FULLREPORT 2

Marie Antoinette Torres

Dec. 13, 2012

A – 20L

Jan. 8, 2013

Group 3

Chemical Equilibrium

I.

Introduction Equilibrium is a state in where no changes could be observed as time goes by. The concentration of reactants and products are constant when a chemical reaction has reached the state of equilibrium. Also, no visible changes occur in the system (Chang, 2007). A reaction reaches chemical equilibrium when the rates of the backward reaction and the forward reaction are equal. The concentration of the reactant and the product must also be equal. There are certain factors that affect chemical equilibrium. As the Le Chatelier’s Principle states, “If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.” Stress in this situation, pertains to changes in concentration, pressure, volume, or temperature (Zumdahl, 2010).

II.

Materials A. Reagents 0.02 M KSCN 0.02 M Fe(NO3)3 0.02 M NaH2PO4 NaNO2 6 M HCl CoCl2 B. Apparatus 4 test tubes syringe rubber stopper 2 250 mL beaker

III.

Procedure Five mL of 0.02 M potassium thiocyanate and 5 mL of water were obtained and mixed in a test tube. Four drops of 0.02 M iron nitrate were added to the solution. The solution was then stirred and observed for any changes.

The solution was then divided into four and placed into four test tubes. The first test tube was used as the control. A small crystal of potassium thiocyanate was added to the solution in the second tube. One drop of 0.02 M iron nitrate was added to the solution in the third and two drops of 0.02 M sodium phosphate was added to the fourth. A pinch of sodium nitrate was placed in a test tube. Three drops of 6 M hydrochloric acid was then added to the tube. Ten cc of the gas evolved were drawn into a syringe. A white background was then placed behind the syringe. A rubber stopper was obtained to prevent the escape of gas. The syringe was placed firmly against the stopper. The plunger was then quickly pushed down to 5 cc. The pressure was maintained for 5 seconds. After, the plunger was pushed in. It was then observed for any changes. Three test tubes were obtained. 0.5 mL of CoCl2 solution was placed in each of the test tubes. The first test tube was placed in a hot water bath and the second tube was placed in an ice-water bath. The third test tube served as the control. The tubes were then observed and after five minutes, switched setups. They were again observed for five minutes for any changes. IV.

Data Table 1.1 Test tube

Contents

A

KSCN solution + Fe(NO3)3

Test Tube A

Contents control

B

KSCN solution + Fe(NO3)3 + KSCN crystals

Observations The solution was homogenous. It was light orange in color.

Table 1.2

C D

Table 1.3 Test Tube A

KSCN solution + 0.02 M Fe(NO3)3 KSCN solution + NaH2PO4

Contents NaNO2 + 6 M HCl

Observations The solution was light orange in color. The solution became a darker shade of orange than the control. The solution became red orange. The solution was pale orange in color.

Observations Initial After pushing After a few seconds

brown gas brown gas light brown gas

Table 1.4 Test Tube A B C

Set-up Hot-water bath Ice-water bath control

Observations after 5 min. blue liquid pink liquid pink liquid

Observations after switching set-ups pink liquid blue liquid pink liquid

V.

Discussion The reaction involved in the first part of the experiment is FeSCN2+(aq) = Fe3+(aq) +SCN –(aq) The reaction produced a homogenous solution that is light orange in color. In the second part of the experiment, the KSCN and Fe(NO3)3 solution was divided into four. The first test tube was used as the control. In the second tube, KSCN crystals were added. The addition of the crystals resulted in the solution turning a darker shade of orange than the control. The change in color could be explained by the increase in concentration of the solution. The iron ions reacted with the added thiocyanate ions and that caused a shift in the equilibrium. The equilibrium shifted from right to left. The same reaction due to an increase in orientation happened in the third test tube. The additional iron ions caused the equilibrium to shift to the left. That reaction produced a red orange solution. In the fourth test tube, the addition of NaH2PO4 caused the equilibrium to shift to the right. Instead of getting darker like the previous set-ups, the fourth solution turned pale orange. In the third part of the experiment, pressure and volume caused the shifts in equilibrium. Initially, a brown gas was observed. After pushing the plunger to 5 cc, the gas still retained its color. A few seconds after that, the gas turned light brown. The color change was caused by the decrease in volume and the increase in pressure. The volume decreased as the plunger was pushed down. This caused the higher concentration of the reactant and the product of the reaction. The equilibrium of the reaction shifted from left to right. Temperature was the factor involved in the fourth part of the experiment. Three test tubes of CoCl2 solution was prepared. The first tube was put into a hot-water bath and the second was put in an ice-water bath. Initially, the solution was pink but after putting it in a hot-water bath, it turned blue. When the solution cooled down, it went back to its initial color. By heating the solution, energy was added to the system. That would cause an equilibrium shift in the direction that uses up energy. In this situation, the equilibrium shifted to the right. By cooling the solution, a shift in the opposite direction would occur.

VI.

Conclusion Shifts in equilibrium occur to compensate for the changes in concentration, volume, pressure, and temperature. A decrease in the volume would cause a system to shift towards the side of the reaction that includes the fewer moles of gas. An increase in temperature

favors an endothermic reaction while a decrease in temperature favors an exothermic reaction. VII.

Literature Cited Chang, R. (2007). Chemistry, 10th Edition. New York, NY: McGraw-Hill Companies, Inc. Zumdahl, S. and Zumdahl, S. (2010). Chemistry, 8th Edition. Stamford, CT: Brooks Cole, Cengage Learning