All Free Chemistry MCQs with Answers

251. In the complex ion [Cu(NH3)4]2+, the ammonia molecules act as

• A. ligands, donating a lone pair of electrons to the central metal ion
• B. reducing agents
• C. counter ions balancing the charge
• D. solvent molecules with no bonding role

Explanation: A ligand is a species with at least one lone pair that forms a dative covalent bond to the central metal ion, and here each nitrogen donates its lone pair, giving copper a coordination number of four. Water, chloride and cyanide act in the same way. The deep blue colour of this complex is the standard test for copper two ions.

252. The coordination number of the central metal ion in a complex is

• A. the charge on the complex ion
• B. the number of ligands in the solution
• C. the number of dative bonds formed between the ligands and the metal ion
• D. the oxidation state of the metal

Explanation: Coordination number counts the bonds to the central ion, so it is six in the octahedral hexaaquairon complex and four in the tetrahedral tetrachlorocuprate ion. It is not the same as the oxidation state, which counts the charge the metal would carry if the ligands were removed with their electron pairs. A single bidentate ligand such as ethylenediamine contributes two to the coordination number.

253. Iron is used as a catalyst in the Haber process, and this catalytic ability of transition metals is largely due to

• A. their high density
• B. their ability to change oxidation state and to adsorb reactants onto their surface
• C. their low melting points
• D. the complete absence of d electrons

Explanation: Variable oxidation states allow a transition metal to accept and release electrons during a reaction, providing an alternative pathway of lower activation energy, and the partly filled d orbitals let gases bond weakly to the metal surface so that their bonds are stretched and weakened. Vanadium pentoxide in the Contact process and nickel in hydrogenation work in the same ways. Density and melting point have nothing to do with it.

254. Which ion would be expected to be colourless in aqueous solution?

• A. Fe3+
• B. Cu2+
• C. Zn2+
• D. Ni2+

Explanation: The zinc ion has a completely filled 3d10 configuration, so no d to d electronic transition is possible and no visible light is absorbed. Iron three, copper two and nickel two all have partly filled d subshells and give yellow brown, blue and green solutions respectively. Scandium three, being 3d0, is colourless for the complementary reason.

255. The highest oxidation state shown by manganese is

• A. plus 2
• B. plus 4
• C. plus 7
• D. plus 6

Explanation: Manganese has five 3d and two 4s electrons, so all seven can be involved in bonding, giving the plus 7 state found in the permanganate ion. That maximum equals the group number, and because the metal is so far from its stable configuration permanganate is a powerful oxidising agent. Plus 4 occurs in manganese dioxide and plus 2 in manganese two salts, which are the most stable.

256. The electronic configuration of the Fe2+ ion, given that iron has atomic number 26, is

• A. [Ar] 3d4 4s2
• B. [Ar] 3d6
• C. [Ar] 3d5 4s1
• D. [Ar] 3d8

Explanation: Neutral iron is [Ar] 3d6 4s2, and forming the two plus ion removes both 4s electrons because 4s lies above 3d once the d subshell is occupied, leaving 3d6. Removing d electrons instead and writing 3d4 4s2 is the classic mistake. Losing one further 3d electron gives the more stable half filled 3d5 arrangement of Fe3+.

257. Transition metals have higher melting points and densities than group I metals because

• A. their atoms are much larger
• B. they contain no free electrons
• C. both the d and s electrons take part in metallic bonding, giving a stronger and more compact lattice
• D. they are non metallic in character

Explanation: A larger number of delocalised electrons per atom means stronger attraction between the cations and the electron sea, so more energy is needed to melt the metal and the atoms pack more closely, raising density. Group I metals contribute only one electron each and are correspondingly soft and low melting. This is why tungsten is used for lamp filaments and sodium can be cut with a knife.

258. Zinc is not regarded as a true transition element because

• A. it is not a metal
• B. it forms no compounds
• C. it has no d electrons at all
• D. its atom and its only ion both have a completely filled 3d subshell

Explanation: Zinc is 3d10 4s2 as the atom and 3d10 as the Zn2+ ion, so it never has a partly filled d subshell and consequently shows only one oxidation state, forms colourless compounds and is a poor catalyst. It sits in the d block by position but fails the chemical test. Scandium is excluded at the other end of the row, since its only common ion, Sc3+, is 3d0.

259. Acidified potassium permanganate acts as a strong oxidising agent and in doing so the purple manganate seven ion is reduced to

• A. the almost colourless manganese two ion
• B. manganese metal
• C. the green manganate six ion
• D. brown manganese dioxide

Explanation: In acidic solution permanganate is reduced from plus 7 to plus 2, and the sharp change from deep purple to almost colourless is what makes it self indicating in titrations. In neutral or alkaline conditions the reduction stops at manganese dioxide, giving a brown precipitate instead. The acid used must be dilute sulphuric acid, since hydrochloric acid would itself be oxidised to chlorine.

260. The magnetic behaviour of many transition metal compounds is explained by the presence of

• A. paired electrons only
• B. unpaired electrons in the d orbitals
• C. an even number of protons
• D. delocalised pi bonds

Explanation: Unpaired electrons give the ion a magnetic moment, so the substance is drawn into a magnetic field and is described as paramagnetic, and the more unpaired electrons the stronger the effect. Ions with all electrons paired, such as Zn2+, are diamagnetic and are weakly repelled instead. Measuring the magnetic moment is therefore a way of counting unpaired electrons and deducing the structure of a complex.