Question

Classify the following redox reactions as combination, decomposition, or displacement: (a) \(\mathrm{P}_{4}+10 \mathrm{Cl}_{2} \longrightarrow 4 \mathrm{PCl}_{5}\) (b) \(2 \mathrm{NO} \longrightarrow \mathrm{N}_{2}+\mathrm{O}_{2}\) (c) \(\mathrm{Cl}_{2}+2 \mathrm{KI} \longrightarrow 2 \mathrm{KCl}+\mathrm{I}_{2}\) (d) \(3 \mathrm{HNO}_{2} \longrightarrow \mathrm{HNO}_{3}+\mathrm{H}_{2} \mathrm{O}+2 \mathrm{NO}\)

Short answer

(a) Combination; (b) Decomposition; (c) Displacement; (d) Decomposition.

Step-by-step solution

Recognize the Reaction Type for (a)

In equation (a), we observe that two reactants, \( \mathrm{P}_4 \) and \( 10 \mathrm{Cl}_2 \), combine to form a single product, \( 4 \mathrm{PCl}_5 \). This is indicative of a combination reaction.

Recognize the Reaction Type for (b)

In equation (b), the single compound \( 2 \mathrm{NO} \) breaks down into two simpler substances, \( \mathrm{N}_2 \) and \( \mathrm{O}_2 \). This suggests a decomposition reaction.

Recognize the Reaction Type for (c)

Here in equation (c), we see \( \mathrm{Cl}_2 \) displacing \( \mathrm{I}_2 \) from \( 2 \mathrm{KI} \), forming \( 2 \mathrm{KCl} \) and \( \mathrm{I}_2 \). This identifies the reaction as a displacement reaction.

Recognize the Reaction Type for (d)

In equation (d), a single compound \( 3 \mathrm{HNO}_2 \) is breaking down into multiple products: \( \mathrm{HNO}_3 \), \( \mathrm{H}_2 \mathrm{O} \), and \( 2 \mathrm{NO} \). It is a decomposition reaction because a complex molecule is breaking into simpler substances.

Key concepts

Combination Reactions

Combination reactions are a fundamental type of chemical reaction where two or more substances combine to form a single, more complex product. These reactions are sometimes also referred to as synthesis reactions. They can occur between elements or compounds.

An example is the reaction: \( \mathrm{P}_4 + 10 \mathrm{Cl}_2 \rightarrow 4 \mathrm{PCl}_5 \). Here, phosphorus \( \mathrm{P}_4 \) and chlorine gas \( \mathrm{Cl}_2 \) react to form phosphorus pentachloride \( \mathrm{PCl}_5 \).

• Characteristics of Combination Reactions: These reactions typically release energy in the form of heat or light.
• Examples include the formation of water from hydrogen and oxygen.
• These reactions are important in various industrial processes, such as the formation of metal oxides from metals and oxygen.

Understanding combination reactions is crucial as they represent the formation of new compounds with unique properties compared to the reactants.

Decomposition Reactions

Decomposition reactions are the opposite of combination reactions. This type of reaction involves a single compound breaking down into two or more simpler substances. Decomposition reactions often require an input of energy in the form of heat, light, or electricity.

Examples from our exercise include: \( 2 \mathrm{NO} \rightarrow \mathrm{N}_2 + \mathrm{O}_2 \) and \( 3 \mathrm{HNO}_2 \rightarrow \mathrm{HNO}_3 + \mathrm{H}_2 \mathrm{O} + 2 \mathrm{NO} \).

• Characteristics of Decomposition Reactions: These reactions are usually endothermic, causing the system to absorb energy.
• They play a vital role in recycling materials in nature, such as in the decomposition of dead organisms.
• Common examples include the breakdown of hydrogen peroxide into water and oxygen.

Decomposition reactions are important for processes like respiration in biology and decomposition of materials in ecological systems.

Displacement Reactions

Displacement reactions, also known as replacement reactions, occur when an element displaces another element in a compound, resulting in the formation of a new element and a new compound. There are two main types: single and double displacement reactions.

In the reaction \( \mathrm{Cl}_2 + 2 \mathrm{KI} \rightarrow 2 \mathrm{KCl} + \mathrm{I}_2 \), chlorine displaces iodine from potassium iodide. This is a common example of a single displacement reaction.

• Characteristics of Displacement Reactions: These reactions generally involve ions, and can occur in aqueous solutions.
• Single displacement reactions involve one element replacing another in a compound.
• They are common in the extraction of metals and purification processes.

Understanding displacement reactions aids in comprehending complex chemical processes including industrial syntheses and metallurgical operations.

Reaction Types

Chemical reactions can be broadly classified based on the nature and changes in the substances involved. Understanding different reaction types helps in predicting the products and conditions needed for a reaction to occur.

Common reaction types include:

• Combination Reactions: Two or more reactants form one product.
• Decomposition Reactions: A single compound breaks down into simpler substances.
• Displacement Reactions: An element displaces another in a compound, forming new products.
• Combustion Reactions: Substances react with oxygen, releasing energy in the form of light or heat.

By identifying and classifying reactions, chemists can efficiently organize chemical information and devise methods for chemical synthesis and analysis. These classifications are foundational concepts for anyone learning chemistry.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They provide a concise and clear way to describe the reactants, products, and the quantitative relationships between them. A chemical equation shows the starting substances, the reactants, and the substances formed, the products.

For instance, the equation \( \mathrm{P}_4 + 10 \mathrm{Cl}_2 \rightarrow 4 \mathrm{PCl}_5 \) tells us:

• The reactants are \( \mathrm{P}_4 \) and \( \mathrm{Cl}_2 \), while \( \mathrm{PCl}_5 \) is the product.
• The coefficients indicate the relative amounts of each substance involved.
• It follows the law of conservation of mass, meaning the mass of reactants equals the mass of the products.

Chemical equations are essential tools in chemistry, allowing for the stoichiometric calculations required to predict yields and optimize reactions for various applications.