Le Chatelier's Principle: Equilibrium Experiments

LE CHATELIER’S PRINCIPLE
EXPERIMENT 2

AMANDA BUCHANAN – SEPTEMEBER 15, 2015
CHEMISTRY 1212- SECTION 50
OBJECTIVES:
The objectives of this experiment are to be able to define equilibrium, equilibrium position, equilibrium constant, reaction quotient and Le Chatelier’s Principle. Another objective is to explain how changes in temperature, pressure and concentration affect the equilibrium position of a reaction. Also, perform chemical equilibrium reactions and manipulate equilibrium positions through concentration and temperature and perform calculations to determine the equilibrium constant (K) and the reaction quotient (Q) of reactions. The last objective is to apply Le Chatelier’s principle to predict changes and explain observed changes in equilibrium position.
DISCUSSION/OBSERVATION
Chemical reactions are reversible and do not always proceed to completions; rather they proceed until they reach a state of chemical equilibrium. The state of chemical equilibrium occurs when the concentrations of all reactants and all products remain constant. Dynamic equilibrium means reactions are still occurring even though the reaction appears to have stopped changing. The balance of product and reactant at chemical equilibrium is called the equilibrium position, and is the point where free energy exists in the lowest possible value. The equilibrium position of a reaction can lay one of three ways: to the left (favoring reactant), to the right (favoring product), or in the middle. The equilibrium constant (K) describes the ratio of concentrations of reactants and products when chemical equilibrium is reached at a specific temperature and pressure. If K is greater than 1, the reaction favors the products. If the value is less than 1, the reaction favors the reactants. Le Chatelier’s principle states that when a change in pressure, temperature, or concentration is imposed on a chemical system, the equilibrium position will shift in the direction that reduces the total effect of change, finding a new chemical equilibrium where free energy exists at the lowest possible value. The reaction quotient equals (Q). When Q equals K, the system remains at equilibrium. When Q is greater than K, the ratio of products over reactants has increased ant the equilibrium position will shift to the left. When Q is less than K, the ratio of products over reactants has decreased and the equilibrium position will shift to the right.
The first part of this exercise was to investigate Le Chatelier’s principle on chromate-dichromate equilibrium. Eight drops of potassium chromate was placed in the well plate. The color was observed (yellow) and recorded. Drops of hydrochloric acid were added just until a color change was observed. This only took three drops. The color changed to orange and was recorded in the data table. Eight drops of sodium hydroxide were added to the reaction until the equilibrium position shifts as noted by a color change (yellow). This was also recorded. A cold water bath was set up and a hot water bath was set up. Eight drops of potassium chromate were put into an empty well and four drops of HCl were added to it. An empty short stem pipet was used to draw up all of the mixture from the well. The color was observed (orange), and recorded. The pipet with the mixture was placed in the cold water bath for 2-3 minutes and then observed for color changes. No changes were observed. The color remained orange. Based on my observation, there is a shift to the right toward the products. The reaction in the pipet was then placed in the hot water bath for 2-3 minutes and observed for color changes. Again, no color change was observed. There is a shift to the right toward the products. I repeated the experiment and obtained the same results.
Exercise two was to investigate Le Chatelier’s principle on ferrocyanide and ferric ferrocyanide. Eight drops of potassium ferrocyanide was placed in an empty well and the color recorded (pale yellow). One drop of iron III nitrate was added to the potassium ferrocyanide and the color of the ferric ferrocyanide recorded (bluish green). Drops of NaOH were added to the reaction until the equilibrium position shifts as noted by the color change and the number of drops recorded (2). The color changed back to pale yellow. This completed the experiment.
Exercise 1: Equilibrium of Chromate and
Dichromate
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
3
ORANGE
8 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
ORANGE
Equilibrium Position
(Left or Right) RIGHT
RIGHT
Explanation of why equilibrium is shifted to the left or to the right PRODUCT SIDE
PRODUCT SIDE Questions
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. I repeated the lab twice but there was no color change. The solution remained orange so it is impossible to determine if the reaction if endothermic or exothermic based solely on this information. Human error could have contributed to the fact that there was no color change.
According to the chart, an increase in temperature of an exothermic reaction causes a shift to the left. An increase in temperature of an endothermic reaction causes a shift to the right. A decrease in temperature of an exothermic reaction causes a shift to the right and a decrease in temperature of an endothermic reaction causes a shift to the left.

B.Write the equation for the equilibrium constant (K) of the reaction studied in this exercise.
2K2CrO4 + 2HCl -> K2Cr2O7 + H2O +2KCl Use the information below to answer Questions C, D, and E: 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.

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.
[NH3]2 / [N2][H2]3= [1.0*10-4]2 / [4.0] [2.5*10-1]3=9.9999*10-9/0.0625=1.599*10-7

Q=0.00000015999 K=0.06

K<Q SO SHIFT TO 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.
[1.0*10-4]2 / [4.0][2.7*10-4]3=9.9999*10-9/7.8731999*10-11=127.01

Q=127.07 K=0.06

Q>K SO SHIFT TO LEFT

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.
[5.6*10-3]2/[4.0]*[1.0*10-2]3=0.000003136/0.000004=7.84

Q=7.84 K=0.06

Q>K SO SHIFT TO LEFT

Footer Header Exercise 2: Equilibrium of Ferrocyanide and Ferric
Ferrocyanide
Data Table 3. Ferric Ferrocyanide.
Color of Potassium
Ferrocyanide
Color of Ferric
Ferrocyanide
Number of drops of
NaOH to shift equilibrium position
Observations when equilibrium position is shifted PALE YELLOW
BLUISH GREEN
2
BACK TO PALE YELLOW

Questions
A.From your observations and data collected in Data Table 3, describe the direction of the equilibrium position shift upon addition of NaOH
It shifted to the left back towards the reactants side.