Determining the Enthalpy of a Chemical Reaction


Introduction

In this experiment, you will measure and compare the quantity of heat involved in three chemical reactions. You will calculate the amount of energy after measuring the temperature change of a known quantity of water. A polystyrene cup will serve as the calorimeter in this experiment. We will assume that the heat of reaction is used to change the temperature of the aqueous solution only. Any small heat losses to the surroundings will be neglected. We will also assume that the aqueous solution has the same heat capacity as water. That is, it takes 1 calorie to change the temperature of 1 g of solution 1⁰C.

You do not need to find the mass of the water since 1.0 mL of water has a mass of 1.0 g and you will measure the volume of water to the nearest tenth of a milliliter. The reactions to be compared are:

Part 1: Solid sodium hydroxide dissolves in water to form an aqueous solution of ions.

NaOH(s)→Na+(aq) + OH-(aq) + ? calories ∆H1 = ?

Part 2: Solid sodium hydroxide reacts with an aqueous solution of hydrogen chloride to form water and an aqueous solution of sodium chloride.

NaOH(s) + H+(aq) + Cl-(aq) → H2O(l) + Na+(aq) + Cl-(aq) + ? calories ∆H2 = ?

Part 3: Solid sodium hydroxide reacts with an aqueous solution of hydrogen chloride to form water and an aqueous solution of sodium chloride.

NaOH(s) + H+(aq) + Cl-(aq) → H2O(l) + Na+(aq) + Cl-(aq) + ? calories ∆H3 = ?

Figure 1

Materials

LabQuest / 0.5 M hydrochloric acid, HCl, solution
Temperature Probe / 1.0 M sodium hydroxide, NaOH, solution
glass stirring rod / 1.0 M hydrochloric acid, HCl solution
Styrofoam cup / Solid sodium hydroxide
ring stand / 250 mL beaker
utility clamp / 100 mL graduated cylinders

PROCEDURE

1. Obtain and wear goggles.

2. Connect the Temperature Probe to LabQuest and choose New from the File menu.

3. Use a utility clamp to suspend the Temperature Probe from a ring stand as shown in Figure 1.

Part I Determination of the heat of reaction 1

4. Nest a Styrofoam cup in a beaker, as shown in Figure 1. Measure out 200 mL of tap water into the foam cup. Lower the Temperature Probe into the solution. Record the temperature.

5. Measure out about 4 grams of solid sodium hydroxide. Record the mass to the nearest 0.10 g. CAUTION: Handle the sodium hydroxide with care.

6. Pour the solid sodium hydroxide into the water in the Styrofoam cup. Stir gently and continuously until the sodium hydroxide is dissolved. Record the extreme temperature that is reached.

7. Rinse and dry the Temperature Probe, Styrofoam cup, and the stirring rod. Dispose of the solution as directed.

Part II Determination of the heat of reaction 2

8. Repeat steps 4-6, but substitute 200 mL of 0.50 M HCl for the tap water in step 4. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin.

9. Rinse and dry the Temperature Probe, Styrofoam cup, and the stirring rod. Dispose of the solution as directed.

Part III Determination of the heat of reaction 3

10. Measure 100 mL of 1.0 M HCl into the Styrofoam cup and 100 mL of 1.0 M NaOH into a 250 mL beaker. Record the temperature of each solution.

11. Add the sodium hydroxide solution to the hydrochloric acid solution in the styrofoam cup. Mix quickly and record the extreme temperature reached.

12. Rinse and dry the Temperature Probe, Styrofoam cup, and the stirring rod. Dispose of the solution as directed.

Data Analysis:

1.  For each reaction, calculate:

a.  The change in temperature

b.  Amount of heat absorbed by the solution

c.  The number of moles of NaOH used

d.  The amount of heat evolved per mole of NaOH

2.  Express the above results as heats of reaction.

3.  Write the net ionic equations for reactions 2 and 3.

4.  In reaction 1, ∆H1 represents the heat evolved as solid sodium hydroxide dissolves. Look at the net ionic equations for reactions 2 and 3 and make similar statements as to what ∆H2 and ∆H3 represent.

5. 

a.  Compare ∆H2 with (∆H1 + ∆H3) and explain in terms of your answers to question #4.

b.  Calculate the percentage difference between ∆H2 and (∆H1 + ∆H3).

6.  Using the appendix table of ∆Hf⁰ and the species included in your net ionic equations calculate the ∆Hrxn for each of the three net ionic equations. Use these three net ionic equations as the mechanism with which to determine the ∆Hrxn. Calculate a percent error between your values and those calculated for ∆H2.

7.  Suppose you had 8 grams of NaOH in reaction ; use increase, no change, or decrease to answer each of the following. Justify your choice.

a.  How would this have affected the change in temperature?

b.  How would this have affected the relative number of joules evolved in your experiment?

c.  What effect would this have had on your calculations of ∆H1, the heat evolved per mole of NaOH?