Question

Which of the following reactions is a formation reaction? If it is not a formation reaction, explain why. a) \(\mathrm{Fe}(\mathrm{g})+1 / 2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{FeO}(\mathrm{s})\) b) \(\mathrm{Hg}(\ell)+1 / 2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{HgO}(\mathrm{s})\)

Short answer

Reaction (b) is a formation reaction.

Step-by-step solution

Understand Formation Reaction Definition

A formation reaction is a chemical reaction where one mole of a compound is formed from its elements in their standard states.

Analyze Reaction (a)

The reaction given is \( \mathrm{Fe}(\mathrm{g})+1 / 2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{FeO}(\mathrm{s}) \). Iron (Fe) is in gaseous form rather than its standard state, which is solid (\( \mathrm{s} \)). Hence, this reaction is not a formation reaction.

Analyze Reaction (b)

The reaction given is \( \mathrm{Hg}(\ell)+1 / 2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{HgO}(\mathrm{s}) \). Mercury (Hg) is in its standard liquid state and oxygen (\( \mathrm{O}_2 \)) is in its standard gaseous state. The product \( \mathrm{HgO} \) forms one mole. Therefore, this is a formation reaction.

Key concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, transform into new substances, called products. These reactions can vary greatly. Some may involve simple changes, such as the phase change of a substance (e.g., ice melting). Others may involve complex processes that fundamentally change the molecular structure of the substances involved. In the context of formation reactions, a very specific type of chemical reaction occurs. Here, the objective is to form one mole of a compound from its constituent elements. This formation must specifically occur with each element in its most stable physical form at standard conditions, also known as their standard states. This type of reaction is important in thermodynamics for determining the standard enthalpy change. Key aspects include determining the correct reactants in their elemental forms and ensuring that the resultant product is indeed one mole of a compound. Understanding the intricacies of chemical reactions provides foundational knowledge useful for chemistry students and is central to interpreting formation reactions correctly.

Standard States

When discussing formation reactions, the concept of standard states becomes essential. The standard state of an element refers to its most stable physical form at a pressure of 1 bar and a specified temperature, typically 298 K (25°C). For example, the standard state of oxygen is as a diatomic gas, \(\mathrm{O}_2(\mathrm{g})\), while carbon's standard state is graphite. Iron, in most circumstances, is found naturally in a solid state, hence its standard state is solid iron (Fe(s)). However, in the original exercise, gaseous iron was used, which deviates from its standard form, thereby rendering it incorrect for a formation reaction. Standard states ensure consistency in chemical equations and reactions, especially in thermodynamic calculations such as forming reliable enthalpy values. It's crucial to remember that not all elements have intuitive standard states; for example, elements like bromine are liquids under standard conditions. Ensuring elements are in their standard states is fundamental when attempting to classify a reaction as a formation reaction.

Mole Concept

The mole concept is a fundamental principle in chemistry that provides a bridge between the atomic world and macroscopic measurements. One mole of a substance contains Avogadro's number of entities, which is approximately \(6.022 \times 10^{23}\) atoms, molecules, or ions, depending on the substance. In formation reactions, it's essential to form exactly one mole of the compound. This precision is required to align with the thermodynamic conventions used to express enthalpies of formation, which are specified per mole of product. For accurate stoichiometry, reactions must be balanced. The reactants must provide exactly the right number of atoms or molecules to form precisely one mole. In exercise example (b), \(\mathrm{Hg}(\ell) + \frac{1}{2} \mathrm{O}_2(\mathrm{g}) \rightarrow \mathrm{HgO}(\mathrm{s})\), it should create exactly one mole of mercury(II) oxide, a perfect illustration of applying the mole concept accurately in a chemical equation. Understanding the mole concept allows students to navigate and manipulate chemical equations confidently.