What is an example of an endothermic reaction?
A Any reaction where the chemical bonds in the reactants contain more energy than the chemical bonds in the products
B The combustion of coal dust when exposed to a spark
C The reaction between glucose and oxygen molecules
D Photosynthesis
The correct answer and explanation is:
The correct answer is D: Photosynthesis.
An endothermic reaction is one that absorbs energy, typically in the form of heat, from its surroundings. In the case of photosynthesis, plants absorb energy from sunlight to convert carbon dioxide and water into glucose and oxygen. This process is a classic example of an endothermic reaction because energy is required for the reaction to occur.
In photosynthesis, light energy is absorbed by chlorophyll in the chloroplasts of plant cells. This absorbed energy is used to convert the chemical bonds in carbon dioxide and water into glucose (C₆H₁₂O₆), a type of sugar that the plant uses as a source of energy. Oxygen is released as a byproduct. The overall chemical reaction for photosynthesis can be represented as: 6CO2+6H2O+light energy→C6H12O6+6O26CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2
The key factor in photosynthesis being endothermic is the absorption of light energy. The plants essentially “capture” energy from the sun to power the formation of chemical bonds in glucose, and since energy is absorbed to break bonds and form new ones, it is considered endothermic.
The other options are not endothermic reactions:
- A: The statement that a reaction where the reactants have more energy than the products is not specific to endothermic or exothermic reactions; it applies to both, but in endothermic reactions, energy is absorbed, not released.
- B: The combustion of coal dust is an exothermic reaction because it releases energy as heat.
- C: The reaction between glucose and oxygen (cellular respiration) is also exothermic, releasing energy to the organism.
Therefore, photosynthesis is the correct example of an endothermic reaction.