Lab # 4: Le Chatelier's Principle

Lab 4: Le Chatelier’s Principle
Name: Cammey Mahowald
Lab Partners: None
Date of Experiment: March 16, 2015
Course: CHE 112

Abstract:
The purpose of this experiment is to understand the components of a reaction at chemical equilibrium and use Le Chatelier’s principle to predict the direction an equilibrium position will shift upon changes in the concentration, temperature, and pressure. It was determined that in the equilibrium of chromate and dichromate it is an exothermic reaction and in the equilibrium of ferrocyanide and ferric ferrocyanide that it is an equilibrium shift to the left after adding NaOH.

Experiment and Observation:
The purpose of this experiment was to observe the color change and the concentration to determine the
direction shit of the equilibrium.

In the first part of the experiment we observed a color change that took place in room temperature, in an ice bath, and in a warm water bath. The room temperature was used as a constant, in which I first observed the color of chromate and the amount of drops of HCl that it took to reach equilibrium (a color change). This color was observed as orange compared to the initial yellow. After the observation was recorded, I added drops of NaOH until the equilibrium shifted, resulting in a color change back to yellow, all of which was recorded in Data Table 1.

I then proceeded to create a new mixture of 8 drops of potassium chromate and 4 drops of HCl into a clean well and used the pipet to draw up the mixture. After doing this I submerged the pipet in a cold-water bath and left it for 2-3 minutes and observed the color, what direction it shifted, as well as the fact it left behind a precipitate, all which can be seen in Data Table 2. I repeated this process, except this time I used a warm water bath and recorded the color change and the direction it shifted.

Data Table 1. Chromate-Dichromate.
Color of Chromate
Number of drops of HCl to reach equilibrium
Color of Dichromate
Number of drops of NaOH to shift equilibrium position
Yellow
2
Orange
5

Data Table 2. Endothermic and Exothermic Equilibrium Position.

Reaction at Room Temperature
Reaction in Cold Water Bath
Reaction in Hot Water Bath
Color
Orange
Orange- Precipitate
Orange
Equilibrium Position
(Left or Right)

Right
Right
Explanation of why equilibrium is shifted to the left or to the right

According to the initial, when the solution is orange it shows the concentration of HCl has increased
According to the initial, when the solution is organge it shows the concentration of HCl has increased

Once I finished with exercise A, I moved on to exercise B which involved different chemicals. The chemicals used were potassium ferrocyanide and ferric ferrocyanid. I put 8 drops of potassium ferrocyanide and recorded the color in Data Table 3. I then added only 1 drop of ferric ferrocyanide, which changed the color to a dark blue/green and left a precipitate. After observing the color change, I then added drops of NaOH until I noticed the color changing back to the original pastel yellow, showing the shift in the equilibrium; it took 10 drops of NaOH to noticed a difference. Using the information gathered, I concluded that the equilibrium shift was to the left after adding NaOH.

Data Table 3.
Color of Potassium Ferrocyanide
Color of Ferric Ferrocyanide
Number of drops of NaOH to shift equilibrium position
Observations when equilibrium position is shifted
Pastel Yellow
Blue/Green
10
The color returns to yellow, however there is a dirty brown looking precipitate on the bottom of the well.

Calculation and Error:
There were no major calculations that were done in this experiment.
Sources of error came from the drop sizes varying from the pipet and the droppers. When collecting the solutions in the empty pipet, we may have not collected all of the solution, however I do not believe this would make a huge different. Another source of error is not leaving the solution in the warm water bath and cold-water bath long enough to observe the change in reaction and or color. The thing I seemed to struggle with is noticing a huge color difference. It was apparent that chromate was yellow and dichromate was orange, although I did not see a darkening of the orange to indicate a change.
Questions for Exercise A:
A. Use your results to determine if the forward reaction in the potassium chromate/HCl reaction endothermic or exothermic. Explain your answer, using Table 1 to help construct your thoughts.
According to table, I would assume the reaction is exothermic. I would assume this because when the chromate/HCl were in warmer temperatures it was solid orange, then when adding the chromate/HCl in the ice bath it formed a precipitate. The precipitate forming shows that there is an addition of crystals, meaning a shift to the
right.

B. Write the equation for the equilibrium constant (K) of the reaction studied in this exercise.

2CrO42- + 2H+  Cr2O72- + H2O

Use the information below to answer Questions C, D, and E:

The equilibrium constant (K) of the reaction below is K = 6.0 x 10-2, with initial concentrations as follows: [H2] = 1.0 x 10-2 M, [N2] = 4.0 M, and [NH3] = 1.0 x 10-4M.
C. If the concentration of the reactant H2 was increased from 1.0 x 10-2 M to 2.5 x 10-1M, calculate the reaction quotient (Q) and determine which way the equilibrium position would shift.
(1.0x10-4 mol/L)2 NH3 / (1.0x10-2) N2 (2.5x10-1 ) 3 H2=
.000000008= Q < K=.06
The reaction will shift to the right.

D. If the concentration of the reactant H2 was decreased from 1.0 x 10-2 M to 2.7 x 10-4M, calculate the reaction quotient (Q) and determine which way the equilibrium position would shift.
(1.0x10-4 mol/L)2 NH3 /(4.0) N2 (2.7x10-4)3 H2 =
Q= 2.05x10-13 < K=.06
The reaction will shift to the right.
E. If the concentration of the product NH3 was decreased from 1.0 x 10-4 M to 5.6 x 10-3M, calculate the reaction quotient (Q) and determine which way the equilibrium position would shift.

(5.6x10-3 mol/L)2 NH3/(4.0) N2 (1.0x10-2)3 H2 =
Q= 7.84x10-12 < K=.06
The reaction will shift to the right .

Questions for Exercise B:
A. From your observations and data collected in Data Table 3, describe the direction of the equilibrium position shift upon addition of
NaOH.
The direction of the equilibrium shift upon adding NaOH is to the left, this is because the NaOH decreases the concentration of the Fe3+ ions and shifts the color back to a yellow rather than a blue/green.
Additional Questions:
1. The following reaction has achieved equilibrium. Predict the effect on the formation of phosphorous pentachloride (increase, decrease or no effect) for the stated changes to the system:
PCl3(g) + Cl2(g) <--> PCl5(g)
a) The concentration of phosphorous trichloride is increased.
The concentration of phosphorus trichloride will shift where the pressure is decreasing, meaning it will move to the right.
b) The concentration of chlorine is decreased.
The concentration will shift to the place where it will regain equilibrium by forming more Cl, meaning it will shift to the left.
c) The volume of the container in which the reaction is taking place is suddenly made smaller.
The volume of the container in which the reaction is taking place being made smaller means that the total pressure will increase, moving the moles of gas to where they are less than in order to gain the low pressure, moving the reaction to the left.
d) The pressure of the system is increased by adding one atmosphere of gaseous nitrogen to the container.
The pressure of the system increasing by adding one atmosphere of gaseous nitrogen to the container means no change will occur, because N2 is not present in the reaction.
2. Newton’s third law of motion can be stated like this: “For every action, there is an equal and opposite reaction.” How are Le Chatelier’s Principle and Newton’s third law similar? How are they different?
Newton’s 3rd law: To every action there is an equal and opposite reaction
Le Chatelier’s Principle: If a chemical system at equilibrium experiences a change in concentration, temperature, or pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established. The two are similar in the way that the both are trying to achieve balance, there is a reaction that will oppose another. The two are different in the fact that Newton’s is for forces while Chatelier’s if for chemicals.