H2 CHEMISTRY

Plaxo SG

This is an A level H2 Chemistry 2009 Paper 1 Qn.

It is regarding the entropy change during reverse osmosis.

Enthalphy of Dilution is given:
NaCl(l) + nH2O->NaCl(aq), enthalpy is said to be zero

During reverse osmosis, NaCl(aq) is separated in to pure water, when water molecules pass through the permeable membrane. In other words, it is the opposite of the dilution reaction (eqn given in the qn). Since there should be a drop in the no. of molecules when water molecules get separated from aqueous solution, entropy has to be negative as disorderliness decreases.

As entropy is negative and enthalphy change is zero, according to Gibbs Free Energy Formula, change in Gibbs free energy has to be positive.

However, the answer given in the ten yr series book was the opposite meaning entropy is positive and Gibbs Energy Change was negative.

Why? Someone please explain.

Thanks in advance.

UltimaOnline SG

The answer given by the TYS book is wrong.

Entropy is negative, as you've correctly understood, and therefore Gibbs free energy is positive, meaning this process is NOT spontaneous and feasible on it's own. It requires tremendous mechanical force and expensive partially permeable membranes to occur, which is why reverse osmosis is an expensive process. (If it were a natural, spontaneous process, why would corporations and governments need to spend millions of dollars on it?)