enthalpy, entropy and gibbs free energy the first law of thermodynamics energy can neither be...
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ENTHALPY, ENTROPY AND GIBBS FREE ENERGY
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The First Law of ThermodynamicsEnergy can neither be
created or destroyedThe energy of the universe
is constant, but it can change forms.
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Energy book keeper
First Law accounts for energy, but it does not tell us why a particular process occurs in a given direction
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SpontaneityDOES NOT MEAN FAST!!!
Means that the process occurs without any outside intervention
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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
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When ΔH is negative H products < H reactants and the reaction is exothermic.
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When ΔH is positive H products > H reactants and the reaction is endothermic.
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Energy released or absorbed by a chemical reaction can be represented by a potential energy diagram.
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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.
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Catalysts
A catalyst provides an alternate reaction pathway, which has a lower activation energy than an uncatalyzed reaction.
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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
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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
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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
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States of matter
Ssolid < Sliquid << Sgas
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Which has more entropy?
1. Solid or gaseous phosphorus
2. CH4(g) or C3H8(g)
3. NaCl(s) or NaCl(aq)
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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
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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)
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Three S’s
Ssys = system
Ssurr = surroundings
Ssys + Ssurr = Suniv
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Suniv
If it is +, the entropy of the universe is increasing
Process is spontaneousIf it is negative, the process
is not spontaneous
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Change of state
H2O(l) H2O(g)
What happens to the S of the water?
Ssys= +
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What about surroundings?
Heat is flowing from the surroundings to the system
Random motion of particles decreases
Ssurr = -
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Is it spontaneous?
Need to look at Suniv
Which S controls the situation?
DEPENDS ON TEMP
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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
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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)
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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