The pattern in which the question papers are being set for a particular subject is quite different from the manner in which the course structure is defined actually. Though the course curriculum is predefined, there are certainly some gaps between the question paper formed and the content of the course. Henceforth, students need to identify these gaps and catch up where the textbooks left off, if they want to get acquainted with the examination pattern. And, they can do it easily by checking out previous year question papers and their solutions. This not only helps them prepare for exams in a better manner but also leads them to the depth which the topic should be studied with.
Some solved questions from CBSE Class 12th Chemistry Solved Question Paper 2011 (Set-1) are given here
Q.Define 'order of a reaction’.
Ans.It is defined as the sum of powers of the concentration of the reactants in the rate law expression.
Q.What is meant by 'shape selective catalysis’?
Ans.Shape selective catalysis: It is a catalytic reaction which is dependent on the size of the reactant, the product molecules & the pore structure of the catalyst. Example of this catalysis is catalysis by zeolites. Range of the pore size present in the zeolites is 260-740 pm. Molecules having a pore size more than this cannot enter the zeolite and undergo the reaction.
Q.Differentiate between a mineral and an ore.
Ans.Minerals are naturally occurring chemical substances containing metals. They are found in the Earth’s crust and are obtained by mining. An ore is a mineral from which the metal can be extracted economically. For example, bauxite & clay both are the sources of aluminium. But bauxite is used for commercial extraction of aluminium but not clay. Thus, bauxite is ore & both of them are mineral. All ores are minerals, but all minerals are not necessarily ores.
Q.What is meant by ‘lanthanoid contraction'?
Ans.On moving along the lanthanoid series, atomic number increases. It means that the number of electrons and protons present in an atom also increases by one. Here, electrons are adding into the same shell because of which the effective nuclear charge increases.The increase in nuclear attraction due to the addition of proton is more pronounced than the increase in the interelectronic repulsions due to the addition of electron.Because of increase in number of electrons in the 4f orbital, shielding effect of 4f electrons is not that much effective as it should be. Thus, the effective nuclear charge experienced by the outer electrons increases. Consequently, the electrostatic force of attraction of nucleus for the outermost electrons increases. This results in a steady decrease in the size of lanthanoids. This is known as lanthanoid contraction.