Which one of the following is the correct order of reactivity of Mg, Al, Zn and Fe with HCI?
Mg > Al > Zn > Fe
Understanding Reactivity Order of Mg, Al, Zn, Fe with HCl
This question asks us to determine the correct order of reactivity for Magnesium (Mg), Aluminum (Al), Zinc (Zn), and Iron (Fe) when they react with Hydrochloric Acid (HCl).
Reactivity Explained
The reactivity of metals refers to how readily they undergo a chemical reaction. When metals react with acids like HCl, they typically displace hydrogen from the acid, forming a metal salt and releasing hydrogen gas (\(H_2\)). The speed at which this reaction occurs indicates the metal's reactivity. More reactive metals react faster.
The general reaction is:
Metal + Hydrochloric Acid \(\rightarrow\) Metal Chloride + Hydrogen Gas
The Reactivity Series
Scientists arrange metals in a list called the reactivity series (or electrochemical series) based on their tendency to lose electrons and react. Metals higher up in this series are more reactive than those lower down.
The typical order of reactivity for the metals mentioned, starting from the most reactive, is:
- Magnesium (Mg)
- Aluminum (Al)
- Zinc (Zn)
- Iron (Fe)
So, the general reactivity order is Mg > Al > Zn > Fe.
Analyzing Reactivity with HCl
Let's look at how each metal behaves with HCl:
- Magnesium (Mg): It is a highly reactive metal and reacts vigorously with dilute HCl, producing hydrogen gas quickly.
- Aluminum (Al): Aluminum is also quite reactive. However, it often reacts slowly with HCl at room temperature because it has a thin, protective layer of aluminum oxide (Al₂O₃) on its surface. This layer prevents the acid from reaching the underlying metal immediately. Once the oxide layer is breached or reacts, the underlying aluminum reacts more readily. Despite this initial effect, its position in the electrochemical series is quite high, making it more reactive than Zn and Fe.
- Zinc (Zn): Zinc reacts steadily with dilute HCl, releasing hydrogen gas at a moderate rate, slower than Magnesium.
- Iron (Fe): Iron is less reactive than Zinc. It reacts slowly with dilute HCl, producing hydrogen gas at the slowest rate among these four metals.
Summary Table
Here's a summary of the relative reactivity with HCl:
| Metal | Reactivity with HCl (Decreasing Order) |
|---|---|
| Magnesium (Mg) | Most Reactive |
| Aluminum (Al) | Reactive (initial layer may slow reaction) |
| Zinc (Zn) | Moderately Reactive |
| Iron (Fe) | Least Reactive (among these four) |
Correct Order
Based on the reactivity series and their behavior with HCl, the correct order of reactivity from most reactive to least reactive is Magnesium > Aluminum > Zinc > Iron.
Therefore, the correct option is Mg > Al > Zn > Fe.
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