Chemical Reactions and Equations

When heat is written on the products side, it means that heat is released during the reaction. A reaction that releases heat is called an exothermic reaction. In contrast, an endothermic reaction absorbs heat, so heat is written on the reactants side.

Absorption of heat with the reactants shows that the reaction is endothermic. The breakdown of a single compound into two or more products is the characteristic of a decomposition reaction. Therefore, it is an endothermic decomposition reaction. In contrast, in a combination reaction, two or more reactants generally form one product.

In combustion, oxidation of a fuel releases energy, mainly as heat and often as light. In respiration, glucose undergoes controlled oxidation and releases energy for life processes. Therefore, both are exothermic processes. In contrast, a requirement for light is associated with processes such as photosynthesis, not with both combustion and respiration.

During photosynthesis, plants absorb sunlight and store it as chemical energy in glucose. In contrast, respiration releases energy during the oxidation of glucose, and part of this energy is captured in ATP. Therefore, option D is correct. Option B is incorrect because respiration does not require absorption of external light energy. Exam tip: remember photosynthesis as energy storage and respiration as energy release.

During mechanical chewing, food is broken into smaller pieces; its size and surface area change, but no new substance is formed. In chemical digestion, enzymes convert complex food substances into simpler substances, so new substances are formed. Therefore, option A is correct; option C is incorrect because chemical digestion is not merely a physical change.

In option A, the ions of two ionic compounds exchange partners. \(\mathrm{Pb^{2+}}\) ions combine with \(\mathrm{I^-}\) ions to form yellow, insoluble \(\mathrm{PbI_2}\) as a precipitate, while \(\mathrm{KNO_3}\) remains in solution. In contrast, option B is a single displacement reaction because zinc displaces hydrogen.

Hydrogen has oxidation number 0 in \(\mathrm{H_2}\) and +1 in \(\mathrm{H_2O}\), so hydrogen is oxidised. Copper has oxidation number +2 in \(\mathrm{CuO}\) and 0 in \(\mathrm{Cu}\), so copper(II) oxide is reduced. Thus, this is a redox reaction because oxidation and reduction occur together.

In option A, oxygen in \(H_2O_2\) has an oxidation number of \(-1\). One part is reduced to \(-2\) in \(H_2O\), while another part is oxidised to \(0\) in \(O_2\). Thus, \(H_2O_2\) undergoes both oxidation and reduction; this is a disproportionation reaction. Option B is also a redox reaction, but zinc is oxidised and copper ions are reduced, so different substances undergo the two changes.

In a chemical equation, a downward arrow indicates the formation of an insoluble solid that settles down, called a precipitate. An upward arrow indicates that a gas is evolved during the reaction. Option B reverses these two meanings. Exam tip: remember ↓ for precipitate and ↑ for evolved gas.

In CuO, copper has oxidation number +2 and becomes elemental copper with oxidation number 0, so CuO is reduced. Hydrogen in H₂ has oxidation number 0 and becomes +1 in H₂O, so hydrogen is oxidised. Thus option A correctly identifies both changes. A decrease in oxidation number means reduction, while an increase means oxidation.

In \(H_2O_2\), oxygen has an oxidation number of \(-1\). In the products, it becomes \(-2\) in \(H_2O\) and \(0\) in \(O_2\). Thus, one portion of oxygen is reduced while another is oxidised, making this a disproportionation reaction. In contrast, in option B, zinc is only oxidised and copper ions are only reduced.

Iron is more reactive than copper, so it displaces copper from copper sulphate: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). The displaced copper forms a reddish-brown coating on the nail, and the iron sulphate formed makes the solution green. In contrast, an unchanged blue solution would not indicate displacement.

In Zn + CuSO₄ → ZnSO₄ + Cu, zinc displaces copper from its salt, so it is a displacement reaction. Zinc changes from oxidation state 0 to +2 and loses electrons, while Cu²⁺ changes to Cu with oxidation state 0 and gains electrons. Thus it is also redox. Option D is redox but not displacement; B is decomposition and C is neutralisation.

In option A, zinc, which is more reactive than copper, displaces copper from copper sulphate. The oxidation state of zinc changes from \(0\) to \(+2\), while that of copper changes from \(+2\) to \(0\). Thus, zinc is oxidised and copper is reduced, making this both a displacement and a redox reaction. Option B is a double-displacement (precipitation) reaction, not a displacement reaction.

The formation of a substance with new properties shows that a new substance was produced; therefore, the process is a chemical reaction. The equality of total mass before and after the reaction in a closed container is consistent with the law of conservation of mass. Options A and D describe physical changes, which do not produce a new substance, while option B contradicts the observation.

The reaction is \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\). In a sealed container, the \(\mathrm{CO_2}\) produced remains inside, so the total mass of the products includes both \(\mathrm{CaO}\) and \(\mathrm{CO_2}\). By the law of conservation of mass, this total equals the mass of the original \(\mathrm{CaCO_3}\). Formation of a gas does not reduce total mass; it would decrease only if the gas escaped from the container.

Subscripts in a chemical formula show the number of atoms of each element in a compound. Changing a subscript changes the compound’s composition and chemical identity; for example, changing \(\mathrm{H_2O}\) to \(\mathrm{H_2O_2}\) changes water into hydrogen peroxide. To balance an equation, coefficients placed before formulae are changed, not subscripts.

One \(\mathrm{H_2O}\) molecule contains 2 hydrogen atoms. Therefore, \(2\mathrm{H_2O}\) contains \(2 \times 2 = 4\) hydrogen atoms. In a balanced equation, the number of atoms of each element is the same on both sides, so the reactant side must also contain 4 hydrogen atoms. The option 2 applies to only one water molecule.

The balanced equation is \(2\mathrm{Mg}+\mathrm{O_2}\rightarrow2\mathrm{MgO}\). \(2\mathrm{MgO}\) contains a total of 2 oxygen atoms, and one \(\mathrm{O_2}\) molecule also contains 2 oxygen atoms. Therefore, the coefficient of \(\mathrm{O_2}\) is 1. Choosing “two” would provide 4 oxygen atoms, so the equation would not be balanced.

A thermal decomposition reaction occurs when heat breaks one compound into two or more simpler substances. Its general pattern is AB → A + B, with the heat symbol Δ over the arrow. Option A names both the energy condition and the reaction type. A combination reaction produces one compound, while displacement and double displacement involve replacement of atoms or ions rather than heat-driven breakdown.