A neutralization reaction is the reaction between an acid and a base to produce water and a salt. A salt is another name for an ionic compound and the specific salt produced depends on the acid and base involved.

The product solution is neutral while the starting solutions are acidic and basic respectively. A complete neutralization reaction requires an exact stoichiometric ratio of the reactants and is termed the "equivalence point".  In the example above, neutralization requires a ratio of 2 moles of HCl to react with exactly 1 mole of Mg(OH)₂ at the equivalence point.  An excess of either the acid or base would result in a solution which is not neutral and not at the equivalence point.

All reactions involve changes in energy.  Neutralization reactions typically give off relatively large amounts of energy as heat since the products are more stable (i.e. lower in energy) than the reactants. The purpose of this lab is to ascertain, if possible, the equivalence point of a neutralization reaction for an unknown acid and base by examining the energy changes involved.

Technique:

Thermochemistry is the study of heat exchange or heat transfer. You will use a simple calorimeter to measure the heat transfer (q) for a series of reactions. This data will be examined to determine the equivalence point for a neutralization reaction.

A calorimeter can be used to determine the energy given off by a particular reaction. The temperature inside the calorimeter is monitored and can be related to the energy produced by the reaction. Any energy given off by the reaction will be used to heat any substances in contact with the reaction such as the solvent and the calorimeter.    This is expressed mathematically below.

The heat gained or lost is related to the mass of the material, the specific heat of that substance, and the change in temperature observed. This can be expressed mathematically (below) for a reaction occurring at constant atmospheric pressure.

We can assume the mass and composition of the calorimeter will remain constant throughout the experiment. The above equation then simplifies to q = C_cal * DT for the heat exchange with the calorimeter.  For the solutions you will be analyzing, the heat exchange will also involve the bulk solution.  The sum of these two components will be equal to the heat  generated by the reaction of interest.

The Experiment:

1. Calibration of the calorimeter:  Use the procedure for determining the heat capacity of a calorimeter provided on the CHEM 1102 web page under " Enthalpy Change for a Chemical Reaction" and discussion provided therein.  The method is described immediately under the "Procedure" heading in this document.  Note that you will be working alone and you will use the calibrated "cool water" calorimeter for the remainder of this experiment.  Follow the directions up to step #1 in that procedure and then continue on with step #2 below.
   
2. Accurately measure out about 30 mL of water and place it into the calorimeter. (What measuring device is appropriate for measurement of volume in this experiment?   Consider accuracy and rate of delivery.) Cover the calorimeter with the top provided and record the temperature of the water once it has stabilized at room temperature.
   
   Accurately measure out 20 mL of a single unknown acid, record the I.D. and concentration provided on the bottle, allow it to stabilize at room temperature ("initial temp."), and add it to the calorimeter.   WARNING:  Always add acid or base to water and not the reverse!  Record the temperature of the solution every 60 seconds until the temperature stablilizes.  Be sure to gently swirl the calorimeter solution to distribute the heat throughout the calorimeter or the temperature readings may be misleading.  Note that thermometers are fragile and make terribly expensive stirring rods in terms of time and money!  Plot temperature against time and use a straight line to extrapolate your results back to the time of mixing (time = 0 sec.). This is the "final temperature".   Calculate the heat exchange for this dissolution process. What physical constants are needed and where can you find these values?  Can any helpful assumptions or approximations be made?
   
   
3. Repeat step 2 using 20 mL of a single unknown base in place of the acid.  How do these values compare to the acid results?
   
   
4. Decide on a series of at least 5 trials that will allow you to probe the equivalence point of the reaction between the acid and the base you chose in steps 2 and 3.   These trials should be done as similar as possible to the dissolution reactions  performed in steps 2 and 3 above.   Keep the total volume of solution constant at 50 mL (vol. acid + vol. base + vol. water). Choose differing volumes of  acid and  base such that the ratio of acid/base covers a reasonably wide stoichiometric range and the volume of acid +volume of base is always 20 mL. (What is the purpose of these experimental restraints?)  Create a table showing the volume of acid, volume of base, and volume of water to be used in each trial.

   Now, carry out your designed trials and determine the heat exchange (q) for each.  It will be difficult to mix three solutions (acid +base+water) at once.  How should you combine the solutions to make a safe and accurate measurement?   If you first dissolve the acid or base with the largest heat of dissolution in water and allow the temperature to stabilize, you will only have to consider heat contributions from the second dissolution reaction and the neutralization reaction.  In addition, the second dissolution reaction will likely only make a very small contribution to the total heat exchange.
   
   

   qdissolve + qneutral = Ccal * DT + m * Csp * DT

    

5. Calculate the heat, q,  for each trial reaction you carried out in step 4.  Make a graph of q_{neutralization} versus the ratio of (moles acid) / (moles base) and determine the equivalence point molar ratio. What do you anticipate the shape of this curve should look like? Why?
   
   
6. For your conclusions, report your equivalence point ratio of moles of acid / moles of base for the unknowns you used.  How does q for the neutralization reaction differ from q of the dissolution reactions done in steps 2 and 3?  What effect do these values have on determining the equivalence point?  Does the ratio of acid / base you obtained make chemical sense in terms of real molecules? Why or why not?  There are many sources of error in this experiment.  What sources of error might have affected your results?

Created by Shane Phillips.
Last edited on 10/30/98.