Which of the following is a precipitation reaction? And we’ve been given five halves of reactions, where we’ve only got two reactants, all in the aqueous phase.
The first reaction is that between magnesium chloride and sodium chloride. The second reaction is that between hydroiodic acid or hydrogen iodide and lithium hydroxide. The third is the reaction of lithium chloride and potassium iodide. The fourth is lead nitrate plus calcium chloride. And the final reaction is that between potassium nitrate and methanoic acid, otherwise known as formic acid.
Now, we need to identify the precipitation reaction, which is a type of reaction where a solid is produced from solution. A good example of a precipitation reaction is what happens when we mix together sodium carbonate and calcium chloride solutions. Combine the two solutions and they’ll rapidly produce a white precipitate of calcium carbonate, the main constituent of limestone. The calcium carbonate solid is produced when the calcium ions from the calcium chloride come into contact with the carbonate ions from the sodium carbonate. They quickly bond together, forming a growing lattice, which falls out of solution.
In our five candidates, we’ll be looking for similar products that form from a combination of ions in their solutions and don’t have high solubilities. Let’s start with magnesium chloride plus sodium chloride. These two salts share the chloride anion. And magnesium and sodium are quite stable in their cationic form. So we’re not going to see any redox behavior. So magnesium chloride and sodium chloride won’t react together. They’ll remain and mix in solution. But we won’t see a precipitate.
So what about hydroiodic acid and lithium hydroxide? Well, the big clue here is that we’ve got an acid mixing with a base. So the anticipated products here are a salt plus water. The salt in this case would be lithium iodide. As with all common lithium salts, lithium iodide is soluble. So again, no precipitate. When we mix lithium chloride and potassium iodide together, we also don’t get a precipitate. We could write the products as lithium iodide and potassium chloride. But those salts are also soluble. And what we’d be getting is a solution where lithium, chloride, potassium, and iodide ions are constantly moving around each other. Since all the salts will be soluble, we wouldn’t get a precipitate.
That’s not the case with lead nitrate and calcium chloride. If we swap around the anions, we get lead chloride and calcium nitrate. Calcium nitrate, like all nitrates, is soluble. But lead chloride is one of the few chlorides that’s insoluble, like mercury chloride and silver chloride. This insolubility is prone to happen when the chloride is paired up with a large, low-charge counterion. If we mix lead nitrate and calcium chloride solutions together, we’d see a white precipitate. So we’re definitely dealing with a precipitation reaction. The combination of potassium nitrate and methanoic acid, on the other hand, would be unremarkable. We might get small amounts of nitric acid and potassium methanoate. But neither of these are insoluble. So you wouldn’t get a precipitate.
So our conclusion is, out of the five equations given, the only example of a precipitation reaction is the combination of lead nitrate and calcium chloride solutions.