All Free Chemistry MCQs with Answers

261. An alloy such as stainless steel forms readily between transition metals because

• A. the metals react chemically to form a compound
• B. their atoms are of similar size and can substitute for one another in the lattice
• C. they have identical melting points
• D. they are all radioactive

Explanation: Atomic radii across a transition series change very little, since the added electrons enter an inner d subshell and shield the extra nuclear charge, so one metal can replace another in the lattice without disrupting it. The resulting alloy is a mixture rather than a compound. Chromium and nickel added to iron in this way give stainless steel its corrosion resistance.

262. Across the first transition series from scandium to zinc, the atomic radius

• A. increases sharply
• B. decreases sharply
• C. changes very little, because the added d electrons shield the increased nuclear charge well
• D. first increases then decreases sharply

Explanation: Each element adds a proton and a d electron, and because that electron enters an inner subshell it shields the outer 4s electrons effectively, so the net attraction on them barely changes. This near constancy is what makes transition metals so readily interchangeable in alloys. The contrast with the sharp contraction across a p block period is the point of the question.

263. Haemoglobin is a naturally occurring complex in which the central metal ion is

• A. magnesium
• B. iron
• C. cobalt
• D. copper

Explanation: Each haem group holds an iron two ion coordinated to four nitrogen atoms of the porphyrin ring, with the remaining site available to bind oxygen reversibly. Magnesium is the central ion of chlorophyll and cobalt that of vitamin B12, both of which are offered here as the parallel biological cases. Carbon monoxide is toxic because it binds the same iron site far more strongly than oxygen.

264. Vanadium pentoxide, V2O5, is used industrially as a catalyst in the

• A. Haber process
• B. hydrogenation of vegetable oils
• C. Contact process for making sulphuric acid
• D. cracking of long chain alkanes

Explanation: Vanadium pentoxide catalyses the oxidation of sulphur dioxide to sulphur trioxide, working by being reduced and then reoxidised in a cycle, which is a classic use of variable oxidation state. Iron is the catalyst for the Haber process and nickel for hydrogenation. Cracking uses zeolites, which are aluminosilicates rather than transition metal oxides.

265. Alcohols have much higher boiling points than alkanes of similar molar mass because

• A. their molecules are hydrogen bonded to one another
• B. they are ionic compounds
• C. their covalent bonds are stronger
• D. they have larger molecules

Explanation: The hydroxyl group allows hydrogen bonding between molecules, and a great deal of extra energy is needed to break those bonds before the liquid can boil, so ethanol boils at 78 degrees Celsius while propane, of similar mass, boils at minus 42. The same hydrogen bonding makes the shorter alcohols completely miscible with water. Boiling point does not depend on the strength of bonds inside the molecule.

266. Oxidation of a primary alcohol with acidified potassium dichromate, distilling the product as it forms, gives

• A. a ketone
• B. an aldehyde
• C. a carboxylic acid
• D. an alkene

Explanation: Distilling removes the aldehyde as soon as it is produced, before it can be oxidised further, whereas heating under reflux keeps it in the flask and the reaction continues to the carboxylic acid. The orange dichromate turns green as chromium is reduced from plus 6 to plus 3. Secondary alcohols stop at the ketone, since no hydrogen remains on the carbonyl carbon.

267. A tertiary alcohol is not readily oxidised because the carbon carrying the hydroxyl group

• A. carries a positive charge
• B. is attached to a halogen
• C. has no hydrogen atom attached to it
• D. is part of a benzene ring

Explanation: Oxidation of an alcohol involves removing the hydroxyl hydrogen together with a hydrogen from the same carbon, and in a tertiary alcohol that carbon holds three alkyl groups and no hydrogen at all. It can only be oxidised under conditions vigorous enough to break carbon to carbon bonds. This difference is the basis of using acidified dichromate to distinguish the three classes of alcohol.

268. Dehydration of ethanol with concentrated sulphuric acid at about 170 degrees Celsius gives

• A. ethane
• B. ethanoic acid
• C. ethoxyethane
• D. ethene

Explanation: At the higher temperature water is eliminated from one molecule to give the alkene, while at about 140 degrees two molecules condense instead and the product is the ether ethoxyethane. Aluminium oxide at 400 degrees achieves the same dehydration catalytically. Controlling temperature to select between two products is a recurring theme in this topic.

269. Ethanol reacts with sodium metal to give

• A. sodium ethoxide and hydrogen
• B. sodium ethanoate and water
• C. ethene and sodium hydroxide
• D. no reaction at all

Explanation: The hydroxyl hydrogen is weakly acidic, so sodium displaces it to give an ionic alkoxide and hydrogen gas, though the reaction is far gentler than that of sodium with water because an alcohol is a weaker acid. The steady effervescence without ignition is a useful test for a hydroxyl group in a dry sample. Alkoxides are strong bases and are hydrolysed by water back to the alcohol.

270. The reaction of ethanol with ethanoic acid in the presence of a little concentrated sulphuric acid produces

• A. an aldehyde
• B. an ester with a fruity smell
• C. an alkene
• D. a soap

Explanation: Esterification joins the acid and the alcohol with the loss of water, giving ethyl ethanoate, and the sulphuric acid acts both as catalyst and as a dehydrating agent to shift the equilibrium to the right. The reaction is reversible, so the yield is never complete. Esters of this kind are used as flavourings and solvents.