AP Unit II C 2

1 / 14

# AP Unit II C 2 - PowerPoint PPT Presentation

AP Unit II C 2. Laws of Thermodynamics Ref: Chapter 12. a) Students should know how to apply the first law of thermodynamics, so they can: (1) Relate the heat absorbed by a gas, the work performed by the gas, and the internal energy change of the gas for any of the processes above.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## AP Unit II C 2

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### AP Unit II C 2

Laws of Thermodynamics

Ref: Chapter 12

a) Students should know how to apply the first law of thermodynamics, so they can:
• (1) Relate the heat absorbed by a gas, the work performed by the gas, and the internal energy change of the gas for any of the processes above.
• (2) Relate the work performed by a gas in a cyclic process to the area enclosed by a curve on the PV diagram.
b) Students should understand the second law of thermodynamics, the concept of entropy and heat engines and the Carnot cycle, so they can:
• (1) Determine whether entropy will increase, decrease, or remain the same during a particular situation.
• (2) Compute the maximum possible efficiency of a heat engine operating between two given temperatures.
• (3) Compute the actual efficiency of a heat engine.
• (4) Relate the heats exchanged at each thermal reservoir in a Carnot cycle to the temperatures of the reservoirs.
1st Law of thermodynamics.
• This is a statement of the conservation of energy.
• An insulated container filled with an ideal gas rests on a heat reservoir. The container is fitted with a snug but frictionless weighted piston that can be raised or lowered. The confined gas is the system and the piston and heat reservoir are the surroundings
• Since F = PA and A Δs = V we have
• W = -P ΔV
• and vice versa.
• W is negative when the system does work against its surroundings, and positive when the surroundings do work on the system
• isochoric - constant volume, isobaric - constant pressure, isothermal - constant temperature, adiabatic - no heat exchanged
First Law Thermodynamics
• The system’s internal energy ΔU = Q (heat energy) + W (work done on the gas)
• ΔU = Q + W
• A 0.5 mol of an ideal gas (CV = 12.5J/mol K, Cp = 20.8 J/mol K) is brought from state a to state b along the path shown in the following P-V diagram

What are the values of each of the following (a) Temperature at a and b b) Work done by the gas during ab

c) Heat added to the gas during ab d) change in internal energy of gas

P

a

b

1.5 x 105 Pa

30

10

V (x10-3 m3

R in Physics is 8.31 j/mol K. In chemistry they use J/liters K. If 1 mol of gas is 22.4 liters, what will be R in chemistry?

• 2. A gas expand from 4 m3 to 85 m3 at constant pressure of 1 atmosphere. How much work does it do.
• 3. A gas increases in pressure from 1 atmosphere to 3 atmospheres at a constant volume of 4 m3. What is the work done?
2nd Law Thermodynamics
• Heat Engine (E.g. Internal combustion engine) are Cyclic Engines ΔU = 0. They have a hot reservoir (QH) and a cold reservoir (QC). Qnet = -W = QH – QC.
• Efficiency = work out/work in

= (QH – QC)/QH = 1- QC/QH

Efficiency is always less than one!!!!!!!

Carnot Engine
• Theoretically most efficient engine
• Since Q is proportional to absolute temperature
• Efficiency = work out/work in

= (TH – TC)/TH = 1- (TC/TH)

Entropy
• The change in entropy of a system (ΔS) is equal to the heat flowing into or out of the system (ΔQ) divided by the absolute temperature (T). Entropy is a measure of the disorder of the universe. Reactions tend to go in the direction of increasing Entropy.
• ΔS = ΔQ/T
• Therefore the second law of thermodynamics can be stated in two ways:
• 1. Heat will not flow spontaneously from a cold object to a hot object.
• 2. No heat engine operating in a cycle can absorb thermal energy from a reservoir and perform an equal amount of work
1. If the heat flowing into a system is 200 Joules at 373K, what is the entropy of the system?
• 2. If the Carnot internal combustion engine operates at 800°C and exhausts to 25 °C , what is the theoretical efficiency of the engine?
• 3. If 330 ml of steam at 1 x 105 Pa is cooled from 100 °C to 0 °C, at constant volume, what is the change in pressure?