# Should I include water of crystallisation in the product of a precipitation reaction with silver nitrate?

I've deduced that a certain compound has an ionization isomerism. I just want to know which of the following reactions is correct.

I think this one is the more likely:

$$\ce{[Cr(H2O)5Cl]Cl2 \cdot H2O + 2AgNO3 -> [Cr(H2O)5Cl](NO3)2\cdot H2O + 2AgCl}$$

But I would just like to make sure that it's not

$$\ce{[Cr(H2O)5Cl]Cl2 \cdot H2O + 2AgNO3 -> [Cr(H2O)5Cl](NO3)2 + 2AgCl + H2O}$$

Basically the question asked me to find an empirical formula from elemental analysis. Then it said 2 compounds have the same empirical formula but why are reaction I and II different (not the above reactions). I know its cause ionization isomerisation. So I just need clarification that my first equation is correct.

The only thing I know about the reaction is that one mole of $\ce{[Cr(H2O)5Cl]Cl2\cdot H2O}$ reacts with $\ce{AgNO3}$ to form $\ce{2AgCl}$.

• So basically, you're asking "Will $\ce{[Cr(H2O)5Cl](NO3)2}$ have a water of crystallization or not?" – Pritt says Reinstate Monica Oct 27 '17 at 15:40
• To make a long story short yes :) – Patrick Moloney Oct 27 '17 at 15:41
• After the reaction - I know it has one before. – Patrick Moloney Oct 27 '17 at 15:49
• This is not a well defined question because you didn't provide more details on the reaction. If you carry out the reaction with heat, you might drive off the water of hydration. You're implying that the chromium complex and the nitrate are bound but you're concerned about the water of hydration. But this is all in solution anyway... – Zhe Oct 27 '17 at 16:37
• So it doesn't matter? I can pick either? – Patrick Moloney Oct 27 '17 at 16:46

## 1 Answer

It will be very unlikely that you are performing the reaction in solid state. Solid state reactions tend to react only on the surface area and will take very long to complete. (On the upside, they have no entropic terms so the yield is generally $\pu{100\%}$ and thermodynamics dictate the direction.) So likely you will be performing this in aquaeous solution.

If you are, you have previously dissolved the reactant and the product will not precipitate. Thus, you should more accurately write the reaction as:

$$\begin{multline}\ce{[Cr(H2O)5Cl]^2+ (aq) + 2 Cl- (aq) + 2 Ag+ (aq) + 2 NO3- (aq)} \\\ce{-> [Cr(H2O)5Cl]^2+ (aq) + 2 NO3- (aq) + 2 AgCl v (s)}\end{multline}\tag{1}$$

Note that I have neither included the water of crystallisation of the starting complex nor any other water molecules except for the (aq) descriptors showing aquaeous solution. You are unable to distinguish which of the water molecules in solution derives from the salt and which were added as a liquid.

And finally, adding a water of crystallisation to a product in solution is incorrect because there is no crystallisation water associated with it. If you wanted to determine a hydrate content, you would first need to crystallise the product salt and then determine its water content subseqently. There is no way to predict a priori whether it contains one, two, three or more water molecules per crystallisation unit.