Question

Unit 6 Lab - Temperature Effects of Equilibrium

Photo Documentation

In order to earn full credit for the participation portion of this activity, you must submit at least 2 photos. Required photos are outlined in the procedure below.

Procedure

IMPORTANT SAFETY CONSIDERATIONS:

Avoid getting NaOH on your skin. Should you come in contact with this chemical, immediately wash with water. Review all MSDS associated with this lab.

Any broken or chipped glassware should be replaced.

Clean up spills immediately.

Create a buffer solution such that the pH=pKa, by adding half the amount of 1.00 M NaOH (prepared from the 6M NaOH provided in your kit) needed to reach the endpoint of 10 mL of vinegar.

This can be performed by titrating a 15.0 mL aliquot of vinegar with 1.00 M NaOH

Record the endpoint volume of NaOH

Add half the endpoint volume of NaOH to another 15.0 mL aliquot of vinegar into the test tube

The solution created in the previous step is a buffer solution such that the pH=pKa

Place the test tube in the 600 mL beaker with enough warm tap water to cover the solution in the test tube.

Place the thermometer in the test tube. Read the temperature and adjust the water until the temperature is near 30°C. (Take a photo)

When the temperature has stabilized record the temperature. Remove the thermometer and insert the pH probe and record the pH.

Insert the thermometer into the test tube and add ice to the bath to lower the temperature by 5°C. (Take a photo)

Repeat steps 4 and 5 in 5°C increments until the solution reaches 10°C.

Download and record all the data in the table linked here: Data Analysis Table. You can also recreate the table and record the data in your lab notebook.

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Unit 6 Lab – Temperature Effects of Equilibrium

Data Analysis Table

pH	T (°C)	Ka	T(K)	ln(Ka)	1/T (K-1)

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Unit 6 Lab - Temperature Effects of Equilibrium

Analysis

Reflect upon the question(s) below and record your response(s) in your lab notebook. Completion of this analysis will be beneficial as you prepare for the post-lab quiz.

Your instructor will notify you if submission of this analysis is required.

Calculate the data for all the columns in the table you downloaded/created during the procedure. Remember the pK_a = pH for these buffer solutions.

Plot the data for the last two columns on the right. Plot ln(K_a) on the y-axis and 1/T on the x-axis.

From the plot or from linear regression determine the slope of the best fit line.

From the slope determine the ∆H° for this reaction.

∆H°=_______________________________________________

Was the value of ∆H° what you expected (i.e. exothermic/endothermic)? Explain.

What is the effect on the equilibrium when the temperature is changed? How is this related to ∆H°?

Answer 1

H is negative (exothermic) for the reaction between NaOH and vinegar(CH3COOH) ,acid-base neutralization system.

NaOH + CH3COOH ---> CH3COONa + H2O

CH3COONa +H2O <--->CH3COOH + OH- +Na+(buffer equilibrium)

In the equilibrium step ,gibbs free energy,G is negative and a reaction takes place to minimize G futher .

G=-RTlnK .k=equilibrium constant

Also

G=H-TS=-RTlnK,so enthalpy changes at equilibrium must be negative(exothermic) as for any spontaneous reaction. like a reaction driven towards equilibrium

2)

H-TS=-RTlnK

or,lnK=-H/RT+S/R..................vant hoff equation .It is a linear equation of the form y=mx+c with slope=m=-H/R ,when y=lnK and x=1/T

When temperature is changed, the equilibrium constant decrease exponentially so that its natural logarithm changes linearly with decrease in temperature.

∆H° or enthalpy changes is negative and relatively constant (,if range in temperature is small enough).Else it decreases with decrease in temperature.

∆H°=G+ TS