Video Transcript
For which of the following reactions is a precipitate likely to form? (A) Pb(NO3)2 aqueous plus MgSO4 aqueous. (B) Ca(OH)2 aqueous plus two HCl aqueous. (C) Na2CO3 aqueous plus H2SO4 aqueous. (D) HNO3 aqueous plus NH3 aqueous. Or (E) CuO solid plus H2SO4 aqueous.
We must identify the reaction where one of the products is a precipitate. A precipitate is an insoluble product that separates from a solution. A precipitate can be formed from a reaction with reactants that are often both aqueous or, in other words, dissolved in water that when mixed together react to form the insoluble compound that is the precipitate and most likely another aqueous compound. We can use the water solubility rules to predict which product will be insoluble in water. Compounds that are generally water-soluble and would not form a precipitate contain alkaline metal cations, ammonium cations, nitrate anions, bicarbonate anions, chlorate anions, acetate anions, halide anions, or sulfate anions.
Compounds containing halides or sulfate ions have some exceptions to their solubility in water. For compounds containing halides, if the compound also contains silver, mercury(I), or lead(II) ions, the compound would then be insoluble in water. Sulfates that contain silver, calcium, strontium, barium, mercury(I), or lead(II) ions would be insoluble in water. Compounds that are insoluble in water contain carbonate anions, phosphate anions, sulfide anions, or hydroxide anions, with the following exceptions. Compounds containing alkali metal cations or ammonium cations are always soluble in water. And sulfides or hydroxides also containing calcium, barium, or strontium cations would be soluble in water.
Let’s predict the products of each of our answer choices to determine which would form a precipitate. Answer choices (A), (B), (C), and (E) are all examples of double-replacement reactions, while answer choice (D) is an example of an acid–base reaction. In double-replacement reactions, cations and anions dissociate in water and recombine to form new compounds. Let’s start with answer choice (A). We can predict the products by first identifying the cations and anions present in each reactant. Our first reactant Pb(NO3)2 contains lead(II) cations and nitrate anions, and MgSO4 contains magnesium cations and sulfate anions. These ions will recombine to form new compounds. So our first product will be composed of lead(II) cations and sulfate anions, forming PbSO4, with our next product containing magnesium cations and nitrate anions to form Mg(NO3)2.
Let’s predict the solubility of our products by checking the water solubility rules. First, for PbSO4, we can see that compounds containing sulfate anions are generally water-soluble, unless the compound also contains lead(II) cations, which our product does contain. This means PbSO4 is insoluble in water and is, therefore, likely a precipitate. We can represent this using a state symbol of s standing for solid. Our second product Mg(NO3)2 contains nitrate anions and is, therefore, soluble in water, which we can represent using the state symbol aq for aqueous. It would seem we have already found the reaction in which a precipitate is likely formed in answer choice (A).
Let’s briefly check the remaining answer choices just to be sure. In answer choice (B), calcium cations from Ca(OH)2 and chloride ions from two HCl will form CaCl2, which according to our water solubility rules will be soluble in water. And the second product formed between hydrogen and hydroxide ions will be liquid water. Answer choice (B) will not form a precipitate. In answer choice (C), sodium cations from Na2CO3 and sulfate anions from H2SO4 will recombine to form Na2SO4, which will be soluble in water and therefore aqueous, since the compound contains sulfate anions and is not one of the listed exceptions, with our second product containing carbonate anions and hydrogen cations to form H2CO3, which would be soluble in water.
It is worth mentioning that in this reaction H2CO3 would actually further break down into liquid water and carbon dioxide gas, but still not forming a precipitate. Answer choice (D) would result in an acid–base reaction where a hydrogen ion from HNO3 would be donated to NH3 to form nitrate anions, which are water-soluble, and ammonium cations, which are also water-soluble. Answer choice (D) would not likely form a precipitate. Finally, answer choice (E), where copper(II) cations from CuO would recombine with sulfate anions from H2SO4 to form CuSO4, which is water-soluble, and the second product forming between hydrogen cations and oxide anions to form liquid water. Answer choice (E) would not form a precipitate.
We have confirmed that the only reaction in which a precipitate is likely to form is answer choice (A) reaction between Pb(NO3)2 aqueous and MgSO4 aqueous.
