CBSE Class 12 Chemistry Worksheet - Elements. CBSE issues sample papers every year for students for class 12 board exams. Students should solve the CBSE issued sample papers to understand the pattern of the question paper which will come in class 12 board exams this year. The sample papers have been provided with marking scheme. It’s always recommended to practice as many CBSE sample papers as possible before the board examinations. Sample papers should be always practiced in examination condition at home or school and the student should show the answers to teachers for checking or compare with the answers provided. Students can download the sample papers in pdf format free and score better marks in examinations. Refer to other links too for latest sample papers.
Q. 1. Give chemical reaction in support of the statement that all the bonds in PCI5 molecule are not equivalent.
Ans. Due to greater bond pair-bond pair repulsions, the two axial P — Cl bonds are less stable than the three equatorial P — Cl bonds. It is because of this reason that when PC15 is heated, the less stable axial bonds are Broken to form Cl2.
Q. 2. The tendency for catenation decreases down the group 14. Or Why carbon shows catenation but silicon does not
Ans. As we move down the group 14, the atomic size increases and hence the strength of M—M bond decreases steadily. Consequently the tendency for catenation decreases down the group.
Q. 3. PCI5 is known but NC15 is not known. Explain.
Ans. Electronic configuration of P is 1s2 2s2 2p6 3s2 3px1 3py1 3pz1 . Thus phosphorus has empty 3d orbitals in which the elecron of the 3s-orbital can be excited but nitrogen has no d-orbitals as its electronic configuration is 1s2 2s2 2px1 2py1 2pz1.
Q. 4. Why molecular nitrogen is not reactive ?
Ans. The bond dissociation energy for the triple bond in N = N is very high because of small size of N-atoms and hence small internuclear distance.
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