Question

1. Why do some combinations of ionic compounds form a precipitate while others do not?

2. Solutions of lead(II) nitrate and potassium iodide were combined in a test tube. The results of this reaction are shown below.
a. Write a formula equation for the reaction.




b. Which of the possible products is the precipitate, and how do you know?




c. Write a complete ionic equation for the reaction and identify the spectator ions.




d. Write a net ionic equation for the reaction between lead(II) nitrate and potassium iodide.

Answer 1

1. Some combination of ions form a solid precipitate because it is not favorable for the ions to become solvated (dissolved). Large and lowly charged ions tend to form precipitates, especially metals such as lead, barium, and silver.

2.
a. Pb(NO3)2 + 2KI -> 2KNO3 + PbI2

b. PbI2 is a precipitate because no other combinations of cations and anions will make an insoluble compound. KI, KNO3, and Pb(NO3)2 are all soluble.

c.

`Pb^(2+)(aq) + 2NO_3^(-)(aq) + 2K^+(aq) + 2I^-{aq} = > PbI_2(s) + 2NO_3^(-)(aq) + 2K^+{(aq)}\\\\`

is the ionic equation. Spectator ions are NO3- and K+

d.

`Pb^(2+)(aq)+ 2I^-{aq} = > PbI_2(s) \\` is the net ionic equation

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