All Free Chemistry MCQs with Answers

101. The melting point of a solid is a measure of

• A. the mass of its particles
• B. the strength of the forces holding its particles together
• C. the size of its crystals
• D. its electrical conductivity

Explanation: Melting requires enough thermal energy to overcome the forces locking the particles in the lattice, so covalent network and ionic solids melt very high, metals vary widely and molecular solids melt low. Comparing melting points is therefore a quick way to infer the type of bonding in an unknown solid. Crystal size affects appearance but not the temperature at which melting occurs.

102. Metals are malleable and ductile because

• A. their atoms are held in rigid directional covalent bonds
• B. they contain no free electrons
• C. layers of positive ions can slide over one another while the sea of delocalised electrons holds them together
• D. their ions are held by hydrogen bonds

Explanation: Metallic bonding is non directional, so displacing one layer of cations does not break the structure because the delocalised electrons continue to bind whatever arrangement results. An ionic crystal shatters instead, because sliding brings like charges opposite each other and the layers repel. The same free electrons account for electrical and thermal conductivity.

103. A Born Haber cycle is used to determine lattice energy because it

• A. measures lattice energy directly in a calorimeter
• B. applies Hess's law to relate lattice energy to quantities that can be measured
• C. predicts the shape of the crystal
• D. measures the melting point of the ionic solid

Explanation: Lattice energy cannot be measured in an experiment, so the cycle links it to the enthalpy of atomisation, ionisation energy, electron affinity and enthalpy of formation, all of which can. Because enthalpy is a state function the total around the cycle is zero regardless of the route, which is Hess's law applied to ionic compounds. A large discrepancy between calculated and experimental values indicates covalent character.

104. Which of the following solids is held together by dispersion forces only?

• A. Solid iodine
• B. Sodium chloride
• C. Diamond
• D. Copper

Explanation: Iodine molecules are non polar, so the only attraction between them is the temporary induced dipole interaction known as the dispersion force, which is why iodine sublimes readily on gentle warming. Sodium chloride is ionic, diamond covalent network and copper metallic, all of which involve far stronger bonding. Dispersion forces still grow with molecular size, which is why iodine is a solid while chlorine is a gas.

105. Oxidation is best defined as

• A. loss of electrons and an increase in oxidation number
• B. gain of electrons and a decrease in oxidation number
• C. gain of hydrogen only
• D. loss of oxygen only

Explanation: The modern definition is in terms of electrons, so oxidation is loss and reduction is gain, remembered as OIL RIG. Definitions based on adding oxygen or removing hydrogen are older special cases that fail for reactions in which neither element is present. Every oxidation must be accompanied by a reduction, since the electrons lost by one species are gained by another.

106. The oxidation number of sulphur in sulphuric acid, H2SO4, is

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

Explanation: Hydrogen contributes plus 1 each and oxygen minus 2 each, so 2 plus x minus 8 equals zero and x is plus 6. The value plus 4 belongs to sulphur in sulphur dioxide and in sulphurous acid, which is why it is the closest distractor. Sulphur at plus 6 is already at its maximum oxidation state, so concentrated sulphuric acid can act as an oxidising agent but not as a reducing one.

107. In the reaction Zn + CuSO4 gives ZnSO4 + Cu, the substance oxidised is

• A. the sulphate ion
• B. copper ions
• C. zinc
• D. copper metal

Explanation: Zinc goes from an oxidation number of zero as the metal to plus 2 in the sulphate, so it has lost electrons and is oxidised, which also makes it the reducing agent. The copper ions gain those electrons, fall from plus 2 to zero and are reduced. The sulphate ion is a spectator, unchanged on both sides of the equation.

108. An oxidising agent is a substance that

• A. loses electrons and is itself oxidised
• B. gains electrons and is itself reduced
• C. neither gains nor loses electrons
• D. always contains oxygen

Explanation: An oxidising agent oxidises something else by taking its electrons, so it is itself reduced in the process, and the same reasoning in reverse defines a reducing agent. Potassium permanganate and acidified potassium dichromate are common oxidising agents that contain oxygen, but chlorine is a strong oxidising agent with none, which disposes of the last option.

109. The oxidation number of oxygen in hydrogen peroxide, H2O2, is

• A. minus 2
• B. zero
• C. minus 1
• D. plus 2

Explanation: Hydrogen is plus 1, so the two oxygens must total minus 2 between them, giving minus 1 each, which is the exception to the usual rule of minus 2 for oxygen. This intermediate value is why hydrogen peroxide can act both as an oxidising agent, going to minus 2, and as a reducing agent, going to zero. In compounds with fluorine, oxygen takes a positive oxidation number.

110. A reaction in which the same element is both oxidised and reduced is called

• A. a neutralisation reaction
• B. a displacement reaction
• C. a precipitation reaction
• D. a disproportionation reaction

Explanation: In disproportionation one species with an intermediate oxidation state changes into two products, one at a higher and one at a lower state, as when chlorine reacts with cold dilute alkali to give chloride and hypochlorite. The element must therefore have an accessible oxidation state on both sides of its starting value. This is why sodium, always plus 1 in compounds, cannot disproportionate.