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When calculating heat in a calorimetry problem, should the mass of the dissolved solid be included in the calculation?   

Started about 2 years ago by Nicolle DiPasquale.


Also, would someone be able to share how they explain the different sign for the dissolving process and the energy change of the solution?


6 Comments

  • Adrian Dingle

    Posted about 2 years ago

    If the data references 'the SOLUTION' (shc's of SOLUTION, ∆T of SOLUTION etc.) then the answer is that masses should ABSOLUTELY be included. I wrote about this at the bottom of this post. https://www.adriandingleschemistrypages.com/ap/the-confounded-mess-that-is-signs-when-it-comes-to-q-m-c-delta-t/

  • Jennifer Bishoff

    Posted about 2 years ago

    If this is an honors/AP class, I really drive home the seriousness of being accurate and precise, so I do include the mass of the solid. If it is a relatively small amount in relation to the liquid, you can point that out to your students and ignore it. For example, if I have 100 g water and .5 g of solid, I may just leave it out, depending on the class.

  • Kimberly Duncan

    Posted about 2 years ago

    Adding to my comment below, the relationship between q for the solution and delta H for the dissolving process is also discussed in the "2016 AP Chemistry Exam Results" webinar.

  • Kimberly Duncan

    Posted about 2 years ago

    This past October (2016) AACT hosted a webinar presented by Roger Kugel, Chief Reader for the AP Chemistry Exam (2016 AP Chemistry Exam Results). FRQ #1 from the exam included calorimetry calculations (this starts at slide 29 – 16 minutes into the webinar). He explained that a common error/misconception is using the wrong mass. The total mass of solute and solvent should be used.

  • Kaleb Underwood

    Posted about 2 years ago

    The mass used should be the total mass of the solution (dissolved solid + water).

    Let’s use an exothermic dissolution for example, such as CaCl2(s) --> Ca2+(aq) + 2Cl-(aq). We first identify the system: the calcium and chloride ions. The temperature of the solution increases when the salt is dissolved. Since the thermometer is part of the surroundings, the temperature change is that of the surroundings. This means that the surroundings have gained energy lost by the system. For this reason, the sign of ΔHsol is negative.

    I advise students to “work positive” in all calculations and apply sign conventions and temperature changes at the end of a problem. This works well for the vast majority of students.

  • Ryan Johnson

    Posted about 2 years ago

    The mass of the dissolved solid is absolutely necessary! Firstly, most calorimetry problems are looking for heat in mass-based units, such as Joules/gram or Joules/mole, so mass is necessary for that. And then, as calorimetry is demonstrating the energy flow out of or into a system, the mass certainly effects that measurement. In my classes, we spend a whole day discussing energy flow and sign conventions for thermodynamics...SO many kids assume that cold things feel cold because they are "giving off cold", whereas a deeper understanding of energy flow leads them to understand that they feel cold because of absorption of heat away from the hand, etc. So my students are tasked with proposing 5 common situations where heat transfer is occurring and then present a schematic diagram on white boards to the class, showing the heat flow involved and whether the system is endo- or exothermic, along with a justification.