The reactivity series

AQA·Chemistry (Combined Science)·5.4.1.2·Trilogy — Foundation

Determining the reactivity series from reactions with water and acids

Metals differ in how readily they react. By comparing their reactions with water or dilute acids at room temperature, metals can be arranged in a reactivity series, from most reactive to least reactive.

A more reactive metal generally reacts more readily and vigorously. A less reactive metal reacts more slowly or may show no observable reaction under the same conditions. Useful observations include whether bubbles appear, how quickly they form and how vigorous the reaction is.

The reactivity series

The following series places the named metals in order. Carbon and hydrogen are included as reference points; neither is a metal.

Most reactive

  • Potassium
  • Sodium
  • Lithium
  • Calcium
  • Magnesium
  • Carbon
  • Zinc
  • Iron
  • Hydrogen
  • Copper

Least reactive

Comparing reactions with water

At room temperature, potassium, sodium and lithium react vigorously with water. Calcium also reacts readily, but less vigorously than lithium. Magnesium reacts only very slowly with cold water, while zinc, iron and copper show no significant reaction under these conditions.

The differences can be used to order metals that react with water. For example, sodium reacts more vigorously than lithium, so the observations place sodium above lithium. A lack of visible reaction with cold water does not, by itself, show that a metal is below every metal that also appears unreactive.

Comparing reactions with dilute acids

Metals above hydrogen in the series generally react with dilute hydrochloric acid or dilute sulfuric acid to produce a salt and hydrogen gas. Bubbles of hydrogen provide visible evidence of the reaction. Copper is below hydrogen and does not react with these dilute acids at room temperature.

Magnesium reacts more rapidly with dilute acid than zinc, and zinc reacts more rapidly than iron. These observations place magnesium above zinc and zinc above iron. The very reactive metals potassium, sodium and lithium react dangerously with dilute acids, so they should not be tested this way.

Hydrogen provides a reference for whether a metal reacts with these dilute acids. Carbon is also a reference point in the series, but its position is not determined by comparing these reactions with water and dilute acids.

Using experimental evidence to order metals

To make a fair comparison, use the same acid concentration and temperature, and metal samples with similar exposed surface areas. Observe the reactions over the same period. Differences in surface condition, such as a coating on a metal, can also affect the observed rate.

For example, if metal A produces bubbles rapidly in dilute acid, metal B produces them slowly, and metal C shows no observable reaction, the evidence suggests the order A, then B, then C, from most to least reactive. Further tests may be needed to distinguish metals that show no observable reaction in the same test.

Metal reactivity and positive ion formation

When a metal reacts, its atoms lose electrons and become positive ions. An atom is electrically neutral, but after losing electrons it has more positive charge than negative charge. The electrons lost by the metal are transferred to another substance in the reaction.

A metal that forms positive ions more easily is generally more reactive. This helps explain why some metals react very readily with water or dilute acids, while others react slowly or not at all under the same conditions.

The reactivity series arranges metals from more reactive to less reactive. A metal higher in the series generally loses electrons and forms positive ions more easily than one lower in the series. The order therefore reflects a difference in the tendency of metal atoms to form positive ions, not just a list of observed reactions.

Displacement and relative reactivity

A more reactive metal can take the place of a less reactive metal in one of its compounds. This is called a displacement reaction: the more reactive metal forms a compound, while the less reactive metal is released.

The reaction happens because the more reactive metal forms positive ions more readily. Its atoms become positive ions in the compound, while positive ions of the less reactive metal become metal atoms.

Displacement of copper by zinc

When zinc is added to copper sulfate solution, zinc displaces copper:

Zn(s)+CuSOX4(aq)→ZnSOX4(aq)+Cu(s)\ce{Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)}

Zinc atoms form positive ions in zinc sulfate, and copper ions become copper metal. A reddish-brown copper deposit appears, and the blue copper sulfate solution becomes paler as copper ions are removed from it.

Using displacement to compare reactivity

A displacement reaction provides evidence about the metals’ positions in the reactivity series. Because zinc displaces copper, zinc is more reactive than copper and is placed above it in the series.

