Chapter 19: Chemical Thermodynamics

Chapter 19: Chemical ThermodynamicsRelating energy and extent of reaction

What do we already knowDefine thermodynamics

Define enthalpy

How is enthalpy related to the first law of thermodynamics?

To be, or not to beWhy can we not use the value E to tell us whether or not a process is favored?

Define spontaneous and non-spontaneous reactions.

GENERALLY SPEAKING: processes that are spontaneous in one direction are non-spontaneous in anotherDetermining SpontaneityPredict which experimental conditions are important in determining spontaneity of reaction.

Temperature: IE: Ice Melting at -10 degrees Celsius, water freezes spontaneouslyPressure:

NOTE: A spontaneous reaction can be very fast or very slow. Thermodynamics tells us direction and extent but not speed.

Cleachtais (Irish)Predict whether or not each process is spontaneous as described, spontaneous in reverse, or in equilibrium.Water at forty degrees Celsius gets hotter when a piece of metal heated to one hundred and fifty degrees Celsius is addedWater at room temperature decomposes into hydrogen and oxygen gasBenzene vapor at a pressure of 1 atm condenses to liquid benzene at the normal boiling point of benzeneAt 1 atm pressure, solid carbon dioxide sublimes at -78 degrees Celsius. Is this process spontaneous at -100 degrees Celsius? Reversible, IrreversibleIdeal Engine:Sadi Carnot- analyzed factors that determine how efficiently a steam engine can convert heat to workImpossible to convert energy content of a fuel completely to work- Why?Ideal engines operates under ideal set of conditions in which all processes are reversible.Define Reversible ProcessDefine Irreversible ProcessReversible change produces the maximum amount of work that can be done by a system on its surroudingsReversible, IrreversiblePredict if flow of heat is a reversible process. Justify your reasoning.

Reversible Processes are those that reverse direction whenever an infinitesimal change is made in some property of the systemReversible, IrreversibleDefine Isothermal

Suppose an ideal gas is confined to a cylinder with a removable piston. The partition is removed and the gas expands. Is this process reversible?

The path that restores the system to its original state requires a different value of w than the original path. The same path cant be followed- thus it is irreversible.To sum things up!ALL REAL PROCESSES ARE IRREVERSIBLE

A NONSPONTANEOUS PROCESS CAN ONLY OCCUR IF THE SURROUNDINGS DO WORK ON THE SYSTEM.

ANY SPONTANEOUS PROCESS IS IRREVERSIBLEINSERT INTRO TO ENTROPY SLIDES HEREApril 4th, 2013DO NOW:AP Practice Question: Lewis Structures and PE DiagramsBarrons Book Chapter 3 MCPlease take out homework and compare answers with your neighbor.2nd Law of ThermodynamicsAny irreversible process results in an increase in total entropy, whereas any reversible process results in no overall change in entropy.

Irreversible Process: Suniv= Ssys + Ssurr > 0Reversible Process: Suniv= Ssys + Ssurr = 0

Entropy of the universe increases in any spontaneous process!!Molecular Motions and EnergyOn the molecular level, how are molecules affected when a substance is heated?

Types of molecule movement:Translational Motion: movement in one direction.

Vibrational Motion: atoms within molecule move periodically toward and away from one another.

Rotational Motion: molecules spin on an axis

Different forms of motion are ways in which molecule can store energy (AKA motional energy)Application to EntropyCompare and contrast the possible motions of atoms to those of molecules.

Number of microstates possible for a system increases with: increase in temperature, increase in number of molecules. Any of changes increase possible positions and KE of molecules in system.2 DescriptionsENTROPY IS:

RANDOMNESS OR DISORDER OF SYSTEMDISPERSION OF ENERGYPredicting Entropy QualitativelyBased on the images above and your knowledge of entropy, explain the differences in entropy of the different states of water.

Entropy in Chemical Reactions2 examples:How does the entropy of the system change when an ionic solid is dissolved in water?

Chemical Reactions:2 NO(g) + O2 (g) 2 NO2 (g)

In SummaryWe generally expect entropy of a system to increase for processes in which:

Gases form from either solids or liquidsLiquids or solutions form from solidsNumber of gas molecules increases during a chemical reactionPraticare (Italian)Predict whether S is a positive or negative for each process (assuming constant Temp)Liquid water becomes water vaporSilver and Chlorine ions form Silver ChlorideSolid Iron is oxidized to form Fe2O3Nitrogen and Oxygen gases form NO gas.Practicare Cont.In each pair, choose the system that has the greater entropy and explain your choice1 mol of NaCl(s) or 1 mol of HCl(g)2 mol of HCl(g) or 1 mol of HCl(g)1 mol of HCl(g) or 1 mol of Ar(g)Third Law of ThermodynamicsAs we reach absolute zero, predict what will happen to the entropy of the system?

Entropy of a pure crystalline substance at absolute zero is zero: S(0 K) = 0.Entropy Changes in Chemical ReactionsNo finite method of measuring S for a reactionSince third law establishes a zero point, experimental measurements can determine absolute value of the entropy

Molar EntropyMolar entropies for substances in their standard states are known as standard molar entropies and denoted as S

With a partner, make observations based on the table.

April 5th, 2013DO NOW:

Calculate the standard entropy of the system, S, for the Haber Process.MotivationHow do we determine the spontaneity of a reaction?

Spontaneity is based off of 2 concepts- enthalpy and entropy.

What does this suggest for our aim?Entropy of the SurroundingsPredict how we would calculate the change in entropy in the surroundings.

How would we calculate the enthalpy change for the system?

Why is the positive value for entropy of the surroundings expected?Gibbs Free EnergyJosiah Willard GibbsAmerican Mathematician (1839-1903)First person to be awarded a Ph.D. in science from an American university

New State Function: Gibbs free energy (G)G = H TST = absolute temperature

Gibbs Free EnergyFor an isothermal process, how would we represent the change in the free energy of a system?

How can we derive our equation for an isothermal and isobaric reaction?

What information does the value of G give us?GIf both T and P are constant

G < 0, reaction is spontaneous in forward directionG = 0, reaction is at equilibriumG > 0, reaction is spontaneous in reverse direction

Why is using G as a criterion for spontaneity better than using entropy?

IN ANY SPONTANEOUS PROCESS CARRIED OUT AT CONSTANT PRESSURE AND TEMPERATURE, FREE ENERGY ALWAYS DECREASES.

(Japanese)Calculate the standard free-energy change for the formation of NO from N2 and O2 at 298 K.H=180.7 kJS= 24.7 J/K

Is this reaction spontaneous?

Free Energy and TemperatureWhy cant we use the G from appendix C to calculate all reactions?

Lets consider the melting of ice:H2O (s) H2O (l) Predict the sign of H and S

Generally, H and S values dont vary much with temperature. How does the G change?

In summary

Free energy under nonstandard conditionsMost chemical reactions occur under nonstandard conditions

G= G + RT lnQR = ideal gas constantT = absolute tempQ= reaction quotient (Chapter 15) (Q=1 @ standard conditions)At standard conditions, G= ?

Relationship between G and KUsing the previous equation, determine the equation to0 solve for K.

ALWAYS BEWARE UNITS.

If G is negative, K > 1If G is positive, K