Unit 11B: Thermodynamics Thermodynamics: the area of Chemistry that explores energy relationships

Thermodynamics We will be examining chemical reactions further in this topic So far we know what contributes to the following: How rapidly a reaction occurs (Kinetics) 2) How far toward completion a reaction will go (Equilibrium)

Enthalpy a thermodynamic quantity equivalent to the total heat content of a system Heat is transferred between system and surroundings when there is an energy during a process Table I: Heat of Reaction for several reactions sign indicates type of change (-) is exothermic (+) is endothermic

Heat of Formation/Standard Enthalpy Values amount of heat absorbed or evolved at 25° C (77° F ) and at one atmosphere pressure when one mole of a compound is formed from its constituent elements, each substance being in its normal physical state ( Hf for elements in stable state =0) Determined and given by the National Institute for Standards and Technology Table I is shows heats of reaction, which are derived from Heats of formation for each substance Hess’ Law: the total enthalpy change for the reaction is the sum of all changes ∆Horxn = Σ nHfo (products)  - Σ nHfo (reactants)

Practice Calculate the heat of combustion of methanol, i.e., ∆Horxn for CH3OH(g) + 3/2 O2(g) --> CO2(g) + 2 H2O(g) ∆Horxn = Σ nHfo (prod) - Σ  nHfo (react)

Using Standard Enthalpy Values CH3OH(g) + 3/2 O2(g) --> CO2(g) + 2 H2O(g) ∆Horxn = Σ nHfo (prod) - Σ  nHfo (react) ∆Horxn =  {1Hfo (CO2) + 2 Hfo (H2O) } – {Hf (CH3OH) + 3/2 Hf (O2)    = (-393.5 kJ) + 2 (-241.8 kJ) - {0 + (-726.0 kJ)} ∆Horxn = -115.9 kJ per mol of methanol *** Remember elements have Hf of zero, and other values may be found on the tables given***

Spontaneous : Processes that occur without outside intervention First law of thermodynamics: the internal energy of a system deals has to do with the work being done on the system plus or minus any heat that flows into or out of the system. It is an extension of the law of Conservation of Energy, where energy isn’t created or destroyed but will be transferred or converted to another form In nature, systems tend toward lower energy, so it is expected that the lowest enthalpy (exothermic) would be favored However, there are processes that occur in nature which don’t favor an exothermic condition, yet they occur anyway

Spontaneous Which of these processes are spontaneous? In reverse? At equilibrium? a) A piece of metal is heated to 150ºC is transferred to water at 40ºC and the water gets hotter b) Water at room temperature decomposed to H2 and O2 c) Benzene vapor in a closed system condenses to liquid at its boiling point at 80.1 º C

Reversible Processes Reversible Processes – the reactants can be returned to their original state by EXACTLY reversing the change Example: ice to water, water to ice Chemical systems in equilibrium are reversible between reactants and products Spontaneous reactions are NOT reversible Thermodynamics tell us about direction and extent but not about speed

ENTROPY and SECOND LAW OF THERMODYNAMICS Second Law of Thermodynamics:  systems gravitate towards thermodynamic equilibrium, also known as a state of maximum entropy, or disorder The overall entropy of the universe will always be greater than any decrease in entropy of the system https://www.youtube.com/watch?annotation_id=ann otation_2975400339&feature=iv&src_vid=hNSD0YDsPs E&v=MALZTPsHSoo ΔS = entropy

Example: Expansion of a Gas