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Topic: Help with Le Chatlier's Principle lab  (Read 433 times)

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Offline Cepheides

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Help with Le Chatlier's Principle lab
« on: September 30, 2020, 10:18:23 PM »
In this lab, I have to answer questions using pictures of colors of different reactions and Le Chatlier's principle.

Here are the lab questions with my responses italicized.

A. Put 50 mL of 0.0005 M KSCN solution in a beaker and observe the appearance.  Add 5 drops of 0.2 M Fe(NO3)2 solution and observe the appearance.  What evidence is there for a chemical reaction?

KSCN originally appears clear and Fe(NO3)2 originally exhibits an orange color. The product (the right beaker in the right slide) has a different color, a shade of red, than either of the two reactants.

The observed reaction is Fe2+ (aq) + SCN- (aq) → FeSCN+ (aq)
Identify the color of each reactant and product.

Fe2+ (aq) is orange, SCN- is transparent and has no color, and FeSCN+ is red.

B. Fill three large test tubes 1/3 full with the solution from part IA.  Keep one test tube unchanged to use as a standard for comparison.  To the second test tube, add a few crystals of solid KSCN (source of SCN-) .  Add one drop of  Fe(NO3)2 (source of Fe2+) to the third test tube.  Record your observations.

The first test tube that maintains the same solution in 1A keeps its red-orange color. The second test tube with the crystals of KSCN exhibits a very dark, red color. The third test tube with the drop of  Fe(NO3)2 has a more prominent and slightly darker shade of red than the first test tube.


C. Consider the reaction Fe2+ (aq) + SCN- (aq) → FeSCN+ (aq)
Is this reaction 100 % complete with one limiting reactant, or is it fractionally complete (at equilibrium) with neither reactant limiting?  Explain your reasoning in detail.

No answer right now :(


I am stuck on answering part C. Do the different colors in test tubes 2 and 3 indicate that the reaction is NOT complete? Before answering, could you also verify my repsonses for questions A and B?

Offline chenbeier

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Re: Help with Le Chatlier's Principle lab
« Reply #1 on: October 01, 2020, 02:03:23 AM »
First its iron-III-nitrate Fe(NO3)3. Under the pictures the formula  is correct.
The red color come mainly from Fe(SCN)3.

Normally in this experiment you don't have same amount of reactant.

The first tube with solid thiocyanate gets dark red, means still free Fe3+ is present.
The other tube change a little, also some free Thiocyanate is present, but not much. The complex dissociate is the reason. To add either iron-III or SCN- change the equilibrium seen in the change of color.

Offline Borek

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Re: Help with Le Chatlier's Principle lab
« Reply #2 on: October 01, 2020, 03:31:19 AM »
I am not convinced C can be answered with the information (experiment result) given. At best there is a slight hint.

If the mixture were stoichiometric, adding excess of one reagent and observing the color change would be an obvious signal reaction went further. Trick is, mixture most likely contains an excess of Fe3+ from the start (there are about 20-25 drops per millilitre, so you added around 0.2-0.25 mL of the the Fe(NO3)3 solution, check what that tells you about the Fe3+ concentration).

Taking that into account, can you try to predict what should happen in solutions? Assume reaction went to completion and you add KSCN/Fe(NO3)3 - what effect would you expect? Assume reaction didn't went to completion - what effect would you expect then? Does the observation fit either scenario?
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Offline Cepheides

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Help with Le Chatelier's lab (General Chemistry)
« Reply #3 on: October 02, 2020, 02:52:30 PM »
I am completing a general chemistry lab based on Le Chatelier's Principle and I have to answer questions based on observing the colors of different solutions. I was wondering if someone could check my answers to the following questions of the lab. I have added photos that relate to the questions.

A. Put 50 mL of 0.0005 M KSCN solution in a beaker and observe the appearance.  Add 5 drops of 0.2 M Fe(NO3)2 solution and observe the appearance.  What evidence is there for a chemical reaction?

KSCN originally appears clear and Fe(NO3)2 originally exhibits a light orange color. The product (the right beaker in the right slide) has a different color, a shade of red, than either of the two reactants.

The observed reaction is Fe2+ (aq) + SCN- (aq) → FeSCN+ (aq)
Identify the color of each reactant and product.

Fe2+ (aq) is orange, SCN- is transparent and has no color, and FeSCN+ is red-orange.

Fill three large test tubes 1/3 full with the solution from part IA. Add a small amount of NaF (source of F-) to test tube #2.  The amount should be about the volume of a pencil eraser or less.  Mix gently and record your observations.

After adding NaF to the 2nd test tube, the test tube loses its orange color and becomes transparent.


Put a few mL of test tube #2 into each of the other two test tubes.  Add a few mL of KSCN solution to test tube #1 and 3-6 drops of Fe(NO3)2 to test tube #3.  Record your observations.

In test tube #3, where the 3-6 drops of Fe(NO3)2 were added after NaF, the test tube has a lighter, clearer, and paler shade of orange than before. In test tube #1, where a few mL of KSCN solution was added after the NaF, the test tube has lost its orange color and became transparent, although it is slightly more orange than test tube #2.


F. Fluoride ion (F-) reacts with Fe2+ to form FeF63- ion.  Knowing this, offer an explanation for all the data obtained in part IE.

We know that the 1A solution, FeSCN+ (aq), is at equilibrium. When we add NaF to the test tube #2, the test tube becomes clear because the system is shifting in the reverse direction. When the Fluoride ion (F-) reacts with Fe2+ to form FeF63- ion, a product has been added to the system, so the system shifts to the left in the reaction that formed the 1A solution
Fe2+ (aq) + SCN- (aq) → FeSCN+ (aq)
to remove the product, and thus the system shifts to the color of one if its reactants (the clearness of SCN- (aq)).
When NaF, then KSCN is added to the 1A solution in test tube #1, this situation is similar to test tube #2, except that now we have added a reactant (KSCN) to the system. The system responds by shifting forward slightly to remove the reactant, which is why a hint of orange away from complete clearness is observed.
I cannot explain what is happening in test tube #3 :(


Offline chenbeier

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Re: Help with Le Chatelier's lab (General Chemistry)
« Reply #4 on: October 02, 2020, 03:18:41 PM »
You asked this questions already, in your new Post you didnt changed the errors. Its Iron- III and not iron-II.
You Add flouride behaviour. Flouride is a stronger complex agent as thiocyanate.

Offline billnotgatez

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Re: Help with Le Chatlier's Principle lab
« Reply #5 on: October 02, 2020, 05:27:08 PM »
All threads  by Cepheides on this topic have been merged

Offline Cepheides

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Re: Help with Le Chatlier's Principle lab
« Reply #6 on: October 03, 2020, 04:21:19 PM »
Hi, I still do not understand why those two test tubes in the last pic I sent those colors. Why is the addition of NaF then KSCN clear? Doesn't adding a reactant force the system to shift in the forward direction, so the test tube will look more orange and less clear??? Is the same happening with Fe????

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