ENTHALPY, ENTROPY AND GIBBS FREE ENERGY

The First Law of ThermodynamicsEnergy can neither be

created or destroyedThe energy of the universe

is constant, but it can change forms.

Energy book keeper

First Law accounts for energy, but it does not tell us why a particular process occurs in a given direction

SpontaneityDOES NOT MEAN FAST!!!

Means that the process occurs without any outside intervention

Energy released or absorbed during a chemical reaction (heat of reaction) is equal to the difference between the potential energy of the products and the potential energy of the reactants. In a chemical reaction; reactants --> productsΔPE = PE products - PE reactants PE can be thought as heat energy (H)Therefore, ΔH (kJ) = H products - H reactants

When ΔH is negative H products < H reactants and the reaction is exothermic.

When ΔH is positive H products > H reactants and the reaction is endothermic.

Energy released or absorbed by a chemical reaction can be represented by a potential energy diagram.

Activation Energy The activation energy is

the minimum energy required to start a chemical reaction by providing colliding molecules with enough energy for effective collisions to occur.

The activated complex is the short-lived and unstable intermediate species located at the highest of the activation energy.    

Catalysts

A catalyst provides an alternate reaction pathway, which has a lower activation energy than an uncatalyzed reaction.

Look at Table A-6 Notice the following:

Substances are in alphabetical orderΔHf (enthalpy of formation) is in kJ/mole

Free elements have a ΔHf = 0 (they are not compounds formed from elements)

The enthalpy of reaction is equal to the sum of the enthalpies of formation for the products – the sum of the enthalpies of formation for the reactants Σ

ΔHr = ΔHf products - Σ ΔHf reactants

ENTHALPY CALCULATIONS2CO(g) + O2(g) 2CO2(g)

ΔHr = Σ ΔHf products - Σ ΔHf reactants

=[2 mole(-393.509kJ/mole)] -

[2mole(-110.525kJ/mole) + 1mole(0kJ/mole)]

= [-787.018kJ] - [-221.050kJ]

=-565.968 kJ

The reaction is exothermic

ENTROPY Entropy is the degree of disorder Represented with the symbol S Matter changes from a more ordered

to less ordered state2H2O 2H2 + O2

H2(l) H2(g) A positive ∆S means an increase in

entropy

States of matter

Ssolid < Sliquid << Sgas

Which has more entropy?

1. Solid or gaseous phosphorus

2. CH4(g) or C3H8(g)

3. NaCl(s) or NaCl(aq)

Second Law of ThermodynamicsIn any spontaneous process

there is always an increase in the entropy of the universe

The entropy of the universe is constantly increasing

ENTROPY CALCULATIONS If the reaction increases entropy, ∆S

is positive and the reaction is said to be ENTROPY-FAVORED

Calculate the entropy change(∆S) for the following reactionCH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Three S’s

Ssys = system

Ssurr = surroundings

Ssys + Ssurr = Suniv

Suniv

If it is +, the entropy of the universe is increasing

Process is spontaneousIf it is negative, the process

is not spontaneous

Change of state

H2O(l) H2O(g)

What happens to the S of the water?

Ssys= +

What about surroundings?

Heat is flowing from the surroundings to the system

Random motion of particles decreases

Ssurr = -

Is it spontaneous?

Need to look at Suniv

Which S controls the situation?

DEPENDS ON TEMP

Exothermic ProcessAlways increases entropy of

surroundingsBut, its significance depends on the

temp at which the process occursEnergy transfer will be more

significant at lower temps

GIBBS FREE ENERGY

Gibbs free energy (∆G) is a measure of the chemical reaction potential of a system If ∆G is negative, the reaction is spontaneous If ∆G is positive, the reaction is not spontaneous

Calculate the change in free energy for the following reaction

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Gibbs Free Energy Enthalpy and Entropy can be combined to

predict reaction spontaneity∆G = ∆H - T∆S

∆H ∆S ∆G Comments on Reaction

- + - Always spontaneous

+ + + or - Spontaneous at high temperatures

- - + or - Spontaneous at low temperatures

+ - + Never spontaneous