Transition metal reactions

What are transition metals?

Transition metals are elements located in the d-block of the periodic table. The definition is:

Transition metals are elements with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell

The chemical properties of transition metals can be summarised by looking at a few examples that display typical properties of transition metals.

Vanadium Chemistry

The many colours of transition metals are illustrated by vanadium. Aqueous vanadium ions show four different oxidation states and colours.

The colour of transition metal compounds arises because they absorb certain colours of the visible spectrum. The colour that is seen is made up of the parts of the visible spectrum that aren’t absorbed. For example, a blue compound will absorb all frequencies of the spectrum apart from blue light, which is transmitted. The colours absorbed are complementary to the colour observed

Chromium Chemistry

We can use the oxidation-reduction reactions of chromium to illustrate redox behaviour as another aspect of transition metals properties. If we know the standard electrode potential values between different chromium compounds we can use them to predict whether a redox reaction will take place or not.

For example, using the electrode potentials enables us to determine that the dichromate(VI) ion, Cr₂O₇²⁻ can be reduced to Cr³⁺ and Cr²⁺ ions using zinc in acidic conditions. We can also deduce that the dichromate(VI) ion, Cr₂O₇²⁻ will be produced by the oxidation of Cr³⁺ ions using hydrogen peroxide in alkaline conditions. To find out how, check out these notes on "Chromium Chemistry"

Ions in Aqueous Solution

Transition metal ions in solution can react with aqueous sodium hydroxide and aqueous ammonia and form brightly coloured precipitates. You can find complete tables of all the colours and precipitates in our revision note on "Substitutions with Hydroxide & Ammonia".

An example of how the precipitate forms can be seen in the formation of iron(II)hydroxide:

[Fe(H₂O)₆]²⁺ (aq) + 2OH^– (aq) → [Fe(H₂O)₄(OH)₂] (s) + 2H₂O (l) 

However, some of these precipitates will dissolve in an excess of sodium hydroxide or ammonia to form complex ions in solution.

[Cu(H₂O)₄(OH)₂] (s) + 4NH₃ (aq) → [Cu(NH₃)₄(H₂O)₂]²⁺ (aq) + 2H₂O (l) + 2OH^– (aq)

Catalysts

Catalysts can be homogeneous or heterogeneous. Heterogeneous means a substance is in a different physical state (phase) from the reactants. Heterogeneous catalysts are usually solids whereas the reactants are gaseous or in solution

The chemical reaction occurs at active sites on the surface of the catalyst.

Catalytic converters are used in car exhaust boxes to reduce air pollution. They usually consist of a mixture of finely divided platinum and rhodium supported on a ceramic base.

• More information: "Catalysts"

Homogeneous Catalysis

A homogeneous catalyst is in the same physical state (phase) as the reactants. This means they are either all gases, or more often, all in an aqueous solution. Homogenous catalysts tend to be much less common than heterogeneous catalysts.

A key feature of a homogeneous catalyst is the ability to form an intermediate species which has a lower activation energy than the uncatalysed transition state. For example, iron(III) ions can catalyse the reaction between iodide ions and peroxodisulfate ions

An energy profile showing the alternative reaction pathway provided by iron(II) catalyst in the reaction between iodide ions and peroxodisulfate ions

To learn more about these types of transition metal reactions check out our revision notes on "Homogeneous Catalysis"

Autocatalysis

Autocatalysis literally means self-catalysing. It is a reaction in which one of the products of the reaction acts as a catalyst for the reaction. Most chemical reactions slow down as the reaction proceeds, so a graph of concentration against time is a curve that gets shallower and shallower as the reactants get used up.

However, in an autocatalytic reaction, the rate graph makes a reverse S shape. The reaction actually speeds up and then slows down!

To learn more about autocatalytic reactions check out our revision notes on "Autocatalysis".

Frequently asked questions about transition metals:

Q. Where are the transition metals on the periodic table?

• They are located in the first row of the d-block of the periodic table. Find out more about them here: "Transition Metals"

Q. Why are transition metals good catalysts?

• In incomplete d-subshell enables transition metals and their ions to temporarily accept electrons and facilitate reactions as "Homogenous and Heterogeneous Catalysts" by providing alternative reaction pathways

Q. Why are transition metals compounds coloured?

• Transition metal ions have an incomplete d-sub shell, which can split into a lower and higher energy level. Electrons absorb energy in the visible spectrum to jump from a lower to a higher level. You can find more detail in "Colour in Aqueous Ions"

What keyword definitions do I need to know for transition metal reactions? 

Some keyword definitions you need to know are:

• Transition metal - an element with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell
• Catalyst - a chemical that speeds up the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy
• Homogeneous - In the same physical state
• Heterogeneous - In a different physical state 

This is a quick summary of some key concepts on transition metal reactions - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!