MYP Chemistry 5.11 Reactivity Series Notes

Building the Reactivity Series from Observations

1. We can compare how different metals react with water, acids and oxygen.
2. From these observations, we can arrange metals from most reactive to least reactive.
3. This list is called the reactivity series.

Metal Reactions with Water

Metals show very different behaviours with water:

1. Very reactive metals (e.g. potassium, sodium, lithium, calcium):
   1. General equation (for Group 1 metals): $$2 \mathrm{M}(\mathrm{~s})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{MOH}(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g})$$
   2. React vigorously with cold water.
   3. Produce hydrogen gas and a metal hydroxide.
   4. Often produce heat and may ignite the hydrogen.
2. Moderately reactive metals (e.g. magnesium, zinc, iron):
   1. Little or no reaction with cold water.
   2. React with steam to form a metal oxide and hydrogen gas.
3. Unreactive metals (e.g. copper, silver, gold):
   1. Do not react with water or steam.

Metal Reactions with Acids

1. Metals can also be compared by how they react with dilute acids like HCl or H₂SO₄.
2. Metals above hydrogen in the reactivity series react with dilute acids to produce salt and hydrogen gas.
3. General equation: $$\text { Metal }+ \text { Acid } \rightarrow \text { Salt }+\mathrm{H}_2(\mathrm{~g})$$
4. Metals below hydrogen (e.g. copper, silver, gold) do not react with dilute acids at room temperature.

Metal Reactions with Oxygen

Most metals react with oxygen to form metal oxides, but at very different rates.

1. Highly reactive metals (e.g. potassium, sodium):
   1. React rapidly with oxygen in air.
   2. May need to be stored under oil to stop them reacting.
2. Moderately reactive metals (e.g. magnesium, iron):
   1. Burn in air when heated strongly, forming a metal oxide.
   2. Iron slowly reacts with oxygen and water over time → rust (iron oxide).
3. Unreactive metals (e.g. copper, silver, gold, platinum):
   1. React only very slowly or hardly at all.
   2. Gold and platinum are described as noble metals because they resist oxidation.

Constructing the Reactivity Series

1. By comparing all these reactions, we can order metals from most reactive to least reactive.
2. A simplified reactivity series (with hydrogen included for reference) is illustrated below:

Displacement Reactions and the Reactivity Series

What Is a Displacement Reaction?

Rule: A metal higher in the reactivity series will displace a metal lower in the series from its compound. A metal lower in the series cannot displace a more reactive metal.

General pattern: $$\text { More reactive metal + metal salt solution → new metal salt + less reactive metal }$$

Real-World Uses of the Reactivity Series

The reactivity series is not just a school concept; it helps us understand and solve real problems in the world around us.

Corrosion and Rusting

1. Iron is relatively reactive → it rusts easily to form hydrated iron(III) oxide.
2. Zinc is more reactive than iron → it can be used to protect iron.
3. Gold and platinum are very unreactive → they do not corrode easily.

Preventing Corrosion

We can use the reactivity series to slow down corrosion:

1. Protective coatings – paint, oil, plastic, or plating create a barrier between metal and environment.
2. Galvanisation – coating iron or steel with zinc.
   1. Zinc is more reactive than iron.
   2. If the coating is scratched, the zinc acts as a sacrificial metal, reacting first and protecting the iron.
3. Sacrificial anodes – blocks of a more reactive metal (e.g. zinc or magnesium) attached to steel structures such as pipelines or ship hulls.

Metal Extraction from Ores

Many metals are found in nature as compounds in ores, not as pure metals. The reactivity series helps decide how to extract them.

1. Very reactive metals (e.g. K, Na, Ca, Mg, Al):
   1. Are high in the series.
   2. Their compounds are very stable.
   3. Extracted by electrolysis of molten salts (e.g. aluminium from Al₂O₃).
2. Moderately reactive metals (e.g. Zn, Fe):
   1. Extracted by chemical reduction, often using carbon or carbon monoxide.
   2. Carbon is placed above zinc and iron in the series, so it can remove oxygen from their oxides.
3. Unreactive metals (e.g. Au, Ag):
   1. Low in the reactivity series.
   2. Can often be found in their native (elemental) form.
   3. Require little or no chemical extraction.

Other Real-World Applications

1. Galvanisation and construction
   1. Steel bridges, ships and cars are often protected by zinc coatings.
   2. The reactivity series predicts that zinc will corrode instead of the iron.
2. Batteries and electrochemical cells
   1. In a simple cell, the metal higher in the reactivity series acts as the negative electrode (anode) and tends to lose electrons more easily.
   2. The potential difference (voltage) depends on the difference in reactivity between the two metals used.
3. Jewellery and electronics
   1. Gold and silver are at the bottom of the reactivity series and resist corrosion.
   2. This makes them ideal for jewellery and for electrical contacts in electronics (where corrosion would cause poor connections).