Enthalpy of Neutralization

 

Introduction

            In the course of most physical processes and chemical reactions there is a change in enthalpy, DH. If the reaction is exothermic, heat is given off and DH has a negative value.  When heat must be added in order for a reaction to occur, the reaction is endothermic and DH has a positive value.  The change in enthalpy, DH, is a measure of the change in heat content and can be measured by subtracting the heat content of the reactants from the heat content of the products.

DHreaction  = S DHproducts  - S DHreactants

 

The heat content of a compound cannot be measured directly, but the change in enthalpy that occurs during a chemical reaction can be measured.

 

The chemical reaction for which you will determine DH is a neutralization reaction.  When an acid and a base react, the net result is the production of a salt and water.  If the salt is water soluble, the net ionic equation for the reaction is

 

H+ (aq)     +     OH- (aq)

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H2O

                        A calorimeter is a container used to measure the heat change.  You will be using a variation of the calorimeter: double Styrofoam cups.   The heat capacity of the calorimeter usually would be calculated first; however, we have found that the heat capacity of the cups is so small that it can be neglected.  Therefore:

 

Heat (q) released by the reaction  =  - heat (q) absorbed by the solution 

Heat released = - SH x mass x DT  =  SH x Vol x Density x ( T initial - T final )

 

            The specific heat (SH) of the solution is given on a later page, as is the density.  You will record the temperatures of the acid and base solutions before mixing to get T initial.  The maximum temperature that is reached after the time of mixing will be recorded to be the solution’s final temperature.  Start monitoring the temperature as soon as the solutions have been mixed.

            Note that q will be negative if T final  is greater than T initial.

 

Equipment

2 Styrofoam cups

Ring stand

Test tube clamp

Thermometer clamp

Thermometer (0-100°C)

Stirring rod

Graduated cylinder

150mL beaker

 

Chemicals 

For students: 2.0 M HCl, 2.0 M NaOH

Spill/Disposal         

Acid/Base Spill/Disposal: B1

Neutralization Product: Spill/Disposal A

 

Heat of Neutralization

1.  Dry the inner Styrofoam cup.  Measure out 50mL (accurately measured) 2.0 M NaOH and pour it into the calorimeter.  Into a dry 150mL beaker, pour 50mL (accurately measured) of 2.0M HCl. Rinse out the graduated cylinder in between measurements.

 

2.  Determine the temperature of the acid to the nearest 0.1°C.  Rinse the thermometer with distilled water, wipe it dry and measure the temperature of the base to the nearest 0.1°C.  (If the temperatures are not the same, take their average.)  Record this initial temperature.

 

3.  Pour the acid into the base quickly and carefully with gentle stirring.  Continue to stir and monitor the temperature.  Record the maximum temperature that the solution reaches.  Take that as T final .

 

4.  Repeat Steps 1 - 3 (above) for Run 2.

 

5.  Determine the average temperature change for the reaction.

 

6.  Calculate q for the reaction you ran and then obtain DH for the enthalpy of neutralization per mole.

 

 

Disposal

            All reactants and products may be disposed of into the sink.

 

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