CHEM 1040 Chapter 18

For a perfect crystal at absolute 0, the absolute entropy is

0 J/ mol x K

The standard enthalpy of formation, delta H f, of an element is

0 Kj/mol

For solids and liquids, the standard state corresponds to the pure solid or liquid in the most stable form at exactly

1 atm pressure and temperature of interest, most of the time at 25 degrees C

The first law of thermodynamics states that

energy cannot be created or destroyed

A nonreversible transformation can be reversed by using

energy to drive the back, nonspontaneous process

The enthalpy change is favorable for

exothermic and unfavorable for endothermic

When the entropy change in a system is unfavorable (negative), the entropy change in the surroundings must be

favorable (positive) and large in order to allow the process to be spontaneous

Entropy is a thermodynamic state function that

growths all the number of energetically equivalent ways of arranging the component increases

Enthalpy change (delta H)

heat involved in a process occurring at constant p

When a process of the system is exothermic

heat is released to surroundings Increase S much more in surroundings than the decrease of system

If the entropy of the system decreases during a spontaneous process, the entropy of the surroundings must

increase by a larger amount

Evaporation of H2O

increase in entropy

Salt dissolving in water

increase in entropy

Above 0 degrees C, the increase in entropy of the surroundings is

insufficient to make delta S positive

The entropy change in the surroundings is also

inversely proportional to the temperature

When a system process is endothermic

it takes heat from surroundings Decreasing surroundings entropy

enthalpy change units

kJ/mol

High entropy means

large dispersal of matter/energy

Bigger more complex molecules usually have

larger entropies They possess more energy states, allowing more dispersal of energy through the states.

-Addition of heat to surroundings at low temperatures has a

larger impact on the entropy than in the case of hot surroundings

The fact that the layers in graphite are not bonded makes graphite

less constrained

Solids have fewer macrostates than

liquids which have fewer macrostates than gases

Solids more ordered than

liquids; liquids more ordered than gases

Random systems are characterized by

lower and dispersed amounts of energy as compared with that of ordered systems

The more ways molecules can move (degrees of freedom), the more

macrostates are possible

When the stopcock is open, the gas atoms distribute themselves in

many energetically equivalent ways

Different motions, degrees of freedom, are possible for

molecules and the number of possibilities increase with molecular complexity

For materials with larger molar masses, the available energy states are

more closely spaced Allows for more dispersal of energy through the states

Processes that result in a randomness increase are

more likely

Changes that increase the entropy are transformations whose products are in a

more random state

If a process is spontaneous in one direction it is

nonspontaneous in other direction

Reversible processes

occur back and forth between two conditions Most involve equilibrium situations. Net result is no change in energy

absolute entropy

of a substance is related to dispersal of energy possessed through all the particles of the material

The PE is the same, whether the molecules are all in

one flask or evenly distributed

Energy can be transferred from

one place to another

The amount of entropy of the surroundings changes depends on

original temperature

Reaction is generally endothermic (+ delta H) if the bonds in the products are

weaker than the bonds in the reactants requiring energy to be absorbed

Entropies are for 1 mol of a substance at 298 K for a particular state, a

particular allotrope, a particular molecular complexity, a particular molar mass, and a particular degree of dissolution

The absolute entropy at 25 degrees C is always

positive

When delta Ssys is negative, delta Ssurr must be

positive and big for a spontaneous process to occur.

Thermodynamics

predicts whether a process will occur under the given conditions

nonspontaneous

processes that need some input to proceed

Spontaneous

processes that occur without intervention

Entropy changes in the surroundings are

proportional to the amount of heat gained or lost

For gases the standard state corresponds to the

pure gas at exactly 1 atm pressure

Liquids exhibit larger entropies than

solids at any particular temperature

Every substance that is not a perfect crystal at absolute 0 has

some energy dispersed

Spontaneity not related to the

speed or the process (not fast or slow)

Energy is not the only considerable aspect because some endothermic processes are

spontaneous

Transport of thermal energy from a hot to colder place is

spontaneous with delta S > 0

Processes like condensation of water vapor are

spontaneous, even though water vapor is more random than liquid water.

