Enthalpy, represented by the symbol $\Delta H$, is a measure of the heat absorbed or released during a chemical reaction at constant pressure. A negative $\Delta H$ indicates an exothermic reaction (releases heat), while a positive $\Delta H$ indicates an endothermic reaction (absorbs heat). Mastering enthalpy calculations is crucial for understanding the energy changes in chemical processes. The problems below will test your understanding of different methods to calculate $\Delta H$, including using standard enthalpies of formation, Hess's Law, and bond energies.
Match each term with its correct definition:
| Term | Definition |
|---|---|
| 1. Enthalpy | A. The heat absorbed or released during a chemical reaction at constant pressure. |
| 2. Exothermic | B. A reaction that releases heat to the surroundings. |
| 3. Endothermic | C. A reaction that absorbs heat from the surroundings. |
| 4. Hess's Law | D. The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process. |
| 5. Standard Enthalpy of Formation | E. The change in enthalpy during the formation of one mole of a substance from its elements in their standard states. |
Matching Answers:
Complete the following paragraph using the words: exothermic, endothermic, positive, negative, enthalpy.
__________ is a state function used to describe the heat content of a system at constant pressure. A reaction with a __________ $\Delta H$ value is considered __________, meaning it releases heat. Conversely, a reaction with a __________ $\Delta H$ value is considered __________, meaning it absorbs heat.
Fill in the Blanks Answers:
Explain how Hess's Law can be used to determine the enthalpy change of a reaction that is difficult or impossible to measure directly.
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