All Free Chemistry MCQs with Answers

241. The functional group present in an ester is

• A. COOR, formed by replacing the acidic hydrogen of a carboxylic acid
• B. OH attached to a benzene ring
• C. NH2 attached to a carbon chain
• D. a carbon to carbon triple bond

Explanation: An ester is formed when a carboxylic acid reacts with an alcohol, water being eliminated, and the resulting group is responsible for the pleasant fruity smells used in flavourings and perfumes. A hydroxyl on a benzene ring makes a phenol and an NH2 group makes an amine. Esterification is reversible and is catalysed by concentrated sulphuric acid.

242. Which pair of compounds are functional group isomers?

• A. Propan-1-ol and propan-2-ol
• B. Butane and 2-methylpropane
• C. Propanal and propanone
• D. Ethane and propane

Explanation: Propanal is an aldehyde and propanone a ketone, yet both have the molecular formula C3H6O, so they differ in functional group. The two propanols differ only in the position of the same hydroxyl group, making them position isomers, and butane with 2-methylpropane are chain isomers. Ethane and propane are homologues, not isomers, since their formulae differ.

243. A nucleophile attacks

• A. a region of high electron density such as a double bond
• B. another nucleophile
• C. an electron deficient carbon atom, donating a lone pair to it
• D. only free radicals

Explanation: Nucleophile means nucleus loving, so these electron rich species, such as hydroxide, cyanide and ammonia, seek out carbon atoms made positive by an attached electronegative group. This is why alkyl halides undergo nucleophilic substitution while alkenes undergo electrophilic addition. Every nucleophile is a Lewis base, since it donates an electron pair.

244. Which compound would show optical isomerism?

• A. CH3CH2OH
• B. CH3CHClCH2CH3
• C. CH3CH2CH3
• D. CH2Cl2

Explanation: In CH3CHClCH2CH3 the second carbon carries a methyl group, a chlorine, a hydrogen and an ethyl group, four different substituents, so it is chiral and the molecule exists as two non superimposable mirror images. In every other option at least two of the four groups on each carbon are identical. Checking each carbon for four different groups is the whole test.

245. A transition element is best defined as one that

• A. lies between groups II and III of the periodic table
• B. forms at least one stable ion with a partially filled d subshell
• C. has a full d subshell
• D. is a hard metal with a high melting point

Explanation: The definition rests on the partially filled d subshell in the element or in one of its ions, because that is what produces variable oxidation states, coloured ions, catalytic activity and complex formation. Zinc is excluded on this test, since it is 3d10 both as the atom and as the Zn2+ ion, which is why it lacks colour and variable valency. Position in the table and physical hardness are consequences rather than the definition.

246. The electronic configuration of chromium, atomic number 24, is

• A. [Ar] 3d4 4s2
• B. [Ar] 3d6
• C. [Ar] 3d5 4s1
• D. [Ar] 4s2 4p4

Explanation: One 4s electron is promoted so that both the 3d and 4s subshells are exactly half filled, an arrangement of extra stability arising from symmetry and reduced electron repulsion. Copper behaves in the same way, adopting 3d10 4s1 to complete the d subshell. Writing 3d4 4s2 by mechanically following the filling order is the expected error.

247. When a transition metal forms a positive ion, the electrons are lost first from

• A. the 3d subshell
• B. the 4p subshell
• C. the innermost shell
• D. the 4s subshell

Explanation: Although 4s fills before 3d, once the d subshell is occupied the 4s level becomes the higher in energy, so it is emptied first on ionisation, and the Fe2+ ion is therefore 3d6 rather than 3d4 4s2. This detail is asked frequently because it appears to contradict the filling order. It explains why almost every transition metal shows a plus 2 oxidation state.

248. Transition metal compounds are usually coloured because

• A. electrons move between d orbitals split by the ligands, absorbing part of the visible spectrum
• B. the metals reflect all wavelengths equally
• C. they contain water of crystallisation
• D. their ions are very large

Explanation: Ligands split the five d orbitals into groups of slightly different energy, and the small energy gap corresponds to visible light, so the complex absorbs one part of the spectrum and appears in the complementary colour. Ions with an empty or completely full d subshell, such as Sc3+ and Zn2+, have no such transition available and their compounds are white. Changing the ligand changes the gap, and hence the colour.

249. Which of the following is NOT a characteristic property of transition elements?

• A. Variable oxidation states
• B. Formation of complex ions
• C. Catalytic activity
• D. Low melting points and softness

Explanation: Transition metals are hard, dense and high melting, because both the 3d and 4s electrons contribute to metallic bonding, and tungsten melts at over 3400 degrees Celsius. The other three properties all follow from the partly filled d subshell. The soft, low melting, low density metals are those of group I, which is the contrast being tested here.

250. Transition metals show variable oxidation states because

• A. their atomic radii vary greatly
• B. the 3d and 4s subshells are close in energy, so a variable number of electrons can be involved in bonding
• C. they always lose all their outer electrons
• D. they have unusually high electronegativity

Explanation: Because the energy gap between 4s and 3d is small, successive ionisation energies rise only gradually and several different numbers of electrons can be removed, giving manganese oxidation states from plus 2 to plus 7. In group I, by contrast, the second ionisation energy is enormous because it breaks into a noble gas core, so only plus 1 is ever seen. The plus 2 state is common to nearly all first row transition metals.