To determine which equation describes a reduction, we recall that in a reduction reaction, a species gains electrons.
Here's a review of each equation step-by-step:
1. [tex]\( Mg (s) \longrightarrow Mg^{2+} (aq) + 2e^- \)[/tex]:
- Magnesium (Mg) in the solid state loses two electrons to form [tex]\( Mg^{2+} \)[/tex].
- This is an oxidation process since Mg is losing electrons.
2. [tex]\( 2 Cl_2 + 2 e^- \longrightarrow 2 Cl^- \)[/tex]:
- Chlorine molecules [tex]\((Cl_2)\)[/tex] gain electrons to form chloride ions [tex]\((Cl^-)\)[/tex].
- Since electrons are being gained, this represents a reduction reaction.
3. [tex]\( Na (s) \longrightarrow Na^+ (aq) + e^- \)[/tex]:
- Sodium (Na) in the solid state loses one electron to form [tex]\( Na^+ \)[/tex].
- This is an oxidation process as Na is losing an electron.
4. [tex]\( Al (s) \longrightarrow Al^{3+} (aq) + 3e^- \)[/tex]:
- Aluminum (Al) in the solid state loses three electrons to form [tex]\( Al^{3+} \)[/tex].
- This is also an oxidation reaction since Al is losing electrons.
From this breakdown, we can see that the second equation:
[tex]\[ 2 Cl_2 + 2 e^- \longrightarrow 2 Cl^- \][/tex]
is the one that describes a reduction reaction, as electrons are being gained by the chlorine molecules.
