MPSC 2024 Prelims Paper-I · Question 68 of 100

Correct — option (1), 'Only (c)', a decomposition reaction. Read the equation as printed: a single reactant, potassium chlorate, stands on the left, and two products, potassium chloride and oxygen gas, stand on the right, with the capital delta on the arrow indicating that heat is supplied and the upward arrow marking oxygen as a gas escaping from the mixture. One reactant breaking down into two or more products is the definition of a decomposition reaction, and because the agent bringing it about is heat, it is more precisely a thermal decomposition. This particular reaction is the standard laboratory preparation of oxygen, usually carried out with a little manganese dioxide added as a catalyst so that the chlorate decomposes at a lower temperature; the oxidation state of chlorine falls from +5 in the chlorate to -1 in the chloride while oxygen is released, so the reaction is also an internal redox change, but the type asked for is the structural one visible in the equation. The general form is worth fixing: AB gives A plus B, as against a combination reaction, in which A plus B gives AB, and a displacement reaction, in which A plus BC gives AC plus B. Decompositions are classified further by the agent that drives them — thermal when heat is used, as here and in the breakdown of calcium carbonate to quicklime and carbon dioxide; electrolytic when an electric current is used, as in the electrolysis of water; and photolytic when light is used, as in the darkening of silver chloride in sunlight.

• (2)Only (b) — A displacement reaction requires a more reactive element to take the place of a less reactive one in a compound, so its equation always shows an element and a compound on the left — iron with copper sulphate, or zinc with hydrochloric acid. The printed equation has a single compound on the left and no free element at all, so no displacement can be occurring. The confusion usually arises from noticing that something has been driven out of the compound; but oxygen leaving of its own accord under heat is decomposition, not displacement, because nothing has taken its place.
• (3)Only (a) and (d) — Both statements in this option fail. The reaction is not photochemical, because the symbol on the arrow is a delta, the conventional mark for heat, and not a light source; the photolytic decompositions in the syllabus are those of silver chloride and silver bromide, which darken in sunlight and underlie photographic film. Nor is the reaction exothermic: heat has to be supplied continuously to keep the chlorate decomposing, which makes the change endothermic. The option is aimed at a candidate who sees a reaction producing a gas and assumes energy must be released, and it also asks for two statements where only one is true.
• (4)Only (a) — This option names the reaction exothermic, which reverses its energetics. The delta printed on the arrow means that heat is being supplied to the reactant, and potassium chlorate must be kept heated for the decomposition to continue; a reaction that consumes heat in this way is endothermic. Most thermal decompositions in the school syllabus are endothermic for exactly this reason — energy is needed to break the compound apart — while combination reactions such as the slaking of quicklime are typically exothermic. The option also ignores the structural type that the question is really asking for.

Chemical reactions are sorted by the pattern their equations show, and five types cover almost everything the syllabus asks about. A combination reaction has two or more reactants forming a single product, as when quicklime takes up water to give slaked lime. A decomposition reaction is its reverse: one reactant breaks into two or more products, and it is named thermal, electrolytic or photolytic according to whether heat, electricity or light drives it. A displacement reaction has a more reactive element ousting a less reactive one from its compound, iron displacing copper from copper sulphate solution being the standard demonstration. A double displacement reaction exchanges the ions of two compounds, and precipitation and neutralisation both belong to it. Oxidation and reduction, finally, cut across all of these, since they classify by the transfer of electrons or of oxygen and hydrogen rather than by the shape of the equation. A separate axis of classification is energetic: exothermic reactions release heat, endothermic reactions absorb it, and the printed symbols carry the information — a delta on the arrow means heat applied, so the reaction is being driven endothermically. Learning to read the notation is half the work: an upward arrow after a formula marks a gas evolved, a downward arrow marks a precipitate, and the state symbols in brackets record solid, liquid, gas or aqueous solution.

MPSC prints an equation and asks for its type reasonably often, and the skill being tested is reading a symbol rather than recalling a fact. The question can be settled in seconds by counting species: one reactant on the left and more than one product on the right means decomposition, whatever the substances involved. The distractors are then designed around the accompanying symbols — the delta invites a guess at exothermic or photochemical, the escaping gas invites a guess at displacement. It is worth adding that this paper flattens a typeset equation into a single line of text; the original prints the delta below a long arrow and sets the subscripts properly, but nothing in the chemistry depends on the typography. The same reading discipline applies to the equations of calcium carbonate, lead nitrate and ferrous sulphate, all of which are thermal decompositions that MPSC and other commissions have used in the same format.

• A reaction in which a single reactant breaks down into two or more products is a decomposition reaction; when heat drives it, it is a thermal decomposition.
• Potassium chlorate on heating gives potassium chloride and oxygen, and this is the standard laboratory preparation of oxygen, usually with manganese dioxide as a catalyst.
• A capital delta printed on the reaction arrow denotes that heat is being supplied; an upward arrow after a formula denotes a gas evolved.
• Thermal decompositions absorb heat and are therefore endothermic, which is why the reactant must be kept heated for the change to continue.
• Decompositions are classified by their driving agent — thermal, electrolytic as in the electrolysis of water, and photolytic as in the darkening of silver chloride in sunlight.

• Reading the delta on the arrow as a sign that heat is released; it means heat is supplied, so the reaction is endothermic
• Calling the reaction a displacement because a gas leaves the compound; displacement needs a free element to take its place
• Confusing photolytic decomposition with thermal decomposition — silver halides in sunlight are the photolytic examples
• Naming the redox aspect when the question asks for the structural type visible in the equation

Equation-typing questions appear in MPSC papers as a printed reaction followed by four candidate labels, and the set of reactions used is small and predictable: the decompositions of calcium carbonate, lead nitrate, ferrous sulphate and potassium chlorate, the combination of quicklime with water, the displacement of copper by iron, and the precipitation of barium sulphate. Each is worth knowing with its type, its energetics and any catalyst involved. The Commission also asks the reverse — name a reaction of a given type — so the examples should be held in both directions.

No directly related past PYQ was found.

• practice — not a real PYQ

  The reaction in which silver chloride turns grey when left in sunlight is an example of :

  • (a)Thermal decomposition
  • (b)Photolytic decomposition
  • (c)Displacement reaction
  • (d)Double displacement reaction

  Answer(b) Photolytic decomposition — light supplies the energy that breaks silver chloride down into silver and chlorine, and the finely divided silver left behind is what gives the grey colour. This is the reaction that underlies photographic film; heat is not involved, so the change is not a thermal decomposition.

• practice — not a real PYQ

  Heating limestone to give quicklime and carbon dioxide is best described as :

  • (a)A combination reaction
  • (b)An exothermic decomposition
  • (c)An endothermic thermal decomposition
  • (d)A displacement reaction

  Answer(c) An endothermic thermal decomposition — a single reactant, calcium carbonate, breaks into two products under heat, and the heat must be supplied continuously, so the change absorbs energy. A combination reaction runs the other way, and no free element is present for any displacement to occur.