Which one of the following is the correct reactivity order of metals reacting with dilute HCl?
Mg > Al > Zn > Fe
Understanding Metal Reactivity with Dilute Acids
The reactivity of metals is often determined by their position in the reactivity series. The reactivity series is a list of metals arranged in order of their decreasing reactivity. More reactive metals can displace less reactive metals from their compounds and also displace hydrogen from dilute acids like HCl.
How Metals React with Dilute HCl
When a metal reacts with a dilute acid like hydrochloric acid (\(\text{HCl}\)), it typically produces a salt and hydrogen gas. The general reaction is:
\(\text{Metal} + \text{Dilute Acid} \rightarrow \text{Metal Salt} + \text{Hydrogen Gas}\)
For example, the reaction of a reactive metal (\(\text{M}\)) with dilute \(\text{HCl}\) is:
\(\text{M(s)} + 2\text{HCl(aq)} \rightarrow \text{MCl}_2\text{(aq)} + \text{H}_2\text{(g)}\) (assuming \(\text{M}\) forms a +2 ion)
A metal can displace hydrogen from dilute acids if it is more reactive than hydrogen in the reactivity series. The question asks about the reactivity order of specific metals: Magnesium (\(\text{Mg}\)), Aluminium (\(\text{Al}\)), Zinc (\(\text{Zn}\)), and Iron (\(\text{Fe}\)) with dilute \(\text{HCl}\).
Reactivity Series of Metals
The approximate order of reactivity for common metals, from most reactive to least reactive, is:
- Potassium (\(\text{K}\))
- Sodium (\(\text{Na}\))
- Calcium (\(\text{Ca}\))
- Magnesium (\(\text{Mg}\))
- Aluminium (\(\text{Al}\))
- Carbon
- Zinc (\(\text{Zn}\))
- Iron (\(\text{Fe}\))
- Hydrogen (\(\text{H}\))
- Copper (\(\text{Cu}\))
- Silver (\(\text{Ag}\))
- Gold (\(\text{Au}\))
Metals above hydrogen in the series can displace hydrogen from dilute acids. The further up a metal is, the more readily it reacts.
Determining the Reactivity Order for Mg, Al, Zn, Fe
Based on the reactivity series, the order of these four metals from most reactive to least reactive is:
Magnesium (\(\text{Mg}\)) > Aluminium (\(\text{Al}\)) > Zinc (\(\text{Zn}\)) > Iron (\(\text{Fe}\))
This means Magnesium is the most reactive among these four, and Iron is the least reactive among these four when reacting with dilute \(\text{HCl}\).
Analyzing the Given Options
Let's examine each option to see which one matches the correct reactivity order: Mg > Al > Zn > Fe.
- Option 1: \(\text{Mg} > \text{Al} > \text{Zn} > \text{Fe}\)
This order matches the reactivity series derived order for these specific metals. - Option 2: \(\text{Mg} < \text{Al} < \text{Zn} < \text{Fe}\)
This order is the reverse of the reactivity series, indicating increasing reactivity from Mg to Fe, which is incorrect. - Option 3: \(\text{Mg} > \text{Zn} > \text{Fe} > \text{Al}\)
This order incorrectly places Al below Zn and Fe. - Option 4: \(\text{Fe} > \text{Mg} > \text{Al} > \text{Zn}\)
This order incorrectly places Fe as the most reactive and Mg below Fe.
Therefore, the correct reactivity order of the metals Magnesium (\(\text{Mg}\)), Aluminium (\(\text{Al}\)), Zinc (\(\text{Zn}\)), and Iron (\(\text{Fe}\)) reacting with dilute \(\text{HCl}\) is Mg > Al > Zn > Fe.
Conclusion
The correct reactivity order of the given metals reacting with dilute \(\text{HCl}\) is \(\text{Mg} > \text{Al} > \text{Zn} > \text{Fe}\). This order reflects their relative positions in the standard metal reactivity series.
Revision Table: Metal Reactivity
| Metal Symbol | Metal Name | Position in Reactivity Series (Relative) | Reactivity with Dilute HCl |
|---|---|---|---|
| Mg | Magnesium | Higher (More Reactive) | Reacts readily |
| Al | Aluminium | Higher (More Reactive, but can be slow initially due to oxide layer) | Reacts |
| Zn | Zinc | Middle | Reacts |
| Fe | Iron | Lower (Less Reactive among the four) | Reacts slowly |
Additional Information: Factors Affecting Reaction Rate
While the reactivity series gives the general order of reactivity, the actual rate of reaction with dilute \(\text{HCl}\) can be influenced by several factors:
- Surface Area: Powdered metals react faster than solid lumps.
- Concentration of Acid: More concentrated acid reacts faster.
- Temperature: Higher temperature increases the reaction rate.
- Presence of Oxide Layer: Aluminium, for example, forms a protective oxide layer on its surface, which can initially slow down its reaction with dilute acids until the layer is removed. However, in the context of the general reactivity series, Al is placed above Zn and Fe.
The reactivity series provides a fundamental basis for predicting whether a reaction will occur and the relative ease with which it does so.
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