Question

Which of the following reactions is a formation reaction? If it is not a formation reaction, explain why. a) \(\mathrm{H}_{2}(\mathrm{~g})+\mathrm{S}(\mathrm{s}) \rightarrow \mathrm{H}_{2} \mathrm{~S}(\mathrm{~g})\) b) \(2 \mathrm{HBr}(\mathrm{g})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{HCl}(\mathrm{g})+\mathrm{Br}_{2}(\ell)\)

Short answer

Reaction (a) is a formation reaction; (b) is not.

Step-by-step solution

Identify Formation Reaction Criteria

A formation reaction involves the formation of one mole of a compound from its elements in their standard states. These elements must be in their most stable form and the reaction should form only a single product.

Analyze Reaction (a)

Check the reaction: \( \mathrm{H}_{2}( ext{g}) + \mathrm{S}( ext{s}) \rightarrow \mathrm{H}_{2} \mathrm{~S}( ext{g}) \). The reactants, \( \mathrm{H}_{2} \) gas and \( \mathrm{S} \) solid, are indeed elements in their standard states, and the reaction produces 1 mole of \( \mathrm{H}_{2} \mathrm{~S} \), which meets the criteria for a formation reaction.

Analyze Reaction (b)

Check the reaction: \( 2 \mathrm{HBr}( ext{g}) + \mathrm{Cl}_{2}( ext{g}) \rightarrow 2 \mathrm{HCl}( ext{g}) + \mathrm{Br}_{2}( ext{l}) \). This reaction involves two reactants that are compounds (\( \mathrm{HBr} \) and \( \mathrm{Cl}_{2} \)) and results in multiple products. It does not meet the criteria for a formation reaction, which requires only elements in their standard states, and only one mole of a compound formed.

Key concepts

Standard States

In chemistry, the concept of standard states refers to the most stable physical form of an element or compound at a specified set of conditions, typically 298 K (25°C) and 1 atmosphere pressure. These conditions are meant to lift any ambiguity regarding phases present during reactions.

Standard states are crucial when representing thermodynamic data, such as enthalpy or Gibbs free energy. For example:

• Gases like oxygen (\(\mathrm{O}_2)\) are found in their diatomic state at room temperature.
• Elements like carbon are in their standard state as graphite rather than diamond.

Standard states allow chemists to have a baseline for calculations, providing consistent starting points for discussing reactions and energy changes. Understanding the standard state of each element is fundamental in various chemical computations.

Stable Form

Stability in chemistry typically refers to an element or compound's propensity to remain in its given state without changing or reacting. An element's stable form is the version of that element that prevails under standard conditions.

Carbon, as mentioned earlier, has graphite as its stable form rather than diamond because graphite's structure is more energetically favorable and resistant to change under normal environmental conditions.

The stable form of an element is inherently tied to its conventional use in formation reactions as it defines how elements combine to create new compounds. Without considering stable forms, the results of chemical reactions would be inconsistent and unpredictable.

Chemical Reactions

Chemical reactions entail the process of substances (reactants) transforming into new substances (products). This can involve the breaking and forming of chemical bonds, observable through changes in temperature, color, or the emission of gases.

Specific types of reactions, such as formation reactions, are noteworthy because they strictly involve converting elements in their standard states to form one mole of a compound as the sole product. A classic example is the formation of water from hydrogen and oxygen:\[\mathrm{H}_2 \text{(g)} + \frac{1}{2} \mathrm{O}_2 \text{(g)} \rightarrow \mathrm{H}_2\mathrm{O} \text{(l)}\]

This requirement differentiates formation reactions from others, like combination or decomposition reactions, by their simplicity and specific conditions. Understanding these elements helps differentiate and classify reactions effectively.