only one of each of the following pairs of reactants undergoes a reaction. Complete and balance the equation for the reaction that occurs.

NaCl (aq) + BaCl2 (aq) -->
Na2SO4 (aq) + BaCl2 (aq) -->

NO3^- (aq) + OH^- (aq)+ Al (s) -->
CO3^2- (aq) + OH^- (aq) +Al (s) -->

K^+ (aq) + Cl^- (aq) -->
Ag^+ (aq) + Cl^- (aq) -->

can you also explain WHY the one pair undergoes a reaction? Thanks so much!!

The first pair:

You look at the table of solubilities.
Reactions occur for three primary reasons.
a. an insoluble ppt formed.
b. a gas formd
c. a weak electrolyte formed.
Here is a simplified table of solubilities.
http://www.files.chem.vt.edu/RVGS/ACT/notes/solubility_rules.html
For the first pair you will note BaSO4 is not soluble.
Third pair: AgCl is insoluble.
Second pair: I think someone goofed. Both equations will go because Al + OH^- gives H2 + Al(OH)3.

1) NaCl (aq) + BaCl2 (aq) --> No reaction occurs because all the reactants are soluble salts in water, and no insoluble product is formed.

2) Na2SO4 (aq) + BaCl2 (aq) --> A reaction occurs. The balanced equation is:

Na2SO4 (aq) + BaCl2 (aq) --> BaSO4 (s) + 2NaCl (aq)

In this reaction, an insoluble product, BaSO4 (barium sulfate), is formed, which can be observed as a white precipitate. The reaction is a double replacement reaction, where the sulfates and chlorides switch partners.

3) NO3^- (aq) + OH^- (aq) + Al (s) --> A reaction occurs. The balanced equation is:

Al (s) + 3OH^- (aq) + NO3^- (aq) + H2O (l) --> Al(OH)3 (s) + HNO3 (aq)

In this reaction, aluminum hydroxide (Al(OH)3) is formed as an insoluble product. Aluminum reacts with hydroxide ions to form aluminum hydroxide, while nitrate ions (NO3^-) react with water to produce nitric acid (HNO3).

4) CO3^2- (aq) + OH^- (aq) + Al (s) --> No reaction occurs because aluminum does not react with carbonate ions to form a solid or gas.

5) K^+ (aq) + Cl^- (aq) --> No reaction occurs because both potassium (K^+) and chloride (Cl^-) ions are soluble salts, and no precipitate or gas is formed.

6) Ag^+ (aq) + Cl^- (aq) --> A reaction occurs. The balanced equation is:

Ag^+ (aq) + Cl^- (aq) --> AgCl (s)

In this reaction, silver chloride (AgCl) is formed as an insoluble product, which appears as a white precipitate. This reaction is a double replacement reaction, where the silver ions (Ag^+) and chloride ions (Cl^-) combine to form the solid silver chloride.

To determine which pair of reactants undergoes a reaction, we need to use our knowledge of the solubility rules and the activity series.

1) NaCl (aq) + BaCl2 (aq)-->
The combination of NaCl (aq) and BaCl2 (aq) does not yield a reaction. Both NaCl and BaCl2 are soluble in water, which means they dissociate into their respective ions, Na+ and Cl- for NaCl, and Ba2+ and 2Cl- for BaCl2. Since there is no driving force for a reaction to occur, we can leave the equation unbalanced and unchanged.

2) Na2SO4 (aq) + BaCl2 (aq)-->
In this case, a reaction occurs. Na2SO4 (sodium sulfate) is soluble in water and dissociates into Na+ and SO4^2-. BaCl2 (barium chloride) is also soluble and dissociates into Ba2+ and 2Cl- ions. In the reaction, the sulfate ion (SO4^2-) reacts with the barium ion (Ba2+) to form an insoluble compound, barium sulfate (BaSO4). The balanced equation is:

Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)

3) NO3^- (aq) + OH^- (aq) + Al (s)-->
This combination does not yield a reaction. The nitrate ion (NO3^-) and hydroxide ion (OH^-) are both soluble in water, so they exist as separate ions. Aluminum (Al) is a metal, and as such, it does not readily react with non-metals such as ions. Therefore, no reaction occurs, and the equation remains unbalanced and unchanged.

4) CO3^2- (aq) + OH^- (aq) + Al (s)-->
In this case, a reaction occurs. The carbonate ion (CO3^2-) and the hydroxide ion (OH^-) are both soluble in water as separate ions. Aluminum (Al) is a metal that is more reactive than hydroxide (OH^-). The aluminum reacts with the hydroxide ions, displacing them and forming aluminum hydroxide (Al(OH)3), a white solid. The balanced equation is:

Al (s) + 3OH^- (aq) + CO3^2- (aq) → Al(OH)3 (s) + CO3^2- (aq)

5) K+ (aq) + Cl^- (aq)-->
No reaction occurs in this case. Both potassium (K+) and chloride (Cl^-) ions are soluble and exist as separate ions in solution. Since they are both stable ions and do not have any tendency to form new compounds, the equation remains unbalanced and unchanged.

6) Ag+ (aq) + Cl^- (aq)-->
In this case, a reaction occurs. Silver (Ag) is less reactive than hydrogen (H) but more reactive than Cl. The silver ion (Ag+) reacts with the chloride ion (Cl^-) to form a white precipitate of silver chloride (AgCl) since it is an insoluble compound. The balanced equation is:

Ag+ (aq) + Cl^- (aq) → AgCl (s)

In summary, the pairs that undergo reactions are:

Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
CO3^2- (aq) + OH^- (aq) + Al (s) → Al(OH)3 (s) + CO3^2- (aq)
Ag+ (aq) + Cl^- (aq) → AgCl (s)

The reactions occur due to the solubility rules and activity series, which dictate whether a compound is soluble, whether ions will actively react, and the formation of an insoluble compound.