[purleyperfect]

Hey!

I'm doing a practical write up at the moment investigating chemical equilibrium in the following reversible reaction

FeCl₃+ 3NH₃CNS  <- -> Fe(CNS)₃   + 3NH ₄Cl

Basically we have to add ammonium thioynate sol to one bottle, iron (III) chloride sol to another and ammonium chloride to another and then describe using le Chatelier's principle why colour changes are observed. I get this bit however, we added distilled water to one of the bottles and then heated it. My tutors given me a bit of a hint telling me this:

It did change colour, didn't it. I saw it go lighter. As the forward reaction is exothermic heating the solution, according to Le Chatelier, would make the reaction move to reduce any increase in temp, ie move in the endothermic direction. In this case using up the iron thiocyanate and so losing colour.

but what I am having trouble explaining in my write up is the exothermic and endothermic bit, how do we know that the forward reaction is exothermic? Or do we just know?! I thi nk I'm being a bit thick here but I cant get my head round it! *delete me*

[0000000]

how do we know that the forward reaction is exothermic? Or do we just know?!

Generally we know does some reaction is exo or endothermic according to the reaction enthalpy dH, if it's value is positive then id endo, if is negative then is exo.
You can calculate this value using tables of standard entails, but this will be right at standard state conditions, even if the conduits are not standard you wouldn't know the exact value but you will know if it's exo or endo.