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Which of the following circumstances will result in a reaction that is spontaneous only at low temperatures?

A.) positive enthalpy change and positive entropy change
B.) negative enthalpy change and negative entropy change
C.) positive enthalpy change and negative entropy change
D.) negative enthalpy change and positive entropy change

Answer :

D) negative enthalpy and positive entropy,
since the formula G=H-TS when a reaction is spontaneous then G must be negative, and if values of t are low then the only time it is guaranteed to be a negative G value is if H is positive and S is negative

Answer: B.) negative enthalpy change and negative entropy change

Explanation:

According to Gibb's equation:

[tex]\Delta G=\Delta H-T\Delta S[/tex]

[tex]\Delta G[/tex] = Gibbs free energy  

[tex]\Delta H[/tex] = enthalpy change

[tex]\Delta S[/tex] = entropy change  

T = temperature in Kelvin

[tex]\Delta G[/tex]= +ve, reaction is non spontaneous

[tex]\Delta G[/tex]= -ve, reaction is spontaneous

[tex]\Delta G[/tex]= 0, reaction is in equilibrium

Thus when [tex]\Delta H[/tex] = -ve

[tex]\Delta S[/tex] = -ve

[tex]\Delta G=(-ve)-T(-ve)[/tex]

[tex]\Delta G=(-ve)(+ve)=-ve[/tex]   only at low temperature

Reaction is spontaneous only if temperature is low.

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