CALORIMETRY LAB — Oct 13, 15, or 17

If you want to print this (but I'll bring copies to lab), try the print-friendly PDF version.

 

There is a PRELAB-QUIZ in Learn@UW with electronic submission, so you don't print anything to hand in.  There is a DEADLINE for this because the quiz closes an hour before the lab-time you're officially registered for, so be sure you do the quiz before then or you can't do it at all.

two options for prelab preparation
You can:  1) make a procedural outline, or  2) mark your lab manual carefully and thoughtfully
to help you preserve what you learn while you're reading it, which will make it easier when you're doing the lab.  Either way is fine, and will get you the same credit:  OK, or marginally-OK (but it should be better and if you do this again it will be -1 point),  or not-OK (-1 point immediately).
 
During the whole semester, this is one of the two labs that is most closely tied to lecture/exam material.  It's a discussion-based lab so you won't hand in a lab-report so you can ignore most of the Blue Pages which are pages 7-5 and 7-6.  Instead, we'll discuss ideas and calculations in lab — I'll have a "discussion grid" and each time we discuss something you'll get an X, and when all topics have an X you're free to leave, or you can stay to ask me other questions — that will help you master the concepts and problem-solving strategies in Chapter 5 for Exam 2.

We'll re-label Reactions 1 2 3 4 (page 7-3) as "Reactions A B C D"
because this will avoid confusion with "Questions 1 2 3 4 5" below,
which are based on the old lab-book (used for > 5 years) before it was revised (and "dumbed down") for Fall 2008.
I think you'll be able to understand everything below, but if not then try to get what you can from a first reading, and then ask me (in lab) about the rest.
 
LABWORK — For Reactions A-D you can just use the "maximum amounts" of solid they list in the lab book (on page 7-3), and (as suggested in the old lab manual) 50-100 mL of a liquid that will be water (in Reaction A) or HCl solution (in B C D).  But if you were doing "real science" that includes the designing of your own experiments, you would use the logic described in Problem 8 near the end of this web-page.


DISCUSSION — The 9 questions below will be in the columns of my "discussion grid" that will be filled with X's as you show me "what you've done" for each question, and we discuss what you did, plus any additional questions (coming from you or me) about it.
 
      Question 1:  Using information in the first 5 rows of the "blue-page table" (Mass of Reactant, Mass of Liquid, Final Temp, Initial Temp, Temp Change), calculateDH in kJ "for each reaction as you performed it." (i.e., with the amounts you actually used in lab) not the whole-number amounts (1 mole,...) in your reaction-equations in Question 2.  If you want, if you know how to do Question 1 based on what you know (from high school, lecture, textbook, and what we covered in class) you can ignore the next few paragraphs and skip to Question 2.  Or you can read what's below:

      In lab you'll measure Ti and Tf, plus either the milliliters of water or grams of water.
      Question 1 is mainly Cat-15, which is just Cat-9 with basic conversion factors, as you did for Exam 1, applied to a new situation.  Your calculation will be:
Qsolution = (__ mL)(__ g/mL)(__ J/gK)(Tf - Ti),  where 3 of these 5 variables are measured in lab;  or
Qsolution = (__ g)(__ J/gK)(Tf - Ti)where 3 of these 4 variables are measured in lab.
      What about the variables you don't measure?  You should assume that your solutions are "close enough to pure water" that you can use "1.00 g/mL" and "4.184 J/gK", adding the mass of solvent and solute together (as in Example 5.6 in KTT, pages 230-231) to get the mass of solution.


      What about the +- sign for DH of the reaction?  Use "Qreaction + Qsolution = 0" (as explained in class) to find Qreaction.  Then, as usual, check by comparing the +- sign of this "algebra answer" with the +- sign you get from your "intuitive common sense answer" — an EXplosion reaction (causing T increase in the surrounding solution) is EXOthermic and has a DH that is negative;  and if T decreases due to an ENDOthermic reaction, DT of the reaction is positive.
      To improve your chemical intution, you can think of this CAUSE-EFFECT logic:  the chemical reaction (either exothermic or endothermic) causes the solution's temperature to change.  (Here, the reaction is "the boss" because it's making things happen.)
 
  
      Question 2 for each reaction (to get 2A 2B 2C 2D) is in two parts:
      First, to do the first part of 2A you'll write a molecular equation for Reaction A;  then for 2B, 2C, and 2D, write a molecular equation for Reactions B, C, and D.
      To help you get started without getting immediately stuck in a rut, I'll give you the equation for 2A, which is "so easy it's difficult" and is just "NH4Cl (s) --> NH4Cl (aq)".   Then, 2B-2C-2D are similar to what you did in writing molecular equations for Exam 1.

