enthalpy lab document copy

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Enthalpy of Neutralization Name: Iris Diaz Hand-In, Chem 150L Partner: Meghan Swanson 1. (1 point) Are the reactions you observed in this experiment exothermic or endothermic? Explain based on your observations . To determine if this reaction is endothermic or exothermic you have to look at the enthalpy change ( ∆࠵? ). If it is positive then it is a endothermic reaction (absorbs heat). However, if the enthalpy change is negative then, it is a exothermic reaction (releases heat). Reaction #4= NaOH(aq) + HNO 3 (aq). In this reaction, NaOH(aq) and HNO 3 (aq) react together and produce NaNO 3 and water ࠵? ! ࠵? . When reactions produce water they are typically exothermic reactions. Additionally, the reaction between NaOH(aq) and HNO 3 (aq) is a acid- base neutralization reaction, which is also exothermic most of the time. So, this reaction would be exothermic. Reaction #5 = KOH(aq) + HNO 3 (aq). This reaction is exothermic because looking at our observation we saw that it released heat because when we touched the beaker it was hot. It is also a acid-base neutralization reaction and it also involves the formation of water which are usually exothermic reactions. Reaction #6= KOH(aq) + HCl(aq). This is also a exothermic reaction because it released heat. It also produces water, and it is also a acid-base neutralization reaction. And even reaction #1, #2, and #3 are also exothermic because they produce heat. So, in conclusion, all the reactions in this experiment are exothermic because they all release heat. 2. (5 points) Attach your Temperature vs. Time graph below for Reaction #2 only . No graph, no points. Also include a typed version of your group’s data table for Reaction #2 (containing rows a–l of measured and calculated values). Show and briefly explain ALL the calculations for any value in the table that is calculated (i.e., moles of each reactant, T f , Δ T , q rxn , and Δ H 2 ). Be sure you are using Equation Editor for showing your calculations, and round all results to the appropriate number of significant figures.

1. Concentration of HCl (M) à 2.02 M 2. Volume of HCl used (ml) à 100ml 3. Moles of HCl used à 0.202 mol HCl 4. Temperature before reaction, Ti ℃ à 20.9 ℃ 5. Extrapolated final temperature of mixture, Tf ℃ à 51.21 ℃ 6. Temperature rise of the solution, ∆࠵? ℃ à 30.31 ℃ 7. Heat produced by the reaction, qrxn (J) à -12.68 J 8. Mass of NaOH used (g) à 5.556 g 9. Moles of NaOH used à 0.1389 mol 10. Litmus test result after reaction à red/acid 11. Limiting reagent (LR) identity à NaOH 12. AH per mol LR, A ࠵? ! (KJ/mol) à -91.29 KJ/mol Calculations: - Finding the moles of HCl used: moles = ࠵? × ࠵? = 0.1 × 2.02 = 0.202 ࠵?࠵?࠵? ࠵?࠵?࠵? - Extrapolated temperature = ࠵? × ࠵? + ࠵? = (−0.01174)(18.5) + (51.43) = 51.21℃ - Qrxn = (−100)(4.184)(30.31) = −12681.70 ÷ 1000 = −12.68 J - AH = "#!.%& ’.!’! = −91.29 ࠵?࠵?/࠵?࠵?࠵? 3. (4 points) a. Write the balanced chemical equation (molecular, NOT net ionic) for each of the first three reactions studied. Be sure to include phase labels. After each reaction, indicate with a word or phrase what is happening in that reaction at the atomic/molecular level. Rxn #1: NaOH(s) ® NaOH(aq) When NaOH is added to water, it produces interactions that weaken the ionic bonds between Na+ and OH-, so due to this, the reaction turns into a aqueous reaction. Rxn #2: NaOH(s) + HCl(aq) ® NaCl(aq) + ࠵? ! ࠵? (l) When NaOH is added to HCl, the OH- ions and the H+ ions from HCL react together to form water ( ࠵? ! ࠵? ). Also. The Na+ and the Cl- recat together to form NaCl in the aqueous solution. Rxn #3: NaOH(aq) + HCl(aq) ® NaCl(aq) + ࠵? ! 0 (l) When NaOH(aq) and HCl(aq) are mixed together, the OH- ions and the H+ ions from HCL react together to form water ( ࠵? ! ࠵? ), and the Na+ and Cl- ions are dissolved in the aqueous solution. b. How are these three reactions related to each other? Explain. (Note: This question is not asking about the enthalpies for these reactions, but only the chemical reactions [reactants and products] themselves. Do not talk about the enthalpies or heats in this answer. Do not confuse this question with the next one.) The 1 st reaction is the dissolution of NaOH (solid to aqueous). In the 2 nd reaction NaOH and HCl react together to produce water and NaCl (salt). In the 3 rd reaction happens the same thing as in the 2 nd reaction but in a more aqueous form.

