Chemical Reactions and Equations

On heating, lead nitrate breaks down because of heat into lead oxide, nitrogen dioxide, and oxygen: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Therefore, it is thermal decomposition. Photolytic decomposition requires light, whereas electrolytic decomposition requires an electric current.

In the presence of light, silver chloride undergoes photochemical decomposition: \(2\mathrm{AgCl}(s) \xrightarrow{\text{light}} 2\mathrm{Ag}(s)+\mathrm{Cl}_2(g)\). The silver metal formed is grey, so the white silver chloride appears grey. This is not due to the formation of silver oxide; light causes silver ions to be reduced to silver metal.

Silver chloride is photosensitive. On exposure to light, it undergoes photochemical decomposition to form metallic silver and chlorine: \(2\mathrm{AgCl}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Cl_2}\). Therefore, it is kept in the dark. Option D is different because light, not heat, is the main cause of decomposition here.

The balanced equation is \(2\mathrm{H_2O}\rightarrow 2\mathrm{H_2}+\mathrm{O_2}\). At the same temperature and pressure, gas volumes are proportional to their moles, so \(\mathrm{H_2}:\mathrm{O_2}=2:1\). Therefore, for 18 mL of hydrogen, the oxygen volume is \(18\div2=9\) mL. The 18 mL option is incorrect because the two gases are not produced in equal volumes.

The overall reaction for electrolysis of acidified water is \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). Oxygen is produced at the anode and hydrogen at the cathode. At the same temperature and pressure, gas volume is proportional to the number of moles, so the volume ratio of hydrogen to oxygen is \(2:1\). Therefore, the oxygen collected at the anode has the lower volume. This difference is due to the stoichiometric ratio in the equation, not to molar mass.

Pure water has a very low concentration of ions due to self-ionisation, so it is a poor conductor of electricity. Adding a dilute acid increases the number of mobile cations and anions that carry charge through the solution. Therefore, electric current passes more easily. Option B is incorrect because acid does not convert water molecules into electrons; conduction in the solution occurs through ions.

When steam is passed over hot iron, magnetite, \(\mathrm{Fe_3O_4}\), and hydrogen are formed. The correct balanced equation is \(\mathrm{3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2}\). Therefore, writing \(\mathrm{FeO}\) as the product is incorrect. Unlike option B, hydrogen from steam is released as hydrogen gas.

Option B is a redox reaction. The oxidation state of Zn changes from \(0\) to \(+2\), so Zn is oxidised. In \(\mathrm{CuSO_4}\), Cu changes from \(+2\) to \(0\), so Cu is reduced. In contrast, option C is a neutralisation reaction, in which oxidation states do not change.

In \(\mathrm{CuO}\), copper changes from oxidation state \(+2\) to \(0\) in the product \(\mathrm{Cu}\). A decrease in oxidation number indicates reduction, so \(\mathrm{CuO}\) is reduced. In contrast, hydrogen in \(\mathrm{H_2}\) changes from \(0\) to \(+1\) in \(\mathrm{H_2O}\), so \(\mathrm{H_2}\) is oxidised.

A precipitation reaction forms an insoluble solid. In chemical equations, an insoluble solid is written with the state symbol \(\text{(s)}\). Therefore, \(\text{(s)}\) is used to show formation of the precipitate.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Black copper(II) oxide loses oxygen and changes into reddish-brown copper. Loss of oxygen is reduction, so copper(II) oxide is reduced. In contrast, oxidation of copper would involve copper gaining oxygen to form an oxide.

A redox reaction involves transfer of electrons. The substance that loses electrons is oxidised, and the substance that gains electrons is reduced. Electron loss and electron gain occur simultaneously in the same reaction, so option B is correct. A colour change may sometimes indicate a reaction, but it is not the definition of oxidation or reduction.

A paint coating acts as a barrier between iron and the environment. At a scratch, iron becomes exposed to oxygen from air and moisture. In their presence, iron can undergo oxidation to form hydrated iron(III) oxide, or rust. Option B is incorrect because paint does not produce oxygen; the scratch merely removes the protective barrier.

In option A, calcium oxide and water combine to form a single product, calcium hydroxide, so it is a combination reaction. Heat is released when quicklime forms slaked lime; therefore, the reaction is also exothermic. In contrast, option B is a photodecomposition reaction in which one compound breaks down into simpler substances in sunlight.

Ghee contains fats. Oxygen in open air oxidises these fats, and warmth increases the rate of this reaction. This leads to rancidity, which can produce an unpleasant smell and taste. Nitrogen is relatively unreactive, so option B is not the cause. Exam tip: Link spoilage of oils and fats with oxidation and rancidity.

Oils and fats react with oxygen in air and undergo oxidation, causing rancidity. Filling the packet with nitrogen greatly reduces the available oxygen, so oxidation occurs more slowly and the food remains fresh for longer. Unlike option C, nitrogen does not reduce or saturate oils and fats under normal packaging conditions.

A cooling container may indicate that the process has absorbed heat from its surroundings, so it may be endothermic. Gas evolution has also been observed, but these two observations alone cannot identify the exact type of chemical reaction. For example, gas evolution does not by itself prove that a reaction is exothermic, so option B is not justified.

Heat written on the products side shows that heat is released during the reaction. A warm reaction vessel also indicates that heat is being transferred to the surroundings, so the reaction is exothermic. In contrast, an endothermic reaction absorbs heat from the surroundings, and heat is written on the reactants side.

Heat written on the reactant side shows that the reaction absorbs heat, so it is endothermic. The breakdown of one compound into two or more products is the defining feature of a decomposition reaction. Therefore, it is an endothermic decomposition reaction. In contrast, a combination reaction usually forms one product from two or more reactants.

In respiration, glucose is oxidised in many enzyme-controlled steps. Therefore, energy is released gradually rather than suddenly as heat and light, and part of it is captured in ATP. In contrast, combustion is usually a rapid oxidation process in which energy is released quickly, mainly as heat and sometimes light.

Photosynthesis is an endothermic process in which light energy is used to store chemical energy in glucose formed from carbon dioxide and water. In contrast, cellular respiration oxidises glucose and releases energy, which is used to form ATP. Option B is incorrect because photosynthesis does not release energy from glucose; it stores energy in glucose.

When food is chewed only by teeth, it is broken into smaller pieces without changing its chemical composition; therefore, this is mainly a physical change. During enzyme-mediated digestion, complex food molecules such as starch and proteins are chemically converted into simpler substances. Thus, digestion is a chemical change. Option B is incorrect because digestion does not merely reduce particle size; it also forms new, simpler substances.

During conversion of milk into curd, bacterial action produces new substances such as lactic acid and permanently changes the properties of milk. Hence, it is a chemical change. In contrast, freezing water changes only its state; ice is still water and returns to water on melting. Thus, ice formation is a physical change, so option A is correct.

A colour change can be an indication of a chemical reaction, but it is not conclusive by itself. For example, mixing coloured substances or dissolving a coloured substance can change the colour without forming a new substance. Confirmation of a chemical reaction requires evidence of a change in composition or formation of a new substance. Option B is incorrect because colour change does not always prove a chemical change.

\(\text{(aq)}\) is used when a species is dissolved in water (aqueous solution). \(\text{(l)}\) is used when a species is present as a liquid, regardless of whether it is dissolved in water. So \(\text{(aq)}\) specifies aqueous solution; \(\text{(l)}\) specifies liquid state generally.