Question

which describes the enthalpy change associated with an endothermic reaction?
○ it is negative because the enthalpy of the products is greater than the enthalpy of the reactants.
○ it is positive because the enthalpy of the products is greater than the enthalpy of the reactants.
○ it is negative because the enthalpy of the reactants is greater than the enthalpy of the products.
○ it is positive because the enthalpy of the reactants is greater than the enthalpy of the products.

Explanation:

To determine the correct option, we recall the definition of enthalpy change ($\Delta H$) in a reaction: $\Delta H = H_{products} - H_{reactants}$. For an endothermic reaction, the system absorbs heat, so the enthalpy of the products ($H_{products}$) is greater than the enthalpy of the reactants ($H_{reactants}$). Thus, $\Delta H = H_{products} - H_{reactants}$ will be positive (since a larger number minus a smaller number gives a positive result). Now let's analyze each option:

• Option 1: Claims $\Delta H$ is negative when $H_{products} > H_{reactants}$—this is incorrect.
• Option 2: States $\Delta H$ is positive because $H_{products} > H_{reactants}$—this matches the definition of endothermic (as $\Delta H = H_{products} - H_{reactants}$ will be positive here).
• Option 3: Says $\Delta H$ is negative because $H_{reactants} > H_{products}$—this describes an exothermic reaction, not endothermic.
• Option 4: Claims $\Delta H$ is positive because $H_{reactants} > H_{products}$—but if $H_{reactants} > H_{products}$, $\Delta H$ would be negative (exothermic), so this is incorrect.

Answer:

B. It is positive because the enthalpy of the products is greater than the enthalpy of the reactants. (Assuming the second option is labeled B; adjust label based on actual question's numbering if needed.)