Group 2: Alkaline earth metals

Read our general notes on Risk Assessment

This question is regarding the thermal decomposition of Group 2 carbonates and nitrates. Why does the ease of decomposition get harder as you go down the group? Does the increasing atomic radii have any influence on this and if so, how?

Jenkin replies
 
Chemists seek to explain the trends in the stabilities of the carbonates or nitrates of groups 1 and 2 in terms of:

• the charge on the metal ion, and
  
• the size of the metal ion.

Down group 2 the carbonates and nitrates become more stable. The ions all have the same charge: 2+. The ions differ in size becoming larger down the group. So it appears that for a given charge, the carbonates get more stable as the ions get larger.
 
Polarising power of the ions
A qualitative explanation is possible in terms of the polarising power of the ions.
 
A small cation, such as Mg²⁺, has a more ‘concentrated’ charge than a large one such as Ba²⁺ where the charge is, if you like, more spread out. This is sometimes referred to as a difference in charge density.
 
An ion having a high charge density will tend to polarise the C-O bonds in the carbonate ions more strongly and hence make them more likely to break, forming oxide ions and releasing CO₂.
 
Energy changes
A more systematic and quantitative explanation is possible by considering the energy changes involved in the decomposition of a group 2 carbonate.
 
Drawing up energy cycles helps to show that the two key energy quantities are:

• the energy needed to break the carbonate ion into an oxide ion and carbon dioxide, and
  
• the energy given out as the 2+ and 2- ions get closer together when the larger carbonate ion breaks up into a smaller oxide ion.

Lattice energy are inversely proportional to the sum of the ionic radii of the positive and negative ions. Down group 2 lattice energies becomes less negative from one compound to the next as the metal ions get larger. For a large anion, such as the carbonate ion, this trend is less marked than for a small anion, such as the oxide ion. It turns out that the difference in the lattice energies (which provides the energy needed to break up the carbonate ion) is greater for the smaller ions and less for the larger ions. So MgCO₃ has, if you like, more to gain by changing into MgO than BaCO₃ has by changing into BaO.

Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website. For further guidance see our tutorial on Risk Assessment.

back to Group 2: Alkaline earth metals

---

Rate this page or react
Share your views on this page, 33 ratings so far , rated at

---

updated: 28 April 2004