Chapter 19 - Chemical Thermodynamics
- Spontaneous Processes
- Energy is conserved - First Law
- D/N address spontaneity
- Spontaneous in one direction, not spontaneous in the other
- nothing about rate (kinetics)
- Things that make reaction tend to be spontaneous
- increase 'randomness'
- Spontaneity, Entropy, and Enthalpy
- Systems tend to minimize energy
- Enthalpy not the only form of energy considered!
- Entropy (S)
- Every spontaneous endothermic process has an increase in randomness!
- Ice melting
- NH4NO3 dissolving in water
- remember also accompanied by increase in order of water
- More disordered, larger entropy!
- State function
- Second Law of Thermodynamics
- Increase in entropy of an isolated system
- For a spontaneous process, the entropy of the Universe is increasing!
- If system increases entropy, surroundings decrease!
- Our discussions will be for Ssystem
- A Molecular Interpretation of Entropy
- Increases in entropy
- Increased volume for gas
- Dissolving a solid
- Reaction to increase moles of a gas
- Decreases in entropy
- dissolving a gas
- Reaction to decrease moles of a gas
- Molecule stores energy in many forms
- As T increases, energy stored in these modes increases!
- Third Law of Thermodynamics
- Entropy of a pure, crystalline substance is 0 at 0K
- Never been at 0K (but very close!)
- Increase in temperature - increase S
- Phase change - sharp change in entropy
- ENTROPY INCREASES (revisited)
- Liquids/Solutions from solids
- Gases from solids or liquids
- #molecules of gas increase during a reaction
- Temperature of a substance is increased
- Calculation of entropy change.
- Srxn = (nS)products - (nS)reactants
- Note units on S (Joules instead of kJ, also has temp unit)
Problems: 7-15 odd
- Gibbs Free Energy
- Compares entropy and enthalpy values
- G = H - TS
- At constant T, G = H - TS
- G sign can tell us about spontaneity!
- G<0, rxn spontaneous as written
- G>0, rxn non-spontaneous (but the reverse reaction is spontaneous!)
- G = 0 at chemical equilibrium (more later)
- Usually concerned with G
- Grxn = (nG)products - (nG)reactants
- G = H - TS
- Conventions for Standard free energy values
- Solids - Pure solid
- Liquids - Pure liquid
- Gas - Present at 1 atm
- Solutions - 1M concentration (simplification)
- Free Energy and Temperature
- For a reaction to be spontaneous their must be either an exothermic process (H<0),
and/or an increase in the entropy (S>0)
- Look at combinations of H and S to predict spontaneity
- H<0, S>0, always spontaneous
- H>0, S<0, never spontaneous
- H<0, S<0, spontaneous at low temperatures
- H>0, S>0, spontaneous at high temperatures
- NOTE THAT A NONSPONTANEOUS REACTION CAN BE ACCOMPLISHED WITH A
CONTINUOUS INPUT OF ENERGY!
- Calculate temperature at which reaction 'reverses direction'
- Approximation because of assumption H constant w/temp
Problems: 25-31 odd
Define the term spontaneity, and apply it in identifying spontaneous processes.
Describe how entropy is related to randomness or disorder.
State the Second Law of Thermodynamics.
Predict whether the entropy change in a given process is positive, negative, or near zero.
State the Third Law of Thermodynamics.
Describe how and why the entropy of a substance changes with increasing temperature or
when a phase change occurs, starting with the substance as a pure solid at 0K.
Calculate S for any reaction from tabulated values of S.
Define free energy in terms of enthalpy and entropy.
Explain the relationship between the sign of the free energy change (G) and whether a
process is spontaneous in the forward direction.
List the usual conventions regarding standard states in setting the values for standard free
Describe the relationship between G and the maximum work that can be derived from a
spontaneous process, or the minimum work required to accomplish a nonspontaneous
Calculate the standard free energy change at constant temperature and pressure (G) for
any process from tabulated values for the standard free energies of formation of reactants
Predict how G will change with temperature given the signs for H and S.
Estimate G at any temperature given S298 and H298.
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