Lab 2: CHEMICAL CHANGES
PART 1: A REACTION WITH COPPER
Some substances react very fast and some are rather slow to react. Solids usually do not react as fast as gases. In the first part of this experiment, you will investigate how the reaction of finely ground copper with oxygen in air proceeds with time.
Wear safely goggles. Mass a dry crucible and add about 1 g of fine granular copper to it. Now find, as accurately as possible, the total mass of the copper and the crucible. Heat the copper on the crucible, above a Bunsen burner for 2 minutes.†† While heating watch the copper carefully and record changes in your lab notebook.
Once the crucible is cool, mass the crucible and its contents. Record your data. Now heat the sample for another 15 minutes. While you do that, record any further changes AND answer the first two postlab questions. Let the crucible and sample cool down again and measure the mass one final time. After massing, break up the contents of the crucible with a scoopula and examine the pieces. Write down your observations in your notebook.
DO NOT DISCARD the black solid in your crucible. Pour the pieces of solid into a test tube and save them for use in the next part of this experiment.
PRELAB QUESTIONS (Part 1)
What is a combustion reaction? Explain thoroughly and clearly. Give an example of a combustion reaction.
Given the above information, what is the chemical equation for this reaction?
Think about what factors may determine whether a reaction takes place slowly or quickly. Write down your hypothesis.
Set up a data table in your lab notebook, that includes all the masses (donít forget units and significant figures.) You will include a polished version of your data table on your final report.
POSTLAB (Part 1)
Did a reaction take place? What makes you say that?
Did the crucible and its contents gain or lose mass?
What can you conclude from the difference in mass between the initial sample of copper and your product?
Do you think that all the copper has reacted? If not, what fraction do you estimate did not react?
What do you think is the product of this reaction?† (The substance which you placed in your test tube.)
THE SEPARATION OF A MIXTURE OF COPPER OXIDE AND COPPER
Suppose all the copper that you started with in the preceding experiment had reacted with oxygen. In that case the black solid present at the end of the experiment would be the pure substance copper oxide. If, however, not all the copper reacted, then the black solid would be a mixture of copper and copper oxide. In order to determine whether the black solid is a mixture (of Copper and Copper Oxide), you will attempt to separate it. This is your goal.
A good way to separate a mixture like copper oxide and copper is to place the mixture in a solvent that will dissolve one of the components but not the other. Copper will not easily dissolve in hydrochloric acid, as we will demonstrate prior to the experiment itself. Copper oxide, however, is soluble in hydrochloric acid.
PROCEDURE (Part 2)
Wear safety goggles, youíre working with an acid! If you accidentally spill acid on your skin or clothes, you must rinse them thoroughly with water.
Retrieve your test tube from part one of the experiment. Add 5 ml of hydrochloric acid and stir with a glass stirring rod for about 8 minutes. (When you take out the stirring rod, rinse it in water, to avoid acid burns.) After letting the solid settle to the bottom, slowly pour off the liquid into another test tube. DO NOT DISCARD THIS LIQUID, you will need it for part 3 of this experiment. Put a rubber stopper on your test tube and label it.
Now wash the remaining solid several times with water and discard the washings.
PRELAB QUESTIONS (Part 2)
What is the objective of this experiment?
Define mixture (do not just copy this from a glossary, think about what a mixture is, research it, define it in your own words.)
What are some ways in which mixtures can be separated into their parts? Explain at least two ways that are different from the procedure described above.
POSTLAB (Part 2)
What does the remaining solid look like?
Is the black substance that you started with a mixture? Explain thoroughly and clearly.
Did you accomplish your objective? Explain thoroughly and clearly.
Part 3: Precipitating Copper
Read the following. This (and the prelab to part 3) is your assignment for the night.
Any reactions, like the reaction of copper with oxygen, are slow. It is difficult in such cases to tell when one of the reacting substances has been completely used up. Because the copper in your crucible changed to a black solid, you may have assumed that all the copper had reacted. This would have been an incorrect assumption, as the presence of copper in the black substance has shown.
