Chemistry 231 Thermochemistry

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 – the study of energy and its transformations Thermochemical changes – energy changes associated with chemical reactions

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

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 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) Examples of state functions  H  G  V  T Examples of path functions work (w) heat (q)

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 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 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 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 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 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 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 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

Boyle's Law V V 1/P P

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 Law V t / C Absolute Zero

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 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 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 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 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 Combine these relationships into a single fundamental equation of state - the ideal gas equation of state

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 They occupy a negligible volume compared with the container volume

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 Define: SATP (Standard Ambient Temperature and Pressure) Temperature - 25.00 °C = 298.15 K Pressure - 1.000 bar (105 Pa) The volume occupied by 1.000 mole of an ideal gas at SATP is 24.79 L!

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

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) 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 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 In the limit of low pressures

The Isothermal Compressibility

Coefficient of Thermal Expansion The coefficient of thermal expansion