Video Transcript
Ammonium chloride decomposes when heated into ammonia gas and hydrogen chloride
gas. If the gases are collected and allowed to cool, then ammonium chloride reforms. Which of the following chemical equations describes these changes? (A) NH3 gas plus HCl gas reacts reversibly to form NH4Cl solid. (B) NH3 gas reacts reversibly to form NH4Cl solid plus HCl gas. (C) NH3 gas reacts to form NH4Cl solid plus HCl gas. (D) NH3 gas plus HCl gas react to form NH4Cl solid. (E) NH4Cl solid reacts to form NH3 gas plus HCl gas.
In this question, we want to know which equation best represents both the
decomposition of ammonium chloride and its reformation from its decomposition
products. Ammonium chloride has the chemical formula NH4Cl, and it is a white solid at room
temperature. We are told that the reactant in this chemical reaction is ammonium chloride and that
it decomposes when heated into ammonia gas, NH3, and hydrogen chloride gas, HCl. The substances that form are called the products. These products can also react together upon cooling to reform ammonium chloride.
A reaction such as this is called a reversible reaction since it proceeds in both
directions. Reactants react to form products, and the products react to reform the reactants. We can represent this reversible process using two half arrows pointing in opposite
directions.
The opposite of a reversible reaction is an irreversible reaction. An irreversible reaction is a reaction that proceeds in one direction only. The products do not react together to reform the reactants. When writing a chemical equation for an irreversible reaction, we use a single
arrow. Using this information, we can eliminate answer choices (C), (D), and (E), as they
all show irreversible reactions.
Answer choices (A) and (B) both have two half arrows, showing reversible
reactions. However, only answer choice (A) has the correct chemical formulas in the correct
places. It shows, from left to right, ammonium chloride forming from ammonia gas and hydrogen
chloride gas. And if we read the equation backwards, it shows the decomposition of ammonium
chloride into ammonia gas and hydrogen chloride gas.
So, which chemical equation correctly describes both the formation of ammonium
chloride from ammonia gas and hydrogen chloride gas, as well as the decomposition of
ammonium chloride? The answer is (A): NH3 gas plus HCl gas react reversibly to form NH4Cl solid.
