The reactivity series is a fundamental concept in chemistry, charting metals in order of their decreasing reactivity. It indicates how readily a metal will react with other substances, reflecting its ability to lose electrons to form positive ions (cations). The more reactive a metal is, the more vigorously it reacts and the more easily it forms these positive ions.
What is the Reactivity Series?
The reactivity series is an arrangement of metals based on their chemical reactivity, from most reactive to least reactive. This order dictates how metals behave in various chemical reactions, such as with water, acids, and other metal salts. A metal's position in the series helps predict whether a reaction will occur and the potential vigor of that reaction.
Here is a common representation of the reactivity series of metals, with the most reactive at the top:
| Metal | Reactivity |
|---|---|
| Potassium (K) | Very high reactivity, reacts vigorously with cold water |
| Sodium (Na) | High reactivity, reacts vigorously with cold water |
| Calcium (Ca) | High reactivity, reacts quite vigorously with cold water |
| Magnesium (Mg) | Moderate reactivity, reacts slowly with cold water, more vigorously with steam |
| Aluminium (Al) | Moderate reactivity, reacts with steam (often coated with inert oxide) |
| Zinc (Zn) | Moderate reactivity, reacts with dilute acids and steam |
| Iron (Fe) | Moderate reactivity, reacts slowly with steam, rusting in moist air |
| Lead (Pb) | Low reactivity, reacts slowly with dilute acids |
| Hydrogen (H) | (Non-metal, included as a reference point for acid reactions) |
| Copper (Cu) | Very low reactivity, does not react with dilute acids or steam |
| Silver (Ag) | Very low reactivity, does not react with dilute acids or water |
| Gold (Au) | Extremely low reactivity, does not react with most common chemicals |
| Platinum (Pt) | Extremely low reactivity, does not react with most common chemicals |
Note: Hydrogen is included as a reference point to show which metals are reactive enough to displace hydrogen from acids.
How is a Reactivity Series Established?
The reactivity series is established through various experimental observations, primarily by comparing the reactions of different metals with common substances like water, dilute acids, and solutions of other metal salts. These experiments reveal the relative ease with which metals lose electrons and form ions.
The key methods used include:
1. Reaction with Water and Steam
Different metals react with water at varying temperatures (cold water, hot water, or steam) to produce hydrogen gas and a metal hydroxide or oxide.
- Cold Water: Highly reactive metals (like potassium, sodium, calcium) react vigorously with cold water.
- Example:
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
- Example:
- Hot Water: Less reactive metals (like magnesium) might react slowly with cold water but more vigorously with hot water.
- Steam: Even less reactive metals (like zinc, iron) will only react with steam at high temperatures.
- Example:
Zn(s) + H₂O(g) → ZnO(s) + H₂(g)
- Example:
- No Reaction: Metals below hydrogen in the series (e.g., copper, silver, gold) do not react with water or steam.
By observing the vigor and conditions required for these reactions, metals can be ordered by their reactivity.
2. Reaction with Dilute Acids
Metals above hydrogen in the reactivity series will react with dilute acids (like hydrochloric acid or sulfuric acid) to produce hydrogen gas and a metal salt.
- Vigorous Reaction: Highly reactive metals (e.g., potassium, sodium, calcium) react explosively with dilute acids, often too dangerous to conduct.
- Moderate Reaction: Metals like magnesium, aluminium, zinc, and iron react at decreasing rates with dilute acids.
- Example:
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
- Example:
- No Reaction: Metals below hydrogen (e.g., copper, silver, gold) do not react with dilute acids to produce hydrogen.
The speed and effervescence (bubbling) of hydrogen gas production provide a clear indication of a metal's relative reactivity.
3. Displacement Reactions
This is a crucial method for establishing the exact order within the series. A more reactive metal will displace a less reactive metal from its salt solution.
- When a more reactive metal is placed in a solution containing ions of a less reactive metal, the more reactive metal loses electrons to become ions, while the less reactive metal ions gain electrons and deposit as solid metal.
- Example: If a piece of zinc is placed in copper(II) sulfate solution, zinc is more reactive than copper, so it displaces copper:
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
(Zinc metal dissolves, and copper metal deposits) - Conversely: If copper is placed in zinc sulfate solution, no reaction occurs because copper is less reactive than zinc.
- Example: If a piece of zinc is placed in copper(II) sulfate solution, zinc is more reactive than copper, so it displaces copper:
By performing a series of displacement reactions, chemists can precisely determine the relative positions of metals within the reactivity series. This directly illustrates the concept of a more reactive metal more easily losing electrons.
4. Oxidation and Reduction Tendencies (Electrochemical Series)
At a more fundamental level, the reactivity series is linked to the ease with which a metal loses electrons (undergoes oxidation) to form positive ions. Metals that lose electrons more easily are more reactive. This is directly related to their standard electrode potentials, which form the basis of the electrochemical series.
- Metals higher in the reactivity series have more negative standard electrode potentials, meaning they have a stronger tendency to be oxidized (lose electrons).
- This electrochemical data provides a quantitative and highly precise way to establish and confirm the reactivity order observed through chemical reactions.
In summary, the reactivity series is built upon empirical observations of how metals interact with common reagents, confirming their relative tendencies to lose electrons and undergo chemical change.
