I.          Title:  Heats of Reaction

II.        Purpose

Text Box: The purpose of this lab experiment is to record the highest temperature of a mixture of an acid and a base, to calculate the heat of solution and heat of neutralization, and to relate that heats of reaction to changes in energy and what causes this. In addition to this, it will be shown that heats of reaction can be additive.

III.      Hypothesis

Text Box: This experiment will show that heat will be from all reactions, and that heat released from reactions one and two together will be equal to the heat released in reaction three.

IV.       Materials

Text Box: styrofoam/plastic cup stirring rod spatula balance/scale, centigram thermometer, -10 to 110 C watch glass graduated cylinder, 100 mL NaOH pellets

V.         Procedure

Text Box: Reaction 1 1. 50 mL of distilled water were measured (to the nearest mL) in a 100 ml graduated cylinder, and poured into a cup (plastic), the temperature was recorded to the nearest 0.1 șC. 2. The mass of a watchglass was determined, and then two grams of NaOH were added to the mass and recorded to the nearest 0.01g. 3. The pellets were then placed into the cup with the distilled water, and stirred gently. Reaction 1 (con't) 4. The solution began to increase in temperature and the highest temperature was recorded to the nearest 0.1oC. 5. The solution was disposed of in the sink with running water. Reaction 2 1. After cleaning, 25 mL of 2.0 M HCl (to the nearest mL) was added to the cup. The volume was (to the nearest mL) and the temperature (to the nearest 0.1oC) were recorded. 2. 25 mL of 2.0 M NaOH (to the nearest mL) was obtained and the temperature was measured (to the nearest 0.1 oC). 3. The NaOH was gently poured into the cup containing the 2.0 M HCl, and gently stirred. 4. The temperature of the new solution was recorded at its peak to the nearest 0.1 oC 5. The solution was disposed of in the sink with running water.

VI.       Data/calculations/results

Data Table 1

Mass of watch glass

20.9

g

Mass of watch glass plus NaOH pellets

22.9

g

Volume of H2O

50

mL

Initial temperature of water

21.5

oC

Final temperature of water

31.5

oC

Data Table 2

Volume of 2.0 M HCl

25

mL

Initial temperature of HCl

22.0

oC

Volume of 2.0 M NaOH

25

mL

Initial temperature of NaOH

22.0

oC

Final temperature of water

34.0

oC

Data Table 3

Volume of 1.0 M HCl

50

mL

Temperature of 1.0 M HCl

22.0

oC

Mass of watch glass plus NaOH pellets

22.9

g

Highest temperature of solution

44.0

oC

Calculations Table** (see attached lab for calculations)

 

Reaction

Δt

(oC)

Heat Released

(J)

Moles NaOH

(mol)

ΔH per Mole NaOH

(kJ/mol)

1

10.0

2090

0.05

41.8

2

12.0

2510

0.05

50.2

3

22.0

4600

0.050

92.0

ΔH1 +  ΔH2 = _______92.05________ (kJ/mol)

Percent Discrepancy = ____0_________ %

VII.     Conclusions

Text Box: This experiment was conclusive in supporting the hypothesis stated above. It can be stated that "heats of reaction can be additive". This is demonstrated using the values obtained from the calculations table above. The sum of the ΔH for reactions 1 and 2 are equal to the ΔH for reaction three (41.8 kJ/mol + 50.2 kJ/mol = 92.0 kJ/mol). This is also demonstrated by the percent discrepancy calculated in the lab packet, equal to 0 %. It also holds that since the sum of net ionic reactions for reactions 1 and 2 are equal to the net ionic reaction for reaction 3 (calculation 6 in lab packet), that the ΔH's would be additive also. The heat of solution, or the amount of heat absorbed or released when a specific amount of solute (in this case 2.0g NaOH) dissolves in a solvent (water or HCl). These values are calculated as the ΔH for reactions 1 and 3 (see calculations table). The heat of neutralization, or amount of heat released or absorbed during a reaction of H+ ions (acids) and OH- ions (bases) to form water, is equal to the ΔH for reactions 2 and 3 (see calculations table). In some instances, such as reaction 3, the heat of solution and heat of neutralization are the same value, since the solute contained OH- ions, and the solvent contained H+ ions. Both of these recorded values are different forms of heats of reactions. In each of the reactions, heat was released from the compounds into the environment/solutions. This is evident due to the rise in solution temperature during each of the reactions, before the change in heat had even been calculated. Had heat/energy been absorbed, then the temperatures of the solutions would have decreased. This heat was released by the breaking of the ionic bonds in the compounds involved, as they reacted with the

 

VIII.   Error analysis

Text Box: The possibility for error exists in any laboratory setting, and any experiment. In this particular lab, the results deviated from accepted values due to a few variables. First, since no Styrofoam cups were available, a plastic cup was substituted. This allowed for greater quantities of heat to be lost to the environment since plastic, especially the thin plastic of the cup, is not as good of an insulator as the Styrofoam would have been. This extra loss of heat, when applied to each of the first two reactions, is rather small, but when the two reactions are added together, the heat lost becomes quite significant, when compared to the amount lost in reaction three. In order to contain, or limit the amount of heat lost, one could perform the reactions inside of a calorimeter, or similar container with a greater capacity for insulation.