February 21, 2024

Find comprehensive solutions for Class 12 Science Chemistry Chapter 7 – “The P Block Elements” in the NCERT book. These step-by-step explanations are highly sought after by Chemistry students, aiding in homework completion and exam preparation. The popularity of these solutions among class 12 Science students is attributed to their effectiveness. Access free answers to all questions from the NCERT Book of Class 12 Science Chemistry Chapter 7, ensuring a valuable resource for your studies. Page No 169: Question 7.1: Why are pentahalides more covalent than trihalides? ANSWER: In pentahalides, the oxidation state is +5 and in trihalides, the oxidation state is +3. Since the metal ion with a high charge has more polarizing power, pentahalides are more covalent than trihalides. Page No 169: Question 7.2: Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements? ANSWER: As we move down a group, the atomic size increases and the stability of the hydrides of group 15 elements decreases. Since the stability of hydrides decreases on moving from NH3 to BiH3, the reducing character of the hydrides increases on moving from NH3 to BiH3. Page No 170: Question 7.3: Why is N2 less reactive at room temperature? ANSWER: The two N atoms in N2 are bonded to each other by very strong triple covalent bonds. The bond dissociation energy of this bond is very high. As a result, N2 is less reactive at room temperature. Page No 172: Question 7.4: Mention the conditions required to maximise the yield of ammonia. ANSWER: Ammonia is prepared using the Haber’s process. The yield of ammonia can be maximized under the following conditions: (i) High pressure (∼ 200 atm) (ii) A temperature of ∼700 K (iii) Use of a catalyst such as iron oxide mixed with small amounts of K2O and Al2O3 Page No 172: Question 7.5: How does ammonia react with a solution of Cu2+? ANSWER: NH3 acts as a Lewis base. It donates its electron pair and forms a linkage with metal ion. Page No 173: Question 7.6: What is the covalence of nitrogen in N2O5? ANSWER: From the structure of N2O5, it is evident that the covalence of nitrogen is 4. Page No 177: Question 7.7: Bond angle in is higher than that in PH3. Why? ANSWER: In PH3, P is sp3 hybridized. Three orbitals are involved in bonding with three hydrogen atoms and the fourth one contains a lone pair. As lone pair-bond pair repulsion is stronger than bond pair-bond pair repulsion, the tetrahedral shape associated with sp3 bonding is changed to pyramidal. PH3 combines with a proton to form  in which the lone pair is absent. Due to the absence of lone pair in, there is no lone pair-bond pair repulsion. Hence, the bond angle in is higher than the bond angle in PH3. Page No 177: Question 7.8: What happens when white phosphorus is heated with concentrated NaOH solution in an inert atmosphere of CO2? ANSWER: White phosphorous dissolves in boiling NaOH solution (in a CO2 atmosphere) to give phosphine, PH3. Page No 178: Question 7.9: What happens when PCl5 is heated? ANSWER: All the bonds that are present in PCl5 are not similar. It has three equatorial and two axial bonds. The equatorial bonds are stronger than the axial ones. Therefore, when PCl5 is heated strongly, it decomposes to form PCl3. Page No 178: Question 7.10: Write a balanced equation for the hydrolytic reaction of PCl5 in heavy water. ANSWER: Page No 180: Question 7.11: What is the basicity of H3PO4? ANSWER: H3PO4 Since there are three OH groups present in H3PO4, its basicity is three i.e., it is a tribasic acid. Page No 180: Question 7.12: What happens when H3PO3 is heated? ANSWER: H3PO3,on heating, undergoes disproportionation reaction to form PH3 and H3PO4. The oxidation numbers of P in H3PO3,PH3, and H3PO4 are +3, −3, and +5 respectively. As the oxidation number of the same element is decreasing and increasing during a particular reaction, the reaction is a disproportionation reaction. Page No 183: Question 7.13: List the important sources of sulphur. ANSWER: Sulphur mainly exists in combined form in the earth’s crust primarily as sulphates [gypsum (CaSO4.2H2O), Epsom salt (MgSO4.7H2O), baryte (BaSO4)] and sulphides [(galena (PbS), zinc blends (ZnS), copper pyrites (CuFeS2)]. Page No 183: Question 7.14: Write the order of thermal stability of the hydrides of Group 16 elements. ANSWER: The thermal stability of hydrides decreases on moving down the group. This is due to a decrease in the bond dissociation enthalpy (H−E) of hydrides on moving down the group. Therefore, Page No 183: Question 7.15: Why is H2O a liquid and H2S a gas? ANSWER: H2O has oxygen as the central atom. Oxygen has smaller size and higher electronegativity as compared to sulphur. Therefore, there is extensive hydrogen bonding in H2O, which is absent in H2S. Molecules of H2S are held together only by weak van der Waal’s forces of attraction. Hence, H2O exists as a liquid while H2S as a gas. Page No 185: Question 7.16: Which of the following does not react with oxygen directly? Zn, Ti, Pt, Fe ANSWER: Pt is a noble metal and does not react very easily. All other elements, Zn, Ti, Fe, are quite reactive. Hence, oxygen does not react with platinum (Pt) directly. Page No 185: Question 7.17: Complete the following reactions: (i) C2H4 + O2→ (ii) 4Al + 3O2→ ANSWER: (i)  (ii)  Page No 187: Question 7.18: Why does O3 act as a powerful oxidising agent? ANSWER: Ozone is not a very stable compound under normal conditions and decomposes readily on heating to give a molecule of oxygen and nascent oxygen. Nascent oxygen, being a free radical, is very reactive. Therefore, ozone acts as a powerful oxidising agent. Page No 187: Question 7.19: How is O3 estimated quantitatively? ANSWER: Quantitatively, ozone can be estimated with the help of potassium iodide. When ozone is made to react with potassium iodide solution buffered with a borate buffer (pH 9.2), iodine is liberated. This liberated iodine can be titrated against a standard solution of sodium thiosulphate using starch as an indicator. The reactions involved in the process are given below. Page No 189: Question 7.20: What happens when sulphur dioxide is …

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