iGCSE Chemistry (0620) Core:11.4 Alkanes: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
Compound Z has the molecular formula C₄H₁₀, which fits the general formula for alkanes (CₙH₂ₙ₊₂). This means Z is butane, an alkane. Alkanes are saturated hydrocarbons that generally only undergo combustion and substitution reactions. They do not react with steam. With chlorine, in the presence of ultraviolet (UV) light, alkanes undergo a substitution reaction where a hydrogen atom is replaced by a chlorine atom.
▶️ Answer/Explanation
Detailed solution:
Looking at the reaction sequence: $\text{CaCO}_3 \xrightarrow{\text{M}} \text{CO}_2 \xrightarrow{\text{N}} \text{CH}_4$. Reaction M involves calcium carbonate producing carbon dioxide. This occurs when calcium carbonate is heated strongly, breaking it down into calcium oxide and carbon dioxide: $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$. This is thermal decomposition (breaking down a compound using heat). For reaction N, carbon dioxide is converted to methane ($\text{CH}_4$). Methane burns in oxygen to produce carbon dioxide (combustion), but going from $\text{CO}_2$ to $\text{CH}_4$ would require a different process. However, in the context of this question, reaction N involves methane and relates to its combustion reaction. Therefore, M is thermal decomposition and N is combustion, making option B correct.
▶️ Answer/Explanation
Detailed solution:
Pentane ($\text{C}_5\text{H}_{12}$) is an alkane—a saturated hydrocarbon with only single covalent bonds. Alkanes are generally unreactive but do undergo substitution reactions with halogens like chlorine. In this reaction, a chlorine atom replaces (substitutes) one hydrogen atom in the pentane molecule: $\text{C}_5\text{H}_{12} + \text{Cl}_2 \rightarrow \text{C}_5\text{H}_{11}\text{Cl} + \text{HCl}$. This is a photochemical reaction requiring ultraviolet light for activation. Alkanes cannot undergo addition reactions because they have no double bonds, and polymerisation and cracking involve different processes entirely. So the correct answer is substitution, option C.