Write the balanced chemical equations for (a) the complete combustion of acetic acid \(\left(\mathrm{CH}_{3} \mathrm{COOH}\right)\), the main active ingredient in vinegar; \((\mathbf{b})\) the decomposition of solid calcium hydroxide into solid calcium(II) oxide (lime) and water vapor; (c) the combination reaction between nickel metal and chlorine gas.

Chemical reactions are essential to understanding the world around us, and one of the most fundamental types is the combustion reaction. In its essence, a combustion reaction involves a substance (typically a hydrocarbon) reacting with oxygen to release heat and form new products.
A classic example, as given in the exercise, is the combustion of acetic acid \(\text{CH}_3\text{COOH}\). Here, the acetic acid reacts with oxygen, resulting in carbon dioxide \(\text{CO}_2\) and water \(\text{H}_2\text{O}\). To ensure the reaction accurately represents the law of conservation of mass, a balancing step is crucial, where coefficients are added before reactants and products to balance the number of atoms of each element on both sides of the equation.

Real-World Applications

Combustion reactions are not just academic exercises; they occur in everyday life. For example, burning fossil fuels for energy, operating vehicles, and even cooking on a gas stove all involve combustion reactions. These reactions are also exothermic, which means they release heat, a principle exploited in heaters and internal combustion engines.
Moreover, understanding and correctly balancing combustion reactions is critical for fields such as environmental science, where the focus might be on reducing harmful emissions resulting from these reactions.

While combustion reactions are marked by the formation of new substances through the combination of reactants, decomposition reactions take the opposite route. They involve the breakdown of a compound into simpler substances or elements. Specifically, it's an important type of chemical reaction where a single compound separates to form two or more simpler substances.
The exercise provided offers the decomposition of calcium hydroxide \(\text{Ca}(\text{OH})_2\) into calcium(II) oxide \(\text{CaO}\) and water vapor \(\text{H}_2\text{O}\) as an example. Notably, the original compound, calcium hydroxide, contains chemical bonds that are broken in this process, thereby creating simpler compounds.

Importance in Various Fields

Decomposition reactions are a cornerstone of understanding chemical processes like the digestion of food in our bodies, the decomposition of organic matter in composting, and even the breakdown of rocks and minerals in geology. In industry, these reactions are pivotal in the production of lime for construction materials and the processing of metals. In environmental contexts, decomposition plays a role in the lifecycle of substances and their eventual return to simpler forms in nature.

Opposite to decomposition reactions, combination reactions involve two or more substances coming together to form a single product. These reactions are fundamental to both the formation of compounds and an array of synthesis processes across chemistry.
In the exercise given, nickel metal \(\text{Ni}\) reacts with chlorine gas \(\text{Cl}_2\) to form nickel(II) chloride \(\text{NiCl}_2\). This reaction showcases how elements can combine to form a more complex compound. Similar to other types of reactions, it is paramount to balance the equation to comply with the law of conservation of mass. This is done by adjusting coefficients to equalize the number of atoms participating in the reaction from reactants to products.

Significance in Technology and Industry

Combination reactions have wide-ranging applications, such as in manufacturing processes where new materials are synthesized. They also occur in the human body, where elements like calcium and phosphate combine to form bone. In the production of alloys and compounds crucial for electronics and other advanced technologies, understanding these reactions is essential. Consciously managing these reactions allows scientists to develop new materials and technologies, underlining the importance of combination reactions in innovation and progress.