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Chemistry 231 . Introduction and Gases . Physical Chemistry. Physics - study of the properties of matter that are shared by all substances Chemistry - the study of the properties of the substances that make up the universe and the changes that these substances undergo

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chemistry 231

Chemistry 231

Introduction and Gases

physical chemistry
Physical Chemistry

Physics - study of the properties of matter that are shared by all substances

Chemistry - the study of the properties of the substances that make up the universe and the changes that these substances undergo

Physical Chemistry - the best of both worlds!

thermodynamics and thermochemistry
Thermodynamics and Thermochemistry

Thermodynamics – the study of energy and its transformations

Thermochemical changes – energy changes associated with chemical reactions

studying systems
Studying Systems
  • Interested in the numerical values of the state variables (defined later) that quantify the systems at that point in time.
  • Systems can be either
    • macroscopic
    • microscopic
3 types of systems
3 types of Systems

open system ® exchanges mass and energy

closed system ® exchanges energy but no mass

isolated system ® no exchange of either mass or energy

state of a system
State of a System
  • Described by variables such as
    • temperature (T)
    • pressure (P)
    • volume (V)
    • energy (U)
    • enthalpy (H)
    • Gibbs energy (G)
state and path functions
State and Path Functions
  • State Variables
    • system quantity whose values are fixed at constant temperature, pressure, composition
  • State Function
    • a system property whose values depends only on the initial and final states of the system.
  • Path Functions
    • system quantity whose value is dependent on the manner in which the transformation is carried out.
state and path functions continued
State and Path Functions (Continued)
  • Examples of state functions
    • H
    •  G
    •  V
    •  T
  • Examples of path functions
    • work (w)
    • heat (q)
equilibrium vs metastable
Equilibrium vs. Metastable

Metastable - the progress towards the equilibrium state is slow

Equilibrium state - state of the system is invariant with time

reversible and irreversible
Reversible and Irreversible

Reversible transformation - the direction of the transformation can be reversed at any time by some infinitesimal change in the surroundings

Irreversible transformation - the system does not attain equilibrium at each step of the process

the definition of a gas
The Definition of a Gas

Gas - a substance that is characterised by widely separated molecules in rapid motion

Mixtures of gases are uniform. Gases will expand to fill containers.

examples of gaseous substances
Examples of Gaseous Substances
  • Common gases include - O2 and N2, the major components of "air"
  • Other common gases - F2, Cl2, H2, He, and N2O (laughing gas)
the definition of pressure
The Definition of Pressure

The pressure of a gas is best defined as the forces exerted by gas on the walls of the container

Define P = force/area

The SI unit of pressure is the Pascal

1 Pa = N/m2 = (kg m/s2)/m2

the measurement of pressure
The Measurement of Pressure
  • How do we measure gas pressure?
  • We use an instrument called the barometer - invented by Torricelli
  • Gas pressure conversion factors
    • 1 atm = 760 mm Hg = 760 torr
    • 1 atm = 101.325 kPa = 1.01325 bar
    • 1 bar = 1 x 105 Pa (exactly)
the gas laws
The Gas Laws
  • Experiments with a wide variety of gases revealed that four variables were sufficient to fully describe the state of a gas
    • Pressure (P)
    • Volume (V)
    • Temperature (T)
    • The amount of the gas in moles (n)
boyle s law
Boyle's Law
  • The gas volume/pressure relationship
  • The volume occupied by the gas is inversely proportional to the pressure
  • V 1/P
    • note temperature and the amount of the gas are fixed
charles and gay lussac s law
Charles and Gay-Lussac's Law

Defines the gas volume/temperature relationship.

V  T (constant pressure and amount of gas)

Note T represents the temperature on the absolute (Kelvin) temperature scale

charles and gay lussac s law1
Charles and Gay-Lussac's Law

V

t / C

Absolute Zero

(-273C = 0 K)

the kelvin temperature scale
The Kelvin temperature scale

Lord Kelvin – all temperature/volume plots intercepted the tc axis at -273.15°C).

Kelvin termed this absolute 0 – the temperature where the volume of an ideal gas is 0 and all thermal motion ceases!

the temperatures scales
The Temperatures Scales
  • T (K) = [ tc (°C) + 273.15°C] K/°C
    • Freezing point of water: tc = 0 °C; T = 273.15 K
    • Boiling point of water: tc = 100 °C; T = 373.15 K
    • Room temperature: tc = 25 °C; T = 298 K
    • NOTE tc = °C; T (K) = K NO DEGREE SIGN
amonton s law
Amonton’s Law

The pressure/temperature relationship

For a given quantity of gas at a fixed volume, P  T, i.e., if we heat a gas cylinder, P increases!

avogadro s law
Avogadro’s Law

The volume of a gas at constant T and P is directly proportional to the number of moles of gas

V  n => n = number of moles of gas

the ideal gas equation of state
The Ideal Gas Equation of State
  • We have four relationships
    • V  1/P; Boyle’s law
    • V  T; Charles’ and Gay-Lussac's law
    • V  n; Avogadro’s law
    • P  T; Amonton’s law
the ideal gas law
The Ideal Gas Law
  • Combine these relationships into a single fundamental equation of state - the ideal gas equation of state
the definition of an ideal gas
The Definition of an Ideal Gas

An ideal gas is a gas that obeys totally the ideal gas law over its entire P-V-T range

Ideal gases – molecules have negligible intermolecular attractive forces and they occupy a negligible volume compared with the container volume

standard temperature and pressure
Standard Temperature and Pressure
  • Define: STP (Standard Temperature and Pressure)
    • Temperature - 0.00 °C = 273.15 K
    • Pressure - 1.000 atm
    • The volume occupied by 1.000 mole of an ideal gas at STP is 22.41 L!
standard ambient temperature and pressure
Standard Ambient Temperature and Pressure
  • Define: SATP (Standard Ambient Temperature and Pressure)
    • Temperature - 25.00 °C = 273.15 K
    • Pressure - 1.000 bar (105 Pa)
    • The volume occupied by 1.000 mole of an ideal gas at SATP is 24.78 L!
partial pressures
Partial Pressures

2

1

2

2

1

1

2

1

2

2

1

1

1

2

Let's consider two ideal gases (gas 1 and gas 2) in a container of volume V.

partial pressures1
Partial Pressures
  • The pressure exerted by gas #1
    • P1 = n1 RT / V
  • The pressure exerted by gas #2
    • P2= n2 RT / V
  • The total pressure of the gases
    • pT = nT RT / V
  • nT represents the total number of moles of gas present in the mixture
partial pressures continued
Partial Pressures (continued)
  • P1 and P2 are the partial pressures of gas 1 and gas 2, respectively.
    • PT = P1 + P2 = nT (RT/V)
    • PT = P1 + P2 + P3 = j PJ
    • note Pj is known as the partial pressure of gas j
dalton s law of partial pressure
Dalton's Law of Partial Pressure
  • Gaseous mixtures - gases exert the same pressure as if they were alone and occupied the same volume.
  • The partial pressure of each gas, Pi, is related to the total pressure by Pi = Xi PT
  • Xj is the mole fraction of gas i.
    • Xj= nj / nT
ideal gas temperature scale
Ideal Gas Temperature Scale

In the limit of low pressures

the isothermal compressibility
The Isothermal Compressibility

The Isothermal Compressibility

coefficient of thermal expansion
Coefficient of Thermal Expansion

The coefficient of thermal expansion