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a Answers to www #12:

What is the HEAT OF conFUSION?

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Earlier in the semester, you were asked a question: "Does adding heat to a substance always raise the temperature?" To answer this question, think about heating a cup containing ice mixed with water. The temperature of the ice-water will stay at 0oC until all of the ice has melted. Although heat was added to the system, there was no temperature change. Why? Because the heat energy was used to change the state of the solid water (ice) to liquid water. Why does this require energy? Because the intermolecular forces in ice had to be disrupted in order for the solid water to become liquid.

The amount of heat absorbed or released when a substance changes state can be calculated using the heat of fusion for substances at the melting point or the heat of vaporization for substances at the boiling point. (See section 8.15, page 220-222, in your text for more information.)

1. How much energy does it take to melt 225 g of ice (an amount about the size of a baseball) at 0oC? To figure this out you need to use the heat of fusion of water, 79.7 cal/g.
  225g (79.7 cal/g) = 17932 cal or 17.9 kcal
2. Is this process endothermic or exothermic? (Hint: Are you adding heat or removing heat?) What is the sign of DH?
  Heat is added when ice, H2O (s), becomes water, H2O (l). This is an endothermic process. The sign of DH is positive.
3.

In the process of melting, is the system becoming more disordered or more ordered? What is the sign of DS? (Hint: Which state has more disorder: solid or liquid?)
  The system is becoming more disordered.

Why are steam burns so nasty? Sure, the steam is really hot, but there is more to it than just the temperature of the steam. Water vapor is condensing to liquid on your skin. When water condenses, heat is released from the change of state! Think about it. If you boil water to form steam, you need to add heat. When the reverse happens, steam condenses to liquid, and heat is released as a result of the change of state. That heat is absorbed by your skin. Ouch!

4. Calculate the amount of heat released when 35 g of steam, H2O (g),  condenses on your skin. The heat of vaporization for water is 540 cal/g.
  35 g (540 cal/g) = 18900 cal or 18.9 kcal

The previous exercises examined heat absorbed or released in changes of state. Earlier in the semester, you calculated the heat required to raise the temperature of a substance when there wasn't a change of state.

heat = mCDT

In this equation, m is the mass, C is the heat capacity (or specific heat) of the substance, and DT is the change in temperature.

5. Calculate the amount of heat needed to raise the temperature of a baseball-sized hunk of sodium metal (225 grams) from 20.oC (room temperature) to its melting point of 98oC. (The heat capacity of solid sodium metal is 0.293 cal/goC.)
  heat = mCDT = 225 g (0.293 cal/goC) (98oC – 20oC) = 5100 cal or 5.1 kcal
6. Now calculate the heat required to melt the same amount of sodium (225 g) at its melting point. The heat of fusion of sodium is 27.0 cal/g.
  225 g (27.0 cal/g) = 6100 cal or 6.1 kcal
Note: Temperature is not used to determine the heat in a change of state calculation.
7. Using your answers from questions 5 & 6, compare the amount of heat needed to melt this hunk of sodium to the amount of heat needed to raise the temperature of the same amount of sodium by 78oC. Does this surprise you? Why?
  nickname: pop star
More heat is needed to melt the amount of sodium. This does not surprise me because the intermolecular forces had to be disrupted.
nickname: Jupiter
The thing that surprises me in this problem is seeing that it takes more heat to melt the sodium (turning it from a solid to a liquid) rather than raising it 78 degrees. I thought that more heat would be required to heat the sodium 78 degrees than to change its state from solid to liquid. Obviously the bonds are strong in sodium and that's why it's so difficult to change its state.

 

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