Read Sections 5.1 and 5.2 before viewing the slide show.
Unit 17
Chemical Equations
•Writing Chemical Equations
•Balancing Chemical Equations
•Volume Relationships from Chemical Equations
Writing Chemical Equations (5.1)
•Verbal descriptions of what happens during a
chemical reaction can become quite complex, thus a
shorthand notation is desirable
• The use of chemical formulas greatly simplifies the
expressions that are used to describe chemical
changes
•Prior to exploring the shorthand notation, a note
about the identification of diatomic elements is in
order
Diatomic Elements
•There is an important set of seven diatomic elements that
occur in nature
•Diatomic means that, as elements, they exist as two atoms
bonded to each other. The seven diatomic elements are
indicated in the periodic chart with a black border– notice how
they sort of form the number “7”.
2
13
14
15
16
17
18
•In formula form, the seven 1
He
are H2, N2, O2, F2, Cl2, Br2, H
Li
Be
B
C
N
O
F
Ne
and I2
Na
Mg
Al
Si
P
S
Cl
Ar
•These are gases under
K
Ca
Ga
Ge
As
Se
Br
Kr
normal conditions except
for Br2 which is liquid and
Rb
Sr
In
Sn
Sb
Te
I
Xe
I2 which is solid.
Cs
Ba
Tl
Pb
Bi
Po
At
Rn
Fr
Ra
←In the throes of discovery→
Writing Chemical Equations (5.1)
•Consider the following statement:
•Sodium solid and chlorine gas react to form solid sodium chloride
•The statement can be abbreviated somewhat by using chemical formulas in place of words (recall
that Cl2 is diatomic):
•Na solid and Cl2 gas react to form solid NaCl
•To streamline even more, the physical states of the substances involved may be represented by
using the symbols (s) for solid, (ℓ) for liquid, (g) for gas, and (aq) for a water-based solution. Then:
•Na(s) and Cl2 (g) react to form NaCl(s)
•And, to finally get rid of the words altogether, replace the “and” by a “+” and the “react to form” by
“→”.
•Na(s) + Cl2 (g) → NaCl (s)
•This equation is not quite done, but it is now in symbolic form. Notice the number of Cl atoms is
different on the left-hand side and the right-hand side – atoms are always conserved in a chemical
change so that will need to be fixed.
Balancing Chemical Equations (5.1)
•The equation from the previous slide is:
Na(s) + Cl2 (g) → NaCl (s)
•Substances to the left of the arrow are called reactants and those to the right
products
•The reactant side contains 1 Na atom and 2 Cl atoms, while the product side
contains 1 Na atom and 1 Cl atom. Atoms are not created or destroyed during a
chemical change, so the equation will have to be “balanced” to ensure the numbers
of atoms of each kind are the same on each side.
•Balancing is done by placing coefficients in front of the chemical substances as
needed – the subscripts are never changed.
•Placing a 2 in front of NaCl will give two Cl atoms, but also gives 2 Na atoms. To
get 2 Na atoms on the left, place a two in front of Na as well. The balanced
equation becomes:
2 Na(s) + Cl2 (g) → 2 NaCl (s)
More Balancing Chemical Equations (5.1)
•The approach we using for balancing is pretty much trial-and-error. Let’s try a few
more examples:
___ LiOH (s) + ___CO2 (g) → ___Li2CO3 (s) +
___H2O (ℓ)
•(For space buffs, this is the reaction used to remove carbon dioxide from
spacecraft cabins made famous – sort of – in the movie Apollo 13)
•Notice the oxygen atoms are distributed through each reactant and product. A key
is to not worry about them first as they will be taken care of when everything else is
balanced.
•Notice there is one Li on the left and two on the right. It might be reasonable to
start by balancing them. The lines above are drawn to stress that we can only put
numbers IN FRONT of the compounds. We cannot mess with the existing
subscripts – remember you are experts at writing formulas and that process does
not depend on what reaction they are involved in.
•Continued on next page.
More Balancing Chemical Equations (5.1)
•So, let’s put a 2 in front of LiOH.
_2_ LiOH (s) + ___CO2 (g) → ___Li2CO3 (s) +
___H2O (ℓ)
•Now there are 2 Li atoms on the left and 2 Li atoms on the right.
•Look at H. There are now 2 on the left (2 x LiOH gives 2 H) and 2 on the right. H
is now balanced.
•Look at C. There is 1 carbon on the left and 1 carbon on the right. They are
balanced
•Finally, check the oxygen atoms. There are 4 oxygen atoms on the left (2 x LiOH
= 2 oxygen atoms plus 1 x CO2 = 2 oxygen atoms) and 4 oxygen atoms on the
right (1 x Li2CO3 = 3 oxygen atoms plus 1 x H2O = 1 oxygen atoms). They are
balanced.
•If you have to work too hard at balancing these, you have probably miscounted
somewhere. The equations we consider will typically balance fairly easily.
Simulation Practice for Balancing
Equations
•If you would like to get some practice using simulations:
•Go to http://phet.colorado.edu
•Click on “Play with sims”
•Choose Chemistry/General Chemistry
•Select the “Balancing Chemical Equations” simulation
•Select either the Save or Run Now option
•The first tab lets you look at three reactions. You change the coefficient in front of
each compound to achieve a balanced equation and will get a big “smiley” face if
you are successful.
•The second tab lets you play a balancing game.
Gay-Lussac’s Law of Combining Volumes (5.2)
•In the 1700’s Joseph Louis Gay-Lussac proposed the law of combining volumes:
•When all measurements are made at the same temperature and pressure, the volume
of gaseous reactants and products are in a small whole number ratio.
•As an example, consider the reaction
2 H2 (g) + O2 (g) → 2 H2 O (g)
•The balanced equation says that 2 molecules of hydrogen react with one molecule of oxygen
to form 2 molecules of water. Molecules are pretty small – we only see the reaction with
massive numbers of hydrogen and oxygen molecules reacting.
•Gay-Lussac’s law says that in this reaction we will always see 2 volumes of hydrogen
reacting with volume of oxygen to produce 2 volumes of gaseous water as long as the
temperature and pressure remain constant.
•For example, 2 L of hydrogen gas react with 1 L of oxygen gas to produce 2 L of gaseous
water; 10 gallons of hydrogen gas react with 5 gallons of oxygen gas to produce 10 gallons of
gaseous water; 200 cups of hydrogen gas react with 100 cups of oxygen gas to produce 200
cups of water, etc.
•As a side note, if you reacted 2 L of hydrogen gas with 8000 L of oxygen gas, you would still
only get 2 L of gaseous water because you would run out of hydrogen gas before oxygen
gas.
Avogadro’s Hypothesis (5.2)
•Based on Gay-Lussac’s law and experimental observations,
Amedeo Avogadro in 1811 explained the law of combining
volumes by stating that equal volumes of all gases, when
measured at the same temperature and pressure, contain the
same number of molecules.
•Further studies by Avogadro led to the next step in
understanding the relationships between masses of material
and the number of molecules, covered in the next Unit.