Metal + Oxygen Reactions Chemistry Tutorial

Key Concepts

All metals react with oxygen except silver (Ag_(s)), platinum (Pt_(s)) and gold (Au_(s)).
In general, metals react with oxygen to form metal oxides.

metal + oxygen → metal oxide

Group 1 metals, except lithium, produce compounds containing more oxygen:

Sodium produces sodium peroxide (Na₂O₂)

Potassium, rubidium and cesium produce metal superoxides (MO₂)

In general, metal oxides are ionic compounds^#.

Metal atoms, M, lose electrons, ne^-, to form positive ions (cations), Mⁿ⁺:
M_(s) → Mⁿ⁺ + ne^-

Oxygen gas, O_2(g), is made up of 2 oxygen atoms, O.
Oxygen atoms, O, gain 2 electrons, 2e^-, to form negative ions (oxide anions), O^2-:

O + 2e^- → O^2-

Mⁿ⁺ are attracted to O^2- because they have opposite charges. This is called an ionic bond.
A compound containing only ionic bonds is called an ionic compound.

The ease with which a metal reacts is known as its activity.

A more active metal will react more readily with oxygen than a less active metal.

In general, Group 1 (IA or alkali) metals and Group 2 (IIA or alkaline earth) metals are more active than transition metals.

Silver, gold and platinum are not active metals. They are known as noble metals^## (or as inactive metals).

Reactions of Some Common Metals with Oxygen Gas

Possible Reaction					Reaction Conditions	Activity
metal	+	oxygen	→	metal oxide	Reaction Conditions	Activity
K_(s)	+	O_2(g)	→	KO_2(s)	Rapid reaction at room temperature.	most active
Ba_(s)	+	½O_2(g)	→	BaO_(s)	Rapid reaction at room temperature.	↓
Ca_(s)	+	½O_2(g)	→	CaO_(s)	Rapid reaction at room temperature.	↓
2Na_(s)	+	O_2(g)	→	Na₂O_2(s)	Rapid reaction at room temperature.	↓
Mg_(s)	+	½O_2(g)	→	MgO_(s)	Slow reaction at room temperature, vigorous reaction if heated.	↓
2Al_(s)	+	³/₂O_2(g)	→	Al₂O_3(s)	Slow reaction at room temperature, vigorous reaction if heated.	↓
Zn_(s)	+	½O_2(g)	→	ZnO_(s)	Slow reaction at room temperature, vigorous reaction if heated.	↓
Fe_(s)	+	½O_2(g)	→	FeO_(s)*	Slow reaction at room temperature, vigorous reaction if heated.	↓
Sn_(s)	+	½O_2(g)	→	SnO_(s)*	Slow reaction only occurs if heated.	↓
Pb_(s)	+	½O_2(g)	→	PbO_(s)*	Slow reaction only occurs if heated.	↓
Cu_(s)	+	½O_2(g)	→	CuO_(s)*	Slow reaction only occurs if heated.	↓
2Ag_(s)	+	½O_2(g)	→	Ag₂O	No reaction even if heated.	↓
2Au_(s)	+	½O_2(g)	→	Au₂O*	No reaction even if heated.	least active

* compounds with other formulae are possible.

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Example 1

Question: Rubidium metal reacts rapidly with oxygen gas from the atmosphere at room temperature.
Is rubidium a very active metal, a moderately active metal or a noble (inactive) metal?

Response: The more active a metal is, the more readily it reacts with oxygen.
Rubidium, like potassium, reacts rapidly with O_2(g), so rubidium, like potassium, is a very active metal.

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Example 2

Question: A student has been given a sample of an unknown silvery metal to identify.
It is thought that the sample of the metal is either magnesium, silver, or tin.
The student places a small, clean, piece of the sample in a test tube and then performs each of the experiments below and tabulates the results as observations:

	experiment	observations
(a)	sample left for 1 hour exposed to the atmosphere	no changes observed
(b)	sample heated gently over a bunsen burner for 15 minutes	the surface of the metal appears less shiny

Which metal is in the unknown sample?

Response: Place the possible metals in order of their activity and note the reaction you would expect:

activity	metal	reaction with O_2(g) in air
most active	magnesium	vigorous reaction on heating
↓	tin	slow reaction only if heated
least active	silver	no reaction even if heated

Locate the position of the unknown sample by comparing its observed to reaction to the reaction expected by each possible metal:

activity	metal	reaction with O_2(g) in air	comparison with unknown sample	conclusion
most active	magnesium	vigorous reaction on heating	sample did not react vigorously on heating	not Mg
↓	tin	slow reaction only if heated	sample reacted slowly on heating	possibly Sn
least active	silver	no reaction even if heated	sample reacted slowly on heating	not Ag

The reaction of the unknown sample with oxygen is most similar to the reaction expected using tin (Sn).
The unknown sample is most likely to be tin.

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Example 3

Question: Copper, magnesium and zinc are all good conductors of electricity as well as being ductile and malleable, so it is possible to use any of these as electrical wires in your house.
Why would it be more common to use copper, rather than magnesium or zinc, as electrical wires?

Response: Place each of these metals in order of their activity with oxygen gas from the atmosphere:

activity	metal	reaction with O_2(g) in air
most active	magnesium	Slow reaction at room temperature, vigorous if heated.
↓	zinc	Slow reaction at room temperature, vigorous if heated.
least active	copper	Slow reaction only if heated

Consider the most likely product of any reaction that occurs at room temperature:

activity	metal	reaction with O_2(g) in air	nature of product	electrical conductivity of product
most active	magnesium	Mg_(s) + ½O_2(g) → MgO_(s)	MgO_(s) ionic solid	non-conducting solid
↓	zinc	Zn_(s) + ½O_2(g) → ZnO_(s)	ZnO_(s) ionic solid	non-conducting solid
least active	copper	no reaction

Copper is more commonly used for electrical wires because it is less active than either magnesium or zinc so a copper wire will be useful for longer.
As magnesium and zinc react with oxygen gas in the air, they form ionic solids which do not conduct electricity, which means that as they react the electrical wires would cease to conduct electricity!
(If the oxides dissolve in water from the atmosphere, yes, the solution would conduct electricity, but it would also be free to drip off the wires!).

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^#Many transition metals oxides show considerable covalent character as does the Al-O bond..

^##Other Noble Metals are; mercury, ruthenium, rhodium, palladium, osmium and iridium.
As you might expect, these noble metals all lie within the same small area of the periodic table!

Metal + Oxygen Reactions Chemistry Tutorial

Key Concepts

Reactions of Some Common Metals with Oxygen Gas

Possible Reaction

Reaction Conditions

Activity

metal

+

oxygen

→

metal oxide

Example 1

Example 2

Example 3