4.1.3.1,4.1.3.2 - Transition metals and their compounds

Transition-metal physical properties

Transition metals share typical metallic properties but differ markedly from the soft, light and low-melting Group 1 metals.

The named examples are chromium $\ce{Cr}$, manganese $\ce{Mn}$, iron $\ce{Fe}$, cobalt $\ce{Co}$, nickel $\ce{Ni}$ and copper $\ce{Cu}$, found in the middle block of the periodic table. They conduct electricity and heat and have shiny surfaces when clean. Do not apply the main-group number shortcut to deduce all their ion charges.

Compared with lithium, sodium and potassium, these metals generally have higher melting points, greater densities, greater hardness and greater strength. Density is mass per unit volume; it does not mean simply that a large piece is heavy. Hardness describes resistance to scratching or indentation, while strength concerns resisting deformation or breaking under a force. These are different properties, even if they often increase together in this comparison.

These are general comparisons with Group 1, not a claim that the six metals have identical values. A material's use needs the right property: electrical conductivity and ductility explain copper wire, while strength and a high melting point matter for structural applications. Alloys can change the properties of the pure metals, so a steel property is not automatically the same as a pure iron property.

Reactivity compared with Group 1

The named transition metals are generally less reactive than the Group 1 metals with oxygen, water and halogens.

Lithium, sodium and potassium react readily with cold water, producing hydroxides and hydrogen, and rapidly tarnish in air. Iron and copper do not show that vigorous cold-water reaction. Iron can rust slowly in moist air, whereas a fresh sodium surface changes much more quickly. A slow corrosion process is still a chemical reaction; “less reactive” does not mean incapable of reacting.

Transition metals can react with oxygen when heated. Iron forms iron oxides and heated copper forms black copper(II) oxide, $\ce{CuO}$. Chromium can develop a protective oxide layer that limits further attack. Manganese, cobalt and nickel also form oxides under suitable conditions. The products and rates depend on the metal, temperature and surface condition. An oxide-coated piece may appear less reactive because the coating restricts contact with reactants.

With chlorine and other halogens, transition metals can form metal halides, but often need stronger heating or otherwise react less readily than alkali metals. Heated iron can form iron(III) chloride, $\ce{FeCl3}$, and copper can form copper(II) chloride, $\ce{CuCl2}$. In contrast, sodium reacts vigorously to form $\ce{NaCl}$. The chloride formulae also suggest an important difference: transition-metal ions need not all have the single +1 charge of Group 1.

A fair comparison uses comparable metal amount or surface area, temperature and reactant conditions. Comparing powdered iron in a flame with a large sodium piece in cold air would confound metal identity with temperature and surface area. Observations such as slower gas production or needing heating support a comparison under the stated conditions; no single observation gives an exact universal rate for every reaction.

Variable ion charges and coloured compounds

Many transition elements can form ions with different charges and compounds with characteristic colours.

Iron provides a clear charge example: $\ce{Fe^2+}$ and $\ce{Fe^3+}$ are different ions of the same element. The Roman numerals in iron(II) and iron(III) specify the charge of the iron ion in the compound. Thus iron(II) chloride is $\ce{FeCl2}$, while iron(III) chloride is $\ce{FeCl3}$, because chloride has charge −1. The numerals do not count iron atoms, and the ions do not have different proton numbers.

Copper can form $\ce{Cu+}$ and $\ce{Cu^2+}$. Chromium can form $\ce{Cr^2+}$ and $\ce{Cr^3+}$, manganese $\ce{Mn^2+}$ and $\ce{Mn^3+}$, cobalt $\ce{Co^2+}$ and $\ce{Co^3+}$, and nickel $\ce{Ni^2+}$ and $\ce{Ni^3+}$. Which ion forms depends on the compound and conditions.

“Many” matters: not every transition-metal compound is coloured, and one element does not have one universal colour. Charge, the surrounding species and hydration can affect appearance. Cobalt chloride is a useful warning that even a related compound can change colour when water is gained or removed. Colour can support chemical identification, but later qualitative-test lessons show why it should be combined with other evidence.

Transition metals as catalysts

Many transition metals and their compounds speed up chemical reactions while remaining chemically unchanged overall.

An iron catalyst is used in the Haber process, where nitrogen and hydrogen make ammonia. Nickel catalyses hydrogenation, the addition of hydrogen to unsaturated organic compounds. Manganese(IV) oxide, $\ce{MnO2}$, catalyses the decomposition of hydrogen peroxide into water and oxygen:

The manganese compound is a catalyst, not a reactant supplying the oxygen product. It is therefore not consumed in the balanced equation. A faster appearance of oxygen with it present shows a rate effect under comparable conditions; it does not show that a different overall reaction has occurred.

Catalytic usefulness does not mean that every transition-metal compound catalyses every reaction. The material and reaction must be matched, and surface area, temperature and contact with reactants affect performance. This helps explain using suitable metal surfaces or finely divided catalysts rather than choosing a substance only because it is in the transition block.

BASF began industrial ammonia production at Oppau, Germany, in 1913. Its Haber–Bosch process is a real application of an iron catalyst: nitrogen and hydrogen form ammonia at the catalyst surface, while the iron remains available for further reaction. The catalyst is not a source of nitrogen or hydrogen, and it is not part of the ammonia formula. This distinction connects a material's useful catalytic role with the reactants actually consumed.

For an illustrative comparison, identical hydrogen peroxide samples are kept at the same temperature. Adding a small amount of manganese(IV) oxide to one sample produces oxygen more quickly than the sample without it. Recovering the solid unchanged overall supports its catalytic role, while the common water-and-oxygen products show that the overall chemical change is the same.