Thermometric Titration of an Unknown Acid

Objectives

Discussion
[This laboratory exercise was adapted from "The Stoichiometry of the Neutralization of Citric Acid", Journal of Chemical Education, November 1995]

In this laboratory exercise you will attempt to determine the number of acidic hydrogen's in a weak organic acid (HnA) by monitoring the heat produced during neutralization. Hess's Law states that if a process can be considered to be the sum of several stepwise processes, the enthalpy change for the total process equals the sum of the enthalpy changes for the various steps. For the neutralization of a weak acid the important steps are:

Success of this laboratory investigation rest on the assumption that enthalpies for the successive neutralizations of the unknown acid are similar and highly exothermic. This assumption will be correct if the enthalpies associated with the successive ionizations of the weak acid are similar.

The investigation will be simplified by keeping the total volume of the reactants equal and assuming that the specific heat of all trials of the reaction mixture is essentially the same as the specific heat of water (4.184 Joules per gram degree Celsius). This will allow us to "determine" the heat produced by monitoring the rise in temperature of the reaction mixture. The data collected will be treated by plotting the change in temperature vs either the volume of acid used or the volume of base added.

Procedure
You will mix together various volumes of 1 M NaOH and 1 M acid. One or more trials will be assigned by your instructor to investigate. Your results will be shared with all members of the class and the composite data treated. Record the rise in temperature of the reaction mixtures for each assigned trial. The reactants are to be mixed in a styrofoam cup. If the reactants are not at the same temperature, use a weighted average of each reactant's temperature as the initial temperature of the reaction mixture.

Data Table
Trial mL acid mL NaOH Temp change (trial 1) Temp change (trial 2) Temp change (ave.)
1 20.0 40.0      
2 25.0 35.0      
3 30.0 30.0      
4 35.0 25.0      
5 40.0 20.0      
6 43.0 17.0      
7 45.0 15.0      
8 48.0 12.0      
9 50.0 10.0      
10 55.0 5.0      

Place a 20 mL sample of your reaction mixture in a stoppered, labeled test tube and display it as per the directions of your instructor.

Questions

  1. Plot the rise in temperature vs the volume of base added.
  2. Plot the rise in temperature vs the volume of acid added.
  3. From either of your graphs above, determine the number of hydrogens in each molecule of your unknown acid.
  4. Secure the structural formula of the unknown acid from your instructor and determine the following:
    1. the equivalent weight of your unknown acid.
    2. the acid hydrogens in the molecule.

  5. The heat of enthalpy for reaction (2) in the discussion above is -54 kJ/ mol of water produced. Using this value and the specific heat of water, 75.31 J/mol-C , determine the enthalpy for the complete ionization of the unknown acid.
  6. Write the ionization equation, including the enthalpy value, for each step in the complete ionization of the unknown acid.


Teacher Notes

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The unknown acid is Citric Acid, which has three acidic hydrogens. If other acids are used the data table will have to be redesigned.


 
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