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.
Tip
When writing half-equations, always check that the number of electrons lost in oxidation matches the number gained in reduction.
Example
Reaction 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.
Hint
- 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.
Common Mistake
- 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.
Example
- 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.
Note
Hydrochloric acid and sulfuric acid are commonly used in these reactions because they are strong acids that readily release hydrogen ions ($H^+$).
Experimental Observations and Tests for Hydrogen Gas
- In the laboratory, you can observe this reaction through the release of bubbles (effervescence) as hydrogen gas is produced.
- To confirm the gas is hydrogen, perform the "pop test":
- Collect the gas in an inverted test tube.
- Bring a lit splint close to the mouth of the test tube.
- If the gas is hydrogen, it will ignite with a characteristic "pop" sound.
Tip
Always ensure the reaction vessel is open or vented to prevent pressure buildup from hydrogen gas.
Predicting and Writing Balanced Equations
Let’s practice predicting the products of metal-acid reactions and writing balanced equations.
Example
Reaction of Magnesium with Hydrochloric Acid
Magnesium reacts vigorously with hydrochloric acid:
$$
Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)
$$
- Oxidation: $Mg(s) \rightarrow Mg^{2+}(aq) + 2e^-$
- Reduction: $2H^+(aq) + 2e^- \rightarrow H_2(g)$
Self review
- Write the balanced equation for the reaction between aluminum and hydrochloric acid.
- Identify the oxidation and reduction half-equations.
Common Mistake
Not all acids react with metals to release hydrogen gas. For example:
- Weak acids like acetic acid react slowly.
- Concentrated oxidizing acids like nitric acid often produce nitrogen oxides instead of hydrogen gas.