     
Second, calculate (and then write to the right of your molecular equation, as in Category 15 on J-page) each DH for the amounts indicated in your molecular equation, with all equation-coefficients representing moles.
      To calculate each DH for "the amounts indicated in your molecular equation" (for the second part of 2A 2B 2C 2D) use a miles/hour strategy by dividing (for the specific "chunk of reaction" you did in the experiment) the reaction-DH (you calculated these in 1A 1B 1C 1D) by the moles of limiting reactant.

     
Do you see why "moles of limiting reactant that react" is important, and why "moles of excess reactant" doesn't really matter?  But after you calculate DH of a reaction correctly, this DH will apply to everything (reactant or product) in the reaction-equation.  For both Questions 1 and 2, your four answers will be in ___ kJ, but for Q2 the energy will be associated with the whole equation (as a "package deal") so you can make several kJ/mole conversion factors out of it, as shown in Category-15 at the top of the J-page.
     
As an example of a reaction-as-written, if we write "2 C2H6 (g) + 7 O2 (g) --> 4 CO2 (g) + 6 H20 (l)" the delta H (= -2857.3 kJ) is the energy change that you calculate (based on your experimental data) would occur for the combustion of 2 moles of C2H6, because 2 moles is the amount in the equation that is written.  But if you wrote the reaction as "1 C2H6 (g) +..." the associated DH would be 1428.7 kJ.
      When you finish 2A-2D (either part of it or all of it) show me your numerical results and we'll compare your results with the "typical experimental results" (below) and with the theoretical predictions (using Cat-18b calculations) in Question 9.

 
      Question 3:  In lab, tell me why DT should be kept in a range of 5-10 C, as suggested in the old lab manual.  For example, imagine two extreme situations, with a T-change of 70 C (very large) or 1 C (very small):  What bad things (physical or statistical) will happen if the DH is either 70 C or 1 C, so (if the initial T is 21 C) the final T is either 91 C or 22 C ?    hint: Different bad things will happen in each situation, and the actual experimental design (aiming for a change of 5-10 C) is a compromise, it's a balance between optimizing the experiment to minimize one of these bad results or the other.
 
      Question 4a (in the old manual) is now on the second Blue Page, "Write the thermochemical equation...may help."
      This is a cat-18a problem.  Think about how you can use some (but not all) of the available SOURCE-reactions (A-D plus the reaction given in Question 4a, which is "H2 (g) +...") to get the GOAL-reaction that you'll write by converting the words of Question 4a — "the thermochemical equation for the formation of CaO (s)" — into a reaction equation.
      Question 4b (How does your value compare..."):  Get the "literature value" from Appendix L in the back of KTT, beginning on page A-29;  I'll bring photocopies of these pages to lab, since many of you won't bring your book to lab.

      Question 5 (in lab manual) should be skipped.
 
• Questions 6a-6c, 7a-7c, 8a-8b and 9 are "extras" that I think will be useful for helping you learn Chapter 6 for the exam.  You can do these before lab if you want, or during lab when help (from other students and me) is available.
 
      Question 6 (Category 18-b)
Using data from Appendix L of KTT, find DH for these three reactions.
6a.  HCl (aq) + NaOH (aq) --> HOH (l) + NaCl (aq)
6b.  NaOH (s) --> NaOH (aq)
6c.  HCl (aq) + NaOH (s) --> HOH (l) + NaCl (aq)
 
Answers for 6a-6c are:  -56.79 kJ,  -43.22 kJ,  -100.01 kJ.
 
      Question 7 (Category 18-a)
To help you understand that 18b (using "final - initial") is just a shortcut for 18a (in which you "combine equations") as explained in Sections 6.7 and 6.8 of KTW, do these three exercises in which you "do 18a" using the results of 18b in Question 6:
    7a. find DH for Reaction 6a by using 18a — by combining equations (and their DHs) — using 6b and 6c (as source-reactions) to get 6a (as the goal-reaction);
    7b. then find DH for 6b by combining 6a and 6c;
    7c. and find DH for 6c by combining 6a and 6b.
    You already know (if you think about it) what the answers should be for 7a, 7b, and 7c.


      Question 8
      Here is the context:  In this lab, you can DECIDE what is limiting and what is excess, in contrast with problems (from textbook, quiz, exam,...) where amounts are "given" and you can only UNDERSTAND (based on your logic and calculations) what is limiting and excess.
      And here is the strategy:  When you're running reactions and you want to get an excess of one reactant, the usual strategy is to CHOOSE an amount of one reactant and CALCULATE the amount of the other reactant that is exactly enough to react with it, and then ADJUST the amounts (for either of the reactants, so there are two adjustment strategies) to be sure you have an excess of what you want to be the excess reactant.
  For practice, you can use this strategy for 8a, and then 8b asks you to think about other factors in the design of this experiment:
 
     
8a:  Calculate the exact volume of 1.02 M HCl solution that is needed to react with 0.2795 g of Ca in Reaction B.  Then choose a volume that makes HCl be in excess and is also consistent with the suggestions on page 7-2.
 
