Refer to the figure below, which shows an oxidation-reduction reaction that occurs spontaneously. Reduced compound A (reducing agent) Oxidized compound B (oxidizing agent) A is oxidized, having lost electrons. B is reduced, having gained electrons. Oxidized compound A Reduced compound B Which statement correctly compares the free energy stored in molecules shown in the figure?
The oxidized form of B has greater free energy than the reduced form of B.
The oxidized form of B has greater free energy than the reduced form of A.
The reduced form of A has greater free energy than the oxidized form of A.
The oxidized form of A has greater free energy than the reduced form of B.
The reduced form of B has greater free energy than the reduced form of A.
The correct answer and explanation is :
The correct answer is: The reduced form of A has greater free energy than the oxidized form of A.
Explanation:
Let’s break it down carefully:
In the figure described, we have an oxidation-reduction (redox) reaction that happens spontaneously. In redox reactions:
- Oxidation = loss of electrons
- Reduction = gain of electrons
Compound A starts in a reduced form (meaning it has extra electrons) and becomes oxidized (loses those electrons).
Compound B starts in an oxidized form (missing some electrons) and becomes reduced (gains electrons).
In spontaneous redox reactions, electrons naturally flow from the substance with higher free energy to the one with lower free energy. This is key!
Now, think about energy:
- Reduced molecules generally store more free energy than their oxidized forms.
- This is because electrons are “energy-rich”; losing electrons (oxidation) releases stored energy.
Thus, the reduced form of A has more free energy than the oxidized form of A.
In other words, before A loses electrons, it holds more chemical energy. After losing electrons (oxidized), A is in a lower-energy, more stable state.
This matches the answer: The reduced form of A has greater free energy than the oxidized form of A.
Now, why not the other choices?
- The oxidized form of B has greater free energy than the reduced form of B → Wrong. Reduced molecules have more free energy.
- The oxidized form of B has greater free energy than the reduced form of A → Wrong. B gaining electrons (being reduced) means it started with lower free energy.
- The oxidized form of A has greater free energy than the reduced form of B → Wrong. Oxidized forms generally have less free energy.
- The reduced form of B has greater free energy than the reduced form of A → Wrong. A being the reducing agent means it started with higher free energy.
Thus, the best match is:
✅ The reduced form of A has greater free energy than the oxidized form of A.