HEATS OF REACTION
Pre-Lab Discussion:
The energy involved in a chemical reaction is expressed in terms of the amount of heat released or
absorbed during the course of the reaction. The heat of reaction, ( H), is measured in calories per mole. By
convention, when H is included as part of the equation for a reaction, it is placed on the product side of an
exothermic reaction and on the reactant side of an endothermic reaction.
To illustrate, consider the equations for the synthesis (exothermic reaction) and decomposition
(endothermic reaction) of 1 mole of liquid water:
(Synthesis) H2 (g) + ½ O2 (g) H2 (l) + 284 kJ
(Decomposition) H2O (l) + 284 kJ H2 (g) + ½ O2 (g)
Note that when this convention is followed, the value of H in an equation is always positive. However,
when heat of reaction is considered separately---that is, when it is not an integral part of an equation---some method
must be used to tell whether the heat is released or absorbed. For his purpose, H of an exothermic reaction (heat
released) is given a minus sign; H of an endothermic reaction (heat absorbed) is given a plus sign. Thus, the
heat of formation of 1 mole of water is given as – 284 kJ. The heat of decomposition of 1 mole of water is + 284
kJ.
In this experiment, you will study three related exothermic reactions involving sodium hydroxide (NaOH),
a strong base. In the first reaction solid sodium hydroxide will be dissociate into ions as it dissolves in water. The
heat produced by this reactions ( H1) is called the Heat of solution of NaOH. In the second reaction, an aqueous
solution of NaOH will be reacted with an aqueous solution of hydrochloric acid (HCl). The heat of this reaction (
H2) is called the heat of neutralization of NaOH. In the third reaction, solid NaOH will be reacted with an aqueous
solution of HCl. This reaction is actually a combination of the first two reactions. The solid NaOH will dissociate
into ions as it dissolves in the acid solution and then is neutralized by the acid. Thus, the heat of this reaction (
H3) should be equal to H1 + H2. Using calculations based on the data collected in this experiment, you will
attempt to verify the additive nature of heats of reaction.
This experiment should provide a better understanding of heats of reaction and reinforce your knowledge of
calorimetry.
Purpose
Determine heats of reaction of three related exothermic reactions. Show the additive nature of the heats of
reaction of these reactions.
Equipment
Balance Thermometer
Styrofoam cup Safety goggles
Graduated cylinder (100 ml) Lab apron
Microspatula
Materials
0.5 M HCl NaOH pellets
1.0 M HCl 1.0 M NaOH
Distilled water
Safety
Solid sodium hydroxide and concentrated aqueous solutions of sodium hydroxide are highly corrosive to
the skin and eyes.
Procedure
PART A
1. Measure exactly 100.0 ml of distilled water into a graduated cylinder. Pour the water into a clean dry,
Styrofoam cup and allow it to stand until it reaches room temperature.
2. Using a microspatula to handle the pellets measure out 2.00 g of sodium hydroxide (NaOH) onto the
balance.
3. Measure the temperature of the water in the Styrofoam cup. Record this as Ti in part A of the data table.
4. Add the NaOH pellets to the water in the cup. Use the thermometer to stir the mixture until all the
NaOH has dissolved and the temperature stops rising. Record the highest temperature as Tf in Part A of the data
table.
5. Discard the solution. Rinse off the thermometer and rinse and dry the Styrofoam cup.
PART B
6. Measure out exactly 50.0 ml of 1.0 M HCl and pour it into the Styrofoam cup. Allow the acid to stand
until it reaches room temperature. Record this temperature as Ti in part B of the data table.
7. Add exactly 50.0 ml NaOH solution to the HCl solution. Stir carefully with the thermometer. Record
the highest temperature as Tf in Part B of the data table.
8. Discard the solution. Rinse off the thermometer and rinse and dry the Styrofoam cup.
PART C
9. Measure out exactly 100.0 ml of 0.5 M HCl and pour it into the Styrofoam cup. Allow it to stand until it
reaches room temperature.
10. Measure out about 2.00 g of NaOH crystals as in step 2.
11. Measure the temperature of the acid in the cup. Record this as Ti in part C of the data table.
12. Add the NaOH pellets to the acid and stir the mixture with the thermometer. Record the highest
temperature as Tf in part C of the data table.
Observations and Data
DATA TABLE
PART A
Mass of 100 ml of H2O 100 g
Mass of NaOH pellets ______ g
o
Ti C
o
Tf C
PART B
Approximate mass of
50 ml NaOH(aq) + 50 ml HCl(aq) 100 g
o
Ti C
o
Tf C
PART C
Approximate mass of 100 ml of HCl(aq) 100 g
Mass of NaOH pellets _______g
o
Ti C
o
Tf C
Calculations
PART A
1. Find T: T = Tf – Ti o
C
2. Find the number of joules absorbed by the H 2O
(released by the NaOH) J
no. of joules = ______mass(in grams) of H2O x T x 4.18 J/g-oC
3. Find the number of joules released per gram of NaOH
_________ joules( from#2)
J/g NaOH = = _________J/g
____ gNaOH
4. Find H1 in kJ/mole of NaOH kJ/mole
H1 = ______J/g NaOH x 40 g NaOH/mole NaOH x 0.001 kJ/J
PART B
5. Find T: T = Tf - Ti o
C
6. Find the number of joules produced by the reaction
of NaOH(aq) and HCl(aq): J
no. of joules = ______mass of solution(in grams) x T x 4.18 J/g-oC
7. Find H2 in kJ/mole NaOH (NOTE: 50 ml of 1.0M NaOH(aq)
contains 0.050 mole NaOH):
kJ/mole
_________ joules( from#6)
H2 = x 0.001 kJ/J
0.05moleNaOH
PART C
8. Find T: T = Tf - Ti o
C
9. Find the number of joules absorbed by the HCl
solution(released by the NaOH) J
no. of joules = _____mass(in g) of HCl(aq) x T x 4.18 J/g-oC
10. Find the number of joules released per g of NaOH: J/g
________ joules( from#9)
J/g NaOH = = ______J/g
____ gNaOH
11. Find H3 in kJ/mole NaOH: kJ/mole
H3 = ______ J/g NaOH x 40 g NaOH/mole NaOH x 0.001 kJ/J
Questions:
1. Write ionic equations for the three reactions observed in this experiment:
Part A:
Part B:
Part C:
2. In your own words, describe the process(or processes) that produced the three heats.
3. Write the algebraic equation, using the symbol H with subscripts, to show the relationship
between the heats of reaction of the three reactions in this experiment.
4. Does your experimental data verify the equation in question 3?
5. What are some possible sources of error in this experiment?
6. Does this experiment illustrate the law of conservation of energy?
Conclusion:
How does the data collected support Hess's Law of Additively? How does this relate to conservation of
energy?