Question

ction to be non-spontaneous at any temperature, which of the following В. Tur a chemical reaction to b conditions must be met? AS > 0, AITO AS > 0, AIT <0 AS <0, AHP <0 AS <0, AHP>0 D. 2. Place the following in on lowing in order of decreasing standard molar entropy. N204(8) NO(g) NO2(g) 204 > NO2 > NO B) NO > NO2 > N204 E) NO2 > NO > N204 C) N204> NO > NO2 D) NO > N204 > NO2 3S(g) → S3(g) 3. Consider the following reaction: One would expect that this reaction: A) will be spontaneous at any temperature. C) will be spontaneous at low temperatures. E) will never happen. B) will be spontaneous at high temperatures. D) will not be spontaneous at any temperature. Cl2 F2 *. Place the following in order of decreasing molar entropy at 298 K. H2 A) H2> Cl2 > F2 B) C12 > H2>F2 C) F2> Cl2 > H2 D) H2>F2 > Cl2 E) CI2 > F2 > H2 5. Consider the following reaction. Cao(s) + CO2(g) +CaCO3(s) If this reaction is endothermic, which one of the following statements is true at 298 K and 1 atm? A) AGº is positive at all temperatures. B) AG is negative at low temperatures. C) AGº is negative at high temperatures. D) AS is positive. E) AH° is negative. 6. For the reaction 25O2(g) + O2(g) 2503(g) AHx= 198 kJ at 298 K. This reaction will A) be driven by the enthalpy. B) not be spontaneous at high temperatures. C) not be spontaneous at any temperature. D) be spontaneous at high temperatures. E) be spontaneous at all temperatures. For a chemical reaction to be spontaneous anly at hioh temneratures, which of the com conditions must be met? A. AS' > 0, AH°> 0 AS > 0, AHP < 0 AS <0, AHP <0 AS <0, AH® > 0 AGⓇ>0
the first one is supposed to be meant spontaneous. thanks

Answer 1

General Concept

Reactions are either driven by positive entropy
(AS">0)
or negative enthalpy
(ΔΗ <0)
. If both of these are true then the reaction will always be spontaneous.

Further, if entropy driven reaction i.e.
(AS">0)
, have unfavourable enthalpy i.e.
(ΔΗ° > 0)
, then they are possible only at higher temperature.

Similarly, enthalpy driven reaction i.e.
(ΔΗ <0)
, if they have
(AS" < 0)
, then they are feasible only at a lower temperature.

Reactions in which both enthalpy and entropy are unfavourable i.e.
(ΔΗ° > 0)
and
(AS">0)
are always non-spontaneous.

This is summarised in this table. I will refer this table in many questions.

	(AS">0)	(AS" < 0)
(ΔΗ° > 0)	Spontaneous at high temperature	Non-spontaneous at all temperature
(ΔΗ <0)	Spontaneous at any temperature	Spontaneous at low temperature

1)

Here, in the question it is written that the reaction must be non-spontaneous at all temperatures. But you are saying that it should be read as spontaneous. In that case from the table, it can be seen that the answer will be

B)
(AS">0)
and
(ΔΗ <0)

2) Greater the atoms in a molecule and heavier the molecule, more will be its molar entropy. Thus, the answer will be

A) N₂O₄ >NO₂ > NO

3)

In this reaction, new bonds are formed, so energy is released. Thus,  .

(ΔΗ <0)

However, three gaseous molecules are combining into 1. Thus, entropy is decreasing. Thus, . From the Table, it can be seen that this reaction is feasible only at low temperatures.

(AS" < 0)

Answer will be (C) spontaneous at low temperatures

4)

Higher is the molar mass, more is the molar entropy. Thus, the answer will be

E) Cl₂ > F₂ > H₂

5)

The reaction is endothermic. This means that

(ΔΗ° > 0)

Further, 1 gaseous molecule and a solid molecule are combining. Thus, entropy is decreasing. Hence,  . From the Table, it can be seen that this reaction non-spontaneous at all temperatures.

(AS" < 0)

For non-spontaneous reactions free energy change is positive i.e.,

(AG > 0)

Thus, A) $\Delta G^o$ is positive at all temperatures

6)

Here, 3 molecules are combining into 2. Thus,

(AS" < 0)

Further, it is given that

(ΔΗ° > 0)

Thus, from the table, it can be seen that this reaction is (C) not spontaneous at any temperatures.

7)

From the table, it can be seen that for a reaction to be spontaneous only at high temperature, the conditions will be,

A)
(ΔΗ° > 0)
and
(AS">0)