[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.
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.
|Trial||mL acid||mL NaOH||Temp change (trial 1)||Temp change (trial 2)||Temp change (ave.)|
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.
This document was formatted to display properly with Netscape v2.0. If your browser does not support the Netscape extensions, specifically subscript and superscripts, you may not see this document as intended, that is, you may have weird looking characters on the screen.
The unknown acid is Citric Acid, which has three acidic hydrogens. If other acids are used the data table will have to be redesigned.