Video Transcript
The diagram shows the electromotive
series. Which of the following statements
about the electromotive series is not true? (A) Zinc metal is a better reducing
agent than aluminum metal. (B) The more negative the standard
reduction potential is, the stronger a reducing agent the metal is. (C) Lithium metal is a better
reducing agent than gold metal. (D) Silver ions are better
oxidizing agents than calcium ions. (E) The more positive the standard
reduction potential is, the stronger an oxidizing agent the metal ion is.
In this question, we want to know
which of the answers in the question is not true when investigating the
electromotive series. The electromotive series is a
sequence of elements that generally consists of metals that are arranged by their
standard reduction potentials. Hydrogen is listed in the middle of
the sequence with a value of 0.00 volts as it is the standard reference electrode
used to determine the reduction potentials of other systems.
The standard reduction potential is
measured under specific standard conditions. These are one-molar electrolyte
solution concentrations, one-atmosphere pressure, and typically a temperature of 298
kelvin.
If we look at the direction of the
arrow and the label, we can see that the elements above hydrogen have negative
values and the ones below hydrogen in the sequence have positive standard reduction
potential values.
The more negative or the lower the
reduction potential of an element is, the less likely it is to be reduced. Lithium is more likely to be
oxidized when in a galvanic cell with any element below it. And thus, lithium is a strong
reducing agent.
The more positive or the higher the
reduction potential, the more likely an element is to be reduced. In other words, a higher reduction
potential indicates a greater capacity to receive electrons and be reduced. The elements below hydrogen, for
example, silver, are more likely to be reduced when in a galvanic cell with any
element above it in the series. Thus, these elements are weak
reducing agents.
We can use this information to
examine our answer choices.
Answer choice (A) states that zinc
metal is a better reducing agent than aluminum metal. However, aluminum is above zinc in
the electromotive series, which would mean that aluminum has a more negative
standard reduction potential and is thus a stronger reducing agent than zinc. This statement is incorrect and is
thus our answer, as we were asked which statement is not true. Let’s quickly check this by
examining the remaining answers.
In answer choice (B), we are told
that the more negative the standard reduction potential is, the stronger the
reducing agent the metal or element is. This is a true statement, as we
have seen. We can eliminate answer (B).
Statement (E) — the more positive
the standard reduction potential is, the stronger an oxidizing agent the metal ion
is — is the converse of statement (B) and thus must be true. So we can rule out answer (E) as
well.
Answer choice (C), lithium metal is
a better reducing agent than gold metal, is also true. We can see this in the circled
portions. So we can eliminate this answer
too.
Answer (D) says that silver ions
are better oxidizing agents than calcium ions. Note, here we are talking about
ions, not neutral metal atoms. This statement makes sense. Since elements lower on this list
are weak reducing agents, silver ions will accept electrons more readily than
calcium ions and are thus better oxidizing agents. Answer (D) is true, and we can
eliminate it.
Finally, which statement about the
electromotive series is not true? The answer is (A): zinc metal is a
better reducing agent than aluminum metal.
