Unit 9: The Reactivity Series of Metals
Ranking metals by their chemical reactivity to predict reactions and understand their behavior.
9.14 Overview & Ranking of Metal Reactivity
As we've seen, metals vary widely in their chemical reactivity. The reactivity series is an arrangement of metals (and some non-metals like hydrogen and carbon) in order of their reactivity, from most reactive at the top to least reactive at the bottom. This series is a powerful predictive tool in chemistry.
A simplified reactivity series is shown below:
| Element | Symbol | Reactivity |
|---|---|---|
| Potassium | K | React with cold water |
| Sodium | Na | |
| Calcium | Ca | |
| Magnesium | Mg | |
| Aluminum | Al | React with acids |
| (C) | ||
| Zinc | Zn | |
| Iron | Fe | |
| Lead | Pb | |
| (H) | Do not react with dilute acids | |
| Copper | Cu | |
| Silver | Ag | |
| Gold | Au | Highly unreactive |
The position of a metal in this series determines its general reactions:
- Metals above hydrogen will react with dilute acids to produce hydrogen gas.
- Metals below hydrogen will not react with dilute acids.
- The most reactive metals (at the top) will react vigorously with water and oxygen.
- The least reactive metals (at the bottom) show little to no reaction with water or oxygen.
Solved Examples:
-
Based on the reactivity series, which metal would react most vigorously with
dilute hydrochloric acid: Zinc, Iron, or Magnesium?
Solution: Magnesium (Mg) is the highest of the three in the reactivity series, so it would react the most vigorously. -
A metal does not react with cold water but does react with steam. It also
reacts with dilute acids. Where would you place it in the reactivity
series?
Solution: It would be placed below Calcium but above Hydrogen. This describes metals like Magnesium, Aluminum, Zinc, and Iron.
9.15 Metal Displacement Reactions
One of the most important applications of the reactivity series is predicting displacement reactions. A more reactive metal will displace a less reactive metal from a solution of its salt. This is a type of redox reaction where the more reactive metal is oxidized (loses electrons) and the less reactive metal ion is reduced (gains electrons).
General Rule: Metal A + Salt of Metal B → Salt of Metal A + Metal B (This only happens if A is more reactive than B).
Example: Zinc and Copper Sulfate
If a piece of zinc metal is placed in a blue solution of copper(II) sulfate, the following occurs:
- Zinc is higher in the reactivity series than copper, so it will displace the copper.
- The blue color of the solution fades as the copper(II) ions ($Cu^{2+}$) are removed.
- A reddish-brown solid (copper metal) coats the surface of the zinc.
Solved Examples:
-
Will a reaction occur if a piece of copper is placed in a solution of silver
nitrate ($AgNO_3$)? If so, write the equation.
Solution: Yes, a reaction will occur because copper is more reactive than silver. Copper will displace the silver.
$Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)$ -
Will a reaction occur if an iron nail is placed in a solution of magnesium
sulfate ($MgSO_4$)? Explain why.
Solution: No, a reaction will not occur. Iron is less reactive than magnesium, so it cannot displace magnesium from its salt solution. -
The Thermite reaction involves reacting aluminum powder with iron(III) oxide
to produce molten iron. Write the equation and explain why it
works.
Solution: $2Al(s) + Fe_2O_3(s) \rightarrow Al_2O_3(s) + 2Fe(l)$. The reaction works because aluminum is more reactive than iron and can displace it from its oxide. The reaction is extremely exothermic, producing enough heat to melt the iron.
9.16 Reactivity & Standard Electrode Potentials
The reactivity series is not just an empirical observation; it has a quantitative basis in standard electrode potentials ($E^\circ$). The standard electrode potential measures the tendency of a substance to be reduced (gain electrons) under standard conditions.
For metals, we look at the potential for the reduction half-reaction $M^{n+}(aq) + ne^- \rightleftharpoons M(s)$.
The key principle is: The more negative the $E^\circ$ value, the more easily the metal is oxidized, and therefore the more reactive it is.
| Half-Reaction | $E^\circ$ / V | Reactivity |
|---|---|---|
| $Mg^{2+}(aq) + 2e^- \rightarrow Mg(s)$ | -2.37 | High (More Reactive) |
| $Al^{3+}(aq) + 3e^- \rightarrow Al(s)$ | -1.66 | |
| $Zn^{2+}(aq) + 2e^- \rightarrow Zn(s)$ | -0.76 | |
| $Fe^{2+}(aq) + 2e^- \rightarrow Fe(s)$ | -0.44 | Medium |
| $Pb^{2+}(aq) + 2e^- \rightarrow Pb(s)$ | -0.13 | |
| $2H^+(aq) + 2e^- \rightarrow H_2(g)$ | 0.00 | (Reference) |
| $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$ | +0.34 | Low (Less Reactive) |
| $Ag^+(aq) + e^- \rightarrow Ag(s)$ | +0.80 |
A metal with a more negative $E^\circ$ will displace a metal with a less negative (or positive) $E^\circ$ from solution. For example, since Zn (-0.76 V) has a more negative $E^\circ$ than Cu (+0.34 V), zinc will displace copper.
Solved Examples:
-
Using the table, predict whether lead (Pb) can displace zinc (Zn) from a
solution of zinc nitrate.
Solution: No. The $E^\circ$ for Pb (-0.13 V) is less negative (more positive) than the $E^\circ$ for Zn (-0.76 V). This means lead is less reactive than zinc and cannot displace it. -
Which is the stronger reducing agent: Magnesium or Iron? Explain using
$E^\circ$ values.
Solution: Magnesium is the stronger reducing agent. A reducing agent gets oxidized. The more negative the $E^\circ$ value, the greater the tendency to be oxidized. Magnesium's $E^\circ$ (-2.37 V) is much more negative than iron's (-0.44 V). -
Can iron (Fe) displace hydrogen from an acid? Use $E^\circ$ values to
justify your answer.
Solution: Yes. The $E^\circ$ for Fe (-0.44 V) is more negative than the $E^\circ$ for Hâ‚‚ (0.00 V). This indicates iron is more reactive than hydrogen and can displace it from an acid. -
A student wants to plate a layer of silver onto an iron object. They place
the iron object into a solution of silver nitrate. Will this work?
Solution: Yes, it will work spontaneously. Iron ($E^\circ$ = -0.44 V) is much more reactive than silver ($E^\circ$ = +0.80 V). The iron will displace the silver from the solution, causing a layer of solid silver to deposit on the iron object. -
Arrange Mg, Fe, and Cu in order of decreasing reactivity based on their
$E^\circ$ values.
Solution: The most negative $E^\circ$ is the most reactive. Therefore, the order of decreasing reactivity is Mg (-2.37 V) > Fe (-0.44 V) > Cu (+0.34 V).