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Alok Bariyar, Hindustan Times
New Delhi, February 19, 2013
Let’s look at three tricky questions in chemistry in which students make a lot of mistakes
Question 1: An organic compound [A] which has a typical odour on treatment with NaOH forms two compounds [B] and [C]. Compound [B] has a molecular formula C7H8O, which on oxidation gives back compound [A]. The compound [C] is the sodium salt of an acid. When [C] is heated with soda lime, it yields an aromatic hydrocarbon [D]. Deduce the structure of [A], [B], [C], and [D].

To answer the above question correctly, the student is required to revise all four chapters of organic chemistry of Class 12. Students tend to get confused while solving questions based on organic conversions. The most common mistake they make is that the oxygen in the molecular formulae suggests that the compound could have been an aldehyde, ketone, alcohol, acid or an ester. They do not to understand that the compound with a smell generally points towards an ester or a benzaldehyde.
 
Similarly, in the above question the typical odour refers to a benzaldehyde because esters have a fruity smell, which is absent in this case. In addition, the student needs to concentrate on the entire question and read it thoroughly before answering it. The second half of the question suggests that there is a Canizzaro reaction going on, which can be given only by aldehydes devoid of alpha hydrogen. In the CBSE exam, word problems (above-mentioned question) and conversions covering properties and reactions of functional groups usually carry five marks. So, it is important that the student doesn’t miss out on practising these types of questions.

Question 2: Why Cu2+ is more stable than Cu+?

The questions related to inorganic chemistry requires the student to memorise a lot of areas, which they are averse to do. Inadequate knowledge leads to confusion while answering the questions. For example, in d and f block elements, the elements exist in variable oxidation states. There are several questions which are asked directly about oxidation state or are related to it – such as the colour, magnetic moment, oxidising and reducing power. In the above question, the student is generally aware that Cu+ has an octet configuration, and therefore, it should be more stable. The electronic configuration confuses the student. The student doesn’t have to only refer to the electronic configuration in the above question, but has to know and refer to the hydration in Cu+ as well as Cu2+. The hydration enthalpy of Cu2+ is far more than Cu+. That is why Cu2+ is more stable than Cu+. So, the hydration enthalpy factor dominates the electronic configuration factor (this is in general true for all such _questions).

Question 3: Calculate the EMF of the concentration cell consisting of zinc electrodes, one immersed in a solution of 0.01 M Zn2+ ions and the other in a solution of 0.1 M Zn2+ ions at 25°C. The two solutions are separated by a salt bridge.

Numericals in physical chemistry make an important part of the Class 12 board examination. Understanding a problem and being able to solve the same is essential. Prior to attempting the numericals, the student must write the proper chemical formulae to be used and the correct units with the calculations. Usually, the student misses to represent the galvanic cell, which can be represented in the following manner: Zn | Zn2+ (0.01 M) Zn2+ (0.1 M) | Zn. Moreover, the above question is tricky in approach as the student is not able to find the standard EMF of the cell. The student can simply do the same by using the following formula: standard EMF of the cell = (SRP) RHE - (SRP) LHE and that will give the requisite value.

The author is HoD, chemistry, HT Studymate , a reputable chain of tuition centres from the house of Hindustan Times which provides quality education to students from Class 9 to Class 12


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