Pure Substances

Pure Substances

Thermodynamics

Professor Lee Carkner

Lecture 5

PAL # 4 First Law Pumping water uphill and then running it back to

produce energy Rate of power imparted to water = W’water = (1000)(2)(9.8)(40) =

But the pump and the turbine are only 75% efficient pump = W’water/W’pump

W’pump = 785/0.75 = Cost per year = (1046)(10)(0.03)(365) = $114500 turbine = W’turbine/W’water

W’turbine = (785)(0.75) = Revenue per year = (785)(10)(0.08)(365) = $171900 Net Yearly Profit =

PAL # 4 First Law Should you build it? No

Pure Substances

A pure substance consists of either a single type of atom or molecule or a homogeneous mixture

Can exist as a mixture of phases Gas

Liquid

Solid

Strong intermolecular forces, high density, fixed molecular position

Types of Phases Compressed liquid

Saturated liquid

Saturated Vapor Saturated liquid and

vapor can coexist together

Superheated vapor

Saturation

A substance changes phase when it is at the saturation temperature for a given pressure

Can be compiled in tables for reference

Tsat increases with Psat

Need more internal energy to become vapor at higher pressure

Change of Tsat with Psat

Enthalpy

Defined as:h = u + PvH = U +PV

Very useful for flow systems such as

turbines

Specific Volume and Quality

Mass of saturated liquid = mf

Mass of saturated vapor = mg

Since both phases often co-exist we define the quality, x:

The quality is the relative fraction of the mixture

in the vapor phase

Quality Relations

The total volume of a mixture is:

the volume occupied by each phase is:

We can use these relationships to write an expression for the combined specific volume of the mixture:

vavg = vf +xvfg

x = (vavg –vf )/vfg

Property Diagrams

We will plot three key variables P, T, and v as:

Each region of the diagram represents a phase or mixture of phases

Pv Diagram

Pv Diagram Properties

Saturation curves define boundaries of liquid-vapor mixture region

Critical point is on the critical isotherm

Densities of both are the same Above the critical point, no amount of pressure

can condense the vapor to a liquid

Regions on a Pv Diagram

PT Diagram Three curves can be drawn on the PT diagram

Fusion curve

Vaporization curve

Sublimation curve

The curves bound three distinct regions, one for each phase

Juncture of the three curves is the triple point where all three coexist

PT Diagram

Other PT Features

An isobar at standard atmospheric pressure intersects the normal boiling and melting points

The critical point is on the vaporization curve

Gas above critical T is called “gas”, below it is called “vapor”

PvT Diagram for Water

PVT Diagram

P, V and T relations can be formed to create a three dimensional diagram

The PT and PV diagrams are formed by projection

Next Time

Read: 3.6-3.8 Homework: Ch 3, P: 48, 54, 75, 86