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Standard XII

Chemistry

Gibbs Free Energy & Spontaneity

When enthalpy...

Question

When enthalpy is positive and entropy is positive :

Δ G

is positive, at high temperature

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Δ G

is negative, at high temperature

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Δ G

is negative, at low temperature

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none of these

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Solution

The correct option is C $Δ G$ is negative, at high temperature
When enthalpy is positive and entropy is positive, $Δ G$ is negative, at high temperature.
$Δ G = Δ H - T Δ S = + v e - T \times + v e = + v e - + v e = - v e$ for high T
Negative free energy change corresponds to spontaneous reaction.

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Similar questions

Which of the following is not correct?
(a) $Δ$ G is zero for a reversible reaction.
(b) $Δ$ G is positive for a spontaneous reaction.
(c) $Δ$ G is negative for a spontaneous reaction.
(d) $Δ$ G is positive for a non-spontaneous reaction.

Q. Assertion :There is no reaction known for which

Δ G

is positive, yet it is spontaneous. Reason: For photochemical reaction,

Δ G

is negative.

Q. When enthalpy is positive and entropy is negative :

Q. Assertion :Reduction of

Z n O

with carbon is done at

1100^{\circ} C

. Reason: At this temperature,

Δ G^{\circ}

is negative thus process is spontaneous.

Q. Free energy,

G = H - T Δ S

, is a state function that indicates whether a reaction is spontaneous or non-spontaneous. If you think of

T S

as the part of the system`s energy that is disordered already, then

(H - T S)

is the part of the system`s energy that is still ordered and therefore free to cause spontaneous change by becoming disordered.
Also,

Δ G = Δ H - T Δ S

From the second law of Thermodynamics, a reaction is spontaneous if

Δ_{t o t a l} S

is positive, non-spontaneous if

Δ_{t o t a l} S

is negative, and at equilibrium if

Δ_{t o t a l} S

is zero.

Since,

- T Δ S = Δ G

and since

Δ G

and

Δ S

have opposite signs, we can restate the Thermodynamic criterion for the spontaneity of a reaction carried out at a constant temperature and pressure.
If

Δ G < 0

, the reaction is spontaneous
If

Δ G > 0

, the reaction is non-spontaneous
If

Δ G = 0

, the reaction is at equilibrium

Read the above paragraph carefully and answer the following questions based on the above comprehension.

A particular reaction has a negative value for the free energy change. Then at ordinary temperature:

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When enthalpy is positive and entropy is positive :

The correct option is C ΔG is negative, at high temperatureWhen enthalpy is positive and entropy is positive, ΔG is negative, at high temperature.ΔG=ΔH−TΔS=+ve−T×+ve=+ve−+ve=−ve for high TNegative free energy change corresponds to spontaneous reaction.