Trends in Properties of Elements in the Periodic Table
Senior 2 · Chemistry
Trends in Properties of Elements in the Periodic Table
Lesson 3 of 4
25 min
An iron roofing sheet left unpainted turns brown with rust within a few rainy seasons, yet an aluminium saucepan can be used for years without corroding, even though aluminium is the more reactive metal. Charcoal burns in a stove to give an invisible gas, while burning sulfur gives a choking gas that helps form acid rain. Farmers spread lime on acidic soils, but never sulfur.
All of these facts come from the chemical properties of metals and non-metals: the way they react with oxygen, water and acids. A chemical property is one that can be observed only when the substance changes into a new substance.
By the end of this lesson you will be able to describe and write equations for these reactions, use the reactivity series, and compare the oxides of metals and non-metals.
Most metals react with oxygen to form metal oxides.
2Mg(s) + O2(g) → 2MgO(s)
Magnesium ribbon burns with a dazzling white flame and leaves a white powder, magnesium oxide. Copper does not burn, but when heated in air it becomes coated with black copper(II) oxide:
2Cu(s) + O2(g) → 2CuO(s)
Iron slowly reacts with oxygen and water to form rust, a hydrated iron(III) oxide. Leaving out the water that becomes part of the rust, the overall change is:
4Fe(s) + 3O2(g) → 2Fe2O3(s)
Metal oxides are basic oxides: they react with acids to form a salt and water. Those that dissolve in water give alkaline solutions, which turn red litmus blue:
Na2O(s) + H2O(l) → 2NaOH(aq)
CaO(s) + H2O(l) → Ca(OH)2(aq)
Why aluminium does not seem to corrode. Aluminium reacts with air immediately, but the thin layer of aluminium oxide (Al2O3) that forms is tough and unreactive. It sticks to the surface and stops oxygen and water reaching the metal underneath. Protecting a metal with its own oxide layer like this is called passivation. Rust, by contrast, is flaky and falls off, exposing fresh iron to attack.
Metals above hydrogen in the reactivity series react with dilute hydrochloric acid or dilute sulfuric acid to form a salt and hydrogen:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
The test for hydrogen: hold a lighted splint at the mouth of the test tube. Hydrogen burns with a squeaky pop. Copper, silver and gold, which lie below hydrogen, do not react with dilute hydrochloric or sulfuric acid.
Putting all these observations together gives the reactivity series, a list of metals from the most to the least reactive. Hydrogen is included as a reference point.
| Metal | Cold water | Steam | Dilute acids |
|---|---|---|---|
| Potassium (K) | Violent | Too dangerous to try | Too dangerous to try |
| Sodium (Na) | Vigorous | Too dangerous to try | Too dangerous to try |
| Calcium (Ca) | Steady fizzing | Vigorous | Vigorous |
| Magnesium (Mg) | Very slow | Burns brightly | Rapid fizzing |
| Aluminium (Al) | None (oxide layer) | Slow once the layer is broken | Slow at first, then steady |
| Zinc (Zn) | None | Reacts when heated | Steady fizzing |
| Iron (Fe) | Rusts very slowly | Reacts when red-hot | Slow fizzing |
| Lead (Pb) | None | None | Very slow |
| Hydrogen (H) | reference point | ||
| Copper (Cu) | None | None | None |
| Silver (Ag) | None | None | None |
| Gold (Au) | None | None | None |
Many non-metals burn in oxygen to form non-metal oxides:
C(s) + O2(g) → CO2(g)
S(s) + O2(g) → SO2(g)
Most non-metal oxides are acidic oxides: they dissolve in water to give acidic solutions, which turn blue litmus red. Carbon dioxide gives weak carbonic acid, and sulfur dioxide gives sulfurous acid, one of the causes of acid rain:
CO2(g) + H2O(l) ⇌ H2CO3(aq)
SO2(g) + H2O(l) → H2SO3(aq)
A few non-metal oxides, such as carbon monoxide (CO) and water (H2O), are neutral: they are neither acidic nor basic.
Metal M does not react with cold water. When heated in steam it gives a white solid and a gas that burns with a squeaky pop. It fizzes steadily in dilute hydrochloric acid. Where does M belong in the reactivity series, and what could it be?
Solution. M is below calcium (no reaction with cold water) but above hydrogen (it releases hydrogen from steam and from acid). It belongs in the magnesium-to-iron region. A metal whose oxide is white and which reacts with steam only when heated is zinc. Zn(s) + H2O(g) → ZnO(s) + H2(g)
Tip
Metals make bases; non-metals make acids. A metal oxide is basic (alkaline if it dissolves); a non-metal oxide is usually acidic. A soluble oxide that turns red litmus blue came from a metal.
Remembering the series: learn it as two parts either side of hydrogen: K Na Ca Mg Al Zn Fe Pb | (H) | Cu Ag Au. Every metal before (H) gives hydrogen with dilute acids; the three after it do not.
Try each item before you read the solution.
Write a balanced equation for calcium burning in oxygen, and state whether the product is an acidic or a basic oxide.
Solution: 2Ca(s) + O2(g) → 2CaO(s). Calcium is a metal, so calcium oxide is a basic oxide; it dissolves in water to give alkaline calcium hydroxide.
Mugisha adds a piece of copper to dilute hydrochloric acid and a piece of zinc to the same acid. Describe what he sees and explain.
Solution: The zinc fizzes and slowly dissolves, giving off hydrogen: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g). Nothing happens to the copper, because copper lies below hydrogen in the reactivity series.
Why is aluminium, a reactive metal, used for window frames that stand in the rain for years?
Solution: A thin, tough layer of aluminium oxide forms on the surface and sticks to it. This layer stops air and water reaching the metal underneath (passivation), so the frame does not corrode further.
Sulfur is burned in a gas jar of oxygen, and a little water and some blue litmus solution are then added. Predict the colour of the litmus and write the two equations.
Solution: The litmus turns red. S(s) + O2(g) → SO2(g), then SO2(g) + H2O(l) → H2SO3(aq). Sulfur is a non-metal, so its oxide is acidic.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
