# How do we know if the reaction is spontaneous?

What does it mean for a reaction to be spontaneous? It means that the reaction will occur on its own, without any outside energy input needed. It does not mean that this will happen fast. In fact, it could take hundreds of years for a spontaneous reaction to occur.

The way to predict spontaneity is by calculation Gibbs Free Energy or deltaG. When deltaG is negative that means the reaction is spontaneous, and positive means nonsponatenous. Let's note that when a reaction is spontaneous in one direction, it will be nonspontaneous in the opposite direction.

To calculate delta G, we can use the formula: deltaG = deltaH- T deltaS, where delta H is enthalpy and delta S is entropy.

Other ways to predict spontaneity are:

1. delta S universe is positive (entropy, or disorder of universe). Delta S universe = deltaS surroundings + delta S system.

2. K is greater than 1, K is the equilibrium constant. Its connected to delta G by the equation: deltaG = -RTlnk

where R is the ideal gas constant and T is temperature in Kelvin (Celsius + 273.15). Ln of a number great than 1 will give us a positive number, and therefore will result in negative delta G.

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