Reaction between Fe(NO3)3 and KSCN

[chemist2b]

Homework Statement

When Fe(NO3)3 and KSCN reated, it immediatly turned red. my teacher says this si the stuff they use for fake blood in movies. My problem is, we must state the reaction type.

Homework Equations

the products, I am assuming, are FeSCN and KNO3 (i know I did not yet balance this).

The Attempt at a Solution

Based on this, i would assume it is a double replacement reaction. However, no precipitate formed and no gas was released. i feel its obvious that this was a chemical reaction, but I am having difficulty classifying it as a double replacement, single replacement, synthesis, decomposition of combustion reaction, becasue it does not seem to match any of their descriptions. (ps I am only in chemistry 1)

 

[Borek]

You need to find what is the red product of the reaction, it should become obvious then. And finding the answer should be pretty easy with google, just try.

 

[chemist2b]

i have looked at google. for several hours. I am coming up with nothing. I've seen some stuff about equillibrium and reversible reacitons, but nothing that i can classify as a definite answer.

 

[Borek]

Oh come on, I googled for "product of reaction Fe SCN" and got answer about product even without clicking to look for details.

Besides, you listed FeSCN as a product - apparently it is wrong, as it has no correct charge, still, it already points at an answer. You just have to think in terms of the NET IONIC reaction.

 

[chemist2b]

okay googled the same as you, just now i mean. the first thing i see in most of them is simply Fe + SCN = Fe SCN with some charge. But i don't understand about the charges, why wouldn't they simply balance themselves out, like Fe+ 3SCN = Fe(SCN)3, to have a net charge of 0?

but this makes it more clear about the reaction type. combination? but then are the K and NO3 simply aqueous byproducts of the reaction? or is it still KNO3 and tht an aqueous byproduct?

 

[Borek]

There is no such thing as combination - take a look at possibilities you listed in your first post.

FeSCN²⁺ is an ion - reaction product doesn't have to be neutral, as long as it is in the solution. This is one of reasons to use net ionic reactions.

Actually there are six possible Fe(SCN)_n^(3-n)+ complexes that are produced in this reaction, and their relative amounts depend on the equilibrium that is achieved in the solution. But as the FeSCN²⁺ dominates diluted solutions, for now you can safely assume it is the only product.

 

[chemist2b]

my commbination i meant synthesis, sorry.

so what is the KN03 considered? becasue it can't just dissapear. this was the reason i was confused between a double replacement reaciton and a synthesis reaction was becasue according to by obviously-limeted knowledge, i thought that becasue the Fe and SCN combined that the K and NO2 would also combine, making it a double replacement. except it couldn't be a double replacement becasue no gas or precipitate formed. the only representation we have been taught of syntheis is a+b=c, so i thought becasue there were two ions as reactant and products that it couldn't be synthesis. so for now, i should just disregard the KNO3? making it a synthesis with random added stuff?

 

[Borek]

K⁺ and NO₃^- don't react - they just stay in the solution. They are called spectator ions. So yes, I would classify it as a synthesis.

 

[chemist2b]

thank you so much you've definitely helped to clarify this alot. final reaction:

Fe(NO3)3 +KSCN = FeSCN(^+2) + K(^+1) + 3NO3(^-1)

 

[Borek]

This is still not correct - you should put reagents on the left hand side as dissociated as well, and cancel out spectators, as they don't change during reaction.

 

[chemist2b]

oh dear... i don't really know what that means.

 

[Borek]

That's a little bit strange - if you have never heard about net ionic reactions and spectators, how are you expected to answer this particular question? No idea.

Imagine AgCl precipitation while mixing silver nitrate and HCl solutions:

AgNO₃(aq) + HCl(aq) -> AgCl(s) + HNO₃(aq)

We know both silver nitrate and hydrochloric acid are dissociated, so in fact they are in the form of ions. If you write tha same reaction showing all dissociated things as ions, you will get:

Ag⁺ + NO₃^- + H⁺ + Cl^- -> AgCl(s) + H⁺ + NO₃^-

Note that H⁺ and NO₃^- are present on both sides of the equation in the identical form - that means nothing happens to them, they don't take part in the reaction, they are just spectators - so they can be safely removed. That yields:

Ag⁺ + Cl^- -> AgCl(s)

as a net ionic reaction for AgCl precipitation. Note, that the same equation will also describe AgCl precipitation when you mix silver nitrate and NaCl solution - just in this case one of the spectators is different (Na⁺ instead of H⁺ - but it can be ignore for exactly the same reasons, it doesn't take part in the reaction).

 

[chemist2b]

Well our teacher i think did not realize that so many of us would actually look into depth into this problem. he wanted us to assume because we know basically nothing about chemistry that based on the two reactants we would assume that it was double replacement. Thank you, again, a lot. This makes a lot more sense that what he was trying to tell us.