Question

What is a synthesis or combination reaction? Give an example. Can such reactions also be classified in other ways? Give an example of a synthesis reaction that is also a combustion reaction. Give an example of a synthesis reaction that is also an oxidation-reduction reaction, but that does not involve combustion.

Short answer

A synthesis or combination reaction is a chemical reaction where two or more simple substances combine to form a more complex product, represented as A + B → AB. An example is the formation of water: \(2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\). Synthesis reactions can also be classified as combustion reactions, such as the burning of methane: CH\(_4 (g) + 2O_2 (g) \rightarrow CO_2 (g) + 2H_2O (g)\), or oxidation-reduction reactions, like the formation of hydrogen chloride: \(H_2 (g) + Cl_2 (g) \rightarrow 2HCl (g)\).

Step-by-step solution

Definition of a Synthesis or Combination Reaction

A synthesis or combination reaction is a type of chemical reaction in which two or more simple substances (elements or compounds) combine to form a more complex product. The general form of a synthesis reaction is: A + B → AB

Example of a Synthesis Reaction

One example of a synthesis reaction is the formation of water from its elements hydrogen and oxygen: \(2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\)

Classification in Other Ways

Yes, synthesis reactions can also be classified in other ways, such as combustion reactions and oxidation-reduction reactions.

Example of a Synthesis Reaction that is also a Combustion Reaction

A combustion reaction involves the reaction of a substance with oxygen to produce energy in the form of heat and light. An example of a synthesis reaction that is also a combustion reaction is the burning of methane in oxygen to produce carbon dioxide and water: CH\(_4 (g) + 2O_2 (g) \rightarrow CO_2 (g) + 2H_2O (g)\) In this reaction, methane combines with oxygen to form the more complex products of carbon dioxide and water, while also releasing heat and light.

Example of a Synthesis Reaction that is also an Oxidation-Reduction Reaction

An oxidation-reduction (redox) reaction involves the transfer of electrons between two substances. A synthesis reaction that is also a redox reaction, but does not involve combustion, is the formation of hydrogen chloride from its elements hydrogen and chlorine: \(H_2 (g) + Cl_2 (g) \rightarrow 2HCl (g)\) In this reaction, hydrogen donates an electron to chlorine, resulting in the formation of the hydrogen chloride molecule.

Key concepts

Combination Reaction

In chemistry, a combination reaction, also known as a synthesis reaction, occurs when two or more reactants unite to form a single product. This process is fundamental to the concept of chemical bonding, where atoms or molecules merge to produce more complex compounds.

Consider the formation of water from hydrogen and oxygen:
\(2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\).

In this reaction, two hydrogen molecules (\(H_2\)) and one oxygen molecule (\(O_2\)) combine to create two molecules of water (\(H_2O\)). The simplicity of the reaction ensures that the key principle of combination reactions is easily understood: substances come together to yield a product with new properties.

Oxidation-Reduction Reaction

Diving deeper into the world of chemistry, an oxidation-reduction reaction, commonly known as a redox reaction, involves electron transfer between substances. Oxidation is the loss of electrons, and reduction is the gain of electrons. In a redox process, there is always one substance being oxidized while another is being reduced.

For example, when hydrogen (\(H_2\)) reacts with chlorine (\(Cl_2\)) to form hydrogen chloride (\(HCl\)), hydrogen is oxidized as it gives up electrons to chlorine, which is reduced because it accepts those electrons:
\(H_2 (g) + Cl_2 (g) \rightarrow 2HCl (g)\).

In this synthesis that doubles as a redox reaction, no combustion takes place. Yet, the transfer of electrons characterizes it as a redox reaction. This demonstrates how synthetic reactions can overlap with other classification types in chemistry.

Combustion Reaction

Lastly, a combustion reaction is a high-energy reaction involving a substance (typically a hydrocarbon) and oxygen, resulting in the production of heat and light. A hallmark of combustion reactions is their exothermic nature, meaning they release energy.

An example can be seen when methane, (\(CH_4\)), burns in oxygen to form carbon dioxide, (\(CO_2\)), and water vapor, (\(H_2O\)):
\(CH_4 (g) + 2O_2 (g) \rightarrow CO_2 (g) + 2H_2O (g)\).

This reaction is not only a synthesis reaction but also a combustion reaction, as methane and oxygen combine to form other chemicals with the evolution of energy. This illustrates how some synthesis reactions can also fall under the category of combustion, displaying the interconnectedness of these chemical processes.