Understanding Ionic Equations: Pb2+ & I- Reactants

[Janiceleong26]

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- ?

Homework Equations

The Attempt at a Solution

 

[BvU]

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".

 

[Janiceleong26]

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]

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]

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?

 

[Janiceleong26]

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!

 

[Janiceleong26]

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