The reverse does not happen: copper cannot displace zinc from zinc sulfate because copper is less reactive than zinc. For two different metals tested under suitable conditions, displacement shows that the added metal is more reactive; no displacement indicates that it is less reactive.

Deducing a reactivity order from experimental results

A reactivity order can be worked out by comparing how substances behave in the same test under comparable conditions. A substance that reacts more readily or more vigorously provides evidence that it is more reactive than one that reacts less readily or less vigorously.

For a fair comparison, conditions that affect the observations should be kept similar. For example, when comparing metals in the same acid, the acid concentration, temperature and amount and surface area of each metal should be comparable.

Combining the comparisons

Suppose three metals are tested with the same acid under comparable conditions:

  • Metal A produces bubbles rapidly.
  • Metal B produces bubbles more slowly.
  • Metal C shows no visible reaction during the test.

A reacts more vigorously than B, so the result places A above B. B reacts while C shows no visible reaction, so the result places B above C. Combining the comparisons gives the experimental order:

A > B > C, where $$>$$ means “is more reactive than”.

A lack of visible reaction places C below a substance that does react in that test; it does not prove that C can never react. If two substances both show no visible reaction, that result alone does not establish which is more reactive.

The order should include only substances that can be placed using the evidence given. Do not assume positions for other substances from a full reactivity series when they were not tested.

Reactions of metals with water

Some metals react with water at room temperature to form a metal hydroxide and hydrogen gas. The hydrogen may be visible as bubbles. How quickly and vigorously a metal reacts provides evidence for comparing its reactivity with other metals.

Comparing reactions with cold water

  • Potassium reacts most vigorously of the three metals potassium, sodium and lithium. It may ignite with a lilac flame.
  • Sodium reacts rapidly. The heat released typically melts it into a ball that moves over the water’s surface.
  • Lithium reacts less vigorously than sodium and fizzes more steadily.
  • Calcium reacts less vigorously than lithium, forming calcium hydroxide and hydrogen.
  • Magnesium reacts very slowly, forming magnesium hydroxide and hydrogen.
  • Zinc, iron and copper show no significant reaction with water at room temperature.

These observations give the order potassium > sodium > lithium > calcium > magnesium for reactions with cold water. The lack of a significant reaction from zinc, iron and copper under these conditions does not establish their order relative to one another.

Equations for reactions with water

For example, sodium reacts with water to form sodium hydroxide and hydrogen:

2 Na+2 HX2O→2 NaOH+HX2\ce{2Na + 2H2O -> 2NaOH + H2}

Calcium forms calcium hydroxide and hydrogen:

Ca+2 HX2O→Ca(OH)X2+HX2\ce{Ca + 2H2O -> Ca(OH)2 + H2}

Potassium and lithium similarly form their own hydroxides and hydrogen. The product formed from the metal therefore depends on which metal reacts.

Reactions of metals with dilute acids

At room temperature, a reactive metal can react with a dilute acid to form a salt and hydrogen gas. The hydrogen is usually seen as bubbles. The metal forms positive ions, while hydrogen ions from the acid form hydrogen gas.

The salt depends on both the metal and the acid. For example, magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen:

Mg+2 HCl→MgClX2+HX2\ce{Mg + 2HCl -> MgCl2 + H2}

With dilute sulfuric acid, magnesium instead forms magnesium sulfate and hydrogen. In each case, the metal determines which metal ion is in the salt; the acid determines whether the salt is a chloride or a sulfate.

Comparing metal reactions with dilute acids

Metals react at different rates. Calcium and magnesium react readily with dilute acids. Zinc reacts less rapidly, and iron reacts more slowly. Copper does not react with dilute hydrochloric or dilute sulfuric acid.

Potassium, sodium and lithium react so violently with dilute acids that these reactions are not normally performed as routine tests. Comparisons with water help establish the order of these most reactive metals.

The usual order of the named metals, from most to least reactive, is:

Potassium → sodium → lithium → calcium → magnesium → zinc → iron → copper