Mini-Lab 6-01: Hot Metal
Purpose:
Transition metals are similar to each other, but very different from the alkali metals and
alkaline earth metals. This experiment illustrates some of those differences.
Think about this:
What is the main difference between a spontaneous and non-spontaneous reaction?
Safety
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Don’t eat or drink anything in the lab.
Always wear eye protection.
Wear protective clothing (lab coats, etc.).
Don’t play around – treat the lab with respect.
Be particularly careful around open flames, and when working with
hot materials.
Questions
Fold a square of copper foil like this:
1) Hold the copper envelope with some crucible tongs and get it very hot in a flame.
After it cools, open the flap and look at the inside surface.
a. Describe your observations. How is the inside different than the outside?
b. Write a balanced chemical equation for what you think happened.
c. Is this reaction spontaneous or non-spontaneous at room temperature?
Explain why you think that.
2) Hold a piece of magnesium tape in some crucible tongs and light it on fire. DO
NOT look directly at the burning magnesium.
a. Describe how this is different than the copper experiment.
b. Write a balanced chemical equation for what you think happened.
c. Is this reaction spontaneous or non-spontaneous at room temperature?
Explain why you think that.
WAIT! Do not write down an answer to the Final question until your Instructor
tells you to.
3) FINAL: Why is the magnesium reaction different than the copper reaction?
What can you determine about the potential of each metal to oxidize?
Instructor’s Page
6-01: Hot metal
Source: Royal Chemistry Society, Classic Chemistry Experiments, #24
Concepts: oxidation potential of metals, chemical reactions, spontaneity of reactions
Materials: Bunsen burner, copper foil, magnesium tape, crucible tongs
Hints: relate the reactivity differences to electronic structure. The spontaneity of the
reactions may not be obvious to them. You might want to point out that the copper
reaction stops when the flame is taken away, but the magnesium reaction does not.
Discussion ideas: This could lead to a discussion of entropy and energy considerations in
determining spontaneity. This is a great way to point out the difference between reactions
that are spontaneous and those that are non-spontaneous. The magnesium continues to
burn (oxidize) without any additional energy input after the flame is taken away. This is
also a great opportunity to remind them that even spontaneous reactions have to be
“started” (i.e. – must be given enough energy to overcome their activation energy). On
the other hand, the copper does not appear to oxidize when it cools down. In fact, copper
does oxidize at room temperature, but very slowly. From the student’s perspective, it
appears that copper does not spontaneously oxidize in this case, but a little thought will
reveal that copper pennies turn brown after many years. It is up to the teacher to decide if
this is an important point in this context. But, what is very obvious at the least is that the
magnesium oxide must have a MUCH bigger driving force creating it, and indeed, that is
the case. Here are the values of ΔG at room temperature for both oxides.
For the formation of:
ΔG˚
CuO(s)
-129 kJ/mol
MgO(s)
-602 kJ/mol
(Values from Chemical Principles, Atkins and Jones, 4th Ed., W.H Freeman and Co.)
Of course, you can also relate this to oxidation potentials for both metals, since they are
both being oxidized. Again, it is obvious that the copper must be much less likely to
oxidize than the magnesium.