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 .
To understand why the entropy change for the autoionization of water is less than zero, let's first define what entropy is. Entropy is a measure of the randomness or disorder in a system. Generally, when a substance goes from a more ordered state to a more disordered state, the entropy tends to increase (ΔS > 0). Conversely, when a system becomes more ordered, the entropy tends to decrease (ΔS < 0).
In the autoionization of water, two water molecules react to form a hydronium ion (H3O+) and a hydroxide ion (OH-), as shown in the equation you provided. Although this reaction involves the formation of ions, it does not lead to a significant increase in randomness or disorder in the system.
The key factor to consider here is the organization of the water molecules in the liquid state compared to the dissociated ions in the solution. In the liquid state, water molecules are highly organized and form a network through hydrogen bonding, which gives water its unique properties. However, when water molecules dissociate, the network is disrupted as the ions become more dispersed in the solution.
As a result, the autoionization of water leads to a decrease in the overall organization or orderliness of the system. This decrease in order corresponds to a decrease in entropy (ΔS < 0). Therefore, despite the formation of ions, the entropy change for the autoionization of water is negative.
To calculate the exact value of the entropy change, you can use thermodynamic equations and experimental data. The standard entropy change (ΔS°) can be determined using tabulated values of standard molar entropies.