4. (2 points) Is there a relationship between the class average values of Δ H for Reactions #1-3 that is consistent (within experimental error) with the relationship between the reactions themselves that you noted in Question 3 above? Explain this relationship, being sure to address experimental error, and show your work if there is a calculation involved. Based on the class average values of ∆࠵? for reactions #1 and #3. Reaction #1 = -37.5066667 and Reaction #3 = -51.13571429. The average enthalpy change for reaction #3 is more negative than reaction #1. As I said before, a negative value in ∆࠵? means that the reaction in exothermic, so it releases heat. So, because Reaction #3 has a more negative number, it is a more exothermic reaction than #1. So, #3 releases more heat that #1. 5. (4 points) List the class average values of D H 3 , D H 4 , ∆ H 5 , and ∆ H 6 (including experimental error, rounded appropriately). How do the values of D H 4 , ∆ H 5 , and ∆ H 6 compare with the value of D H 3 (within experimental error) ? Is this what you would expect considering the net ionic equations for all of these different reactions? Explain your answer, being sure to write the net ionic equation(s) for these reactions. class average values of D H 3 = -52.48 D H 4 = -53.27 ∆ H 5 = -54.9 ∆ H 6 = -54.5 D H 4 is more negative than D H 3 , so it releases a little bit more of heat compared to D H 3. ∆ H 5 is more negative than D H 3 , so it releases more of heat compared to D H 3. ∆ H 6 is more negative than D H 3 , so it releases more of heat compared to D H 3. So, the reaction from D H 4 , ∆ H 5 , ∆ H 6 are more exothermic than the reaction for D H 3. 6. (4 points) Applying the principles learned in this lab, calculate the D H values for Reaction #8 and Reaction #9 below. Use class average data (for Reactions #4-6, as appropriate) and the enthalpy value given below for Reaction #7. Show your work for each calculation, and explain carefully in words (and chemical equations if necessary) the reasoning behind each of your calculations/predictions. Reaction #7 KOH( s ) ® KOH( aq ) Δ H 7 = –57.4 kJ/mol Reaction #8 KOH( s ) + HCl( aq ) ® KCl( aq ) + H 2 O( l ) Reaction #9 KOH( s ) + HNO 3 ( aq ) ® KNO 3 ( aq ) + H 2 O( l ) To calculate Δ H for reaction #8 we can use Hess’s Law, which is when the Δ H of a reaction is the same whether it occurs in one step or in multiple. So, Δ H 8 = Δ H 7 + D H 4 (dissolution of KOH(aq) + the neutralization of KOH(aq) with HCl(aq) Δ H 8 = -57.4 KJ/mol + (-53.26875 kJ/mol) = -110.66875 kJ/mol To calculate reaction #9 we also use Hess’s Law: Δ H 9 = Δ H 7 + D H 6 (dissolution of KOH(aq) + neutralization of KOH(aq) with HNO3(aq)) Δ H 9 = Δ H 7 + D H 6 = -57.4 kJ/mol +(-54.5 kJ/mol) = -111.9 kJ/mol

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7. (4 points EXTRA CREDIT) In the CHEM 150 lecture course, you should have learned how to calculate the standard enthalpy change (∆ H °) expected for a given reaction, using tabulated (appendix) values of ∆ H f ° for reactants and products. See also the last page of the lab procedure. Calculate ∆ H ° for all six reactions that you performed (Reactions #1-6) using the approach you learned in lecture. Show your work for one of these calculations, and list the value for each reaction in the table below. Note: For a dissolved substance such as NaOH( aq ), you will likely need to look up and add together the values for the individual ions, e.g., Na + ( aq ) and OH - ( aq ). {Show sample calculation here} Complete the following table showing the class average experimental ∆ H values for Reactions #1-6 (with error) and the expected/theoretical ∆ H ° values for the reactions calculated in this question using tabulated reactant and product data. Reaction Experimental ∆ H values (± error), kJ/mol ∆ H ° values calculated using tabulated data, kJ/mol #1 #2 #3 #4 #5 #6 Does the ∆ H ° value for Reaction #2, calculated using tabulated data, fall within the experimental error of the class average (measured) value? Explain carefully how you determined this and what it means. {Type answer here} See the attached rubric for more detailed information about grading. Unsatisfactory Borderline Satisfactory Excellent Pts. Q#1 Exo or Endo Incorrect answer or not based on observations. 0 points Answer correct, but poor explanation. 0.5 point Answer correct and based on observations. 1 point 1 pt Q#2 Data and calcs. No graph. 0 points Equation editor not used, -2 pts. 1-2 points Minor errors in calculations, rounding, or units will result in 0.5 pt. deductions. 3-4 points Properly formatted graph, 1 pt. Reasonable data table, 1 pt. Calculations correct and presented in a proper mathematical format with explanations, 3 pts. 5 points 5 pts Q#3 Rxns. 1-3 Errors in all three items listed. Errors in any two of the three items listed. Errors in any one of the three items listed. Correct equations and description of reactions, 1 pt. each Correct identification of relationship between reactions, 0.5 pt. 4 pts

0 points 1-2 point 3-4 points Good explanation of relationship between reactions, 0.5 pt. 4 points Q#4 ∆H 1-3 0 points Inadequate explanation but otherwise correct. 1 point Minor error or no consideration of uncertainty. 1.5 points Correct identification of relationship between enthalpies, 1 pt. Good explanation of relationship between enthalpies, 1 pt. 2 points 2 pts Q#5 ∆H 3-6 0 points 1 point Minor rounding mistakes, -0.5 pt. No consideration of error, -0.5 pt. Missing phase labels, -0.5 pt. 2-3 points ∆H 3 -∆H 6 values listed and rounded appropriately, 1 pt. Correct identification of relationship between ∆H 3 -∆H 6 values, 1 pt. Correct net ionic equation(s), 2 pt. 4 points 4 pts Q#6 ∆H 8/9 No work shown or explanations given. 0 points 1 point Inadequate explanation but otherwise correct. 2-3 points Correct determination of ∆H 8 and ∆H 9 , 2 pts. Clear explanation of calculation of ∆H 8 and ∆H 9 , 2 pt. 4 points 4 pts Q#7 ∆H calc / ∆H expt 0 points Expected values not listed, -2 pts. 1-2 points Errors in calcs. of ∆H° values, -0.5 pt. each. Inadequate discussion, -0.5 pt. 3 points Calculations are clear and correct and table adequate, 3 pts. Valid discussion of agreement between values, 1 pt. 4 points 4 pts E.C. Total 20 pts