GC2LB9 - lab report 9 from the textbook PDF

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General Chemistry 2 SCC202.239B Afsana Abdul Rahim Lab Partners: Sarah, Jacob Experiment 9: Understanding Enthalpy in Neutralization Reactions 05/15/2018 Professor Amit Aggarwal

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Objective: 

To experience the technique of Calorimetry and gain experience in using a thermometer.



To determine the heat of neutralization for a reaction of HCl (a strong acid) and NaOH (a strong base).

Materials required: 

100 mL beaker



250 mL beaker



25 mL graduated cylinder



100 mL graduated cylinder



2 dry Styrofoam cups with lids



Thermometer



1.0 M Sodium hydroxide (NaOH)



1.0 M Hydrochloric acid (HCl)

Methods: 1. Volumes of 1.0 M HCl and 1.0 M NaOH needed for each mixture as shown in Data Table 1 were obtained in graduated cylinders. 2. Vials of acid and base were mixed together in Styrofoam cups to make 100 mL of different product solutions according to the different volume ratios indicated in Data Table 1. 3. The temperature changes was recorded immediately upon mixing in Data Table 1, one at a time. 4. The mixtures were discarded in the proper waste containers when the entire experiment was completed.

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Data: Data Table 1: Temperature Changes during Mixing of Different Volumes of 0.10 M HCl and 0.10 M NaOH Experiment Number 1

2

3

4

5

6

7

8

9

10

11

HCl (mL)

100

90

80

70

60

50

40

30

20

10

0

NaOH (mL)

0

10

20

30

40

50

50

70

80

90

100

Room Temp (°C)

22

22

22

22

22

22

22

22

22

22

22

Temp of mix (°C)

22

23

24

26.8 28

28

27

26

25

24.5

22

∆T (°C)

0

1

2

4.8

6

5

4

3

2.5

0

6

The following calculations are done using the data. 

Mass of NaCl solution = sum of masses of reactants= 100.0g



T of NaCl solution = Tfinal – Tinitial = 6.0C



Heat absorbed by solution, Q= mass of solution x specific heat of the solution x T = 100.0 g x 4.18 J/gC x 6.0C = 2510.6 J



Heat released by reaction = - heat absorbed by solution = -2510.6 J



Moles of NaCl formed: Moles of NaOH used = molarity of NaOH X volume of NaOH in L = 1.0 M NaOH X 0.05L NaOH = 0.05 moles of NaOH

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

Ratio of NaOH: NaCl = 1:1; therefore, 0.05 mol of NaCl formed.



Heat release per mole of NaCl = (1/0.05) x -2.51 x 103 J = -50212 J



Value of heat release per mole = 50.21 kJ



True value of heat of neutralization of HCl and NaOH = 57.62 kJ



% Error in specific heat of metal =

(true value−experimental value) true value

X 100

= 12.86% Figure 1: Graph of temperature change versus volume of NaOH.

Discussion: According to the first law of thermodynamics, energy that is lost by a system must be gained by the surroundings, and vice versa (Zumdahl & Zumdahl, 2008, p. 209). Calorimetry, the measurement of heat flow, is based on observations about temperature changes. The device that is used to measure the heat flow is called a calorimeter. It can be used to derive specific heat of

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substances and enthalpy of a system or reaction. Enthalpy is a thermodynamic quantity that measures the total heat of a system. When substances such as acids and bases in solution are mixed together reactions can occur and this leads to heat being transferred either into or out of the system. When heat is transferred into the system the reaction is absorbing heat and is called endothermic; when heat is transferred out of the system the reaction is releasing heat and is called exothermic. Many reactions are exothermic and one example is a reaction between an acid and a base, called a neutralization reaction that results in the formation of a salt and water. The reaction between NaOH and HCl is represented by the chemical equation below: NaOH (aq) + HCL (aq)  NaCl (aq) + H2O (l) Since strong acids (substances that completely dissociate into ions, including H+ ions) and strong bases (substances that completely dissociate into ions, including OH- ions) are highly reactive, it is predicted that they possess higher potential energy (at molecular level) than water and salt, which are weekly reactive. This is also descriptive of an exothermic reaction in which the potential energy of products is lower than that of the reactants (Jespersen, 167). When HCl and NaOH were mixed in 1:1 volume ratio in the experiment, the temperature of the mixed solution increased maximum (Data Table 1 and Figure 1) because of complete dissociation and higher energy which resulted in large amount of heat release during the chemical reaction. Since the reaction released heat, the reaction was exothermic. Exothermic reactions are indicated by a negative sign for H, which represents heat flow at constant pressure. Based on the temperature change, the specific heat, and the mass of the solution, the heat released by the reaction was -2510.6 J. The amount of heat per mole of NaCl was -50.21 kJ/mol.

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Considering the simplicity of the coffee cup calorimeter, the calorimeter was very effective. For the enthalpy of neutralization of HCl and NaOH, the accepted value is -57.62 kJ/mol (Lide, 2000). Based on the accepted value, the experimental value had a percent error of 12.86%. One possible explanation for the inaccuracy is that the coffee cup calorimeter is not a perfect insulator and thus heat could have escaped, resulting in a slightly lower temperature recording. Even though there was a small percent error, the coffee cup calorimeter was able to provide approximate values for thermodynamic quantities. Errors and precautions taken: 1. Volumetric measurements can have different readings depending on the observer’s estimations and perspective. To minimize errors, volumetric measurements were read at eye level and at the bottom of the meniscuses. 2. Although the coffee cup calorimeter was treated as a perfectly isolated system for the calculations, it was possible for heat to escape the coffee cup. Heat could have escaped from the cup before the lid was placed on the cup or it could have escaped through the small gap between the thermometer and the straw hole. In order to account for the imperfect insulation, the temperature of the calorimetric contents was monitored and recorded immediately after the lid was closed.

Conclusion: Calorimetry can be used to determine the heat of a neutralization reaction. Temperature changes, which can be measured with a thermometer, correspond with the heat lost or gained by

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substances. If the surroundings increase in temperature, then the system must have given off heat. For the enthalpy of the neutralization reaction of HCl and NaOH, the experimental value of heat release by the reaction was found to be -2510.6. The heat released per mol of NaCl was -50.21 kJ/mol. The experimental value had a 12.86% error when compared to the accepted value, 57.62 kJ/mol. References: Jespersen, Neil. "Chemistry: Barron's College Review." Barron's Educational Series, Incorporated, 28 Apr. 1997. Web. 19 May 2018. Lide, D. R., (2000). CRC Handbook of Chemistry and Physics. Boca Raton, Fl: CRC Press LLC. Tomoyuki T., Kana, S., and Nobuyoshi K. Experiment 9: Understanding Enthalpy in Neutralization Reactions. LaGuardia Community College, NY. 2018 (lab handouts). Zumdahl, S. S. and Zumdahl, S. A. (2008). Chemistry. (8th ed. pp.244). Belmont, CA: Brooks Cole....