Experiment  # 7
HOW MUCH HEAT IS RELEASED IN A CHEMICAL REACTION?

OBJECTIVE:
In this experiment you will determine the heat of the neutralization reactions between sodium hydroxide and two acids: hydrochloric and sulfuric.

The neutralization reaction is the reaction between the acid and a base that produces water and  a salt ......not  the table salt, of course. This term is commonly used to describe an ionic compound formed from an acid and a base, which contains a metal cation or ammonium ion and an anion.
Example:

              2NaOH(aq) + H2SO4 (aq) ---->  Na2SO4 (aq)+ 2H2O(l)                   Eq.1
                     Base            Acid                         Salt

The reaction between acid and a base produces heat. Therefore, it is an example of an exothermic reaction.
You can calculate the heat  of neutralization  using the following equation:
                                                    DH= -(V x d  x DT x Cp)                               Eq.2
 where:
V=  volume of the reaction mixture, mL ( sum of the volumes of acid and base solutions)
d= density of the reaction mixture, g/mL
DT = temperature change (the maximum mixture temperature after mixing minus the initial temperature of the acid before the two solutions were mixed, oC)
 Cp =  heat capacity of the reaction mixture =
        energy required to increase the temperature of 1 gram of the  solution by 1oC.
For sodium hydroxide/sulfuric acid mixture:                  Cp =3.76 J/g oC; density =1.04 g/mL.
For sodium hydroxide/hydrochloric acid mixture:          Cp= 3.89 J/g oC; density = 1.12 g/mL.

The heat absorbed by the Styrofoam cup (calorimeter) is negligible and does not have to be taken into account for the purpose of this lab.

SAFETY
Be extremely careful when working with 2.0 M solution of NaOH (sodium hydroxide) and HCl ( hydrochloric acid) as well as with 1.0 M solution of H2SO4 (sulfuric acid). All of these solutions are toxic and corrosive, and may cause permanent burns. Sulfuric acid is also an oxidizer!  Use hood to dispense the solutions.

PROCEDURE
1. Prepare a ring stand with a small ring and a clamp. Obtain a 6 oz  Styrofoam cup And a thermometer.Support the cup in a ring, but do not suspend the cup in the air- keep it on the table

2.  Measure  20 ml of  1.0 M sulfuric acid in your 50 ml  graduated cylinder and add the acid  to the cuP.  Secure a thermometer in a clamp and insert the thermometer in the acid, making sure that the thermometer tip does not touch the bottom or walls of the cup.

3. Let the temperature equilibrate for 5 minutes. During this time wash the graduated cylinder with water, dry it well  with a paper towel,  and measure 20 ML of the 2.0 M sodium hydroxide solution.

4. Record the temperature of the acid. Without removing the thermometer from the cup, add the sodium hydroxide solution to the acid, all at once. Gently Stir the solution with the  thermometer . Record the temperature, starting 0.5 minute after the mixing, then  in  1 minute intervals, for additional 5 minutes.

5 . REPEAT THE STEPS 1 THROUGH 4 USING 20  mL of  2.0 m HYDROCHLORIC ACID AND 20 mL of  2.0 MOLAR SODIUM HYDROXIDE.

CALCULATIONS
1. Calculate the volume in mL of the  final reaction mixture:  V= V base + Vacid
2. Calculate  T for each reaction : DT =  Tmax - Tinitial acid
3.  Use Eq. 2 to calculate  the  enthalpy of the  neutralization reaction between the sulfuric acid and sodium hydroxide as well  for the reaction between the hydrochloric acid and sodium hydroxide.

DATA SHEET
PRELAB

POSTLAB
1.If you did not ignore the heat absorbed by a calorimeter, but somehow measured it and used in the calculations, would the calculated heat of reaction be lower or higher?  Hint: will the heat absorbed by the calorimeter have to be added or subtracted from your calculated heat of reaction? Explain.
 
 
 
 
 
 

2. Glass conducts heat easily and is not as good an insulator as styrofoam. If you performed the neutralization reaction in a glass
beaker instead of a styrofoam cup, would you expect to register  more or less heat released by the reaction ? Explain.
 
 
 
 
 

3. How can you tell whether the reaction is exothermic or endothermic based on the temperature measurements of the reaction
mixture before  and after the reaction?
 
 
 
 
 

4. Does the heat of neutralization reaction depend on the number of hydrogen atoms in  a molecule of acid or not? Explain using
your experimental results.