Question

Without doing calculations, decide whether each of the following is exo- or endothermic. (a) the combustion of natural gas (b) the decomposition of glucose, \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6},\) to carbon and water

Short answer

(a) Exothermic, (b) Endothermic

Step-by-step solution

Determine Reaction Type for Combustion

The combustion of natural gas involves the reaction of a fuel (such as methane, CH₄) with oxygen to produce carbon dioxide and water. Combustion reactions are a type of redox reaction where energy is released primarily in the form of heat and light. Therefore, combustion reactions are typically exothermic.

Identify the Decomposition Reaction

The decomposition of glucose refers to the breakdown of glucose into other products. Generally, the breakdown of complex molecules, such as glucose, into simpler molecules, requires energy input, making it an endothermic process unless it is a metabolic reaction like respiration, which is complex and involves energy release in cells.

Key concepts

Endothermic Processes

Endothermic processes absorb energy from their surroundings, usually in the form of heat. These processes require energy input to proceed. A classic example is the melting of ice. As ice absorbs heat from the environment to turn into liquid water, it becomes an endothermic process.
In chemical reactions, if energy in the form of heat is consumed, it's an endothermic process. This is often represented in chemical equations where energy is included as a reactant. The decomposition of glucose is a typical endothermic process in laboratory settings. It involves breaking down a complex molecule like glucose into simpler constituents, which requires an energy input.
One important thing to note is that not all decompositions are purely endothermic. Biological processes like cellular respiration in cells involve the breakdown of glucose but release energy. However, outside of such biological conditions, the decomposition of glucose without using cellular processes tends to require an external energy input, thereby qualifying as endothermic.

Exothermic Reactions

Exothermic reactions release energy to the surroundings, usually in the form of heat, light, or sound. These reactions occur spontaneously when the energy released by new bonds forming exceeds the energy required to break the old bonds in the reactants.
An everyday example of an exothermic process is the burning of wood in a fireplace, where heat and light are released as a result of the combustion reaction. This reaction is exothermic because it releases more energy than it consumes.
In chemistry, when a reaction's products have lower energy than its reactants, the reaction is exothermic. The energy change can be visualized by the enthalpy change, \\(\Delta H\), which is negative for exothermic reactions, indicating that energy flows out of the system.
Exothermic reactions are common in real-life situations ranging from heating homes to powering engines. They are fundamental to many technologies relying on combustion, highlighting their real-world importance.

Combustion Reactions

Combustion reactions are a common type of chemical reaction that typically involve the burning of a fuel with oxygen to produce carbon dioxide, water, and energy. They are a subset of exothermic reactions and are known for releasing a large amount of energy in the form of heat and light.
For example, when natural gas, primarily composed of methane \(\text{CH}_4\), combusts with oxygen, it forms carbon dioxide \(\text{CO}_2\) and water \(\text{H}_2\text{O}\). The equation for this reaction is:
\[ \text{CH}_4 + 2 \text{O}_2 \rightarrow \text{CO}_2 + 2 \text{H}_2\text{O} + \text{energy} \]
This reaction releases energy, making it a powerful and efficient way to generate heat. Combustion reactions are vital in numerous applications, from powering vehicles and heating buildings to generating electricity.
Understanding combustion is crucial because it plays a critical role in energy production and environmental considerations. The energy provided by combustion has fueled human development, yet it also demands attention due to its carbon dioxide emissions, contributing to global warming.