Chemistry - Why do aqueous solutions of sodium chloride and silver nitrate swap ions?

Solution 1:

While solutions of silver nitrate and sodium chloride are in stable equilibrium on their own, once mixed the equilibrium is changed. When the salts are dissolved, you no longer have silver and nitrate or sodium and chloride ions associated with each other, you have a mixture of individual ions in solution.

But silver chloride has very low solubility, about 1 mg can dissolve in 1 L of water. So when a silver ion and chloride ion meet, they quickly fall out of solution. So if we had an equilibrium between the dissolved ions and the small amount of "soluble" silver chloride, that equilibrium would be driven toward the silver chloride by the precipitation. When compounds escape the solution by precipitating or evaporating, they are removed from the equilibrium, so trying to replace them to restore equilibrium requires converting more free ions into silver chloride and pushes the reaction to completion.

Solution 2:

So, we know that:

$$\ce{NaCl (aq) + AgNO3 (aq) -> NaNO3 (aq) + AgCl (s) v}$$

(In other words, the ions swap and a precipitate forms).

Ah yes, that is how it is always written on the blackboard. But remember that the label (aq) actually carries a meaning. It means that upon dissolution, it is wrong to talk about associated sodium and chloride ions in any way, rather they are separated and go diffusing through the solution on their own.

$$\ce{NaCl (s) + H2O -> Na+ (aq) + Cl- (aq)}$$

If you took one pair of $\ce{NaCl}$ ions and coloured them both pink, you would realise that the two pink ions move through the solution entirely individually. The same thing goes for $\ce{AgNO3}$. Sometimes a sodium ion may meet a chloride ion (or a silver ion a nitrate ion) but they’ll just greet and move on.

Now we mix the two solutions. All of a sudden in this chaos of ions diffusing everywhere, silver ions can meet chloride ions. They won’t just greet but they’ll hug each other because they like to hold tight together. And thus they shall precipitate. The reaction is much better written as:

$$\ce{Cl- (aq) + Ag+ (aq) -> AgCl (s) v + H2O}$$

Think about it a different way. Mix a solution of sodium chloride and potassium bromide and then evaporate the water. You will obtain a mixture of $\ce{NaCl, NaBr, KCl}$ and $\ce{KBr}$. Why? Because all ions are equally distributed and upon removal of the solvent, they will all fall together. All salts will form in an entirely statistical distribution.