To determine which substance is the acid in the given reaction, we need to remember the fundamental definition of an acid. According to the Brønsted-Lowry definition, an acid is a substance that donates a proton (H⁺ ion) during a reaction.
Let's analyze the reaction provided:
[tex]$
H _2 SO _4(a q) + Mg (s) \longrightarrow MgSO _4(a q) + H _2(g)
$[/tex]
1. Identify the potential candidates for the acid:
- [tex]\( Mg(s) \)[/tex] is a solid metal.
- [tex]\( H _2(g) \)[/tex] is a diatomic gas.
- [tex]\( MgSO _4(aq) \)[/tex] is an aqueous ionic compound.
- [tex]\( H _2 SO _4(aq) \)[/tex] is an aqueous solution of sulfuric acid.
2. Examine the nature of each substance:
- [tex]\( Mg(s) \)[/tex] is a solid metal and typically does not donate protons. It can, however, be a reducing agent.
- [tex]\( H _2(g) \)[/tex] is a gas composed of diatomic hydrogen molecules. It is generally produced as a byproduct in acid-metal reactions and does not act as an acid.
- [tex]\( MgSO _4(aq) \)[/tex] is a salt formed in the reaction and remains in the aqueous ionized form. It does not participate in proton exchange.
- [tex]\( H _2 SO _4(aq) \)[/tex] is sulfuric acid, a well-known strong acid. It is capable of donating protons (H⁺) readily.
3. Determine the proton donor:
The substance which donates a proton (H⁺) in the reaction is [tex]\( H _2 SO _4 \)[/tex]. During the reaction, [tex]\( H _2 SO _4 \)[/tex] donates protons which react with the metal [tex]\( Mg(s) \)[/tex] to produce [tex]\( MgSO _4(aq) \)[/tex] and release hydrogen gas [tex]\( H _2(g) \)[/tex].
Given the above analysis, the acid in this reaction is clearly [tex]\( H _2 SO _4(aq) \)[/tex].
So, the substance acting as the acid in the reaction is:
[tex]$[/tex]
\boxed{H _2 SO _4(aq)}
```
