Which reactions have a positive ΔSrxn? 2A(g)+B(g)⟶4C(g) 2A(g)+3B(g)⟶4C(g) A(s)+2B(g)⟶C(g) 2A(g)+B(s)⟶3C(g)

Most likely:

(the first choice.)

(the fourth choice.)

Explanation:

denotes the entropy change in a reaction.

The value of is greater than zero if the reaction corresponds to an increase in entropy. In other words, entropy should be higher for the products of this reaction than for its reactants.

The exact entropy of each species depends on both the environment (temperature, pressure, etc.) and on the structure of the species. However, there are some general trends.

Consider one mole of some gaseous molecules and one mole of some solid particles. If that gas and that solid are under the same temperature and pressure, which one would have a higher entropy? Generally, the gas would have a higher entropy.

Also, the entropy of a gas is approximately proportional to the number of particles in that gas.

Therefore, it is likely that would be positive for a reaction if there are more gas particles among the products than among the reactants.

Note that is the state symbol for solids, whereas is the state symbol for gases.

In this question:

First reaction: .

In each unit of this reaction, there are:

Second reaction: .

In each reaction, there are:

Third reaction: .

In each reaction, there are:

Fourth reaction: .

In each unit of this reaction, there are:

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