All Free Chemistry MCQs with Answers

161. When calcium reacts with cold water, the products are

• A. calcium oxide and hydrogen
• B. calcium hydride and oxygen
• C. calcium hydroxide and hydrogen
• D. calcium carbonate and hydrogen

Explanation: Calcium reacts steadily with cold water to give a suspension of slightly soluble calcium hydroxide, known as limewater when filtered, together with bubbles of hydrogen. Magnesium reacts only very slowly with cold water but readily with steam, which gives the oxide instead. Calcium carbonate would require carbon dioxide to be present.

162. The flame colour produced by potassium compounds is

• A. golden yellow
• B. brick red
• C. apple green
• D. lilac

Explanation: Heating excites the outer electron to a higher level and the colour is emitted as it falls back, the energy gap being characteristic of the element, so potassium gives lilac, sodium golden yellow, calcium brick red and barium apple green. The flame test is the simplest analytical use of atomic emission spectra. Sodium contamination easily masks the potassium colour, so it is viewed through blue glass.

163. Which property increases down group II from beryllium to barium?

• A. First ionisation energy
• B. Electronegativity
• C. Solubility of the hydroxides in water
• D. Melting point of the metal

Explanation: Hydroxide solubility rises down the group, so magnesium hydroxide is only sparingly soluble while barium hydroxide dissolves appreciably, whereas the sulphates show the opposite trend and become less soluble. Ionisation energy and electronegativity both fall down a group as the atoms get larger. This contrast between hydroxides and sulphates is a favourite examination point.

164. Carbon differs markedly from the other members of group IV mainly because it

• A. is small, has no available d orbitals and forms strong multiple bonds with itself
• B. has a much larger atomic radius
• C. is a metal
• D. has a full outer shell

Explanation: Its small size lets carbon form stable double and triple bonds and catenate into long chains, which is the basis of the whole of organic chemistry, while the absence of d orbitals limits it to a maximum covalency of four. Silicon and the heavier members can expand their octets using d orbitals, so silicon forms SiF6 2- while carbon cannot form the analogous ion. Carbon has four outer electrons, not a full shell.

165. In an exothermic reaction the enthalpy change is

• A. negative, because the products have less enthalpy than the reactants
• B. positive, because heat is released
• C. always zero
• D. negative, because the products have more enthalpy than the reactants

Explanation: Energy leaves the system as heat, so the products end up at a lower enthalpy and the difference, products minus reactants, is negative. The sign convention is taken from the point of view of the system, which is why released heat carries a minus sign even though the surroundings get warmer. Combustion and neutralisation are the standard exothermic examples.

166. Which of the following is an endothermic process?

• A. Combustion of methane
• B. Photosynthesis
• C. Neutralisation of an acid by an alkali
• D. Condensation of steam

Explanation: Photosynthesis absorbs light energy to build glucose from carbon dioxide and water, so the products store more energy than the reactants and the enthalpy change is positive. Combustion and neutralisation both release heat, and condensation releases the latent heat that was absorbed during evaporation. A useful check is that an endothermic reaction feels cold and usually needs a continuous energy supply.

167. A reaction mixture in a beaker becomes noticeably cold. This shows that the reaction is

• A. exothermic, since heat leaves the system
• B. at equilibrium
• C. endothermic, since heat is absorbed from the surroundings
• D. catalysed

Explanation: The system draws energy from its surroundings, and since the beaker and the solution are those surroundings their temperature falls. Dissolving ammonium chloride or ammonium nitrate in water is the usual classroom demonstration, and it is the principle behind an instant cold pack. An exothermic reaction would make the beaker warm instead.

168. In thermodynamics, the system is defined as

• A. everything in the universe
• B. the container only
• C. the energy of the reaction
• D. the part of the universe under study, separated from the surroundings by a boundary

Explanation: The system is whatever the chemist chooses to study, usually the reacting substances, and everything else is the surroundings, with the boundary between them being real or imaginary. An isolated system exchanges neither matter nor energy, a closed system exchanges energy only and an open system exchanges both. A reaction in an open beaker is therefore an open system.

169. A state function is a property whose value depends on

• A. the path taken to reach the present state
• B. the present state of the system only, not on how it was reached
• C. the time taken for the change
• D. the catalyst used

Explanation: Enthalpy, internal energy, entropy, pressure, volume and temperature are all state functions, so the change in any of them depends only on the initial and final states. This path independence is precisely what makes Hess's law valid and useful. Work and heat, by contrast, are path functions, since their values do depend on how the change was carried out.

170. Internal energy of a system is the

• A. heat supplied to the system only
• B. work done by the system only
• C. total of all kinetic and potential energies of the particles in the system
• D. energy lost to the surroundings

Explanation: Internal energy sums the translational, rotational and vibrational energy of the particles together with the potential energy stored in the bonds and intermolecular forces. Its absolute value cannot be measured, but the change in it can, which is all thermodynamics requires. It is a state function, so the change depends only on the initial and final conditions.