Reactivity series
Metals vary in how readily they lose electrons to form positive ions. This tendency is called reactivity. The more easily a metal loses electrons, the more reactive it is.
The standard reactivity series (from most reactive to least reactive) is:
Potassium > Sodium > Calcium > Magnesium > Aluminium > Carbon > Zinc > Iron > Hydrogen > Copper > Silver > Gold
Note: Carbon and Hydrogen are non-metals included in the series because they act as reference points for extraction (Carbon) and displacement (Hydrogen).
Reactivity is defined by a metal's tendency to form positive ions.
- High reactivity: The metal loses electrons easily (M \rightarrow M^{n+} + ne^-). It forms positive ions readily.
- Low reactivity: The metal holds onto its electrons tightly. It does not form positive ions easily.
This concept is the foundation for predicting displacement reactions. A more reactive metal will displace a less reactive metal from its aqueous compound because it has a stronger tendency to form positive ions.
General Equation:
\text{Metal A} + \text{Salt of Metal B} \rightarrow \text{Salt of Metal A} + \text{Metal B}
This only happens if Metal A is higher in the reactivity series than Metal B.
The position of a metal in the reactivity series determines whether it reacts with cold water, steam, or not at all.
| Metal | Reactant | Observation / Product |
|---|---|---|
| Potassium (K) | Cold Water | Reacts violently. Floats, fizzes, moves rapidly. Forms potassium hydroxide and hydrogen gas. |
| Sodium (Na) | Cold Water | Reacts vigorously. Melts into a ball. Forms sodium hydroxide and hydrogen gas. |
| Calcium (Ca) | Cold Water | Reacts steadily. Bubbles of hydrogen form. Forms calcium hydroxide and hydrogen gas. |
| Magnesium (Mg) | Steam | Reacts with steam (not cold water). Burns with a bright white flame. Forms magnesium oxide and hydrogen gas. |
| Zinc (Zn) | Steam / Hot Water | Reacts slowly with steam. Forms zinc oxide and hydrogen gas. |
| Iron (Fe) | Steam | Reacts very slowly with steam. Forms iron(II,III) oxide (Fe_3O_4) and hydrogen gas. |
| Copper (Cu) | Any Water/Steam | No reaction. |
| Silver (Ag) | Any Water/Steam | No reaction. |
| Gold (Au) | Any Water/Steam | No reaction. |
Key Takeaway: Metals above Magnesium react with cold water. Magnesium reacts with steam. Zinc and Iron react only with steam. Copper, Silver, and Gold do not react with water or steam.
Metals above Hydrogen in the series will displace hydrogen from dilute acids. Metals below Hydrogen will not.
| Metal | Reaction with Dilute HCl? | Product |
|---|---|---|
| Potassium, Sodium, Calcium | Yes (Violent/Dangerous) | Metal chloride + Hydrogen gas |
| Magnesium | Yes (Fast) | Magnesium chloride + Hydrogen gas |
| Aluminium | Yes (After delay) | Aluminium chloride + Hydrogen gas |
| Zinc | Yes (Moderate) | Zinc chloride + Hydrogen gas |
| Iron | Yes (Slow) | Iron(II) chloride + Hydrogen gas |
| Copper, Silver, Gold | No reaction | - |
Note: Aluminium is above Hydrogen but often appears unreactive initially due to its oxide layer (see below).
You are often given experimental data (e.g., 'Metal X reacts with water, Metal Y does not') and asked to order them.
Example Scenario:
- Metal A: Reacts violently with cold water.
- Metal B: Reacts slowly with steam.
- Metal C: No reaction with water or steam.
- Metal D: Reacts with dilute HCl but not water.
Deduction Steps:
- Identify the most reactive: Metal A (reacts with cold water) is more reactive than those reacting only with steam or acid.
- Compare Steam vs. Acid: Metal B reacts with steam, so it is more reactive than metals that only react with acid (Metal D).
- Identify the least reactive: Metal C does not react with water or steam, placing it at the bottom.
- Order (Most to Least): A > B > D > C.
Logic Check: If Metal A displaces Metal B from a salt solution, A is more reactive than B.
We can determine the order of reactivity between specific metals by testing if one displaces another from its aqueous salt solution.
Rule: A metal can only displace a metal below it in the reactivity series.
| Metal Added | Aqueous Solution | Reaction? | Explanation |
|---|---|---|---|
| Zinc (Zn) | Copper(II) sulfate (CuSO_4) | Yes | Zn is above Cu. Zn displaces Cu. Solution turns colorless (Zn^{2+}), red-brown Cu precipitates. |
| Copper (Cu) | Zinc sulfate (ZnSO_4) | No | Cu is below Zn. No reaction. |
| Iron (Fe) | Copper(II) sulfate (CuSO_4) | Yes | Fe is above Cu. Fe displaces Cu. |
| Copper (Cu) | Silver nitrate (AgNO_3) | Yes | Cu is above Ag. Cu displaces Ag. Silver crystals form on the copper surface. |
| Silver (Ag) | Copper(II) sulfate (CuSO_4) | No | Ag is below Cu. No reaction. |
Note: Magnesium and Aluminium are so reactive that if added to aqueous solutions, they often react with the water first rather than displacing the metal ion directly in a simple displacement context, though theoretically, they are more reactive than Zn, Fe, Cu, and Ag.
Learning Objective 5: Explain the apparent unreactivity of aluminium.
Aluminium is high in the reactivity series (above Zinc and Iron), yet it does not appear to react with water or dilute acids immediately.
Reason: Aluminium reacts rapidly with oxygen in the air to form a very thin, tough, and unreactive layer of aluminium oxide (Al_2O_3) on its surface.
- This layer is impermeable and adheres strongly to the metal.
- It prevents water or acid from reaching the underlying pure aluminium metal.
- Therefore, the reaction stops almost immediately after the layer forms.
How to make Aluminium react:
- Scrape/Sandpaper the oxide layer off before use.
- Use ammonia solution (which dissolves the oxide layer) instead of water/acid initially.
- Use mercury(II) chloride to form an amalgam that prevents oxide formation.
Correction: Aluminium is reactive (high in the series). It only appears unreactive due to the protective oxide layer. If the layer is removed, it reacts vigorously with acids and steam.
Correction: Copper is below Hydrogen in the reactivity series. It cannot displace hydrogen from acids. No reaction occurs.
Correction: Magnesium reacts very slowly with cold water (barely visible bubbles). It requires steam to react visibly and form magnesium oxide.
When to use: When asked to write a symbol equation for a displacement reaction (e.g., Zinc + Silver Nitrate).
Why examiners accept this: Examiners look for the correct products and balancing. The key is identifying that the more reactive metal takes the anion's place.
Correct Usage Example:
- Question: Write the equation for Zinc reacting with Silver Nitrate.
- Answer: Zn + 2AgNO_3 \rightarrow Zn(NO_3)_2 + 2Ag
- Tip: Ensure you know that Zinc forms a +2 ion (Zn^{2+}) and Nitrate is NO_3^-. The product is Zinc Nitrate, not Zinc Silver.
When to use: When explaining why Aluminium does not react with water or acid in an experiment.
Why examiners accept this: Examiners specifically look for the phrase 'oxide layer' or 'protective coating'. Simply saying 'it is unreactive' is incorrect because Aluminium is reactive; the layer just hides it.
Correct Usage Example:
- Question: Explain why aluminium foil does not dissolve in dilute HCl immediately.
- Answer: 'Aluminium has a protective layer of aluminium oxide on its surface which prevents the acid from reaching the underlying metal.'