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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