Chemical Reactions and Equations

In a chemical equation, every reactant and product is represented by its chemical formula, such as \(H_2O\) for water and \(CO_2\) for carbon dioxide. Therefore, a substance is identified by its chemical formula. State symbols such as \((s)\), \((l)\), and \((g)\) show only the physical state, not the identity of the substance. Exam tip: Always write correct formulae while writing a chemical equation.

The new substances formed in a chemical reaction are called products, and they are written on the right side of the arrow. For example, in \(2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}\), \(\mathrm{H}_2\mathrm{O}\) is the product and is written on the right. The substances on the left are reactants.

The starting substances in a reaction are called reactants, and they are written on the left side of the arrow in a chemical equation. Products formed by the reaction are written on the right side. For example, in \(2H_2 + O_2 \rightarrow 2H_2O\), \(H_2\) and \(O_2\) are reactants, while \(H_2O\) is the product.

The arrow in a chemical equation separates the reactants from the products and points from the substances that react toward the substances formed. It therefore communicates the direction of the reaction and the formation of products, making option B correct. A colour change may be an observation, but it is not the meaning of the equation arrow; neither weight loss nor a container change is represented by it.

The production of light indicates an energy change and can be a sign that a chemical reaction is occurring. For example, when magnesium burns, it produces a bright white light and forms a new substance, magnesium oxide. In contrast, a change of shape, filtration, and freezing are generally physical changes in which no new substance is formed.

Magnesium reacts vigorously with oxygen in an exothermic reaction: \(2Mg + O_2 \rightarrow 2MgO\). Energy is released as heat and dazzling white light, and white magnesium oxide powder is formed. Unlike option C, the light is not due to magnesium oxide glowing on its own; it is due to energy released during the reaction.

Magnesium oxide reacts with water to form magnesium hydroxide: \(\mathrm{MgO + H_2O \rightarrow Mg(OH)_2}\). Magnesium hydroxide is basic, so the resulting aqueous mixture is basic in nature. A neutral mixture has neither acidic nor basic properties, whereas the hydroxide formed here shows basic behaviour.

Calcium oxide (CaO) reacts with water (H₂O) to form calcium hydroxide (Ca(OH)₂): \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). This is a combination reaction, and the product is called slaked lime. In contrast, calcium carbonate is formed when a calcium compound reacts with carbon dioxide, not simply with water.

A skeletal equation shows the chemical formulae of reactants and products, but the number of atoms of each element need not be equal on both sides. For example, \(\mathrm{H_2 + O_2 \rightarrow H_2O}\) is a skeletal equation; when balanced, it becomes \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\). In a balanced chemical equation, the number of atoms of every element is equal on both sides.

Subscripts in chemical formulae must not be changed while balancing an equation, because changing a subscript changes the identity of the substance. For example, changing \(\mathrm{H_2O}\) to \(\mathrm{H_2O_2}\) changes water into hydrogen peroxide. An equation is balanced only by placing coefficients before formulae, so the number of molecules or moles and the quantitative ratio may change.

According to the law of conservation of mass, mass is neither created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants equals the total mass of the products. Temperature and volume may change, and the physical state of substances may also change.

In a combination reaction, two or more reactants combine to form a single product. Its general form is \(A + B \rightarrow AB\), so combination reaction is correct. In contrast, a decomposition reaction involves one compound breaking into two or more substances. Exam tip: If several reactants give only one product, identify it as a combination reaction.

A decomposition reaction occurs when one compound breaks into two or more simpler substances. Its general pattern is AB → A + B, so option B is correct. This is the reverse pattern of a combination reaction, A + B → AB. In displacement and double-displacement reactions, atoms or ions exchange partners rather than a single compound simply splitting into simpler products.

When calcium carbonate is heated, it breaks down into calcium oxide and carbon dioxide: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The breakdown of a compound into two or more simpler substances by heat is called thermal decomposition. In contrast, freezing water is only a change of state and does not form a new substance.

Electrolytic decomposition uses electrical energy to drive the breakdown of a compound into simpler substances, so option B is correct. For example, passing electricity through water can produce hydrogen and oxygen. Heat would describe thermal decomposition, while light can cause photodecomposition in suitable compounds; sound is not the energy form used in ordinary electrolysis.

In photolytic decomposition, light supplies the energy needed to break a compound into simpler substances. For example, silver chloride decomposes in sunlight to form silver and chlorine: \(2AgCl \xrightarrow{sunlight} 2Ag + Cl_2\). In contrast, decomposition caused by heat is thermal decomposition, so option A is not correct.

Silver chloride undergoes photodecomposition in sunlight according to the balanced equation 2AgCl(s) → 2Ag(s) + Cl₂(g). The metal formed is silver, so option B is correct. Chlorine is also produced, but it is a non-metal gas. Sodium and magnesium are not products of this reaction and are not present in silver chloride.

A more reactive metal displaces a less reactive metal from its salt solution or compound. For example, zinc displaces copper from copper sulphate solution: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). A less reactive metal cannot displace a more reactive metal.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The released copper deposits on the iron nail as a reddish-brown coating. Iron sulphate is formed in the solution, while zinc is not involved in this reaction.

Iron is more reactive than copper. It therefore displaces copper from copper sulphate and forms iron sulphate: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The \(\mathrm{FeSO_4}\) solution is pale green, while copper is deposited on the nail as a reddish-brown solid. Copper oxide would require reaction with oxygen and is not formed in this displacement reaction.