Video Transcript
Which of the following statements
best describes a substitution reaction? (A) A more active metal replaces
hydrogen in another compound. (B) A more active metal replaces
another less active metal in another compound. (C) A less active metal forms a
compound with another more active metal from another compound. (D) A less active metal replaces
another more active metal in another compound. Or (E) a more active metal forms a
compound with another less active metal from another compound.
The word substitution means to swap
or replace. Options (A), (B), and (D) all
involve replacing one metal with another, whereas options (C) and (E) involve
forming a compound from two metals. So as neither option (C) nor option
(E) involve replacing or substituting one substance for another, then they cannot be
used to describe a substitution reaction and therefore cannot be the answer to this
question.
We’ve now narrowed the answers
down, so all statements involve substituting one element for another. But we now need to figure out
whether (A), (B), or (D) is correct. To do this, we should consider the
series of chemical activity. It is a list showing the chemical
activity of metals with the nonmetal hydrogen as a reference.
Option (A) states that a more
active metal replaces hydrogen in another compound. A metal can replace hydrogen in
another compound. For example, an atom of sodium can
replace one of the atoms of hydrogen in a molecule of water, producing sodium
hydroxide and hydrogen. For a metal to replace hydrogen in
another compound, it must be more active than hydrogen, so it must be higher than
hydrogen in the series of chemical activity, which sodium is.
But although a more active metal
can replace hydrogen in another compound, the metal doesn’t just have to replace
hydrogen; it can replace another metal. For example, magnesium can react
with lead nitrate to form magnesium nitrate and lead. As the metal is not replacing
hydrogen, we can rule out option (A).
The difference between the two
remaining answer choices is that one says “A more active metal replaces another less
active metal in another compound” and the other states that a less active metal
replaces another more active metal in another compound. We can see from this example that
magnesium is replacing the lead in a lead compound. Magnesium is higher up in the
series of chemical activity than lead is, so magnesium is more active than lead. Therefore, this substitution
reaction involves a more active metal replacing another less active metal in another
compound. This best matches the statement in
(B). Thus, option (B) is the correct
answer. And option (D) is incorrect.
So the answer to the question
“Which of the following statements best describes a substitution reaction?” is
(B). A more active metal replaces
another less active metal in another compound.
