A student is given the task of determining the enthalpy of reaction for the reac

Question

A student is given the task of determining the enthalpy of reaction for the reaction between HCl (aq) and NaOH(aq). The following materials are available. 1.00 M HCl (aq) 1.00 M NaOH (aq) distilled water 2.00 M HCl (aq) 2.00 M NaOH (aq) goggles insulated cups with covers gloves lab coat thermometer (x 0.1 degree C) stirring rod The student may select from the glassware listed in the table below. The student selects two 100 mL beakers, uses them to measure 50 mL each of 1.00 M HCl (aq) solution and 1.00 M NaOH (aq) solution and measures an initial temperature of 24.5 degree C for each solution. Then the student pours the two solutions into an insulated cup. stirs the mixture, covers the cup, and records a maximum temperature of 29.9 degree C. Is the experimental design sufficient to determine the of reaction of a precision of two significant figures? Justify your answer. List two specific changes to the experiment that will allow the student to determine the enthalpy of reaction to a precision of three significant figures. Explain. A second student is given two solutions. 75 mL of 1.00 M HCl and 75 0 mL of 1.00. M NaOH. each at 25.0 degree C. The student pours the solutions into an insulated cup, stirs the mixture, covers the cup. and records the maximum temperature of the mixture. The student calculates the amount of heat evolved in the experiment to be 4.1 kJ. Calculate the student's experimental value for the enthalpy of reaction, in kJ/mol_ein The student assumes that the thermometer and the calorimeter do not absorb energy during she reaction. Does this assumption result in a calculated value of the enthalpy of reaction that is higher than, lower than, or the same as it would have been had the best capacities of the thermometer and calorimeter been taken into account? Justify your answer. One assumption in interpreting the results of the experiment is that the reaction between HCl(aq) and NaOH(aq) goes to completion. Justify the validity of this assumption in terms of the equilibrium constant for the reaction. A third student calculate, a value for the enthalpy of reaction that is significantly higher than the accepted value. Identify a specific error in procedure made by the student that will result in a calculated value for the enthalpy of reaction that is higher than the accepted value. (Vague statements like "human error" or "incorrect calculations" will not earn credit.) Explain how the error that you identified in part leads to a calculated value for the enthalpy of reaction that is higher than the accepted value.

Explanation / Answer

(2) a (i) , No, the experimental design adopted by the student is erroneous . The enthalpy

of reaction determined will not have a precision upto two significant figures.

Explanation , -

Use of 100 ml beaker to measure solutions ( ie. 50ml of HCl soln & 50 ml NaOH Soln . )

is erroneous .

(ii) The following changes in the experiment should allow the student to determine the

enthalpy of reaction to a precision of three significant figures,

a. Measuring cylinder should be used to measure the solutions separately and after rinsing

it with the solution being used to measure.

b .   The desired volumes of solutions should be poured into the cup directly.

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Second student -----------

b . (i) Enthalpy of reaction in kJ / mole = measured heat evolved / no. of moles reacted

..............................................................= 4.1 / (75 / 1000 )

.............................................................. = 54.67 kJ per mol.

(ii) Wrong assumption ! A part of heat given out by neutralization reaction is taken up by the

cup and thermometer , hence it should also be taken into account . Since the student has

ignored this heat the calculated /experimental enthalpy of reaction would be lower than the

actual value of enthalpy.

(iii) Kc   for the neutalization reaction is given as

........................Kc   = [NaCl] [ H2 O] / [ NaOH] [ HCl]

since , the salt ( NaCl ) is a`strong electrolyte , and HCl , NaOH are strong acids and bases

present in solution they would get completely ionized without further hydrolysis. Hence Kc  can be written as

...................Kc   = [ Na+ ][ Cl-] [ H+ ] [OH- ] / [Na+ ][ OH- ][ H+ ][Cl- ]

......................... = 1

Hence , the reaction goes to completion , and therefore the assumption made by student is correct.

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Third student...........

(i) The enthalpy of reaction determined by this student could be higher than the accepted value beause of inaccurate measurements of the solutions taken / poured into the cup, and also recording of temperatures of solution without proper stirring / mixing.

(ii) Explanation ,-

If a greater volume of solutions happens to get mixed /reacted erroneously the reaction mixture would evolve more heat , resulting into higher value of enthalpy of reaction.

If the solutions taken / mixed in the cup are not properly stirred the reaction mixture would not attain an steady state of temperature . Therefore there are chances of recording a higher temperature leading to calculations giving out higher enthalpy of reaction.

  

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