Question

What do we call a reaction of the form `AB+CD rarr AC+BD`? What occurs when potassium iodide and lead nitrate react?

Answer 1

This type of reaction is called a double replacement reaction, a double displacement reaction, and a precipitation reaction.

Explanation:

The reaction between potassium iodide and lead nitrate in which a yellow solid (precipitate) forms has several names: double replacement, double displacement, and precipitation reaction.

In this type of reaction, the cations and anions in the reactants trade places, forming a precipitate.

`"AB"+"CD"` `rarr` `"AD"+"CB"`, `"where A and C are cations, and B and D are anions"`.

For this reaction specifically, the equation is the following:

`"2KI(aq)"+"Pb(NO"_3)_2("aq")` `rarr` `"2KNO"_3("aq")"+PbI"_2("s")"`

Answer 2

The student has performed a `"metathesis"` or `"partner exchange"` reactions, as evidenced by the precipitation of an insoluble solid.

Explanation:

You know the drill: solubility follows a hierarchy. All nitrates and acetates are soluble. All halides are soluble, EXCEPT for `PbX_2`, `Hg_2X_2`, and `AgX`. I would expect a 1st/2nd year student to know this, because they would have done the experiment in the lab.

We can represent the metathesis reaction that occurred in the test tube by the equation:

`Pb(NO_3)_2(aq) + 2KX(aq) rarr PbX_2(s)darr+2KNO_3(aq)`

As a net ionic equation, (the potassium and nitrate ions are along for the ride):

`Pb^(2+)(aq) + 2X^(-)(aq) rarr PbX_2(s)darr`