Chemistry Class 11 NCERT Solutions: Chapter 11 the p Block Elements Part 4

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Q: 16. Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous is bubbled through. Give reasons.


Hydrogen fluoride (HF) is a covalent compound and has a very strong intermolecular hydrogen-bonding. Thus, it does not provide ions and aluminium fluoride (AlF) does not dissolve in it. Sodium fluoride (NaF) is an ionic compound and when it is added to the mixture, dissolves. This is because of the availability of free . The reaction involved in the process is:

When boron trifluoride is added to the solution, aluminium fluoride precipitates out of the solution. This happens because the tendency of boron to from complexes is much more than that of aluminium. Therefore, when is added to the solution, B replaces Al from the complexes according to the following reaction:

Q: 17. Suggest a reason as to why CO is poisonous.


Carbon monoxide is highly-poisonous because of its ability to form a complex with haemoglobin. The complex is more stable than the complex. The former prevents from binding with oxygen. Thus, a person dies because of suffocation on not receiving oxygen. It is found that the complex is about 300 times more stable than the complex.

Q: 18. How is excessive content of responsible for global warming?


Carbon dioxide is a very essential gas for our survival. However, an increased content of in the atmosphere poses a serious threat. An increment in the combustion of fossil fuels, decomposition of limestone, and a decrease in the number of trees has led to greater levels of carbon dioxide. Carbon dioxide has the property of trapping the heat provided by sunrays. Higher the level of carbon dioxide, higher is the amount of heat trapped. This results in an increase in the atmospheric temperature, thereby causing global warming.

Q: 19. Explain structure of diborane and boric acid.


(A) Diborane

is an electron-deficient compound. has only 12 electrons from atoms and each from atoms. Thus, after combining with atoms, none of the boron atoms has any electrons left. X-ray diffraction studies have shown the structure of diborane as:

2 boron and 4 terminal hydrogen atoms ( ) lie in one plane, while the other two bridging hydrogen atoms ( ) lie in a plane perpendicular to the plane of boron atoms. Again, of the two bridging hydrogen atoms, one H atom lies above the plane and the other lies below the plane. The terminal bonds are regular two-centre two-electron bonds, while the two bridging bonds are three-centre two-electron bonds

(B) Boric acid

Boric acid has a layered structure. Each planar unit is linked to one another through H atoms. The H atoms form a covalent bond with a unit, while a hydrogen bond is formed with another unit. In the given figure, the dotted lines represent hydrogen bonds.