Chapter 20: Periodic Table – Group 2 and 17

Chapter 20: Periodic Table – Group 2 and 17

Chapter 20
Periodic Table – Group 2 and 17

The chapter on Group 2 and 17 elements is one of the simplest and shortest chapters in H2 Chemistry. Similar to the chapter on The Periodic Table, this chapter focuses on the trends and properties of Group 2 and 17 elements. Students are expected to familiarise themselves with the qualitative explanations for each trend.

Group 2 elements are known to be very reactive reducing agents. The table below will highlight the explanations/proof behind such an observation.

Thermal Decomposition

Group 2 compounds can undergo thermal decomposition to form oxides. Depending on the type of Group 2 compound, the products obtained would be different. There are 3 main types of Group 2 compounds that undergo thermal decomposition:

Thermal decomposition temperature increases down the group, which means an increase in thermal stability. Let’s take a look at worked example 1 to showcase the format of presentation expected of students when dealing with qualitative questions regarding thermal decomposition of Group 2 compounds.

State and explain how the temperature of decomposition of BaCO₃ would differ from that of MgCO₃.

Solution:
Ba²⁺ had a lower charge density than Mg²⁺ due to its larger ionic radius. Hence Ba²⁺ has weaker polarising power than Mg²⁺ and distorts the electron cloud of the carbonate ion to a smaller extent. There is less weakening of the C-O bonds in the ion and more heat energy is required for the decomposition. Hence BaCO₃ is more stable to heat and will decompose at a high temperature.

Checklist for comparing thermal stability between 2 compounds:

❏     Compare ionic radius of both cations

❏     Compare charge density and polarising power

❏     Compare extent of distortion of electron cloud of anion

❏     Compare extent of weakening of covalent bond in the anion

❏     Conclude which compound has higher decomposition temperature

Note: The thermal decomposition temperature can also be affected by polarisability of anion in which the more polarisable the anion, the more thermally unstable the compound is.

Next, Group 17 elements (also called halogens) are p-block elements with a characteristic outer-shell configuration of ns²np⁵. Here are some of the trends related to Group 17 elements.
We will not be covering previously mentioned trends such as melting point, boiling point, electronegativity and ionisation energy as the explanations for these trends can be found in previous chapters.

An alternative method to prove decreasing reactivity will require the use of displacement reactions. This happens where a more reactive/oxidizing halogen will displace a less reactive/oxidising halogen from its compound. The table below illustrates the workings of a displacement reaction:

Apart from the properties of halogens, questions related to the properties of halogen halides can be tested as well. A commonly tested property is thermal stability. Firstly, let’s take a look at Worked Example 2 to understand the proper presentation format for thermal stability.

Thermal stability of Group 17 hydrides decreases down the group. Explain why that is so, quoting evidence from the data booklet.

Solution:
Down the group, the size of halogen increases. The valence orbital used for bonding becomes larger and more diffused. There is less effective orbital overlap between the 1s orbital of hydrogen and the valence p orbital of the halogen. Hence, H-X bond strength decreases down the group as shown by the decrease in bond energy from +431 KJ mol⁻¹ in H-Cl to +299 KJ mol⁻¹ in H-I. Since H-X bond is more easily broken down the group, thermal stability of the halogen halide decreases.

Although no longer in the syllabus, it is good to know that the acidity of Group 17 hydrides increases down the group.

Note: the higher the K value, the more acidic the solution

Explain why the acidity of Group 17 hydrides increases down the group

Solution:
To dissolve in water, the H-X bond must be broken. Thus the acidity of the HX depends on the H-X bond strength. The weaker the H-X bond, the greater the ease of breaking the H-X bond in order to produce H⁺ hence the more acidic the HX in aqueous solution. Since bond energy decreases in the order H-F >> H-Cl >> H-Br >> H-I, acid strength thus increases in the order H-I >> H-Br >> H-Cl >> H-F

Note: the highlighted portions in both Worked Example 2 and 3 indicate the important marking points when answering questions related to thermal stability and acidity.