoms in the reactants and products.
3. Make more molecules until the equation is balanced.
4. Write the balanced equation in your notebook.
This reaction helps to pressurize the toxin
spray from the back end of a beetle.
Carbonic acid: Blood buffer
(used to transport CO2 in blood)
H2CO3
H20 + CO2
1. Set out all the atoms needed to make carbonic acid.
Look at the number of holes available and try to work
out the structure.
2. Make the other molecules and balance the equation.
3. Write the equation in your notebooks.
4. This is a decomposition reaction that is important for
maintaining blood pH and telling our brain that we need
to breath!
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Balancing
Equations
____C3H8(g) + _____ O2(g) ---->
_____CO2(g) + _____ H2O(g)
____B4H10(g) + _____ O2(g) ---->
___ B2O3(g) + _____ H2O(g)