The Heat of Reaction
In this experiment you will use simple calorimeter as a reaction vessel to measure the heat evolved or absorbed during the reactions.
You may assume that the heat of reaction will be used to change the temperature of the aqueous solution and of the reaction vessel
and neglect other small losses to the surroundings. Recall that it takes one calorie to change the temperature of one gram of water one degree
Celsius. You do not need to weigh the water used since one milliliter of water has a mass of one gram and you will measure its volume to the
nearest milliliter. When the reactants are added to your reaction vessel, you should note the change in temperature to the nearest 0.2 ˚C.
From the change in temperature and the weight of the reactants you can calculate the number of calories evolved or absorbed. In this
experiment you will measure and compare the quantity of the heat involved in three reactions:
Reaction 1: Solid sodium hydroxide dissolved in water to form an aqueous solution of ions:
NaOH(s)  Na⁺(aq) + OH⁻(aq)
△H₁ =
Reaction 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 + Na⁺(aq) + Cl⁻(aq)
△H₂ =
Reaction 3: An aqueous solution of sodium hydroxide reacts with an aqueous solution of hydrogen chloride to form water and an aqueous
solution of sodium chloride:
Na⁺(aq) + OH⁻(aq) + H⁺(aq) + Cl⁻(aq)  H₂O + Na⁺(aq) + Cl⁻(aq)
△H₃ =
Procedure
To determine the heat of reaction (1):
a. Weigh a clean reaction vessel to the nearest 0.01g.
b. Put 100.0ml of cool tap water into the reaction vessel. Stir carefully with a thermometer until a constant temperature is reached.
Record this temperature to the nearest 0.2˚C.
Weigh about 2g of solid sodium hydroxide, NaOH, to the nearest 0.01g. Weigh quickly, since sodium hydroxide becomes
moist in the open air.
d. Transfer NaOH(s) into the water in the reaction vessel. Stir/swirl the water in the vessel until the sodium hydroxide is
dissolved. Place the thermometer into the vessel and record the extreme temperature reached.
*Before proceeding to reaction (2), rinse the reaction vessel thoroughly with distilled water.
c.
To determine the heat of reaction (2);
Repeat steps a, b, c and d above except in step b substitute 100ml of 0.25M HCl for tap water.
*Before proceeding to reaction (3), rinse the reaction vessel thoroughly with distilled water.
To determine the heat of reaction (3):
a. Prepare 50.0ml of 0.5M NaOH solution. Place 50.0ml of 0.5M HCl in the reaction vessel. Be sure that both of these solutions
are at or slightly below room temperature. You should check the temperature of each solution with a thermometer, being
careful to rinse and dry it before transferring it from one solution to the other. Record the temperatures of both solutions.
b. Add the sodium hydroxide solution to the hydrochloric acid solution. Mix quickly and note the highest temperature reached.
Calculations
1. For each reaction, calculate:
a. The change in temperature
b. The amount of heat absorbed by the solution
c. The amount of heat absorbed by the reaction vessel DO NOT DO THESE
d. The total amount of heat absorbed
e. The number of moles of NaOH used in each reaction
f. The total amount of heat involved per mole of NaOH
2.
Express your results as heats of reaction: △H₁, △H₂, △H₃.
3.
Compare △H₂ with △H₁ + △H₃ and explain.
4.
Calculate the percent difference between △H₂ and △H₁ + △H₃, assuming △H₂ to be correct.
Questions
1.
Using Hess’ Law find the summation of energy, enthalpy , for the reaction NaOH(s) + H⁺(aq) + Cl⁻(aq)  H₂O + Na⁺(aq) +
2.
Cl⁻(aq) using the reactions 1 and 3 please show work.
Suppose you had used 4g of NaOH(s) in reaction (1):
a. What would be the number of kilojoules evolved?
b.
What effect would this have on your calculation of △H₁, the heat evolved per mole?