Chemistry 2012 Jamb Examination

Oxidation-reduction enzymes, also called oxidoreductases, catalyse reactions in which one molecule is oxidised while another is reduced. These enzymes facilitate electron transfer between donor and acceptor molecules. Dehydrogenases are classic examples of this group because they remove hydrogen atoms (oxidation) from substrates and transfer them to acceptors such as NAD\( ^+ \) or FAD, forming NADH or FADH\( _2 \). Amylase, protease, and lipase are hydrolytic enzymes that break down carbohydrates, proteins, and fats respectively, without involving redox reactions.

Simple carbohydrates generally follow the formula C\( _x \)(H\( _2 \)O)\( _x \), where the number of carbon atoms equals the number of water units. For glucose, C\( _6 \)H\( _{12} \)O\( _6 \), x equals y. Starch, on the other hand, is a polysaccharide with an empirical formula (C\( _6 \)H\( _{10} \)O\( _5 \))\( _n \), where x ≠ y because the polymerisation process eliminates water molecules between monomer units. Therefore, glucose and starch do not have equal x and y values.

Assuming 100 g of the compound:
Carbon = 40.0 g → \( \dfrac{40.0}{12} = 3.33 \) mol
Hydrogen = 6.7 g → \( \dfrac{6.7}{1} = 6.7 \) mol
Oxygen = 53.3 g → \( \dfrac{53.3}{16} = 3.33 \) mol

Dividing by the smallest (3.33): C : H : O = 1 : 2 : 1
The empirical formula = CH\( _2 \)O (mass = 30).
Given molecular mass = 180, thus \( \dfrac{180}{30} = 6 \).
Therefore, molecular formula = (CH\( _2 \)O)\( _6 \) = C\( _6 \)H\( _{12} \)O\( _6 \).

Saponification is the alkaline hydrolysis of an ester (usually a fat or oil) to produce glycerol (an alkanol) and soap (a salt of a fatty acid). During this process, the ester bond in the alkanoate is broken by hydroxide ions, forming an alcohol and the carboxylate salt. This clearly represents a hydrolysis reaction, not dehydration or oxidation.

In 2-methylpropan-2-ol, the hydroxyl group \(-OH\) is attached to a carbon atom bonded to three other carbon atoms. This type of structure is characteristic of tertiary alkanols. Primary and secondary alcohols have the hydroxyl group attached to carbons bonded to one and two carbon atoms respectively, while quaternary alcohols do not exist as carbon cannot form five bonds.

When ethanol is heated with concentrated H\( _2 \)SO\( _4 \) above 170°C, a dehydration reaction occurs, removing water from the ethanol molecule to form ethene. The acid acts as a catalyst, and the reaction can be represented as:\[ \text{C}_2\text{H}_5\text{OH} \xrightarrow{\; H_2SO_4,\; 170^{\circ}C} \text{C}_2\text{H}_4 + H_2O \]This is an elimination reaction typical of alcohols at high temperature.

Although zinc does not exhibit variable oxidation states or form coloured ions, it can form complex ions just like other transition metals. Zinc commonly forms complexes such as \([Zn(NH_3)_4]^{2+}\) due to its ability to donate vacant orbitals for coordination. This property aligns it with the general behaviour of transition metals in complex formation, despite it being a d\(^{10}\) element.

Gold (Au) is the least reactive metal among the given options. It resists oxidation, corrosion, and tarnishing because of its stable electronic configuration and strong metallic bonding. This is why gold is often found in its native state and is used in jewellery and electronics due to its chemical inertness. The others—lead, tin, and mercury—are more reactive than gold.

Geometric (cis-trans) isomerism occurs in alkenes when there is restricted rotation around a C=C double bond and each carbon of the double bond has two different substituents. Hex-3-ene meets this condition because each of the carbon atoms in the double bond has two different groups attached. Hexane has no double bond, prop-1-ene and 3-methylbut-1-ene do not satisfy the condition due to identical groups on one of the double-bonded carbons.

Alkanals (aldehydes) give a positive reaction with Fehling's solution because they can be readily oxidised to carboxylic acids, reducing the blue Cu\( ^{2+} \) ions to red Cu\( _2 \)O precipitate. Alkanones (ketones), however, lack a hydrogen atom on the carbonyl carbon and therefore do not reduce Fehling's solution. This difference is a standard qualitative test used to distinguish between aldehydes and ketones.