How do you know which reactions are spontaneous?
For example, the equation I2(s)+2NaF(aq)->F2(g)+2NaI(aq) is spontaneous. Why is this so?
What is Gibbs Free Energy for this reaction?
Determining whether a reaction is spontaneous or not can be done by considering the concept of Gibbs free energy (ΔG). The spontaneity of a reaction depends on the sign of ΔG. If ΔG is negative, the reaction is spontaneous, whereas if ΔG is positive, the reaction is non-spontaneous.
To calculate ΔG for a reaction, you can use the equation:
ΔG = ΔH - TΔS
Where ΔH is the change in enthalpy (heat absorbed or released) of the reaction, T is the temperature in Kelvin, and ΔS is the change in entropy (disorder) of the system.
In the example you provided, I2(s) + 2NaF(aq) → F2(g) + 2NaI(aq), we need to determine if the reaction is spontaneous.
To do this, we will consider the factors that influence ΔG:
1. ΔH: The enthalpy change indicates the heat absorbed or released during the reaction. It depends on the strengths of the chemical bonds being broken and formed. In this case, the reaction involves the formation of strong covalent bonds between F atoms to produce F2, which releases a significant amount of energy. This contributes to a negative ΔH.
2. ΔS: The change in entropy measures the disorder of the system. As the reaction involves the formation of two moles of NaI from one mole of I2 and two moles of NaF, it leads to an increase in the number of particles (i.e., increased disorder). This contributes to a positive ΔS.
Now, plugging these values into the equation ΔG = ΔH - TΔS:
The ΔH value will be negative (exothermic process), and the ΔS value will be positive. Since both ΔH and ΔS contribute to a negative ΔG, the given reaction is spontaneous.
It's important to note that the spontaneity of a reaction can also be determined by comparing the standard Gibbs free energy change (ΔG°) to understand whether a reaction is spontaneous under standard conditions (1 atm pressure and 298 K temperature). If ΔG° is negative, the reaction is spontaneous under standard conditions.