The direction that spontaneous readjust is the direction in i m sorry total entropy increases. Full entropy change, likewise called the entropy adjust of the universe, is the sum of the entropy change of a system and the its surroundings:

DSuniv = DSsys + DSsurr

According come the second law of thermodynamics, the entropy the the universe, Suniv must always increase because that a spontaneous process, the is, DSuniv>0.

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Free energy and complimentary Energy Change—the Gibbs cost-free energy, G, is offered to define the spontaneity of a process.

G = H - TDS

The cost-free energy change, DG is equal to -TDSuniv and it uses just come a device itself, there is no regard because that the surroundings. It is characterized by the Gibbs equation:


For a spontaneous procedure at continuous temperature and also pressure, DG should be negative. In plenty of cases, we have the right to predict the sign of native the signs of DH and also DS.Standard totally free Energy Change, DGo —the standard totally free energy change, DGo deserve to be calculated (1) by substituting standard enthalpies and entropies that reaction and also a Kelvin temperature right into the Gibbs equation or (2) by combining standard cost-free energies of formation through the expression


Summary the Gibbs cost-free energy

Enthalpy change

Entropy change

Gibbs cost-free energy




depends ~ above T, might be + or -

yes, if the temperature is high enough



always negative

always voluntarily



depends ~ above T, may be + or -

yes, if the temperature is low enough



always positive

never voluntary


1) In the Haber process for the to produce of ammonia


Enthalpy change

Entropy change

N2 + 3H2


ΔH = -93Kj / mol

ΔS = -198 j / mol K

At what temperature will the reaction over become spontaneous?

The fact that both terms room negative way that the Gibbs cost-free energy equation is balanced and also temperature dependent:


ΔG = -93000 - (T x -198) note that the enthalpy is provided in kilojoules

if ΔG = 0 then the system is at the border of reaction spontaneity

When ΔG = 0 then (T x -198) = -93000

and T = 93000/198 Kelvin

therefore the reaction i do not care spontaneous when T = 469 K (196 ºC)

below this temperature the reaction is spontaneous.

2) recognize the Delta G under standard conditions using Gibbs free Energies of Formation discovered in a an ideal Thermodynamics table for the adhering to reaction:

4HCN(l) + 5O2(g) ---> 2H2O(g) + 4CO2(g) + 2N2(g)

examine to make certain the equation is well balanced Look up the Standard cost-free Energy of development of H2O(g) and also multiply by its coefficient(2) in the equation.

2 mole ( -237.2 kj/mole) = -474.4 kj = Standard complimentary Energy of development for 2 moles H2O(l)

Look for the Standard totally free Energy of development of CO2(g) and also multiply by its coefficient (4)

4 mole ( -394.4 kj/mole) = -1577.6 kJ = Standard cost-free Energy of development for four moles CO2

Look up the Standard totally free Energy of formation of N2(g) and multiply by its coefficient(2)

2 moles(0.00 kj/mole) = 0.00 kJ = Standard free Energy of development for 2 mole of N2(g)

add the results of procedures 2,3, and 4 to acquire the Standard cost-free Energy for the commodities

(-474.4 kJ) + (-1577.6 kJ) + 0.00 = -2052 kJ = Standard totally free Energy for products

Look increase the Standard free Energy of development for HCN(l) and also multiply by its coefficient(4)

4 moles ( 121 kj/mole) = 484 kJ = Standard cost-free Energy for 4 mole HCN(l)

Look increase the Standard free Energy of formation of O2(g) and multiply by its coefficient(5)

5 mole (0.00 kJ/mole) = 0.00 kJ = Standard totally free Energy the 5 mole of O2(g)

include the outcomes of steps 7 and 8 to obtain the Standard totally free Energy of the reactants

(484) + (0.00) = 484 kJ = Standard cost-free Energy because that Reactants

Subtract the an outcome of step 9 indigenous the result of action 5 to gain the Standrad complimentary Energy readjust for the Reaction

Sum of complimentary Energy of commodities - sum of totally free Energy of reaction = (-2052 kJ) - (484 kJ) = -1568 kJ = Standard cost-free Energy readjust for the Reaction.

