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General Chemistry II Lab #8 – Le Chatelier’s Principle
The Effect of Temperature on Equilibrium
INTRODUCTION
Le Chatelier’s Principle states that if a system at equilibrium is subjected to a change of
conditions, reactions occur in the system that tend to counteract the imposed change. In other
words, the system tends to react in a way that restores the equilibrium. When the imposed stress
is an increase or a decrease in pressure, reactions that involve gases are greatly affected. In
general, an increase in pressure (a decrease in volume) favors the net reaction that decreases the
total number of moles of the gases. A decrease in pressure (an increase in volume) favors the net
reaction that increases the total moles of gas.
In this experiment, you will investigate how changes in temperature affect the position of
equilibrium in a gaseous reaction. A convenient reaction to use is the equilibrium between two
gases: nitrogen dioxide (NO 2 ) and dinitrogen tetroxide (N 2 O 4 ).
2 NO2 (g) ↔ N 2 O4 (g) red-brown colorless
Nitrogen dioxide will be prepared in the hood by the following reaction:
Cu (^) (s) + 4 HNO3 (aq) → Cu(NO 3 ) (^) 2 (aq) + 2 H 2 O (^) ( l ) + 2 NO2 (g)
NO 2 is an odd electron molecule that does not obey the octet rule. It readily dimerizes to
form the colorless N 2 O 4 molecule that does obey the octet rule. The NO 2 -N 2 O 4 equilibrium is
established as soon as the NO 2 is produced and it temperature dependent. The gas mixture is
collected in a cuvette and the amount of NO 2 gas is determined using a spectrophotometer.
Mixtures containing large amounts of NO 2 absorb light strongly over much of the visible range.
Mixtures containing more N 2 O 4 than NO 2 absorb less light and have a lighter color. One can
thus estimate the relative amount of NO 2 in a sample mixture by observing the color of the gas
and determine it quantitatively by measuring its absorbance.
In this experiment, you will gradually change the temperature of a mixture of the two
gases by placing the gas mixture in hot and cold water. The change in amount of NO2 in the
gaseous mixture will be measured at a wavelength of approximately 500 nm. A graph of
absorbance vs. time will illustrate how both raising and lowering the temperature of the system
affects the amount of NO 2 in the mixture at equilibrium.
General Chemistry II Lab #8 – Le Chatelier’s Principle
PROCEDURE Set the spectrophotometer to a wavelength of 500 nm. Adjust the % transmittance to zero with no cuvette and to 100% with a cuvette of distilled water in the cell compartment. Obtain a second clean cuvette and a yellow No. 1 snap cap that fits tightly on the cuvette. Collect some of the NO 2 gas from the setup in the hood. Quickly put the snap cap on the cuvette and further seal it with a layer of Parafilm. Put the cuvette in the Spectronic 20 and read its absorbance. The absorbance should initially read 0.50. If it does, record the % transmittance and proceed to the cold water step of the experiment. If it does not, the wavelength must be adjusted so that an absorbance of 0.50 is obtained. Turn the wavelength dial to wavelengths greater than 500 nm. If the absorbance decreases, turn the dial until the absorbance reads 0.50. If increasing the wavelength increases the absorbance, decrease the wavelength until the absorbance reads 0.50. Record the % transmittance of the mixture at room temperature and the wavelengths used.
Cold Water Place the cuvette in a beaker of ice and water for 5 to 6 minutes to cool the gas mixture. While the mixture is cooling, place the distilled water cuvette in the Spectronic 20 and readjust the %T to 100%. Quickly dry the cuvette from the ice water bath and place it in the cell compartment. Take % transmittance readings every 20 seconds from time zero to 4 minutes and then every minute for a minimum of 10 minutes. Continue taking readings until the transmittance values stabilize. Record for a maximum of 15 minutes. You can put the cuvette back into the ice water bath and redo this part if necessary.
Hot Water Remove the cuvette from the spectrophotometer and place it in a beaker of water at a temperature of between 80-90°C for 5-6 minutes. Quickly dry the cuvette and place it in the cell compartment. If the % transmittance is less than 10%, do not try to read it, start measuring the % transmittance at a value of around 20%. Take % transmittance readings as above until the % transmittance values stabilize or until the 15 minute time is reached.
DATA AND CALCULATIONS
Wavelength selected for transmittance measurements _______________ nm
% Transmittance at above wavelength at room temperature ___________________
Absorbance at above wavelength at room temperature ___________________
Remember that A = - log (%T/100). If you wish to have Excel do the calculations from %T to A for you for the below data, simply leave the Absorbance columns below blank and make sure that you include the data table that shows the absorbance values.
General Chemistry II Lab #8 – Le Chatelier’s Principle
RESULTS
Plot absorbance vs. time for the hot water experiment and the cold water experiment on the same graph.
- Which species is favored at high temperatures? Explain your answer.
- Which species is favored at low temperatures? Explain your answer.
- Is the 2 NO (^) 2 (g) N 2 O4 (g) equilibrium exothermic or endothermic? Explain your answer.
- Does your graph show that the system gradually returns to the original gaseous mixture after it has been heated? After it has cooled? If not, what error could have occurred?
General Chemistry II Lab #8 – Le Chatelier’s Principle
PRESTUDY Given the following equilibrium:
PCl (^) 5 (g) ↔ PCl (^) 3 (g) + Cl (^) 2 (g); ∆H = + 87.9 kJ
- Are products or reactants favored when the following changes are made to the above equilibrium? Explain your answer in each case. a. The temperature is decreased.
b. The external pressure on the system is decreased.
c. A small amount of solid is added to the system that reacts only with Cl 2 gas, removing it from the system.
d. Some of the PCl 5 gas escapes from the system.
e. Simultaneously, the temperature is increased and the external pressure is decreased.
