Homework Problem 8.41

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melissapasao1B
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Joined: Wed Sep 21, 2016 2:56 pm

Homework Problem 8.41

Postby melissapasao1B » Thu Jan 26, 2017 10:41 pm

Hi! I was wondering if anyone can explain how to go about question 8.41 from the textbook? I get confused on what to do about phase changes and how to calculate for the enthalpy.

Thanks!

stephanieyang_3F
Posts: 62
Joined: Wed Sep 21, 2016 2:55 pm

Re: Homework Problem 8.41

Postby stephanieyang_3F » Fri Jan 27, 2017 2:23 am

Basically for this question, you're going to be dealing with a system approaching equilibrium... first thing you wanna know is that:
the heat gained by the water in the ice cube will be equal to the heat lost by the initial sample of hot water. In other words, both these values will be equal to each other. BUT keep in mind that the temperature of the water initially at 45.0 C is going to cool down. This is because that ice was put into a warmer sample of water, so naturally and spontaneously the system together is going to cool down to equilibrate at a final temperature between 0.0 C and 45.0 C.

So the heat gained by the water/ice at 0.0C comes from: 1) phase change from solid state to liquid state, 2) the rest of the heat contributes to a rise in temperature to that final temperature.
The heat lost by the sample of hot water at 45.0C will contribute to only a decrease in temperature... a lower final temperature.

From here you'd calculate the heat from the phase change + heat capacity (nCdelta T=q) formula for the heat gained portion = -(heat lost by initial sample of hot water also using heat capacity formula). This part is negative because one is gaining heat and one is losing heat. If you're confused about the phase change part, just know that HEAT will either affect the temperature or change the phase of the substance. I hope this is clear and helps!

Jaz_Y_1H
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

Re: Homework Problem 8.41

Postby Jaz_Y_1H » Fri Jan 27, 2017 9:26 am

For this type of question when we are finding heat, does the amount stay in grams or does it need to be converted to moles?
I have seen q=nCT calculated using both grams and moles for the "n" term and am curious to know when to use which unit.
Thank you for your help.

stephanieyang_3F
Posts: 62
Joined: Wed Sep 21, 2016 2:55 pm

Re: Homework Problem 8.41

Postby stephanieyang_3F » Fri Jan 27, 2017 7:50 pm

It doesn't really matter. The reason why we sometimes use moles and sometimes use grams is to correspond the units with the type of heat capacity you choose to use. Specific heat capacity is specifically with the amount of heat to raise 1 gram (for liquid water 4.184 J/gC) and molar heat capacity is specifically with the amount of heat to raise 1 mole (for liquid water 75.29 J/Kmol). Hope this makes sense.


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