Question

Which of the following reactions is a formation reaction? If it is not a formation reaction, explain why. a) \(\mathrm{Zn}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{ZnCl}_{2}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})\) b) \(2 \mathrm{Na}(\mathrm{s})+\mathrm{C}(\mathrm{s})+3 / 2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3}(\mathrm{~s})\)

Short answer

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

Step-by-step solution

Understanding Formation Reactions

A formation reaction is where exactly one mole of a compound is formed from its elements in their standard states. The reactants must be pure elements.

Evaluate Reaction (a)

Reaction a is: \( \mathrm{Zn} + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{H}_{2} \). Hydrogen chloride (\( \mathrm{HCl} \)) is not in its elemental form, so this cannot be a formation reaction.

Evaluate Reaction (b)

Reaction b is: \( 2 \mathrm{Na} + \mathrm{C} + \frac{3}{2} \mathrm{O}_{2} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3} \). This reaction forms one mole of a compound (\( \mathrm{Na}_{2} \mathrm{CO}_{3} \)) from elements (sodium, carbon, and oxygen) in their standard states, so it fits the definition of a formation reaction.

Key concepts

Chemical Reactions

Chemical reactions are processes that convert one or more substances into new substances. They entail the breaking and forming of chemical bonds, leading to different arrangements of atoms.
For instance, when zinc (\( \mathrm{Zn} \)) reacts with hydrochloric acid (\( \mathrm{HCl} \)), the chemical bonds in the reactants are broken. The reaction forms new bonds to create zinc chloride (\( \mathrm{ZnCl}_2 \)) and hydrogen gas (\( \mathrm{H}_2 \)).
All reactions have key components:

• **Reactants** - the initial substances involved in the reaction.
• **Products** - the new substances produced by the reaction.
• **Balanced Equation** - a representation of the reaction that ensures the conservation of mass and atoms.

It is essential to refer to both the nature and state of the substances involved when analyzing a chemical reaction.

Elemental States

Elemental states refer to the physical state (solid, liquid, gas) and the form of an element under standard conditions of temperature and pressure (25°C and 1 atm). These states are crucial for determining if a reaction is a formation reaction.
In a formation reaction, all reactants must be in their **standard elemental forms**. This means:

• Sodium (\( \mathrm{Na} \)) is a solid.
• Carbon (\( \mathrm{C} \)) is also a solid.
• Oxygen (\( \mathrm{O}_2 \)) is a gas.

These forms are considered 'standard' because they are the most stable physical forms of the elements at room temperature.
This concept is important because it ensures that the compounds formed in reactions are derived directly from their elemental states, without any prior chemical modification. This is a key feature in identifying formation reactions.

Compound Formation

Compound formation is the process of creating a new chemical compound from two or more elements in their basic forms. In a formation reaction, a compound is synthesized with a unique set of characteristics and a specific formula, resulting from the combination of its elemental components.
For example, in the formation of sodium carbonate (\( \mathrm{Na}_2 \mathrm{CO}_3 \)), the elements sodium, carbon, and oxygen react together. This reaction:\[2 \mathrm{Na} (s) + \mathrm{C} (s) + \frac{3}{2} \mathrm{O}_2 (g) \rightarrow \mathrm{Na}_2 \mathrm{CO}_3 (s) \]creates a compound, ensuring that the product, sodium carbonate, is formed exactly from one mole of the compound in its standard state.
Key considerations in compound formation include:

• **Correct Stoichiometry** - ensuring the right proportions of elements to match the desired compound.
• **Stable Compounds** - resulting compounds should be stable under standard conditions.

This process highlights the transformation of raw elements into complex and sometimes multifunctional compounds.