3
$\begingroup$

I once saw a problem, iron ions(II) titrated with potassium dichromate to produce Fe(II) and Cr(III). If I am to write the net ionic equation of this reaction, I got to make potassium dichromate on the right into dichromate ion since potassium is a spectator.
I got very confused over here. Why should you see potassium as a spectator if K(II) will be balanced as 2K on the right.

$\endgroup$
  • $\begingroup$ I'm not entirely sure what you're asking. But firstly, K(II) will not form, as potassium ionizes to a +1 charge. Potassium is generally deemed a spectator ion because it remains soluble in aqueous solution and will therefore not react to form a precipitate. $\endgroup$ – Shafter Oct 2 '14 at 3:12
4
$\begingroup$

The molecular equation for this titration is $$\ce{K2Cr2O7 + 6Fe(NH4)2(SO4)2 + 7H2SO4 -> 3Fe2(SO4)3 + Cr2(SO4)3 + K2SO4 + 6(NH4)2SO4 + 7H2O}$$

The complete ionic equation is $$\ce{2K+ + Cr2O7^2- + 6Fe^2+ + 12NH4+ + 19SO4^2- + 14H+ -> 6Fe3+ + 14SO4^2- + 2Cr^3+ + 2K+ + 12NH4^+ + 7H2O}$$

When you cross out everybody who shows up on both sides in the same form, you get the net ionic equation for this titration: $$\ce{Cr2O7^2- + 6Fe^2+ + 14H+ -> 6Fe^3+ + 2Cr^3+ + 7H2O}$$

Just as the hydrogens in $\ce{H2SO4}$ become $\ce{2H+}$ so the potassiums in $\ce{K2Cr2O7}$ become $\ce{2K+}$ and they don't change on the other side of the equation where they form $\ce{K2SO4}$.

$\endgroup$
4
$\begingroup$

Well first of all instead of $\ce{K(II)}$ it is always $\ce{K(I)}$

now the use of net ionic equation are:

a) It saves your time in writing

b) It highlights the species that are undergoing change and bringing more clarity

c) spectator ions have little significance in reaction but are necessary for that ion to ion to be stable , also spectator ion are to show that to make that reaction happen you do not need only those spectator ions which are given in your reaction but any other ion of similar properties could do. So

d) you will see better pattern in remembering reaction.

I think instead of remembering these two reaction you would prefer to remember this one reaction only

$\ce{CuSO4 + Zn -> ZnSo4 + Cu}$

$\ce{Cu(NO3)2 + Zn -> Cu + Zn(NO3)2 }$

You would like to remember

$\ce{Cu^{2+} + Zn -> Zn^{2+} + Cu}$

also you could even see from here that the two reactions are clearly the same only spectator ion is changed

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.