 #  State of Balance For the phase changes: As many molecules evaporating as there are condensing.

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 State of Balance For the phase changes: As many molecules evaporating as there are condensing

As the water changes from a liquid to a gas, the energy goes into breaking the bonds that hold one molecule of liquid water to another. Latent heat of vaporization: The energy required to change a particular liquid at its boiling temperature into a gas

 Latent Heat of Fusion: The energy required to change a particular solid at its freezing point into a liquid

So far, we have discussed transfer of energy while heating with out phase change: Q = mcΔT For heat transfer during a phase change, we will be using the following equation: Q= mΔH ΔH is heat of fusion or vaporization, In your text the symbol is L

heat of fusion The value for heat of fusion is used for freezing and melting. Ex. Latent heat of fusion of water is 80 cal/g heat of vaporization The value for heat of vaporization is used for condensation and vaporization.

The heat of fusion of water is 80 cal/g Calculate the energy needed to melt 20 g of ice.

The heat of fusion of water is 80 cal/g Calculate the energy needed to melt 20 g of ice. Q = mΔH

The heat of fusion of water is 80 cal/g Calculate the energy needed to melt 20 g of ice. Q = mΔH Q = (20g)(80 cal/g) Q = 1600 calories

Calculate the energy released by 17 g of 100°C steam that condenses and then cools to 0°C. Heat of Vaporization of water is 540 cal/g.

Calculate the energy released by 17 g of 100°C steam that condenses and then cools to 0°C. Heat of Vaporization of water is 540 cal/g. Q = mΔ H + mcΔT

Calculate the energy released by 17 g of 100° C steam that condenses and then cools to 0° C. Heat of Vaporization of water is 540 cal/g. Q = mΔ H + mcΔT = 17g(540 cal/g) + 17 (1 cal/g) (100°C) = 10880

First Law of Thermodynamics: Whenever heat is added to a system, it transforms to an equal amount of some other form of energy.

Heat added = Change + External Work in Internal done by Energy system Q = ΔU + W

ΔU = Q + W (on the system) ΔU = Q - W (by the system)

 An engine has an input of 15000 J of heat in order to do 2000 J of work. What is the internal energy change of the system?

 ΔU = Q - W (by the system)

 An engine has an input of 15000 J of heat in order to do 2000 J of work. What is the internal energy change of the system?  ΔU = Q - W (by the system)  ΔU =15000- 2000  ΔU = 13000

 A gas sample expands and cools, reducing its internal energy by 2500 J, while no heat is added or lost from the system. How much work is done in this process?

W = Q – ΔU W = 0 J – (-2500 J) = 2500 J

 Compression or expansion in a system in which no heat enters or leaves.

 An increase in temperature leads to an increase in kinetic energy  This leads to increased collisions of the gas with the walls of the container…….  Which leads to an increase in pressure!!!  An increase in pressure leads to an increase in in kinetic energy  This leads to increased collisions between the molecules  Which leads to an increase in temperature!!!!

A gas in a sealed cylinder is heated. Which of the following does not increase as the gas is heated? A. the average number of gas molecules hitting the cylinder walls per second B. the average kinetic energy of the gas molecules C. the average speed of the gas molecules D. the average distance between the gas molecules

The pressure of a gas inside a closed, rigid container will increase when the gas temperature increases. The pressure of the gas increases because the A. density of the gas decreases B. rate of collisions of gas molecules with the surface increases. C. container expands in size when heated D. gas molecules bond

Heat is always transferred from hot to cold!!! A heated gas expands, raising a piston. Which of the following describes the energy exchanges of this process? a. Energy is transferred to the gas by the piston, and to the piston from the heat source. b. Energy is transferred to the gas from the heat source, and to the raised piston from the gas. c. Energy is transferred to the gas in the form of heat and work done by the piston. d. Energy is transferred directly to the piston from the heat source.

 Change internal energy into mechanical work  Absorb heat from a reservoir  Convert some of this into work  Expel the remaining as heat into a lower temperature reservoir

W = Q H – Q L Practice Problem: An engine has an input of heat energy of 10,750 J and does 2420 J of work. What is the heat loss?

 We Will discuss Entropy and Disorder next time in class

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