Reactivity of alkali metals w/water? A CHM 109 homework 2 pt F 09
Due Tuesday, 10/13/09 at start of lecture.
Objectives: 1. Practice describing chemical reactions by writing balanced chemical equations
2. Use balanced equations to determine mole ratios
3. Use mass to mole conversions to determine product yield from a given amount of reactant
4. Practice being skeptical of information obtained from non-peer reviewed sources
1. What is a mole? A mole is 6.022 × 10 23 structural units. If the structural unit is an
atom (as would be the case for He), then a mole of helium is 6.022 × 10 23 He atoms. If the
structural unit is a molecule (like H2O), then a mole of water is 6.022 × 10 23 water molecules.
2. We usually do not determine how many moles of a substance we have by counting
the number of structural units. We usually determine that by weighing the material and then
using the atomic or formula weight to calculate how many moles are present. For example:
How many moles of water are present in 1000.00 g of water?
a) First calculate the formula weight of water using the values from the Periodic Table:
For hydrogen 1.00794 g/mol H × 2 mol H/mol H2O = 2.01588
For oxygen 15.9994 g/mol O × 1 mol O/mol H2O = 15.9994
sum: formula weight (molar mass) of H2O = 18.01528 g/mol H2O
b) Then use the formula weight to determine the number of moles present. The units
will help you if you let them!
1000.00 g × = 55.5084351 ö 55.5084 mol H2O
3. Stoichiometry. Synonym is mole ratios. One of the uses of a balanced chemical equation
is to determine how much product will be produced if a n known amount of reactant is consumed.
For example, if methane combustion is correctly described by the following balanced equation:
CH4 + 2 O2 ö CO2 + 2 H2O
How many moles of water are produced when 172.4 moles of methane (CH4) are combusted?
2 mol H2O
172.4 moles of CH4 × = 348. 4 mol H2O
1 mol CH4
Note: the 2 to 1 ratio derives from the coefficients used in the balanced equation.
You must show calculations/logic throughout to receive full credit.
Go to google.com and search “alkali metal reactivity Brainiac.” You should find multiple
entries for the video we saw in class. Watch/listen to it as needed to answer the questions below.
1. How many moles of Rb are present in 2.000 g of Rb?
2. Write a balanced chemical equation for the oxidation of Rb atoms in water.
Hints: a) Listen carefully to the video clip. The Brainiac people identify the element
contained in the product that is not rubidium. That element is the only other type of element you
need to write in your balanced equation. Remember, even in “pure” water you have chemical
species other than H2O. b) This is similar to reaction #6 you ran in the Chemistry of Copper lab. It
Zn + 2 H+ º Zn2+ + H2
Be careful in identifying the correct stable ionic form of Rb. What remains when H2O loses H+?
3. In your balanced chemical equation above, identify the reactant that is being reduced and the
reactant that is being oxidized. Remember Simba (Leo?)!
4. Assuming that the yield was 100%, (Note: 100% yield means if you started off with 200 atoms of
Rb all 200 of them would be converted to product.) calculate the number of moles of gaseous
product that were produced from the 2.000 g of Rb.
5. I could not tell from the video what weight of Cs was used. Since the alkali metals are normally
sold by weight, we will assume that the same weight of Cs as Rb was used. How many moles of Cs
are present in 2.000 g Cs?
6. Assuming that the yield was 100% calculate the number of moles of gaseous product that were
produced from the 2.000 g of Cs.
7. Write the chemical equation for the reaction that is said to be directly responsible for causing the
explosions. Hints: We have already discussed this reaction in lecture in the context of air travel,
and it does not directly involve an alkali metal.
8. Were more moles of gaseous product produced from the Rb reaction or the Cs reaction? Is this
the answer you expected based on the size of the explosion? Explain briefly. (We will find out
below that both the nature of the events in the video and the factors dictating the size of the
explosion are somewhat complicated.)
Now do another google search. This time search “alkali metal reactivity gray” Go to
Theodore Gray’s web page and read the page and first look at the five associated videos (“Lithium”
through Cesium”) for Alkali Metal Bangs. Now answer the following questions.
9. Who is Ben Goldacre, and what did he have to say (in six sentences or less) about the Brainiac
10. See Gray’s Sodium Party page: http://theodoregray.com/periodictable/Stories/011.2/index.html
View the “click” where he indicates to see “a video of this first explosion.” Gray indicated he used
about 50 g of Na for this explosion in a garbage can.
a) Assume that value 50. has two s.f. Calculate the number of moles of Na that reacted in
the garbage can and the number of moles of H2 produced from 50. g of Na if the yield of H2
produced was 100%.
b) Is this larger or smaller than the number of moles of H2 than was produced by the Rb
reaction performed by the Brainiac group?
c) Does the result obtained by the Brainiac group seem logical, based on your above
calculations? Explain, very briefly.
Now go back to Gray’s Alkali Metal Bangs web page:
and view the videos where he does Rubidium Underwater and Cesium Underwater ampule smashes.
Then answer the questions shown below.
11. a) Which of the elements shown in the five videos gave the biggest explosion, and which gave
b) Did Gray use less, the same amount, or more alkali metal than the Brainiac folks?
12. Gray postulated that the relatively small explosions for some of the alkali metals were a result
of some of the metal being blown out of the water before it could react. How did he change the
experimental conditions to get around the postulated problem?
13. When Gray repeated the experiment with rubidium and cesium using his new experimental
approach (watch the rubidium and cesium videos farther down the page), did he get the same size
explosion as before? If not this is not easy to answer, re-watch the two upper videos (for Rb & Cs)
and the two lower videos (for Rb & Cs) and then re-rate all five alkali metals from least explosive
(left) to most explosive (right).
Extra credit: Who has a more interesting accent, Mr. Tickle or Richard? Elaborate very briefly.