Explain what is meant by the following two terms used in molecular orbital theor

Question

Explain what is meant by the following two terms used in molecular orbital theory. In both cases use molecular hydrogen (H_2) to illustrate your answer. (i) LCAO (ii) HOMO (iii) Differentiate between the LCAO Approximation 1 and LCAO Approximation 2. Illustrate how these two approximations pertain to the orbitals of the homonuclear molecules, di-borane (B_2) and di-fluorine (F_2). Explain the origin of the different energy ordering of the molecular orbitals in these two molecules. b. (i) Calculate the standard entropy change Delta S degree for the following reaction at 25 degree C. Use the data provided in Table 1. S_(rhombic) + O_2(g) rightarrow SO_2(g) (ii) Provide a brief justification for the sign of the Delta S degree of reaction you obtained. (ii) Based on the data shown in Table 2 below. Calculate die standard Gibbs free energy change (Delta G degree) for the combustion of ethane at 25 degree C.

Explanation / Answer

(i)

deltaS0rxn = deltaS0f(SO2) - deltaS0f(S) - deltaS0f(O2)

= 248.5 - 31.88 - 205.0

= + 11.62

(ii) Since solid rhombic sulphur is being converted into gaseous sulphur, the disorderness of the system increases and hence the entropy of the system also increases.

(iii)

2 C2H6 (g) + 7 O2 (g) ------------> 4 CO2 (g) + 6 H2O (l)

deltaG0rxn = 4 deltaG0f(CO2) + 6 deltaG0f(H2O) - 2 deltaG0f(C2H6) - 7 deltaG0f(O2)

= 4 (- 394.4 ) + 6 ( - 237.2 ) - 2 ( - 32.86 ) - 7 (0)

= - 2935.08 kJ

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