Answer:
option D : when ΔH is positive and ΔS isnegative
Step-by-step explanation:
The answer will be explained by the following equation for the gibbs free energy
G = H - TS
Where
G = gibbs free energy
H = enthalpy of a system (heat
T = temperature
S = entropy
So the change in the gibbs free energy at constant temperature can be written as
ΔG = ΔH - TΔS . . . . . . (1)
Where
ΔG = Change in gibbs free energy
ΔH = Change in enthalpy of a system
ΔS = Change in entropy
During a spontaneous reaction the energy given in the form of heat that is ΔH or spontaneous due to disorder of the system that is ΔS are the driving forces of gibbs free energy.
Gibbs free energy is a state function and the change in gibbs free energy is equal to the change in enthalpy minus product of temperature and change in entropy as shown in equation 1
ΔG = ΔH - TΔS . . . . . . (1)
From the above equation it is obvious that ΔG will be positive for any reaction for which ΔH is positive and ΔS is negative. when ΔG is positive the reaction will be non spontaneous and unfavorable.
So,
option D is correct: when ΔH is positive and ΔS is negative