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3.2 Thermal Properties

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Objectives: State the basic definitions of calorimetry including: –Specific heat capacity –Latent heats of fusion and vaporization Understand why temperature stay constant during phase changes Solve calorimetry problems Distinguish between evaporation and boiling

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3.2 Specific heat capacity Does it take as much energy to heat 1 kg of iron as it does to heat 1 kg of silver? Answer: No, it doesn’t The reason is they have different heat capacities. Iron is 470 Joules kg -1 K -1 Silver is only 234 J kg -1 K -1

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3.2 Molar specific heat It is also known it takes about the same amount of energy to raise 1 mol of iron and 1 mol of silver 1 K. I mol of silver has a mass of 107 g, and iron 56 g Thus there are almost twice as many atom in 1 kg of iron that need to be heated.

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3.2 Equation Q = mc T Q – quantity of energy transferred as heat c – specific heat capacity T – Change in temperature (K or o C)

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3.2 Thermal Equilibrium Energy flows from hot (higher average kinetic energy) to cold (lower average kinetic energy When the temperatures are equal the net flow of energy equals zero This is called thermal equilibrium

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3.2 Change of State

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There are two types of energy in molecules. –Kinetic: due to translation, rotation & vibration –Potential: due to bonds and position of molecules Work must be done to separate the molecules when moving from solid to liquid

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3.2 Latent Heat From Ancient Greek (lanthano) "to escape notice, to be unknown, to be unseen, to be unnoticed.“Ancient Greek Energy is added but the is no change in temperature. The energy goes to potential instead of kinetic.

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Latent Heat Equation Q = mL Q – quantity of heat (J) m – mass (kg) L – Latent heat (J kg -1 ) L f – Latent heat of fusion (solid ↔ liquid) L v – Latent heat of vaporization (liquid ↔ gas)

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Phase Change Graph

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3.2 Change of State As we go from left to right energy is (+) added. From right to left energy is (-) removed. mc T takes care of the negative sign when heat is removed because T is negative when T f < T i This is not so with mL, so be mindful to add a negative when removing energy.

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Example How much energy must be removed from 1kg of water at room temperature (22 o C) to make ice at -20 o C, at 1 ATM? Solution: Using mc T and mL we can find the energy. First remove energy to bring the water to its freezing point. After all we can’t have liquid water below that.

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Solution Notice the negative sign in front of the mL term. We are removing energy, so it must be negative. The T term take care of itself

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3.2 Calorimetry To measure specific heat a calori-meter is used. Conservation of energy is the principle. Energy lost by one object must be gained by the others. Q loss + Q gain = O

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3.2 Evaporation Since most liquid molecules have different speeds, the faster molecule may have enough energy to escape if they are near the surface of the liquid. The rate of evaporation increases with surface area and temperature, creating a “vapor pressure” above the fluid.

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