Which of the following describes an endothermic reaction?
A. [tex]$\Delta H_{f, \text { products }}=0$[/tex]
B. [tex]$\Delta H_{ f }$, products $\ \textgreater \ \Delta H_{ f }$, reactants[/tex]
C. [tex]$\Delta H_{ f }$, products $=H_{ f }$, reactants[/tex]
D. [tex]$\Delta H_{ f }$, products $\ \textless \ \Delta H_{ f }$, reactants[/tex]
Answer (2)
Endothermic reactions absorb heat, meaning the products have higher enthalpy than the reactants.
The change in enthalpy, Δ H , is positive for endothermic reactions, so H_{\text{reactants}}"> H products > H reactants .
Option B, Δ H f , products \Delta H_{ f }"> > Δ H f , reactants, correctly describes this relationship.
Therefore, the answer is B.
Explanation
Understanding Endothermic Reactions An endothermic reaction is a chemical reaction that absorbs heat from its surroundings. This means the products have a higher enthalpy (heat content) than the reactants. The change in enthalpy, denoted as Δ H , is positive for endothermic reactions.
Enthalpy Change The change in enthalpy ( Δ H ) is defined as the difference between the enthalpy of the products ( H products ) and the enthalpy of the reactants ( H reactants ): Δ H = H products − H reactants For an endothermic reaction, 0"> Δ H > 0 . Therefore, we have: 0"> H products − H reactants > 0 which implies: H_{\text{reactants}}"> H products > H reactants This means the enthalpy of the products is greater than the enthalpy of the reactants.
Analyzing the Options Now, let's examine the given options:
A. Δ H f , products = 0 : This option states that the enthalpy of the products is zero, which is not necessarily true for an endothermic reaction.
B. Δ H f , products \Delta H_{ f }"> > Δ H f , reactants: This option states that the enthalpy of the products is greater than the enthalpy of the reactants, which is consistent with our understanding of endothermic reactions.
C. Δ H f , products = H f , reactants: This option states that the enthalpy of the products is equal to the enthalpy of the reactants, which would mean there is no heat absorbed or released ( Δ H = 0 ). This is not an endothermic reaction.
D. Δ H f , products < Δ H f , reactants: This option states that the enthalpy of the products is less than the enthalpy of the reactants, which means the reaction releases heat (exothermic reaction), not absorbs it.
Conclusion Based on our analysis, the correct option is B, which states that the enthalpy of the products is greater than the enthalpy of the reactants in an endothermic reaction.
Examples
Endothermic reactions are commonly used in instant cold packs. When you squeeze the pack, it mixes water with a chemical like ammonium nitrate. This reaction absorbs heat from the surroundings, causing the pack to become cold. This principle is also used in some self-cooling beverage containers, where an endothermic reaction cools the drink without needing refrigeration. Understanding endothermic reactions helps in designing and utilizing these cooling technologies effectively.
The correct answer is B, which states that the enthalpy of the products is greater than that of the reactants in an endothermic reaction. Endothermic reactions absorb heat, resulting in a positive change in enthalpy. Therefore, they align with the relationship expressed in option B.
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