Question

Which is a composition reaction and which is not? a) \(\mathrm{H}_{2}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl}\) b) \(2 \mathrm{HBr}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl}+\mathrm{Br}_{2}\)

Short answer

Reaction a is a composition reaction, reaction b is not.

Step-by-step solution

Identify composition reaction

A composition reaction, also known as a synthesis reaction, occurs when two or more substances combine to form a single product. Typically, the general form of such a reaction is \(A + B \rightarrow AB\).

Analyze reaction a

In reaction a), \( \mathrm{H}_2 + \mathrm{Cl}_2 \rightarrow 2 \mathrm{HCl} \), two substances (\(\mathrm{H}_2\) and \(\mathrm{Cl}_2\)) combine to form a single product (\(\mathrm{HCl}\)). This matches the pattern of a composition reaction.

Analyze reaction b

In reaction b), \( 2 \mathrm{HBr} + \mathrm{Cl}_2 \rightarrow 2 \mathrm{HCl} + \mathrm{Br}_2 \), the substances \(\mathrm{HBr}\) and \(\mathrm{Cl}_2\) do not form a single new product but instead rearrange to form \(\mathrm{HCl}\) and \(\mathrm{Br}_2\), which means this is not a composition reaction.

Key concepts

Synthesis Reaction

A synthesis reaction, also commonly referred to as a composition reaction, is one where two or more reactants combine to form a single product. It's like putting two LEGO blocks together to make a single structure. If you ever see a chemical equation that consists of multiple substances on the left and a single substance on the right, you are likely looking at a synthesis reaction. For example, in the reaction \(\mathrm{H}_2 + \mathrm{Cl}_2 \rightarrow 2\mathrm{HCl}\), two simple molecules combine to form hydrogen chloride, a single compound. This pattern – multiple reactants leading to one product – is a hallmark of composition reactions. These reactions are important in various processes such as the formation of compounds in nature, industrial manufacturing, and even our own body's chemical processes.

Understanding synthesis reactions is crucial as it forms the basis for many chemical dynamics. Recognizing these patterns allows scientists and students alike to predict and balance chemical equations with ease.

Chemical Equations

Chemical equations serve as the symbolic representation of chemical reactions. They depict the substances involved in a chemical reaction, including both reactants and products. Reactants are substances that start a reaction, while products are those formed as a result of the reaction. Each substance in a chemical equation is usually represented by its chemical formula. For instance, in the equation \(\mathrm{H}_2 + \mathrm{Cl}_2 \rightarrow 2\mathrm{HCl}\), \(\mathrm{H}_2\) and \(\mathrm{Cl}_2\) are the reactants, and \(\mathrm{HCl}\) is the product.

Balancing chemical equations is essential because it reflects the law of conservation of mass, which states that matter can neither be created nor destroyed in a chemical reaction. This means that the number of atoms of each element must be the same on both sides of the equation. For our example equation, we can confirm that both sides have equal numbers of hydrogen and chlorine atoms.

• Two hydrogen atoms on both sides.
• Two chlorine atoms on both sides.

This balance ensures that the chemical reaction is accurately represented.

Reaction Analysis

Analyzing reactions helps us determine the types of chemical reactions, such as synthesis, decomposition, or displacement reactions. By carefully examining the reactants and products, one can categorize the reaction and predict possible outcomes or products. Let's break down the analysis process with our example reactions:

1. **Reaction \(a\) Analysis:** In the reaction \(\mathrm{H}_2 + \mathrm{Cl}_2 \rightarrow 2\mathrm{HCl}\), we clearly see a synthesis reaction because two different substances combine to form a single new compound, \(\mathrm{HCl}\). This reaction shows a classic example of a composition reaction.
2. **Reaction \(b\) Analysis:** In the reaction \(2\mathrm{HBr} + \mathrm{Cl}_2 \rightarrow 2\mathrm{HCl} + \mathrm{Br}_2\), the reactants do not combine into a single new product. Instead, they rearrange to form different compounds - \(\mathrm{HCl}\) and \(\mathrm{Br}_2\). This characteristic marks it as a displacement rather than a synthesis reaction.

Through analysis, it's easier to understand why some reactions result in new mixtures and align this understanding with known reaction patterns. This essential skill in chemistry helps identify which reactions occur under certain conditions and how they are written and balanced as chemical equations.