3) because that the complying with reaction making use of the Thermodynamics table:

CoCl2(g) ---> CO(g) + Cl2(g)

calculation at 127°C the DG calculation the Temperature when the above reaction is in ~ equilibrium (DG = 0) in ~ what temperature will this reaction be spontaneous? check to watch if the equation is balanced recognize the Delta H of the Reaction utilizing Hess Summation Law and Standard Enthalpies of development

Delta H = sum of Delta Hf of products - sum Delta Hf of reaction

Delta H = < 1(-110.5) + 1(0.00)> - < 1(-220)>

Delta H = -110.5 - (-220) = +110.5 kJ

recognize Delta S for the reaction using typical Molar Entropies and also Hess law of Summation

Delta S = amount Standard Molar Entropies of assets - amount of traditional Molar Entropies of reactants

Delta S = < 1 mole(197.5 J/mole-K) + 1 mole(223) J/mole-K> - < 1 mole(283.7 J/mole-K)>

Delta S = 420.5 J/K - 283.7 J/K = 136.8 J/K

transform Delta S native J/K come kJ/K

136.8 J/K X 1 kJ / 1000 J = 136.8 / 1000 = .1368 kJ/K

transform 127 C to K

K = C + 273 = 127 + 273 = 400 K

Plug outcomes of step 2 and 4 right into Gibbs Helmholtz Equation along with Kelvin Temperature to gain Delta G the the Reaction

Delta G = Delta H - T(Delta S)

Delta G = 110.5 kJ - 400 K(.1368 kj/K)

Delta G = 110.5 - 54.72 kJ = + 55.78 kJ

Because this reaction has actually a confident Delta G it will certainly be non-spontaneous together written.

Name of species Delta Hf(kJ/mole) Delta Gf(kJ/mole) S(J/mole-K)

CO2(g) -393.5 -394.4 213.7

CH3OH(l) -238.6 -166.2 127

COCl2(g) -220 -206 283.7

CO(g) -110.5 -137.2 197.5

C2H2(g) 227 209 200.9

Cl2(g) 0 0 223

H2O(g) -241.8 -228.6 188.7

H2O(l) -285.8 -237.2 69.9

HNO3(aq) -206.6 -110.5 146

N2(g) 0 0 191.5

NO2(g) 33.2 51 239.9

NO(g) 90.3 86.6 210.7

O2(g) 0 0 205

CS2 151

H2 0 0 130.6

CH4 -74.8 -50.8 186.3

H2S -20.17 -33.01 205.6

SO2 248.1

Free Energy and Equilibrium.

Because DG is a measure up of just how favorable a reaction is, it additionally relates come the equilibrium constant.

A reaction v a an adverse DG, is an extremely favorable, therefore it has actually a large K. A reaction v a confident DG is not favorable, so it has actually a little K. A reaction with DG = 0 is at equilibrium. There space several various DG"s. It is crucial to distinguish in between them. DGo (a delta G, v a superscript o), is the cost-free energy adjust for a reaction, with whatever in the standard states (gases in ~ 1 bar, and also solutions at 1 M concentration), and at a particular temperature (usually 25°C) DG (just delta G). This is the cost-free energy adjust for a reaction that is no at the standard state. The DG"s are related as follows. Know THIS EQUATION and understand just how to usage it:

DG =DGo + RT ln Q

Where Q is the exact same Q we provided for calculating equilibrium (K is the special instance for Q when at equilibrium.)

Spontaneity and also Speed the Reactions.

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This section merely points out that there is not any type of direct relationship between DG and the speed of a reaction (kinetics).


Starting through the adjust in complimentary energy at constant temperature: DG° = DH° - TDS°, and also with the relation between DG and also equilibrium constant, K: DG° = -RT lnK, derive a linear equation the expresses lnK together a function of 1/T (a straight equation is of the kind y = mx + b).

(b) The equilbrium constants for the switch of 3-phosphoglycerate come 2-phosphoglycerate in ~ pH 7 is provided: