Reactivity measures how readily a metal forms positive ions
Definition
Reactivity series
A list of metals arranged in order of how readily they react, from the most reactive at the top to the least reactive at the bottom.
Definition
Cation
A positively charged ion, formed when an atom or group of atoms loses electrons.
- Metals are placed in order of how readily they react, with the most reactive at the top.
- The order used in this course runs potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, (hydrogen), copper, silver, gold.
- A metal reacts by losing electrons from its atoms, which leaves positively charged ions behind.
- The more readily a metal's atoms form cations, the higher that metal sits in the order.
- Three kinds of evidence fix a metal's place: its reaction with water, with dilute acid, and with salt solutions.
Key Idea
Position in the series is a summary of what metals are observed to do, not a number read from the periodic table.
Reactions with cold water separate the top of the series
- Potassium reacts violently with cold water and the hydrogen given off burns with a lilac flame.
- Sodium reacts vigorously, melting into a ball that skims across the surface.
- Calcium reacts steadily, giving a stream of bubbles and a cloudy suspension of calcium hydroxide.
- Each of these three metals gives a hydroxide and hydrogen: metal+water→metal hydroxide+hydrogen\text{metal} + \text{water} \rightarrow \text{metal hydroxide} + \text{hydrogen}metal+water→metal hydroxide+hydrogen
- Magnesium barely reacts with cold water, but reacts readily with steam to give magnesium oxide and hydrogen.
- Zinc, iron and copper show no reaction with cold water at all.
Note
- Calcium hydroxide is only slightly soluble, so the mixture turns cloudy rather than staying clear.
- With steam the product is an oxide, because no liquid water is left to form a hydroxide.
Reactions with dilute acid rank the metals below calcium
- A metal above hydrogen in the order reacts with dilute acid: metal+acid→salt+hydrogen\text{metal} + \text{acid} \rightarrow \text{salt} + \text{hydrogen}metal+acid→salt+hydrogen
- Magnesium gives rapid, steady bubbling with dilute hydrochloric acid.
- Zinc bubbles more slowly than magnesium, and iron more slowly still.
- Copper, silver and gold give no reaction with dilute acid, which places them below hydrogen.
- Since the products are the same in every case, it is the rate of bubbling that ranks the metals.
Common Mistake
Potassium, sodium and calcium are never added to dilute acid in a school laboratory, because the reaction is dangerously violent.
Displacement in salt solutions compares two metals directly
Definition
Displacement reaction
A reaction in which a more reactive metal takes the place of a less reactive metal in a compound.
- A more reactive metal takes the place of a less reactive one in a solution of its salt.
- Zinc displaces copper from copper sulfate solution: Zn+CuSO4→ZnSO4+Cu\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}Zn+CuSO4→ZnSO4+Cu
- Copper placed in zinc sulfate solution gives no change, and that one-way answer fixes the order of the pair.
- A colour change in the solution and a coating on the metal are the two observations that show a reaction has happened.
- Testing every metal against every salt solution builds the whole order from pairs.
Example
- Iron in copper sulfate: the blue fades to the pale green of iron(II) sulfate and a pink-brown coating forms on the iron.
- Magnesium in copper sulfate: the blue fades towards colourless, since magnesium sulfate solution has no colour.
- Copper in iron sulfate: nothing happens, because copper is below iron.
Carbon and hydrogen sit in the series as reference points
- Carbon and hydrogen are non-metals, so they are written in brackets within the order.
- Their places come from the same evidence as any metal's: whether they displace a metal, or are displaced by one.
- A metal below carbon can be extracted from its oxide by heating the ore with carbon.
- A metal above hydrogen reacts with dilute acid, and one below hydrogen does not.
- These two markers turn the order into a tool for predicting reactions rather than a list to learn.
Self review
- What does a metal's position in the reactivity series say about its atoms?
- Which three metals react with cold water, and what two products form?
- Why is copper unchanged by dilute hydrochloric acid?
- What is seen when iron is placed in copper sulfate solution?
- Why do carbon and hydrogen appear in a list of metals?