-Standard entropy change

difference in absolute values between the reactants and products under standard conditions

The state with the highest entropy also has the greatest

dispersal of energy

macrostate

defined by variables T, p, V and n of the system, and encompasses all the possible arrangements (or microstates) that result in a specific enthalpy and entropy of the system

Delta E univ = 0 =

delta Esys + delta Esurr

Np =

# mol for each product

Nr =

# mol for each reactant

Delta s o rxn =

(sum of np S o products) - (sum of nr S o reactants)

Randomness increases when delta S is

+

The absolute entropy of substances is always

+

Qsurr =

- q sys

At constant pressure and temperature, the overall relationship is

delta S surr = -qsys/temp (T) = (-) delta H sys/ T

K is the Boltzmann constant =

1.38 x 10 ^ -23 J/K

The clustered flasks only have 1 possible arrangement while the spread-out flasks have

6

For a reversible process, delta Suniv

= 0

For irreversible (spontaneous), delta Suniv

>0

For a process in which the final condition is more random than the initial condition

delta S sys is positive The entropy change is favorable for the process to be spontaneous

Delta Suniv =

delta Ssys + delta Ssurr

For a process in which the final condition is more orderly than the initial condition

Delta S sys is negative. Entropy change is unfavorable for the process to be spontaneous

Delta S sys = delta S rxn =

En (S o products) - En (s o reactants)

2 factors that determine whether a reaction is spontaneous or not are

Enthalpy changes of system. Entropy changes of system.

Molar entropy =

J/ (mol x K)

Delta S measured in

J/k

Delta S > 0 for

Reactions that have larger numbers of product molecules than the reactant molecules Processes leading to increases in temperature. Solids dissociating into ions upon dissolving.

S solid <

S liquid < S gas

Entropy evaluated using Boltzmann equation

S= k ln W

Delta S =

Sfinal - S initial

Delta H measures the

difference between the sum of the internal energy and pV work energy of the products and reactants

Heat released by the system increase

T and the entropy of surroundings

W

a distribution, or number of energetically equivalent ways a system can exist

If a process is spontaneous, yet entropy change is unfavorable, there must have been

a large increase in surrounding entropy

The standard state of a solution consists of the substance dissolved in

a liquid with a concentration of 1 M

Low entropy means

a more ordered system

Some spontaneous processes can proceed from

a state of lower PE to another of higher PE Endothermic

The universe consists of

a system plus surroundings

Energy can be distributed among

all the possible DOF

H2O condensation is

an exothermic process

Entropy is

an extensive physical property of matter

Energy needed to

bring order to the system

Thermodynamics (kinetics)

centered on properties determined under equilibrium

When materials change their physical state, the number of macrostates they have can

change

The spread-out flask has a higher entropy than the

clustered flasks

A way to attempt assessment of spontaneity is by

comparison of the potential energy of the system before the reaction with the energy of the system after the reaction

Total universe energy remains

constant

kinetics

deals with the speed of transformations

S o designates

standard state conditions

Standard state

state of a material at a defined set of conditions, dependent on the form of material

Reaction is generally exothermic (- delta H) if bonds in the products are

stronger than the bonds in the reactants, resulting in energy release

Entropy change (delta S)

the difference between the dispersion of energy (or matter) of the products and that of the reactants

The less constrained the structure of an allotrope is, the larger

the entropy

Water freezes spontaneously below 0 degrees Celsius because

the heat released in this physical change increases the entropy of the surroundings enough to yield a positive delta S

The higher number of possible motions

the higher the entropy

Microstate

the instantaneous arrangement of energy (or position) of the components in a system

The larger the molar mass

the larger the entropy

The higher the original temperature

the less effect addition or removal of heat has

Gases exhibit larger entropies than

the liquid at any particular temperature

Entropy provides a measure of

the state of dispersal of either matter or energy in a system

The second law of thermodynamics

the total entropy change of the universe must be positive for a process to be spontaneous

Second law demands that

the universe entropy increase for spontaneous process

Spontaneous processes are irreversible because

there is a net release of energy when they proceed in a specific direction

If after the reaction the system has less PE than before, the process is

thermodynamically favorable.

Spontaneous processes may occur because

they release energy from the system Most proceed from a state of high PE to another at lower PE. This is exothermic.