Question

When hydrogen sulfide, \(\mathrm{H}_{2} \mathrm{~S}\), gas is bubbled through a solution of lead(II) nitrate, \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\), a black precipitate of lead(II) sulfide, \(\mathrm{PbS},\) forms, and nitric acid, \(\mathrm{HNO}_{3},\) is produced. Write the unbalanced chemical equation for this reaction.

Short answer

The unbalanced chemical equation for the reaction between hydrogen sulfide and lead(II) nitrate is: \(\mathrm{H}_{2}\mathrm{S} + \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \rightarrow \mathrm{PbS} + \mathrm{HNO}_{3}\).

Step-by-step solution

Identify the reactants

The reactants in this reaction are hydrogen sulfide, \(\mathrm{H}_{2}\mathrm{S}\), and lead(II) nitrate, \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\).

Identify the products

The products in this reaction are lead(II) sulfide, \(\mathrm{PbS}\), and nitric acid, \(\mathrm{HNO}_{3}\).

Combine the reactants and products into a chemical equation

The unbalanced chemical equation for this reaction is given by: \(\mathrm{H}_{2}\mathrm{S} + \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \rightarrow \mathrm{PbS} + \mathrm{HNO}_{3}\)

Key concepts

Chemical Reactions

Understanding chemical reactions is fundamental to the study of chemistry. A chemical reaction is a process where reactants transform into products through the breaking and forming of chemical bonds. Revealing the nature of these reactions allows scientists to manipulate conditions to create a vast array of products, from new medicines to sustainable fuels.

In the exercise example, we encounter a reaction where hydrogen sulfide gas reacts with a solution of lead(II) nitrate to produce lead(II) sulfide and nitric acid. Each substance in this reaction has a distinct identity, represented by a chemical formula, such as \( \mathrm{H}_{2}\mathrm{S} \) for hydrogen sulfide and \( \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \) for lead(II) nitrate. These formulas are not arbitrarily given but are derived based on principles centred around the atoms involved and how they bond together.

When writing chemical equations, it's essential to start with the correct formulas for all reactants and products. This ensures that the subsequent steps, like balancing the equation, are done with precision. It's also important for students to recognize that the equation represents a shorthand description of a sometimes complex molecular transformation.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two solutions combine and form an insoluble solid, known as a precipitate. This reaction is fundamental to fields ranging from geochemistry to everyday plumbing.

In our given exercise, the black precipitate of lead(II) sulfide \( \mathrm{PbS} \) formed when hydrogen sulfide and lead(II) nitrate come into contact is a prime example of a precipitation reaction. While the other product, nitric acid \( \mathrm{HNO}_{3} \) remains in the aqueous phase, the insoluble \( \mathrm{PbS} \) separates out as a distinct solid.

One key point of emphasis for students is that not all combinations of ionic compounds lead to precipitation. It requires understanding the solubility rules to predict whether a solid will form. For example, sulfides often form precipitates with heavy metal ions like lead(II), as in the example. Visual observation of a solid forming is a strong indication that a precipitation reaction has occurred, but knowledge of the solubility rules helps us understand the process before the experiment is even conducted.

Stoichiometry

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It's through stoichiometry that chemists calculate the amounts of substances consumed and produced in a reaction.

The exercise shows the unbalanced equation \( \mathrm{H}_{2}\mathrm{S} + \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \rightarrow \mathrm{PbS} + \mathrm{HNO}_{3} \). However, to fully understand the quantities involved, we need to balance this equation. In a balanced chemical equation, the number of atoms for each element is the same on both sides, adhering to the law of conservation of mass.

For students, the art of balancing equations resides in setting up a system where all atoms are accounted for by adding coefficients to the chemical formulas. This step might involve trial and error at first, but with practice, it becomes a more intuitive process. Stoichiometry not only helps us balance chemical equations but also allows predicting the amounts of substances required or produced, thus enabling the practicable application of chemistry in real-world scenarios.