Question

Which of the following reactions is a formation reaction? If it is not a formation reaction, explain why. a) \(\mathrm{H}_{2}(\mathrm{~g})+\mathrm{S}(\mathrm{s})+2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{H}_{2} \mathrm{SO}_{4}(\ell)\) b) \(\mathrm{C}_{3} \mathrm{H}_{8}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 3 \mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{H}_{2} \mathrm{O}(\ell)\)

Short answer

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

Step-by-step solution

Determine the Definition of a Formation Reaction

A formation reaction is a reaction where one mole of a compound is formed from its elements in their standard states. The standard states are the most stable forms of the element at 1 atm and 25 degrees Celsius.

Analyze Reaction (a)

The reaction given is \( \mathrm{H}_{2}(\mathrm{~g})+\mathrm{S}(\mathrm{s})+2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{H}_{2} \mathrm{SO}_{4}(\ell) \). Here, hydrogen gas \( \mathrm{H}_{2}(\mathrm{~g}) \), sulfur in solid form \( \mathrm{S}(\mathrm{s}) \), and oxygen gas \( \mathrm{O}_{2}(\mathrm{~g}) \) are reacting to form sulfuric acid \( \mathrm{H}_{2} \mathrm{SO}_{4}(\ell) \), which is a compound. Furthermore, the elements are present in their standard states and only one mole of the compound is formed. Hence, this is a formation reaction.

Analyze Reaction (b)

The reaction given is \( \mathrm{C}_{3} \mathrm{H}_{8}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 3 \mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{H}_{2} \mathrm{O}(\ell) \). This is a combustion reaction of propane \( \mathrm{C}_{3} \mathrm{H}_{8}(\mathrm{~g}) \) with oxygen, forming carbon dioxide \( \mathrm{CO}_{2}(\mathrm{~g}) \) and water \( \mathrm{H}_{2} \mathrm{O}(\ell) \). It involves a hydrocarbon reacting with oxygen, rather than forming a single compound from elements in their standard states. Therefore, this is not a formation reaction.

Key concepts

Standard States

Understanding the concept of standard states is crucial when analyzing chemical reactions, especially formation reactions. **Standard states** refer to the most stable forms of elements under specific conditions: 1 atmosphere pressure (atm) and 25 degrees Celsius (298 K).
Let's break this down:

• Most stable form: Each element has a form which is its most stable at the given conditions. For instance, oxygen is most stable as \( ext{O}_2( ext{g}) \) under standard conditions, while sulfur is stable as \( ext{S}( ext{s}) \) (solid).
• Implication for reactions: In formation reactions, elements must be in these standard states to meet the specific criteria for the reaction.

By understanding how standard states operate, you can better determine whether a reaction qualifies as a formation reaction based on the presence of elements in their standard states.

Chemical Equations

Chemical equations are a symbolic way of expressing chemical reactions, providing valuable information about the processes occurring at the atomic level. A **chemical equation** must be properly balanced to reflect the conservation of mass and the principle that matter is neither created nor destroyed.
Consider the following key aspects:

• Reactants and Products: In any chemical equation, reactants are written on the left, and products are written on the right, separated by an arrow. For example, in the reaction \( ext{H}_2( ext{~g})+ ext{S}( ext{s})+2 ext{O}_2( ext{~g}) ightarrow ext{H}_2 ext{SO}_4( ext{l}) \), the reactants are hydrogen, sulfur, and oxygen, while the product is sulfuric acid.
• Balancing Importance: Balancing the equation ensures that the number of each type of atom on both sides remains equal. This is essential for correctly representing the physical reality of the reaction.

By examining these equations carefully, one can understand the type and quantity of substances involved and how they transform, which is especially important for identifying reaction types, including formation reactions.

Reaction Types

Different kinds of chemical reactions have distinct characteristics. Among them, **formation reactions** and **combustion reactions** are two crucial types with unique properties:

• Formation Reactions: These involve forming one mole of a compound from its elemental constituents in their standard states. According to our example, the reaction \( ext{H}_2( ext{~g})+ ext{S}( ext{s})+2 ext{O}_2( ext{~g}) ightarrow ext{H}_2 ext{SO}_4( ext{l}) \) is a perfect illustration since sulfuric acid is formed from elements in their standard states.
• Combustion Reactions: These occur when hydrocarbons react with oxygen, producing carbon dioxide and water. An example is the reaction \( ext{C}_3 ext{H}_8( ext{~g})+5 ext{O}_2( ext{~g}) ightarrow 3 ext{CO}_2( ext{~g})+4 ext{H}_2 ext{O}( ext{l}) \)— a combustion reaction as it results in carbon dioxide and water, not a single compound from its elements.

Understanding these types helps in classifying reactions and predicting their outcomes. This knowledge can clarify why some reactions fit into the formation category while others do not.