Write a balanced chemical equation for each chemical reaction. (a) Solid lead(II) sulfide reacts with aqueous hydrochloric acid to form solid lead(II) chloride and dihydrogen sulfide gas. (b) Gaseous carbon monoxide reacts with hydrogen gas to form gaseous methane \(\left(\mathrm{CH}_{4}\right)\) and liquid water. (c) Solid iron(III) oxide reacts with hydrogen gas to form solid iron and liquid water. (d) Gaseous ammonia \(\left(\mathrm{NH}_{3}\right)\) reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

In our journey to understand science, one of the foundational blocks is the concept of a chemical reaction. A chemical reaction is a process where reactants transform into products through the making and breaking of chemical bonds. The breathtaking range of materials and phenomena around us are governed by these reactions, making them crucial to nearly all fields of science and technology.

For students, it's vital to recognize that during these transformations, the type of atoms involved doesn't change, but their rearrangement leads to new substances with different properties. For example, in a simple chemical reaction, hydrogen gas can react with oxygen gas to form water. This process is not only a chemical reaction but a tangible demonstration of the conservation of matter, as the total mass of reactants equals the mass of the products. This conservation is represented in a balanced chemical equation, which is the heart of chemical reactions and a frequent topic in homework assignments.

Stoichiometry is an essential tool in chemistry that involves the calculation of the quantities of substances involved in chemical reactions. It's based on the laws of conservation of mass and the concept of the mole, a unit signifying a specific number of molecules or atoms.

To understand stoichiometry, consider being a chef following a recipe: Just as a recipe requires precise amounts of ingredients to make your favorite dish, stoichiometry provides the coefficients needed to balance a chemical equation. The coefficients tell us the proportion of molecules of each reactant and product needed to satisfy the law of conservation of mass. For example, the balanced equation for the reaction between lead(II) sulfide and hydrochloric acid is expressed as \(PbS(s) + 2HCl(aq) \rightarrow PbCl_2(s) + H_2S(g)\), indicating that one molecule of lead(II) sulfide reacts with two molecules of hydrochloric acid to produce products.

Chemical formulas provide a wealth of information about a substance. They tell us not only which elements are present, but also in what quantities. Not to be confused with chemical names, formulas convey the composition of compounds using symbols and numbers.

Consider water, \(H_2O\), one of the most well-known chemical formulas. From its formula, we can tell water is composed of two hydrogen atoms and one oxygen atom bonded together. Chemical formulas can become more complex for larger molecules and are fundamental when it comes to chemical reactions. For instance, the balanced chemical equation for the reaction of iron(III) oxide with hydrogen gas, \(Fe_2O_3(s) + 3H_2(g) \rightarrow 2Fe(s) + 3H_2O(l)\), highlights the conversion from reactants to products using their chemical formulas. This is a visualization of how we express quantities in a reaction and balance the equation accurately.