Which forms of energy can be released during a chemical reaction?

Aqueous solutions of $Na_{2}CO_{3}$ and $Ca(NO_{3})_{2}$ , 0.10 M each, are combined. A white precipitate is observed in the container after mixing. he precipitate is filtered andcarefully rinsed with distilled water to remove other ions. A sample of the precipitate is added to 100 mL of 0.1 M NaCl. A second sample of the precipitate is then added to 100 mL of 0.1 M HCl. What would be observed in each case?

Observation upon Observation upon

addition of precipitate addition of precipitate

to NaCl(aq) to HCl(aq)

(A) additional precipitates forms no visible reaction occurs

(B) no visible reaction occurs gas is produced and some precipitate dissolves

(C) no visible reaction occurs no visible reaction occurs

(D) additional precipitates forms gas is produced and some

precipitate dissolves

Answer: (B) No visible reaction occurs; Gas is produced and some precipitate dissolves

Explanation: When aqueous solutions of $Na_{2}CO_{3}$ and $Ca(NO_{3})_{2}$ are combined, it reacts according to the following balanced equation:

$Na_{2}CO_{3}+Ca(NO_{3})_{2}$ → $CaCO_{3}_{(s)}+2NaNO_{3}_{(aq)}$

forming calcium carbonate ( $CaCO_{3}$ ), which, as it is insoluble in water, precipitates as a solid of the color white. This process is Precipitation and this reaction is a Precipitation Reaction.

When calcium carbonate reacts with NaCl it produces:

$CaCO_{3}+2NaCl$ → $CaCl_{2}+Na_{2}CO_{3}$

Now, calcium chloride is an inorganic compound very soluble in water, so, in this reaction, there are no precipitate and no visible reaction occurs.

When $CaCO_{3}$ reacts with hydrochloridric acid, the balanced reaction is

$CaCO_{3}+2HCl$ → $CaCl_{2}+H_{2}CO_{3}$

which, also produces calcium chloride and carbonic acid.

Both are soluble in water but, when carbonic acid is in an "aqueous state", carbonic acid, it dissociates, forming carbon dioxide and water. Therefore, gas is produced and some precipitate dissolves.

In conclusion, sentence B is the correct alternative.

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