# Ionic equations

## Homework Statement

An ionic equation is an equation that shows the chemical species that actually take part in a reaction.
Ex. Pb(NO3)2 (aq) + 2KI (aq) -> PbI2 (s) + 2KNO3 (aq)
So the ionic equation for this reaction is
Pb2+ (aq) + 2I- (aq) -> PbI2 (s)
How do we know that Pb2+ and I- takes part in the reaction? What about K+ and NO3- ?

## The Attempt at a Solution

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BvU
Homework Helper
2019 Award
KNO3 is pretty soluble. So is KI. Perhaps Pb(NO3)2 as well. So except the ions that precipitate, the ions in solution don't even notice what's "happening".

KNO3 is pretty soluble. So is KI. Perhaps Pb(NO3)2 as well. So except the ions that precipitate, the ions in solution don't even notice what's "happening".
Ok...then what about NaOH (aq) + HCl (aq) -> NaCl (aq) + H2O (l)
How do we know that H+ and OH- are the ions that react? And not Na+ and Cl-?

BvU
Homework Helper
2019 Award
We know because -- if everything happens in solution, so full ionization (*) and no solids --
the NaOH (aq) on the left stands for Na+ plus OH-
The HCl (aq) on the left for H+ plus Cl-
NaCl (aq) on the right stands for Na+ plus Cl-
And if we leave out the common things on left and right all that happens is H+ + OH- → H2O

So much for the simplistic answer. Reality is a lot more complex, but this case can be untangled by considering it as a set of equilibria:

NaOH (aq) ↔ Na+ + OH-
HCl (aq) ↔ H+ + Cl-
H2O ↔ H+ + OH-
with an equilibrium constant for each of these.
And the k for water is so small compared to the other two that it makes the last equilibrium reaction "shift" to the left.

Borek
Mentor
Knowing what reacts and when is what chemistry is about. Some things you just have to remember - H+ + OH- is a neutralization reaction, quite common.

What do the solubility rules say about NaCl solubility? Do you expect it to precipitate?

We know because -- if everything happens in solution, so full ionization (*) and no solids --
the NaOH (aq) on the left stands for Na+ plus OH-
The HCl (aq) on the left for H+ plus Cl-
NaCl (aq) on the right stands for Na+ plus Cl-
And if we leave out the common things on left and right all that happens is H+ + OH- → H2O

So much for the simplistic answer. Reality is a lot more complex, but this case can be untangled by considering it as a set of equilibria:

NaOH (aq) ↔ Na+ + OH-
HCl (aq) ↔ H+ + Cl-
H2O ↔ H+ + OH-
with an equilibrium constant for each of these.
And the k for water is so small compared to the other two that it makes the last equilibrium reaction "shift" to the left.
Oh I see. Thanks!!

Knowing what reacts and when is what chemistry is about. Some things you just have to remember - H+ + OH- is a neutralization reaction, quite common.

What do the solubility rules say about NaCl solubility? Do you expect it to precipitate?
Nope.. Ok I think I've got it thanks