Question

Endothermic reactions are those in which :

• A. Heat is evolved.
• B. Heat is absorbed.
• C. Temperature increases.
• D. Light is produced.

Hint:

Remember the prefix meanings:
- Endo- \(\to\) Enter/Inside (Heat enters the system; absorbed).
- Exo- \(\to\) Exit/Outside (Heat exits the system; released).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the definition of an endothermic chemical reaction in terms of heat energy exchange.

Step 2: Key Formula or Approach:
Chemical reactions involve energy exchange with the surroundings. This is quantified by the enthalpy change (\(\Delta H\)):
\[ \Delta H = H_{products} - H_{reactants} \]

Step 3: Detailed Explanation:

• In any chemical reaction, energy is required to break chemical bonds in the reactants, and energy is released when new bonds are formed in the products.

• Endothermic reactions: If the energy required to break the existing bonds is greater than the energy released during the formation of new bonds, the reaction must absorb energy from its surroundings to proceed.
- This absorption of heat energy causes the surroundings to cool down, and the enthalpy change of the system is positive (\(\Delta H \gt 0\)).

• Conversely, exothermic reactions are those that release or evolve heat to the surroundings (\(\Delta H \lt 0\)), often causing the temperature of the surroundings to increase.

• Therefore, by definition, endothermic reactions are those in which heat is absorbed.


Step 4: Final Answer:
Endothermic reactions are those in which heat is absorbed.