Acids Reacting with Reactive Metals to Release Hydrogen
- When acids react with reactive metals, the metal atoms lose electrons to form positive ions, while hydrogen ions $H^+$ from the acid gain electrons to form hydrogen gas $H_2$.
- This process is a redox reaction, a reaction involving both oxidation (loss of electrons) and reduction (gain of electrons).
- The general word equation for this reaction is:
$$\text{Metal + Acid → Salt + Hydrogen Gas}$$
Example- When zinc reacts with hydrochloric acid, the products are zinc chloride (a salt) and hydrogen gas: $$
Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)
$$ - Here’s what happens at the atomic level:
- Oxidation: Zinc atoms lose two electrons to form zinc ions ($Zn^{2+}$):
$$
Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-
$$ - Reduction: Hydrogen ions ($H^+$) from the acid gain electrons to form hydrogen gas:
$$
2H^+(aq) + 2e^- \rightarrow H_2(g)
$$
- Oxidation: Zinc atoms lose two electrons to form zinc ions ($Zn^{2+}$):
- Together, these half-equations illustrate the electron transfer from zinc to hydrogen, which is the hallmark of a redox reaction.
When writing half-equations, always check that the number of electrons lost in oxidation matches the number gained in reduction.
ExampleReaction Between Zinc and Sulfuric Acid
- When zinc reacts with sulfuric acid, a salt (zinc sulfate) and hydrogen gas are produced: $$Zn (s) + H_2SO_4 (aq) \rightarrow ZnSO_4 (aq) + H_2 (g)$$
- In this reaction:
- Zinc displaces hydrogen from sulfuric acid due to its higher reactivity.
- Hydrogen gas is released as bubbles.
- Zinc sulfate is formed as an aqueous salt in the solution.
- This is a typical example of an acid reacting with a reactive metal to produce hydrogen gas.
Why Do Only Certain Metals React with Acids?
- Not all metals react with acids to release hydrogen gas.
- The key lies in the reactivity series, which ranks metals based on their tendency to lose electrons and form positive ions.
- Metals higher in the series (e.g., magnesium, zinc, iron) react readily with acids because they are easily oxidized.
- Metals lower in the series (e.g., copper, silver, gold) do not react with acids under normal conditions because they resist oxidation.
- Students often assume that all metals react with acids.
- Remember, only metals above hydrogen in the reactivity series can displace hydrogen ions to form hydrogen gas.
- For example, when iron reacts with sulfuric acid, it forms iron(II) sulfate and hydrogen gas:
$$
Fe(s) + H_2SO_4(aq) \rightarrow FeSO_4(aq) + H_2(g)
$$ - This reaction is used in industrial processes to clean iron surfaces before further processing.
