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# Thermal Interactions and Enthalpy

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Thermal Interactions
Part 1:
In an insulated container, you mix 200. g of water at 80ºC with 100. g of water at 20ºC. After mixing, the temperature of the water is 60ºC.
a. How much did the temperature of the hot water change? How much did the temperature of the cold water change? Compare the magnitudes (positive values) of these changes.
b. During the mixing, how did the heat transfer occur: from hot water to cold, or from cold water to hot?
c. What quantity of heat was transferred from one sample to the other?
d. How does the quantity of heat transferred to or from the hot-water sample compare with the quantity of heat transferred to or from the cold-water sample?
e. Knowing these relative quantities of heat, why is the temperature change of the cold water greater than the magnitude of the temperature change of the hot water.
f. A sample of hot water is mixed with a sample of cold water that has twice its mass. Predict the temperature change of each of the samples.
g. You mix two samples of water, and one increases by 20ºC, while the other drops by 60ºC. Which of the samples has less mass? How do the masses of the two water samples compare?
h. A 7-g sample of hot water is mixed with a 3-g sample of cold water. How do the temperature changes of the two water samples compare?

Part 2:
A sample of water is heated from 10ºC to 50ºC. Can you calculate the amount of heat added to the water sample that caused this temperature change? If not, what information do you need to perform this calculation?

Part 3:
Two samples of water are heated from 20ºC to 60ºC. One of the samples requires twice as much heat to bring about this temperature change as the other. How do the masses of the two water samples compare? Explain your reasoning.
6.30. Enthalpy
a. A 100.-g sample of water is placed in an insulated container and allowed to come to room temperature at 21ºC. To heat the water sample to 41ºC, how much heat must you add to it?
b. Consider the hypothetical reaction,

2X(aq)+Y(l) ? X2Y(aq)

being run in an insulated container that contains 100. g of solution. If the temperature of the solution changes from 21ºC to 31ºC, how much heat does the chemical reaction produce? How does this answer compare with that in part a? (You can assume that this solution is so dilute that it has the same heat capacity as pure water.)

c. If you wanted the temperature of 100. g of this solution to increase from 21ºC to 51ºC, how much heat would you have to add to it? (Try to answer this question without using a formula.)
d. If you had added 0.02 mol of X and 0.01 mol of Y to form the solution in part b, how many moles of X and Y would you need to bring about the temperature change described in part c.
e. Judging on the basis of your answers so far, what is the enthalpy of the reaction 2X(aq)+Y(l) ? X2Y(aq)

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