The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. Isolated systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy.

The sign of the change in entropy (∆S) of a system can be either positive or negative.

1. If ∆S is positive, the system is moving towards equilibrium, which means the reaction is likely to happen spontaneously.
2. If ∆S is negative, the system is moving away from equilibrium, which means the reaction is not likely to happen spontaneously.

The entropy of the surroundings (∆S surroundings) is affected by the heat flow from the system.

1. If heat is released by the system (exothermic reaction), ∆S surroundings is positive.
2. If heat is absorbed by the system (endothermic reaction), ∆S surroundings is negative.

For a reaction to be spontaneous, the total entropy change (∆S total), which is the sum of the entropy change of the system and the entropy change of the surroundings, must be positive (∆S total 0).

So, the combinations of signs for ∆S system and ∆S surroundings that will ensure a reaction happens spontaneously are:

1. ∆S system is positive and ∆S surroundings is positive (++).
2. ∆S system is positive and ∆S surroundings is negative (+-), but only if the absolute value of ∆S system is greater than the absolute value of ∆S surroundings.

The combinations -- and - are not likely to result in a spontaneous reaction because they would result in a negative ∆S total.

Question

The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. Isolated systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy.

The sign of the change in entropy (∆S) of a system can be either positive or negative.

1. If ∆S is positive, the system is moving towards equilibrium, which means the reaction is likely to happen spontaneously.
2. If ∆S is negative, the system is moving away from equilibrium, which means the reaction is not likely to happen spontaneously.

The entropy of the surroundings (∆S surroundings) is affected by the heat flow from the system.

1. If heat is released by the system (exothermic reaction), ∆S surroundings is positive.
2. If heat is absorbed by the system (endothermic reaction), ∆S surroundings is negative.

For a reaction to be spontaneous, the total entropy change (∆S total), which is the sum of the entropy change of the system and the entropy change of the surroundings, must be positive (∆S total > 0).

So, the combinations of signs for ∆S system and ∆S surroundings that will ensure a reaction happens spontaneously are:

1. ∆S system is positive and ∆S surroundings is positive (++).
2. ∆S system is positive and ∆S surroundings is negative (+-), but only if the absolute value of ∆S system is greater than the absolute value of ∆S surroundings.

The combinations -- and - are not likely to result in a spontaneous reaction because they would result in a negative ∆S total.

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