Chemical Reactions and Equations

In chemical equations, an upward arrow (↑) generally indicates that a gas is evolved during the reaction and escapes from the reaction mixture. For example, an acid reacting with a carbonate releases carbon dioxide gas. The arrow does not mean that a solid forms, water is produced, or heat is removed; those changes require different descriptions or symbols.

A downward arrow (↓) in a chemical equation generally indicates the formation of a precipitate, which is an insoluble solid produced in a solution. For example, mixing silver nitrate and sodium chloride forms solid silver chloride. The downward arrow therefore represents a solid settling out, whereas gas evolution is conventionally shown by an upward arrow.

A chemical equation in which the number of atoms of one or more elements differs on the reactant and product sides is called a skeletal equation. In a balanced chemical equation, the number of atoms of every element is equal on both sides; therefore, option B is not a skeletal equation.

A chemical formula shows the elements in a substance and their fixed ratio. Changing a subscript changes the identity of the substance; for example, H₂O is water, whereas H₂O₂ is hydrogen peroxide. Therefore, an equation is balanced by changing coefficients only, because coefficients change the amount of a substance, not its identity.

The addition of oxygen to a substance is called oxidation. For instance, magnesium combines with oxygen according to 2Mg + O₂ → 2MgO, so magnesium is oxidised. Reduction is generally associated with removal of oxygen or addition of hydrogen. Neutralisation involves acid and base, while displacement involves replacement of one element by another.

Removal of oxygen from a substance is called reduction. For example, in the reaction CuO + H₂ → Cu + H₂O, copper(II) oxide loses oxygen and is reduced to copper. Oxidation is the opposite idea when oxygen is added. Neutralisation concerns acid-base reactions, and decomposition describes one compound splitting into simpler substances, so neither is the best answer.

The balanced equation is 2H₂ + O₂ → 2H₂O. Two reactants, hydrogen and oxygen, combine to produce only one compound, water; this satisfies the definition of a combination reaction. A decomposition reaction starts with one compound and gives multiple products. Displacement reactions involve replacement, while double displacement exchanges ions between two compounds.

In photosynthesis, carbon dioxide reacts with water in the presence of sunlight and chlorophyll to form glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Thus, carbon dioxide is a gaseous reactant. Oxygen is mainly released as a product, while nitrogen and hydrogen are not the gaseous reactants in this standard equation.

A rise or fall in temperature indicates an energy change in the reaction. In an exothermic reaction, heat is released, so the temperature may rise; in an endothermic reaction, heat is absorbed, so the temperature may fall. A colour change can also be evidence of a chemical reaction, but a temperature change specifically indicates the release or absorption of heat, that is, an energy change.

An equation that states reactants and products by their names rather than by chemical formulae is called a word equation. For example: magnesium + oxygen → magnesium oxide. In contrast, a balanced chemical equation has the same number of atoms of each element on both sides.

In a balanced chemical equation, the number of atoms of each element is the same on the reactant and product sides. This represents the law of conservation of mass because atoms are neither created nor destroyed in a chemical reaction. In contrast, reaction rate depends on factors such as temperature, concentration, and catalysts; balancing an equation does not increase the rate.

Coefficients are written before chemical formulae and indicate the number of units of each substance involved. They are adjusted to make the number of atoms of every element equal on both sides of an equation, in accordance with the law of conservation of mass. In contrast, changing a subscript within a chemical formula changes the identity of the substance, so option B is incorrect.

Reactants are the starting substances that undergo chemical change and form products. In a chemical equation, they are conventionally written on the left side of the arrow; for example, in 2H₂ + O₂ → 2H₂O, hydrogen and oxygen are reactants. The new substances on the right are products, while a substance that changes reaction rate is a catalyst.

In a chemical reaction, reactants change to form new substances. These new substances are called products. In a chemical equation, products are written on the right-hand side of the arrow. Option B describes reactants, the substances present at the beginning of a reaction, whereas option C describes a catalyst.

On heating, calcium carbonate decomposes: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The breaking of one compound into two or more simpler substances by heat is called a thermal decomposition reaction. In contrast, in a combination reaction, two or more substances form a single product.

The decomposition of water by electric current is represented by \(2H_2O \xrightarrow{\text{electric current}} 2H_2 + O_2\). A decomposition reaction caused by electrical energy is called an electrolytic decomposition reaction, so option A is correct. Thermal decomposition requires heat, whereas photolytic decomposition requires light energy.

A decomposition reaction in which a compound breaks down in the presence of light is called photolytic decomposition. For example, silver chloride decomposes in sunlight: \(2\mathrm{AgCl} \xrightarrow{\text{sunlight}} 2\mathrm{Ag} + \mathrm{Cl}_2\). In contrast, thermal decomposition requires heat, while electrolytic decomposition is caused by electric current.

Silver bromide is a light-sensitive compound. On exposure to light, it undergoes photodecomposition to form silver and bromine: \(2AgBr \xrightarrow{\text{light}} 2Ag + Br_2\). The silver formed is grey, producing a visible colour change. Option C refers to formation of silver oxide by oxidation, not to photodecomposition.

A combination reaction occurs when two or more reactants join to produce one product. For example, 2Mg + O₂ → 2MgO has two reactants and one product, magnesium oxide. A decomposition reaction has the reverse pattern, with one compound producing multiple products. Displacement and double-displacement reactions involve replacement or ion exchange rather than simple formation of one product.

A decomposition reaction is one in which a single compound breaks into two or more simpler substances. For example, on heating, calcium carbonate decomposes as CaCO₃ → CaO + CO₂. This is the reverse pattern of combination, where several reactants form one product. Displacement and double-displacement reactions involve replacement or exchange between substances, not the breakdown of one compound alone.