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Reference:
E. Stelter, Basic Chemistry Laboratory Manual, Experiment 11 — Heat of
Reaction, Part I, p. 57, The Lawrenceville School, 1986.
Discussion: The quantity of heat evolved or absorbed during a chemical reaction is viewed as
the difference in enthalpy between the products and reactants, !H. In an exothermic process,
the products have a lower enthalpy than the reactants as indicated by a certain amount of
thermal energy released. An exothermic process has a negative enthalpy change. In an
endothermic process, a certain amount of heat is absorbed so that the enthalpy of the products
is greater than the enthalpy of the reactants — !H is positive.
In this experiment you will determine the enthalpy changes associated with three exothermic
chemical reactions. A Styrofoam cup will serve as the calorimeter. The heat produced in each
reaction will increase the temperature of a given mass of water by a certain number of degrees:
heat absorbed by water = (mass of water) x (heat capacity of water) x (temperature change)
!H = m · Cp · !T
The heat capacity of a substance is the amount of thermal energy necessary to change the
temperature of one gram of the substance by one degree Celsius. The heat capacity of water is
4.184 J/g °C (or 1.000 cal/g °C).
The following important assumptions are made in order to simplify the calculations:
1. Assume that any heat lost to the surroundings is insignificant.
2. Assume that the reactant solution is essentially water so that
a. The heat capacity of water can be used.
b. The density of water, 1.000 g/mL, can be take as the density of each reaction solution.
Thus, the volume of the solution in milliliters is equal to its mass in grams.
The three reactions are:
A. Dissolving solid sodium hydroxide in water.
NaOH (s) ! Na+ (aq) + OH– (aq) + HEAT
!HA = –!HA(H2O)
B. The reaction of solid sodium hydroxide with aqueous hydrochloric acid.
NaOH (s) + H+ (aq) + Cl– (aq) ! Na+ (aq) + Cl– (aq) + H2O (l) + HEAT
!HB = –!HB(H2O)
C. The reaction of aqueous sodium hydroxide with aqueous hydrochloric acid.
Na+ (aq) + OH– (aq) + H+ (aq) + Cl– (aq) ! Na+ (aq) + Cl– (aq) + H2O (l) + HEAT
!HC = –!HC(H2O)
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Materials Needed:
sodium hydroxide pellets
0.50 M hydrochloric acid
1.0 M hydrochloric acid
1.0 M sodium hydroxide
small Styrofoam cups
TI-83+ and CBL temperature probe
250 mL beaker
100 mL graduated cylinder
Procedure:
1. Determination of the change in enthalpy of reaction A.
CAUTION!! SODIUM HYDROXIDE IS CORROSIVE TO THE SKIN AND MAY CAUSE
BLINDNESS IF IT GETS INTO THE EYES.
Pour 50 mL of water into a Styrofoam cup. Stir gently with a thermometer until constant
temperature is reached. Measure this temperature as precisely as possible. Weigh out about
1.0 g of sodium hydroxide pellets. Use a spatula to handle the reagent and not your fingers.
Since sodium hydroxide deliquesces (absorbs water from the air), the instructor will tell you
the approximate number of pellets in 1.0 g. Quickly transfer the NaOH to the water in the
Styrofoam cup. Put the thermometer into the solution and stir gently and steadily until the
NaOH has dissolved. What might happen is the solution is stirred vigorously? Record the
highest temperature reached. Discard the solution and rinse the cup thoroughly with water.
2. Determination of the change in enthalpy of reaction B.
Repeat the steps in part 1, substituting 50 mL of 0.50 M HCl for the tap water.
3. Determination of the change in enthalpy of reaction C.
Measure 25 mL of 1.0 M HCl into the Styrofoam cup and 25 ml of the 1.0 M NaOH into a
second Styrofoam cup. Both solutions should be no warmer than room temperature. Be sure
to rinse and dry the thermometer before changing from one solution to another. Record the
temperatures. Add the sodium hydroxide solution to the hydrochloric acid solution in the
Styrofoam cup. Mix rapidly and record the highest temperature reached.
68
Experiment #21
DATA SHEET 1—Heat of Reaction and Hess!s Law
Reaction A — Addition of 1g Solid NaOH to 50 mL Water
Trial #1
Trial #2
mass of NaOH (g)
moles NaOH
volume of water (mL)
mass of water (g)
initial T (°C)
final T (°C)
!T
Cwater (J/g·°C)
4.184
4.184
Q (J) = masswater · Cwater · !T
!HA (kJ/mol NaOH)
Reaction B — Addition of 1g Solid NaOH to 50 mL of 0.5 M HCl
Trial #1
Trial #2
mass of NaOH (g)
moles NaOH
volume of "water" (mL)
mass of water (g)
initial T (°C)
final T (°C)
!T
Cwater (J/g·°C)
4.184
4.184
Q (J) = masswater · Cwater · !T
!HB (kJ/mol NaOH)
Reaction C — Addition of 25 mL of 1.0 M NaOH to 25 mL of 1.0 M HCl
Trial #1
Trial #2
vol of NaOH (mL)
conc. NaOH (M)
moles NaOH
vol of HCl (mL)
total volume of "water" (mL)
mass of water (g)
initial T (°C)
final T (°C)
!T
Cwater (J/g·°C)
1.0
1.0
4.184
4.184
Q (J) = masswater · Cwater · !T
!HC (kJ/mol NaOH)
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Experiment #21
DATA SHEET 2—Heat of Reaction and Hess!s Law
Reaction A — Addition of 1g Solid NaOH to 50 mL Water
!HA (kJ/mol)
Trial #1
Trial #2
Trial #3
Average
Reaction B — Addition of 1g Solid NaOH to 50 mL of 0.5 M HCl
!HB (kJ/mol)
Trial #1
Trial #2
Trial #3
Average
Reaction C — Addition of 25 mL of 1.0 M NaOH to 25 mL of 1.0 M HCl
!HC (kJ/mol)
Trial #1
Trial #2
Trial #3
Average
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