The Reactivity Series: The Key to Displacement
For one metal to displace another in a chemical reaction, it must be higher on the reactivity series. The reactivity series is an arrangement of metals in order of their decreasing chemical reactivity. A more reactive metal will lose its electrons more easily, giving it a greater tendency to form positive ions in a solution. This fundamental concept is the basis for understanding if and how a displacement reaction between magnesium and copper will occur.
Why Magnesium is More Reactive
Magnesium's position in the reactivity series is significantly higher than copper's. This means magnesium has a greater tendency to be oxidized, or to lose its valence electrons, compared to copper. When a piece of solid magnesium is introduced into a solution containing copper ions, such as copper(II) sulfate, the magnesium atoms have a stronger pull on the electrons than the copper ions do. The result is a spontaneous transfer of electrons from the magnesium to the copper ions, initiating a displacement reaction.
The Magnesium and Copper Displacement Reaction
The most common and demonstrative example of this reaction involves a strip of magnesium ribbon placed into a blue solution of copper(II) sulfate. The reaction can be summarized by the following chemical equation:
$Mg(s) + CuSO_4(aq) \rightarrow MgSO_4(aq) + Cu(s)$
- Reactants: The reaction begins with solid magnesium ($Mg$) and an aqueous solution of copper(II) sulfate ($CuSO_4$), which is blue in color.
- Process: The magnesium metal displaces the copper from the copper sulfate. As this happens, the magnesium atoms lose electrons and go into the solution as magnesium ions ($Mg^{2+}$). Simultaneously, the copper ions ($Cu^{2+}$) in the solution gain these electrons and are deposited as solid copper metal ($Cu$).
- Products: The final products are colorless aqueous magnesium sulfate ($MgSO_4$) and reddish-brown solid copper ($Cu$).
Visual Evidence of the Reaction
This chemical change is visually striking. Key observations include:
- The bright blue color of the copper(II) sulfate solution begins to fade and eventually turns colorless as the concentration of copper ions decreases.
- A layer of reddish-brown solid copper metal forms and coats the surface of the magnesium strip.
- Over time, the magnesium strip will appear to dissolve as it is consumed in the reaction, replaced by the copper deposit.
Understanding the Redox Process
The reaction between magnesium and copper is a classic example of a single displacement redox (reduction-oxidation) reaction. Redox reactions involve the transfer of electrons between chemical species. The process can be broken down into two half-equations:
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Oxidation Half-Reaction: $Mg(s) \rightarrow Mg^{2+}(aq) + 2e^{-}$ In this step, the magnesium atom is oxidized. It loses two electrons to become a magnesium ion, which dissolves in the solution.
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Reduction Half-Reaction: $Cu^{2+}(aq) + 2e^{-} \rightarrow Cu(s)$ Here, the copper ion is reduced. It gains two electrons to become a neutral copper atom, which precipitates out of the solution as a solid.
Practical Applications of This Displacement
The principle that a more reactive metal displaces a less reactive one has several real-world applications. One common use is in cathodic protection, a technique used to prevent corrosion. For instance, a block of a more reactive metal, like magnesium, is attached to a steel pipeline or ship hull. The magnesium preferentially corrodes (sacrifices itself) by displacing other metals and preventing the iron from rusting. This reaction is also a simple, visual demonstration often used in chemistry classrooms to teach the concepts of metal reactivity and redox chemistry.
Comparison: Magnesium vs. Copper Reactivity
| Property | Magnesium (Mg) | Copper (Cu) |
|---|---|---|
| Position in Reactivity Series | Higher (more reactive) | Lower (less reactive) |
| Electron Loss Tendency | High; readily loses electrons | Low; holds onto electrons more tightly |
| Displacement Ability | Can displace copper from its salts | Cannot displace magnesium from its salts |
| Action in Copper Sulfate | Reacts; displaces copper | No reaction with magnesium sulfate |
| Corrosion Protection | Used as a sacrificial anode | Less effective for protecting more reactive metals |
Key Factors Influencing the Reaction
Several factors can influence the speed and effectiveness of the magnesium-copper displacement reaction:
- Surface Area: A larger surface area of the magnesium, such as using powder instead of a strip, will increase the rate of reaction.
- Solution Concentration: A higher concentration of copper(II) sulfate will lead to a faster reaction rate due to a higher frequency of collisions between magnesium and copper ions.
- Temperature: Increasing the temperature of the solution will increase the kinetic energy of the particles, leading to a faster reaction.
- Purity of Metals: Impurities can sometimes affect the reaction rate or create side reactions.
Conclusion: Answering the Question
Yes, magnesium can and does displace copper in a chemical reaction. The answer is rooted in the metals' relative positions in the reactivity series. Since magnesium is more reactive, it readily loses electrons to copper ions in a single displacement redox reaction. This reaction transforms the reactants (magnesium metal and copper sulfate solution) into new products (magnesium sulfate solution and copper metal), a process clearly observable through the fading of the solution's color and the formation of a copper deposit. This fundamental principle of chemistry has both educational and practical applications, illustrating the inherent difference in reactivity between these two common metals. For more information on metal reactivity and displacement reactions, you can explore resources like the BBC's Bitesize guides. [https://www.bbc.co.uk/bitesize/guides/zpndh39/revision/3]
Summary of Key Learnings
- Magnesium's Reactivity: Magnesium is a more reactive metal than copper due to its higher position in the reactivity series.
- Displacement Confirmation: The displacement of copper by magnesium is visibly confirmed by the fading of the blue copper sulfate solution and the appearance of solid copper.
- Redox Process: The reaction is a redox process where magnesium is oxidized (loses electrons) and copper is reduced (gains electrons).
- Chemical Equation: A key representation of this reaction is $Mg(s) + CuSO_4(aq) \rightarrow MgSO_4(aq) + Cu(s)$.
- Practical Relevance: This chemical principle is used in applications like cathodic protection to prevent corrosion.
