Chemical Reactions and Equations

In a double displacement reaction, ions from two ionic compounds exchange partners. If one of the products is insoluble in water, it separates as a solid precipitate: \(AB(aq)+CD(aq)\rightarrow AD(s)+CB(aq)\). Therefore, this is a double displacement reaction with precipitation. In contrast, a single displacement reaction usually involves an element replacing another element from a compound.

Loss of oxygen is called reduction, whereas gain of oxygen is called oxidation. Since reduction and oxidation occur simultaneously in this reaction, it is a redox reaction. For example, when oxygen is removed from a metal oxide, that oxide is reduced while the substance receiving the oxygen is oxidised.

Oxidation is a process in which oxygen is added to a substance or hydrogen is removed from it. Therefore, a substance is oxidised when oxygen is added to it. In contrast, addition of hydrogen or removal of oxygen is reduction; a decrease in temperature alone does not indicate oxidation.

Rusting is an electrochemical process that requires both oxygen and water. A thin layer of moisture allows ions to move, accelerating oxidation of iron and formation of rust. In contrast, moisture does not block oxygen; iron rusts faster in moist air than in dry air.

Rusting requires iron to come into contact with oxygen and moisture in air. In galvanisation, iron is coated with zinc, whereas in painting it is coated with paint. Both coatings act as barriers between the iron surface and air and moisture, thereby preventing rusting. Option B is incorrect because paint is not a coating of a reactive metal.

In an open bottle, oil remains in contact with oxygen in air. Oxidation can produce unpleasant smell and taste in the oil; this process is called rancidity. Galvanisation is the coating of iron with zinc, so it is unrelated to keeping oil exposed to air.

Rancidity is caused by the oxidation of fats and oils, which produces an unpleasant smell and taste in food. More oxygen provides greater opportunity for oxidation, and a higher temperature increases the rate of this reaction. Galvanisation is the coating of iron with zinc to prevent rusting, so it is not a food-spoilage problem.

In an exothermic reaction, heat is released from the reaction mixture to the surroundings. Therefore, when no external heat is supplied, a rise in the temperature of the mixture and container strongly indicates heat release. Option B may indicate an endothermic reaction, whereas gas evolution or a colour change may show that a reaction is occurring but do not by themselves identify the direction of heat flow.

When one compound breaks down to form two or more products, the reaction is a decomposition reaction. Since heat is absorbed during the reaction, it is endothermic. Hence, it is an endothermic decomposition reaction. In a combination reaction, two or more reactants usually form a single product, so option D does not fit.

In the equation, two reactants, \(A\) and \(B\), combine to form a single product, \(C\); therefore, it is a combination reaction. Since heat appears on the product side, heat is released, so the reaction is exothermic. In a decomposition reaction, one compound generally breaks down into two or more products.

In respiration, food substances such as glucose undergo controlled oxidation and release energy. In combustion, a fuel undergoes relatively rapid oxidation and releases energy. Thus, both are energy-releasing oxidation processes. Option B is incorrect because respiration does not require a flame.

Photosynthesis requires an energy input to form energy-rich glucose from carbon dioxide and water. Plants absorb energy from sunlight to drive this reaction; therefore, sunlight is written above the arrow and the process is energy absorbing. Option C is incorrect because sunlight is not a catalyst: a catalyst generally remains unchanged after a reaction, whereas light is the source of energy for this process.

During digestion, enzymes convert complex food substances such as carbohydrates, proteins, and fats into simpler new substances. The formation of new substances is the characteristic of a chemical change. In contrast, chewing only breaks food into smaller pieces, changing mainly its size and shape without forming a new substance. Therefore, option B is incorrect.

Iron is above copper in the reactivity series. Therefore, iron displaces copper from copper sulphate solution:


\(
\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}
\)

The iron sulphate formed gives the solution a light-green colour, while displaced copper deposits as a reddish-brown coating on the nail. Option D is incorrect because dilution alone cannot produce a solid copper deposit.

A colour change may indicate a chemical reaction, but by itself it does not prove that a new substance has formed. A change in colour can also be observed during physical processes such as mixing or dilution. Therefore, additional evidence, such as gas evolution, precipitate formation, temperature change, or a suitable test for a new substance, is needed. Option B is incorrect because many chemical reactions do not evolve a gas.

Evolution of a gas is an observable sign of a chemical reaction. A cooling container suggests that the reaction may be taking heat from its surroundings, so it may be endothermic. However, these observations alone do not establish that combustion or rusting definitely occurred.

A skeletal equation shows the correct chemical formulae of reactants and products, but the number of atoms may not be equal on the two sides. In a balanced equation, the number of atoms of each element is the same on the reactant and product sides, in accordance with the law of conservation of mass. For example, \(\mathrm{H_2 + O_2 \rightarrow H_2O}\) is skeletal, whereas \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\) is balanced. Only coefficients are changed during balancing; chemical formulae are not changed.

In a chemical reaction, the chemical composition of the reactants changes and new substances are formed. These substances have properties different from those of the original substances, so their formation is a clear indication of a chemical change. In contrast, changing only the position, shape, or container does not form a new substance; these are physical changes.

In \(2Mg + O_2 \rightarrow 2MgO\), magnesium and oxygen combine to form only one compound, MgO; therefore, it is a combination reaction. Magnesium changes its oxidation state from 0 to +2, while oxygen changes from 0 to −2. Thus, magnesium is oxidised and oxygen is reduced, so the reaction is also a redox reaction. Option D is also a redox reaction, but it is a displacement reaction rather than a combination reaction.

In \(H_2O_2\), oxygen has oxidation number \(-1\). One portion becomes \(-2\) in water, \(H_2O\), so it is reduced; another portion becomes 0 in \(O_2\), so it is oxidised. Therefore, this is a disproportionation reaction. In option B, zinc is oxidised and \(Cu^{2+}\) is reduced, but they are different reactants.

In a precipitation reaction, ions from two aqueous solutions combine to form an insoluble solid that separates from the solution. Therefore, option A is the clear indication of precipitation. Option B represents combustion/combination, whereas option C represents decomposition by electrolysis.

According to the law of conservation of mass, mass is neither created nor destroyed in a chemical reaction. Since no substance leaves the closed container, the total mass of the products equals the total mass of the reactants. Therefore, the mass of the products is 12 g. The 24 g option is incorrect because a reaction does not double the mass.

Subscripts show the number of atoms of each element in a chemical formula. Changing the subscript 2 in \(\mathrm{H_2O}\) changes the composition, so the substance is no longer water. An equation is balanced by changing coefficients placed before formulae, not by changing subscripts within them.

The balanced equation for forming water is
0(
02 ext{H}_2+ ext{O}_2
ightarrow2 ext{H}_2 ext{O}
0)
0. The 2 water molecules on the product side contain 2 oxygen atoms altogether. One
0(
0 ext{O}_2
0)
0 molecule also contains 2 oxygen atoms, so 1 oxygen molecule is required on the reactant side. Using two oxygen molecules would not balance the oxygen atoms.

Each \(\mathrm{MgO}\) unit contains one magnesium atom. Therefore, \(2\mathrm{MgO}\) contains two magnesium atoms in total. To conserve atoms on both sides, the reactant side must contain \(2\mathrm{Mg}\). Oxygen is written as \(\mathrm{O}_2\), but this does not change the number of magnesium atoms required.