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Classify the possible combinations of signs for a reaction's ΔH and ΔS values by the resulting spontaneity

A. ΔH is positive and ΔS is negative

B. ΔH is positive and ΔS is positive

C. ΔH is negative and ΔS is positive

D. ΔH negative and ΔS is negative

For A, B, C and D find out which of following they are:

1. Spontaneous as written at all temperatures

2. Spontaneous in reverse at all temperatures

3. Spontaneous as written above a certain temperature

4. Spontaneous as written below a certain temperature

They can only have one answer but 2 can have the same answer

Respuesta :

Answer :

(A) ΔH is positive and ΔS is negative  → (2) Spontaneous in reverse at all temperatures

(B) ΔH is positive and ΔS is positive  → (3) Spontaneous as written above a certain temperature

(C) ΔH is negative and ΔS is positive  → (1) Spontaneous as written at all temperatures

(D) ΔH negative and ΔS is negative → (4) Spontaneous as written below a certain temperature

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

As we know that:

[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

(A) ΔH is positive and ΔS is negative.

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

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

[tex]\Delta G=(+ve)[/tex]

The reaction is non-spontaneous at all temperatures or spontaneous in reverse at all temperatures.

(B) ΔH is positive and ΔS is positive.

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

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

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

[tex]\Delta G=(-ve)[/tex]    (at high temperature)   (spontaneous)

The reaction is spontaneous as written above a certain temperature.

(C) ΔH is negative and ΔS is positive.

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

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

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

The reaction is spontaneous as written at all temperatures

(D) ΔH is negative and ΔS is negative.

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

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

[tex]\Delta G=(+ve)[/tex]    (at high temperature)   (non-spontaneous)

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

The reaction is spontaneous as written below a certain temperature.

The possible combinations of signs for a reaction are:

(A) ΔH is positive and ΔS is negative  → (2) Spontaneous in reverse at all temperatures

(B) ΔH is positive and ΔS is positive  → (3) Spontaneous as written above a certain temperature

(C) ΔH is negative and ΔS is positive  → (1) Spontaneous as written at all temperatures

(D) ΔH negative and ΔS is negative → (4) Spontaneous as written below a certain temperature

Gibb's Equation:

It helps us to predict the spontaneity of reaction on the basis of enthalpy and entropy values directly. When the reaction is exothermic, enthalpy of the system is negative making Gibbs free energy negative.

Hence, we can say that all exothermic reactions are spontaneous.

→ According to Gibb's equation:

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

The temperature in the Gibbs free energy equation is the Kelvin temperature, so it can only have a positive value. When ΔH is negative and ΔS is positive, the sign of ΔG will always be negative, and the reaction will be spontaneous at all temperatures.

Thus, the correct match for possible combinations is given above.

Find more information about Gibb's equation here:

brainly.com/question/4238009

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