      8b:  Why do you think there is a suggestion (on page 7-4) that you use only 0.3 g of Ca, which (as you calculated in 8a) is a lot less Ca than could react in the typical volume of solution that's being used in your calorimetry experiments?  Or, in another way to think about the same question, why do you want to use a lot more liquid than would be "just sufficient to react with" the amount of solid Ca you're using?   (Yes, there is a very good practical reason for using a LARGE excess of solution.)
      a hint: This reason is related to the cause-effect described earlier, with THE REACTION producing T-CHANGE OF SOLUTION, and DT depending on both of the amounts (the reacting-chemicals that are causing DT, and/or the solution-chemicals that are being affected *) and either, or both, can be adjusted to produce a DT in the desired range.   {* Yes, there is some overlap here, with some chemicals performing both roles (as producers of the cause and receivers of the effect) but the practical effects of this overlap are not significant, so it's useful to think of them as being separate, with each doing primarily one role or the other. }

      Question 9
     
If you've already mastered 18b-problems, you can do Question 9 in a few minutes.  And if you don't know 18b well you can practice it now, while you're in lab and you can get help from other students and me.   (Exam 2 will be here soon, about two weeks after your labs, so you might as well learn it now, right?)
 
       You can calculate DH for each of the 4 reactions (ABCD) using "DH of formation" values from Appendix L of KTT plus two more values that aren't in KTT:
      to form Ca+2, -543.0 kJ/mol
      to form Cl-,  -167.4 kJ/mol
And, according to page 126 of KTW, CaCl2 does dissolve-and-dissociate into aqueous ions.
 
Using the information above (and data from 6th Edition, which may differ slightly from data in 7th Edition) you should get these answers:
Reaction A:  + 14.89 kJ

Reaction B:  – 543.482 kJ
Reaction C:  – 194.222 kJ
Reaction D:  – 129.052 kJ
For several reasons, this precision (to nearest .001 kJ) isn't justified;  these are just the results that will appear on your calculator if you do it correctly.
If you want to see "math setups" for each of these 18-b problems, click this link.

Typical experimental results, with the equipment and techniques you're using in this Chem 103 lab, are similar but slightly different from what you calculate using data from KTT:
    Reaction A (dissolving NH4Cl) is +15.6 kJ (plus/minus 2 kJ) for a reaction written with "1 NH4Cl" (1 mole NH4Cl) on left
    Reaction B (for Ca with HCl) is -500 kJ (plus/minus 40 kJ) for a reaction written with "1 Ca" (1 mole Ca) on left
    Reaction C (for CaO with HCl) is -175 kJ (plus/minus 10 kJ) for a reaction written with "1 CaO" (1 mole CaO) on left
    Reaction D (Ca(OH)2 with HCl) is -120 kJ (plus/minus 30 kJ) for a reaction written with "1 Ca(OH)2" (1 mole Ca(OH)2) on left

 
That's it.

 

 



 
OPTIONAL:  This section isn't necessary for doing the lab, but you may find it interesting because you can see my personal perspective on labs, shared with the hope that it will make our labs more fun for us.  Here is an excerpt from my web-page about Discussion-Based Labs (if you're curious, I think you'll find the sections about The Main Idea and Actual Benefits are the most interesting after you click the link):

      I'm fairly shy in many situations, but I enjoy thinking and talking about ideas.  For me, discussion-based labs make interactions with students much easier, more enjoyable, and more effective for teaching.   Why?
      If there is no "reason" to talk with students, and everything depends on my own social intuitions and actions, I often find it difficult to achieve a balance between ignoring students and bothering them with too much attention.
      But with motivation provided by the grid [containing questions to discuss] which must be filled with Xs before they can leave the lab, students initiate conversations.  And our discussions have a clear intellectual focus: their own experiences and "what they can learn" about chemistry concepts and thinking skills.
     Usually, talking about these topics is interesting and educational for all of us, and it also leads to small-talk that produces social and emotional bonding, both student-teacher and student-student.  A discussion-based approach to labs provides a useful organizing structure for interactions that lead to learning and to an improved rapport between everyone in the learning community that we're building.

Unfortunately, a discussion-based structure is not allowed in the grading schemes used by the UW Chemistry Dept.  In the UW Phyics Dept, one semester (in Physics 104) I did a discussion-based lab, and three semesters (in Physics 109) I did no lab-grading even though labs were a central part of the course, because the focus of the labs was LEARNING.  In Physics 104, almost all students said that, compared with their labs in 103, our discussion-based labs were easier, they learned more, and it was more fun — a three-way win, and I liked it better for the same reasons.