Ch 206 Lab Help

LeChatelier's Principle

Some suggested reading: Brown, LeMay Sec 15.6

Part I, Equilibrium of HSO₄^- in Water

For this section, the indicator is thymol blue; this indicator is yellow for pH > 2, and is red for pH < 2. Note that these color changes are *not* especially dramatic - your best bet is to set up an extra tube of NaHSO₄ with indicator to use as a standard for comparison.

We are studying the equilibrium reaction

Notice that, as H₃O⁺(aq) is produced, the pH decreases; thus, any process which shifts this equilibrium to the right will result in a lower pH, and the indicator should shift towards a red color (at least in comparison to a standard solution.) Conversely, any process which shifts this equilibrium to the left (and consumes H₃O⁺(aq)) will result in a higher pH, and a corresponding shift in the indicator to a yellow color.

Pay special attention to steps 7-10. Note that, in an endothermic reaction, heat can be treated as a reactant, while in an exothermic reaction, heat is treated as a product. This makes applying LeChatelier's principle a bit easier when you are trying to understand the effects of added heat on an equilibrium. Be sure to indicate whether the equilibrium reaction is endothermic or exothermic as written (i.e, is the reaction endothermic or exothermic in the forward direction?)

Part II, Equilibrium of a Slightly Soluble Salt, Mg(OH)₂

Here we are studying the equilibrium:

You are using phenolphthalein as an indicator to study this equilibrium; this indicator turns very dark pink at alkaline pH (i.e., high [OH^-]). Therefore, any shift in the above equilibrium which results in the production of more OH^-(aq) will make the indicator turn darker purple. Again, it is a good idea to have a standard set up so you can compare your results to identify color changes. Also, in this equilibrium, we have a solid present; any process which shifts this equilibrium to the left will result in the dissolution of magnesium hydroxide, while any process which shifts the equilibrium to the right will result in the formation of more magnesium hydroxide.

in step 5, notice that the EDTA is a chelating agent; it removes metal ions from solution. (like Mg²⁺) from solution.

Part III, Equilibrium of Hexaaquocobalt(II) Ion

Here, you are on your own to design an experiment to explore the equilibrium

Notice that the [CoCl₄]^2- (in alcohol) is blue, while the [Co(H₂O)₆]²⁺ is pink; this equilibrium is not established until you add some water to the [CoCl₄]^2- (in alcohol). Any shift to the left will result in a blue color, while any shift to the right will result in a pink color. It is a good idea to keep a standard handy.

You have available ethyl alcohol, 0.1M silver nitrate, distilled water, and concentrated HCl (and your hot water / ice bath.) Note that adding ethyl alcohol to the equilibrium has the same effect as removing water, and the silver ions from the silver nitrate will react with aqueous chloride ions to form insoluble AgCl.

When desigining your experiments, try to study the effects of only one process at a time (i.e, don't add HCl and then heat.)

The Report

Include the usual brief intro and methods sections. In the methods section, state what it is you were looking for as evidence of a shift in equilibrium (i.e., thymol blue turns darker in response to a shift to the .... and lighter in response to a shift towards...) Also state what you were using to disturb the equilibria (i.e., SO₄^2-, heat, cold, etc.)

Don't forget to cite the lab manual!

Results

The easiest way to present the results section is with three tables (properly labelled and cited.) For the first equilibrium, here's an example:

Table 1. Equilibrium of a weak acid in water. The equilibrium studied was HSO₄^-(aq) + H₂O(l) = H₃O⁺(aq) + SO₄^2-(aq).

Make a similar table for the other two experiments. Be sure that you have the equilibrium reaction written somewhere so we can tell what you mean by a shift to the right/left!

For the experiments where you heated/cooled the equilibrium mixture, be sure to state whether the reaction was endothermic or exothermic! A really handy place to do this is in the text of the paper in the results section where you refer to tables 1, 2, and 3.

Discussion

There isn't any error analysis here - discuss, in terms of LeChatelier's principle, how a system at equilibrium responds to a disturbance (i.e, if product is added to an equilibrium mixture, then...) Again, sec 15.6 of your lecture text will be very helpful.

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