Kespontanan Reaksi dan EntropiKimia Anorganik FisikLatifah, MSi.Kimia FMIPA UNNES24 September 2010

KespontananSpontaneous reaction = having the potential to proceed without assistance of external agency

Kespontanan ReaksiTermasuk Reaksi spontan ???

Reaksi : Fe + O2 Fe2O3Reaksi spontan ??

Dua faktor yang menentukan terpisahnya atom-atom reaktan dan penataulangannya menjadi produkKekuatan ikatanDerajat ketidakteraturan pada keadaan akhirFungsi Termodinamik : Gibbs EnergyEnergi yang tersedia untuk melakukan kerja

Free Energy ChangeThe free energy change (DG) for a process at constant temperature and pressure is given by the Gibbs equationDGsys = DHsys T.DSsys

Factors Determining GFree energy change depends onenthalpyentropyH = (enthalpy of products) - (enthalpy of reactants)S = (entropy of products) - (entropy of reactants)G = H - TS

Enthalpy DHo = heat released or absorbed during a chemical reaction at standard conditions.Exothermic, (-DH), heat is released.Endothermic, (+DH), heat is absorbed.Reactions favor products with lowest enthalpy (strongest bonds).

Entropy DSo = change in randomness, disorder, freedom of movement.Increasing heat, volume, or number of particles increases entropy.Spontaneous reactions maximize disorder and minimize enthalpy. In the equation DGo = DHo - TDSo the entropy value is often small. =>

Criterion for Spontaneous ChangeEOS

Signs of Thermodynamic Values

NegativePositiveEnthalpy (H)ExothermicEndothermicEntropy (S)Less disorderMore disorderGibbs Free Energy (G)SpontaneousNot spontaneous

Standard Free Energy and Temperature If H < 0 and S > 0, then G is always negative(spontaneous.) If H > 0 and S < 0, then G is always positive(nonspontaneous) If H < 0 and S < 0, then G is negative at low temperatures. If H > 0 and S > 0, then G is negative at high temperatures.NOTE: Even though a reaction has a negative G it may occur too slowly to be observed

Principles of EntropyThe gaseous state is more probable than liquid state, which in turn is more probable than the solid state

Entropy and Temperature

Entropy of Various Substances at 25C

SubstanceSolidLiquidGasNa12,313,8336,71P9,8210,2838,98Si4,4311,2140,12Pb15,517,1441,89H2O-16,7245,14

2. A monoatomic gas is more probable than polyatomic moleculer gas, and hence tends to have higher entropy

H 27,4H2 15,6N 36,6N2 22,9O 38,5O2 24,5O3 19,0F 37,9F2 24,4Si 40,1Si2 17,5P 39,0P2 26,1P4 16,7S 40,1S2 27,3S8 12,9

3. An amorphous solid is more probable than a crystalline solid, and a simple crystalline solid more probable than a more complex crystalline solid

4. Coordination complex is less probable than its separete components

5. Compounds of elements of higher atomic weight, or molecules of the free elements themselves, tend to have higher entropy

Effect of Mass on entropy of Gases at 25C :

XFClBrIHX41,5144,65--NaX51,7054,88--PbX269,3576,6382,4385,91ZrX476,9587,3798,78108,42

Standard Molar Entropies5. In general, the more atoms in its molecules, the greater is the entropy of a substance

6. At ordinary temperature, entropy effects are commonly small enough to have relatively little effect on the direction of reaction unless the difference in total bonding energy between reactants and products is relatively small7. All chemical reactions that increase the entropy occur spontaneously at high enough temperature G = H T.S