Additivity of Heats of Reaction- Hess’s Law Lab Report PDF

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Additivity of Heats of Reaction- Hess’s Law Lab Report...

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Shannon Urmetz Chem 266 sec 01 2702902 Additivity of Heats of Reaction: Hess’s Law Lab Report Introduction In this lab we tested Hess’s law by measuring the heat released in three reactions. Hess’s law states that the total enthalpy change for the reaction, will be the sum of all those changes, no matter how many different steps or stages in the reaction there are (Cohen, 2016). The equations for the reactions in the experiment done are as follows:

(1) NaOH(s) Na+(aq) + OH–(aq) (2) NaOH(s) + H+(aq) ) + Cl–(aq) H2O(l) + Na+(aq) + Cl–(aq) (3) Na+(aq) + OH–(aq) + H+(aq) ) + Cl–(aq) H2O(l) + Na+(aq) + Cl–(aq) Material and Methods 

Obtain and wear goggles

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Connect the temperature probe to your lab quest place Styrofoam cup in 250 mL bigger to steady, measure out 100 mL of Water into Styrofoam cup and place in temperature probe.

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Weight out about 2 g of NaOH and record mass to nearest .01 g. start data collection with temperature probe and obtain initial temp t1, monitor temperature. After three or four readings at same temperature have been obtained add your measured NaOH to the solution. Stir continuously until the temperature reaches its max point and begins to drop. Record the maximum temperature, t2, before the drop occurred.

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Rinse and dry temperature probe, Styrofoam cup, and stirring rod. Dispose of solution and aqueous waste bin.

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Complete all steps the same for reaction 2 and reaction 3. EXCEPT: In reaction 2 use 100 mL of 0.50 M hydrochloric acid, HCL, instead of water. Use the same amount of solid NaOH as before. In reaction 3 measure out 50 mL of 1.0 M HCL instead of water and 50 mL of 1.0 M NaOH solution into a graduated cylinder. After three- four temperature readings for an initial temperature have been taken, add the 1.0 M NaOH.

Results Reaction 1 2.00 g

Reaction 2 1.97 g

Reaction 3 No solid NaOH mass

100.0 g

100.0 g

100.0 g

28.1 C 22.2 C

34.0 C 21.6 C

27.7 C 21.7 C

5.9 C

12.4 C

6.0 C

6. Heat, q

2.4662 KJ

5.1832 KJ

2.508 KJ

7. Change in Heat

-2.47 KJ

-5.18 KL

-2.51 KJ

8. Moles of NaOH

.05 mol

.049 mol

.05 mol

9. Change in Heat/ mol

-49.4 KJ/mol

-105.71 KJ/mol

-50.2 KJ/mol

1. Mass of Solid NaOH 2. Mass (total) of Solution 3. Final Temp, t2 4. Initial Temp, t1 5. Change in Temp

10. Experimental Value: -99.6 11. Accepted Values: -105.71 12. Percent Error: 5.7 % Discussion In this lab we were supposed to use these experiments to prove Hess’s Law. In theory, when combining the steps/ stages of all reactions, we came pretty close to equaling the 2nd equation. Thus proving that the total enthalpy change was (nearly) the sum of all reactions. With all experiments comes the chance for error, in this case erros could be present in measuring out the NaOH, inaccuracies with the stock solution of the 1.0 M HCL or NaOH, problems measuring out our solutions, as well as logger pro or temperature probe issues. Conclusion After this experiment, I feel as though I have an overall better understanding of the principles of Hess’s Law, as well as real world applications of it. With a percent error of 5.7 % I would say the experiment was a success....