Question

Which of the following is not an endothermic reaction? (a) combustion of methane (b) decomposition of water (c) dehydrogenation of ethane or ethylene (d) conversion of graphite to diamond

Short answer

The combustion of methane (a) is not an endothermic reaction.

Step-by-step solution

Define Endothermic vs Exothermic

An endothermic reaction is a chemical reaction that absorbs energy from its surroundings, usually in the form of heat. On the other hand, an exothermic reaction is a chemical reaction that releases energy to its surroundings.

Analyze Reaction (a)

The combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) is an exothermic reaction. It releases energy in the form of heat and light, which is why it's used as a fuel.

Analyze Reaction (b)

The decomposition of water (2H₂O → 2H₂ + O₂) is endothermic. This reaction requires energy input, typically from electricity in the process of electrolysis, to break the bonds in water molecules.

Analyze Reaction (c)

Dehydrogenation of ethane or ethylene is endothermic. It involves the removal of hydrogen (C₂H₆ → C₂H₄ + H₂ or C₂H₄ → C₂H₂ + H₂), requiring energy to break the chemical bonds.

Analyze Reaction (d)

The conversion of graphite to diamond is also an endothermic process. This transformation requires high input energy due to the need to change the structure of carbon atoms from graphite's planar layers to diamond's tetrahedral lattice.

Identify the Reaction Which is Not Endothermic

Among the options, the combustion of methane is the only exothermic reaction as it releases energy. All other reactions require energy input, making them endothermic.

Key concepts

Combustion Reaction

Combustion reactions are chemical processes that produce energy in the form of heat and often light. These reactions occur when a substance, typically a hydrocarbon like methane, reacts with oxygen. Combustion is an exothermic reaction because it releases energy by breaking and forming chemical bonds. For instance, the combustion of methane is represented by the equation: \[ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \]This reaction is widely used as a source of energy in fuel applications due to its ability to produce a significant amount of heat and light. As an exothermic process, combustion is the opposite of endothermic reactions, which absorb energy.

Decomposition Reaction

A decomposition reaction occurs when a single compound breaks down into two or more simpler substances. This type of reaction usually requires energy input, making it an endothermic process. Decomposition is often induced by heat, electricity, or light.The decomposition of water is a classic example. Through electrolysis, water breaks down into hydrogen and oxygen gases:\[ 2H_2O \rightarrow 2H_2 + O_2 \]This process requires electrical energy to break the bonds in the water molecules, underscoring its endothermic nature. In general, decomposition reactions are crucial in various applications, from industrial processes to biological systems, where energy absorption and substance breakdown are essential.

Dehydrogenation Reaction

Dehydrogenation reactions involve the removal of hydrogen from an organic molecule. These reactions are typically endothermic because they require energy to break molecular bonds. Dehydrogenation is important in various chemical industries, particularly for producing alkenes from alkanes.For example, in the dehydrogenation of ethane or ethylene:\[ C_2H_6 \rightarrow C_2H_4 + H_2 \]or\[ C_2H_4 \rightarrow C_2H_2 + H_2 \]This reaction is relevant for producing products like ethylene and acetylene, which are precursors to many industrial chemicals. The energy demand of dehydrogenation highlights its role as an endothermic process, in contrast to its exothermic counterpart, hydrogenation.

Allotropes of Carbon

Carbon exists in several different forms, known as allotropes, including graphite and diamond. These allotropes display distinct physical properties due to their unique atomic structures. Graphite features planar layers, which allow it to act as a good conductor of electricity and makes it feel slippery. In contrast, diamond boasts a strong, tetrahedral lattice structure that results in its renowned hardness and insulating properties. The conversion of graphite to diamond is an example of an endothermic reaction, as it involves altering the atomic arrangement through high energy input. Such transformation requires subjecting graphite to extreme pressure and temperature, illustrating the significant energy needed. Understanding carbon allotropes provides insights into the fascinating versatility and applications of carbon in different fields, from electronics to jewel making.