Video Transcript
Which of the following chemical
equations shows what happens to nitric acid when heated? (A) Four HNO3 liquid reacts to form
four NO2 gas plus four H2O liquid. (B) Four HNO3 liquid reacts to form
four NO2 gas plus O2 gas plus two H2O liquid. (C) HNO3 liquid reacts to form NO
gas plus O2 gas plus H2O liquid. (D) Four HNO3 liquid reacts to form
four NO gas plus four O2 gas plus two H2 gas. (E) Four HNO3 liquid reacts to form
four NO2 gas plus two O2 gas.
When nitric acid is heated, it
decomposes and changes from colorless to an orange-brown color. The reason for this color change is
the formation of nitrogen dioxide gas, or NO2. Since we know that nitrogen dioxide
is a product of the reaction, answer choices (C) and (D) are incorrect because these
equations do not contain NO2 as a product.
Now, we are left with answer
choices (A), (B), and (E). So we will need to determine the
other products of the decomposition reaction. The other products of the
decomposition of nitric acid are oxygen gas and liquid water. Knowing this allows us to eliminate
answer choice (A) because the equation does not contain O2 as a product. We can also eliminate answer choice
(E) because the equation does not contain H2O as a product. This leaves only answer choice (B),
which must be the correct answer.
The correct balanced chemical
equation for the reaction includes a coefficient of four in front of nitric acid and
nitrogen dioxide and a coefficient of two in front of water. When nitric acid is heated, it
decomposes to form nitrogen dioxide gas, oxygen gas, and liquid water. Therefore, the correct chemical
equation for this reaction is answer choice (B). Four HNO3 liquid reacts to form
four NO2 gas plus O2 gas plus two H2O liquid.
