Michael's thermochemistry workshop

[allyssa bradley 1H]

Hi everyone, hoping someone could check my work on the second part of the ice cream problem -- I got the first part right, so I'll include my givens/calculated I know is correct as well as my thought process, and hopefully someone can help me out! This is about enthalpy of fusion.

So, I know my specific heat is 10.1 and the system, which weighs 125 grams, absorbs 234 kJ of heat as it goes from -2 degrees C to melting. I made the equation:


So, plugging in my values: 234 000 = (125 g * 10.05 J/g degC * 2 degC) + (125 g * delta Hfus)

I solved this equation to get 1851.9 J or 1.85 kJ, but the answer key gave an answer that's exactly double my calculated value.

Hopefully my work is pretty easy to follow, and maybe someone will be able to see easily where I went wrong! Here is also a link to the worksheet, I'm talking about the problem at the bottom of page 2: https://docs.google.com/document/d/1uQEaiAyJYalTXMZzsW1d3Ce8VQedo8pz0Hey3ZLccA8/edit?usp=sharing

[Chem_Mod]

You have the exact right line of reason for this problem! The only mistake that you made is that the problem states that only half of the ice cream gets melted. Therefore, the mass you use when you multiply by deltaHfus should be 125/2 g! However, the first mass you use in the mc(deltaT) term is still the entire 125 g.

You can think about it like the entire mass of ice cream gets heated up (the entire 125 g), but only half of the ice cream undergoes melting (125/2 g), which is why two different masses are used in the same equation.

[allyssa bradley 1H]

Ohh that makes a lot of sense. I must have missed that part of the problem, but I'll go back and review it now! Thanks so much!