Group Work 3

Reactions

Reaction A.

1. Write a list of observations about the reaction in the vial. Use the loupe (magnifying glass) to better see whatÕs going on.

-the solution turns blue

-silver crystals form on the copper wire beneath the surface of the liquid.

2. Make some hypotheses about what oxidation and reduction reactions took place in vial A.

Cu wire --> Cu(2+)

The white metal looks like silver. So perhaps Ag+ --> Ag metal

3.Write half reactions for the oxidation and reduction reactions, then a balanced net redox equation for the overall vial reaction.

Cu --> Cu(2+) + 2 e-

[Ag(+) + e- --> Ag] x 2

Cu + 2 Ag(+) --> Cu(2+) + 2 Ag

Reaction B.

1. Pour carefully the contents of vial B into the vial C. Cap the vial loosely. Swirl the contents to mix. Observe for a few seconds, then bring your vial to the front of the classroom and place into the warm water bath. Observe. The reaction should be complete in 3 minutes.

Observations:

2. Vial B contained an aqueous solution of glucose. During the redox reaction that occurred in the warm water bath, glucose was converted into gluconic acid. The structures of glucose and gluconic acid are shown below.

3. What is the type of redox reaction that produces gluconic acid? To help you figure this out, check out how oxidation states of C and H are assigned in organic molecules below, as the number of oxygen atoms around C increases:

Glucose --> gluconic acid is an oxidation; an O atom from water is added.

4. Now, how many electrons are involved in the reaction of glucose to make gluconic acid? To figure this out, you might find it helpful to consider the simpler reaction of an aldehyde to a carboxylic acid (below):

Since the O atoms are ALWAYS -2, what is the NET change in oxidation states of C and any H atoms in this half reaction? How many e- are lost or gained?

There are 2 e- lost when glucose --> gluconic acid; the C goes from +2 to +3 and one H goes from 0 to +1.

5. Based on your observations of the glucose reaction, take a guess as to what redox active species is in vial C. Tell your instructor, who will give you the precise identity of the reactive species in vial C.

It appears that a silver mirror formed on the vial walls.

6. Write a balanced reaction for the reaction of glucose to gluconic acid, using either the net reaction method or the half-reaction method.

C6H12O6 + 2 Ag(+) + H2O ---> C6H12O7 + 2 Ag(0) + 2H+

7. Adjust your balanced reaction to occur in basic solution, which is the actual condition of the reaction you performed.

... add 2OH- to each side to "neutralize" the H+ (this makes 2 H2O):

C6H12O6 + 2 Ag(+) + 2OH- ---> C6H12O7 + 2 Ag(0) + H2O