Chemistry MCQs for NEET — Practice Questions with Answers

The NCERT text clearly states, 'During denaturation secondary and tertiary structures are destroyed but primary structure remains intact.' If a protein has a quaternary structure, it would also be disrupted as it depends on the tertiary structure of individual subunits.

The NCERT explains, 'When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed.'

The NCERT states, 'protein loses its biological activity' due to denaturation. Since enzymes are globular proteins and their biological activity is catalysis, denaturation leads to a loss of this function. Also, from the 'BIOMOLECULES_biology' chapter summary, 'Proteinaceous enzymes exhibit substrate specificity, require optimum temperature and pH for maximal activity. They are denatured at high temperatures.'

I is correct because 'globules unfold and helix get uncoiled' affecting overall folding (tertiary and secondary structures). II is incorrect because the primary structure (amino acid sequence) remains intact. III is incorrect because hydrogen bonds are 'disturbed', not strengthened.

Denaturation is caused by changes in temperature or pH that disturb hydrogen bonds. Maintaining optimum conditions (temperature and pH) is where a protein exhibits its highest activity and is least likely to denature. Extremes of temperature and pH (like strong acids/bases leading to extreme pH) cause denaturation.

The NCERT states, 'There is an increase in ionisation enthalpy along each series of the transition elements from left to right due to an increase in nuclear charge which accompanies the filling of the inner d orbitals.' It also mentions that the 'nuclear charge increases from scandium to zinc but electrons are added to the orbital of inner subshell, i.e., 3d orbitals. These 3d electrons shield the 4s electrons from the increasing nuclear charge somewhat more effectively than the outer shell electrons can shield one another. Therefore, the atomic radii decrease less rapidly. Thus, ionization energies increase only slightly along the 3d series.'

The NCERT states, 'The first ionisation enthalpy, in general, increases, but the magnitude of the increase in the second and third ionisation enthalpies for the successive elements, is much higher along a series.' It further explains that 'A general trend of increasing values of second ionisation enthalpy is expected as the effective nuclear charge increases because one d electron does not shield another electron from the influence of nuclear charge because d-orbitals differ in direction.'

The NCERT mentions, 'The irregular trend in the first ionisation enthalpy of the metals of 3d series, though of little chemical significance, can be accounted for by considering that the removal of one electron alters the relative energies of 4s and 3d orbitals.'

The NCERT explicitly states, 'You have learnt that when d-block elements form ions, ns electrons are lost before (n – 1) d electrons.'

The NCERT lists, 'The three terms responsible for the value of ionisation enthalpy are attraction of each electron towards nucleus, repulsion between the electrons...'. Nuclear charge is inherently part of the attraction of electrons towards the nucleus. Magnetic properties are not listed as a direct factor for ionization enthalpy.