CHEM 105 Workshop – 6/4/09
Workshop 3 – Station 1
Part I
For this first section of the workshop, work in pairs. Each student needs one bag
of M&Ms..
Each letter in the formulas below corresponds to a different color M&M. Arrange
your M&Ms as if they were participants in the chemical reactions below.
Make
sure you balance your reactions!
a) A2 + B2 -------> AB
b) AB4 + D2 -----> AD2 + B2D
The coefficient in a balanced chemical reaction indicates either the number of
molecules that react, or the number of moles that react in that reaction.
Part II
Now, let’s move to actual chemicals. In reaction (b) above, A is carbon, B is
hydrogen, and D is oxygen. First, rewrite reaction (b) in terms of its elements.
Now, let’s solve a problem using the balanced equation.
15.0 grams of methane gas, CH4, react with an excess of oxygen, O2, to produce
carbon dioxide, CO2, and water, H2O. How much H2O in grams, is produced in this
reaction?
Step 1: Convert the number of grams of CH4 reacted to moles, using the molar
mass of CH4. Why do we need to convert to moles?
Step 2: Convert the number of moles of CH4 reacted to moles of H2O produced,
using the balanced chemical equation.
Step 3: Convert the moles of H2O to grams of H2O, using the molar mass of H2O.
Will all of that H2O be produced? Sometimes, less is produced if the reaction
doesn’t go to completion. We can calculate the percent yield by the following
formula: % yield = actual yield (how much H2O was actually produced) / theoretical
yield (the amount we calculated could be produced) *100%.
Calculate the percent yield for the above reaction if 11.5 grams of H2O are
produced.
CHEM 105 Workshop – 6/4/09
Workshop 3 – Station 2
If you have five socks and three pairs of clean underwear, how many days will it be
before you have to do your laundry? Since two socks are needed for every pair of
underwear, the socks are limiting how many days you can go without washing, even
though you have more of them.
In Chemistry, the limiting reagent is the one that there’s
stoichiometrically
less of.
In other words, it’s the one that limits the amount of product formed because it
runs out first. Let’s go back to the first M&M equation. If A is brown M&Ms, and
B is green, which one limits how much AB can be formed? Your answer will depend
on what combination of colors you have in your bag!
A2 + B2 -------> 2AB
If a different reaction occurs, and E is orange M&Ms, which one limits how much
product can be formed?
B2 + E ------> B2E
We will now work a chemical limiting reagent problem:
During the launch of a US space shuttle, high-pressure pumps deliver 4400 kg of
liquid H2 fuel and 31, 000 kg of liquid O2 to each of it main engines in the first
minute. They react to form H2O and a lot of energy!
a) Write a balanced chemical reaction for the process.
b) Without using your calculator, determine which is the limiting reagent.
c) Without using your calculator, estimate the amount, in grams, of water that will
form. There are lots of ways to do this calculation. Try it on your own first, but if
you need a hint: Round all values to 1 sig fig.
d) Calculate how much water, in grams, will form.
