Question

which statement is true about the following reaction equation?
$\ce{n_{2} + 3h_{2} -> 2nh_{3}}$

a the reaction has n and h on both sides, so it
obeys the law of conservation of mass.

b the reaction is not balanced, so it does not obey
the law of conservation of mass.

c the reaction has different types of molecules on
each side, so it does not obey the law of
conservation of mass.

d the reaction is balanced with the same number
of each element on both sides, so it obeys the
law of conservation of mass.

Explanation:

To determine the correct statement, we analyze the reaction \( \ce{N2 + 3H2 -> 2NH3} \) and the law of conservation of mass (mass is conserved in a chemical reaction, meaning the number of each atom is the same on both sides).

• Option a: Just having N and H on both sides isn’t enough—we need the same number of each atom. The law of conservation of mass requires balanced atom counts, not just presence. Eliminate a.
• Option b: The reaction is balanced. On the left: 2 N (from \( \ce{N2} \)) and 6 H (from \( 3\ce{H2} \)). On the right: 2 N (from \( 2\ce{NH3} \)) and \( 2 \times 3 = 6 \) H. So it is balanced. Eliminate b.
• Option c: Different molecules (reactants vs. products) are normal in reactions. The law of conservation of mass depends on atom counts, not molecule types. Eliminate c.
• Option d: The reaction is balanced (2 N and 6 H on both sides), so it obeys the law of conservation of mass. This matches the requirements of the law.

Answer:

d. The reaction is balanced with the same number of each element on both sides, so it obeys the law of conservation of mass.