Which of the following reactions have a positive δsrxn? check all that apply.2a(g)+b(> c(g)a(g)+b(> 3c(g)2a(g)+3b(> 4c(g)2a(g)+b(> 3c(g)

The reactions that have a positive ΔSrxn are the second and the fourth:

Explanation:

ΔSrxn stands for the change in entropy of a reaction.

Since ΔSrxn equals Final entropy - Initial entropy, a positive ΔSrxn (ΔSrxn > 0) means that the entropy increases.

Given the chemical equations for some reactions, you can identify the direction of the change of entropy (increase or decrease) by inspection, using basic definitions and principles.

These are:

With those principles, you get the following results for each of the given systems.

1) 2A(g) + B(g) → C(g)

  Since in the product (right) side there are fewer molecules than in the reactant (left) side, you predict that the entropy decreases, and so the ΔSrxn is negative.

2) A(g) + B(g) → 3C(g)

    Since in the product (right) side there are more molecules than in the reactant (left) side, you predict that the entropy increases, and so the ΔSrxn is positive.

3) 2A(g) + 3B(g) → 4C(g)

    Since in the product (right) side there are fewer molecules than in the reactant (left) side, you predict that the entropy decreases, and so the ΔSrxn is negative.

4) 2A(g) + B(s) → 3C(g)

    Since there are the same number of molecules in both sides, the determining factor of the change of entropy is the physical state of the components.

Since the reactant B (s) is in solid state and the only product is C(g), and it is in gas state, you predict that the entropy increases, and ΔSrxn is positive.

Conclusion: the reactions that have a positive ΔSrxn are the second and the fourth:

the answer is c. nacl