For the autoionisation of water, it looks like
2H2O(l)-> H3O+(aq) + OH–(aq)
I thought that becuase it is going from liquid to ions, wouldnt the Entropy be greater than 0?
However, I have been told and I have calculated the opposite, that entropy is less than 0. Please Explain .
It is an endothermic process. The entropy change is the heat required divided by the temerature, and is negatve because heat energy must be supplied.
It is true that the process occurs spontaneously, but the reverse process is much more likely. That is why so few of the H2O molecules are ionized.
The entropy change in a reaction can be determined by considering the change in the number of particles and their distribution. In the autoionization of water, two water molecules (liquid) react to form one hydronium ion (H3O+) and one hydroxide ion (OH-), both in aqueous solution.
Now, let's analyze the entropy change at the molecular level.
Initially, we have two water molecules in the liquid state. The molecules are relatively close together and have low entropy. As the reaction occurs, the water molecules break apart to form the hydronium and hydroxide ions. These ions are now dispersed in solution and have higher entropy compared to the liquid water.
So, based on this analysis, it might seem logical to conclude that the entropy change (∆S) should be positive (greater than zero) because the system becomes more disordered. However, there is another factor to consider.
During the autoionization of water, hydrogen bonding plays a crucial role. Hydrogen bonding exists between water molecules in the liquid phase, but when water molecules ionize, they break the hydrogen bonds because the ions interact with water through ion-dipole interactions instead. This breaking of hydrogen bonds results in a decrease in entropy.
While the formation of ions increases the randomness in the system, the breaking of hydrogen bonds contributes to a decrease in randomness. Overall, the decrease in entropy due to the breaking of hydrogen bonds outweighs the increase in entropy due to the formation of ions.
Therefore, the net entropy change for the autoionization of water is negative (∆S < 0). The breaking of the more ordered hydrogen bonds dominates the increase in randomness caused by the formation of ions.
To calculate the actual entropy change (∆S) numerically, you would need to consider the entropies of the reactants and products and use standard entropy values. The magnitude of the negative entropy change (∆S) can be determined by these values.