Reactions like the reaction of copper with oxygen are called incomplete reactions, because neither one of the reacting substances, copper or oxygen is completely used up during the reaction. Other reactions are complete reactions.
When two elements come together to form a compound, they do so in a constant ratio. In other words, when hydrogen and oxygen are ignited to form water, they always come together in a ratio of 2/1.† This ratio is a constant. Do all substances combine in a constant proportion when they form compounds?
Chemists strongly disagreed about the relative amounts of substances that react to form compounds. On one side of the argument was a distinguished chemist, Claude Louis Berthollet. He claimed, on the basis of experiments, that a pair of substances can combine in any proportion to form a compound. On the other side was another distinguished French chemist, Joseph Louis Proust. He based his answer on evidence obtained from experiments that showed constant proportion, like the synthesis of water.
Proust suggested a new law of nature, the law of constant proportions. This can be stated as: When two substances combine to form a compound, they combine in a constant proportion. The ratio of the masses that react remains constant, no matter in what proportions the substances are mixed. If there is too much of one of the substances in the mixture, some of it will simply not react.
This is a bit like saying that you need two slices of bread and one slice of cheese to make a sandwich. If you have 10 slices of bread and 5 slices of cheese, you can make 5 perfect sandwiches. If you have 12 slices of bread and 5 slices of cheese, you still can only make 5 perfect sandwiches, but you will have 2 slices of bread leftover.
Answer the following:
What, in your own words, is the law of constant proportions?
In the second sandwich example, which is the limiting factor to making more sandwiches, bread or cheese availability?
If you are making water and you have 40 atoms of hydrogen and 27 atoms of oxygen, which is the limiting reactant?
What is the law of conservation of mass?
Does the law of conservation of mass have anything to do with the law of constant proportions? Why or why not? Explain.
PART 3: PRECIPITATING COPPER
When you heated a sample of finely ground copper in part one, you observed an increase in mass. This indicated that something was being added to the copper. A new substance, copper oxide, was formed. In part two, you dissolved the copper oxide in hydrochloric acid, and from the black powder you produced a blue-green solution.
What happened to the copper in all these reactions? Did it disappear forever, or is it still in the solution in some form? Putting a piece of zinc in the solution will provide you with an answer. Finding this answer is your goal.
PROCEDURE (Part 3)
Wear safety goggles; again, you are working with an acid. Put the test tube containing the solution in a beaker half-filled with water, for safety.
Put a piece of zinc in the solution you saved from part 2 and observe the reaction. Feel the outside of that test tube as the reaction takes place. While the reaction is going on, break up the solid from time to time with a stirring rod.
After the zinc has finished reacting, pour off the solution from the remaining solid and wash the solid several times with water to clean off the acid. Remove the washed solid from the test tube and dry it quickly by pressing it between two layers of paper towel.
Press a scoopula firmly against the paper towel and move it across the dry solid.
PRELAB (Part 3)
What is the objective of the experiment?
Write a chemical equation of what will happen to our solution when we place zinc metal into it.
Research what an exothermic reaction is and explain, in your own words and in simple terms what you found.
Research what an endothermic reaction is and explain, in your own words and in simple terms what you found.
POSTLAB (Part 3)
Did the reaction generate or use-up heat? What is this type of reaction called, then?
Can you identify the dry solid?
What would happen if you heated the solid that you recovered in this experiment?
How do you expect the total mass of solid you recovered in parts two and three compare with the initial mass of copper you heated in part one?
How do you know that water, zinc chloride and baking soda are not elements?
Hydrogen chloride Ėa gaseous pure substance- can be decomposed into two different gases, each of which acts like a pure substance. On the basis of this evidence alone:
a) Can hydrogen chloride be an element?
b) Can either of the other two gases be an element?
c) Can you be sure that any of the pure substances mentioned is an element?
While on a class field trip, a student found a shiny rock that appeared to be a metal. When she returned to the classroom, she heated the rock for a while and found that it lost mass. Further heating did not affect the mass. Could this rock be an element? Explain your answer.
(Adapted from Introduction to Physical Science, experiments 6.6, 6.7, 6.8)