Ch.9 - Thermochemistry: Chemical Energy

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.9 - Thermochemistry: Chemical Energy

Problem 6d

Chapter 9, Problem 6d

Several processes are given in the table and labeled as endo- thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process (d) Rubbing alcohol evaporates from your skin.

Verified step by step guidance

Identify the type of process: Evaporation is a phase change from liquid to gas.

Understand the energy requirement: Evaporation requires energy to overcome intermolecular forces, making it an endothermic process.

Determine the sign of \( \Delta H^\circ \): For endothermic processes, \( \Delta H^\circ \) is positive because energy is absorbed.

Relate the process to the classification: Since rubbing alcohol evaporating is endothermic, it should have a positive \( \Delta H^\circ \).

Verify the classification: Ensure that the process is correctly labeled as endothermic with a positive \( \Delta H^\circ \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Endothermic and Exothermic Processes

Endothermic processes absorb heat from their surroundings, resulting in a decrease in temperature of the environment, while exothermic processes release heat, increasing the temperature of the surroundings. The classification of a process as endothermic or exothermic is determined by the sign of the enthalpy change (∆H°); a positive ∆H° indicates an endothermic reaction, and a negative ∆H° indicates an exothermic reaction.

Enthalpy Change (∆H°)

Enthalpy change (∆H°) is a measure of the heat content of a system at constant pressure. It quantifies the energy absorbed or released during a chemical reaction or physical process. In the context of phase changes, such as evaporation, the enthalpy change reflects the energy required to overcome intermolecular forces, which is crucial for understanding whether a process is endothermic or exothermic.

Evaporation and Heat Transfer

Evaporation is a physical process where liquid molecules gain enough energy to transition into the gas phase. This process is endothermic because it requires heat energy to break the intermolecular forces holding the liquid molecules together. When rubbing alcohol evaporates from the skin, it absorbs heat from the skin, leading to a cooling sensation, which is indicative of an endothermic process.

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Textbook Question

Several processes are given in the table and labeled as endo- thermic or exothermic and given a sign for ∆H°. Which process is labeled with the correct sign of ∆H° and correct classification as endothermic or exothermic? (LO 9.8) Process (c) Water freezes into ice in the freezer.

How much heat is required to raise a 50.0 g piece of iron from 25 °C to its melting point of 1538 °C? The specific heat capacity for iron is 0.451 J/g•°C.(a) 34.1 kJ(b) 168 kJ(c) 12.1 kJ(d) 6.78 kJ

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Textbook Question

A 25.0 g piece of granite at 100.0°C was added to 100.0 g of water of 25.0°C, and the temperature rose to 28.4°C. What is the specific heat capacity of the granite? (The specific heat capacity for water is 4.18 J/(g•°C).) (LO 9.10)(a) 0.563 J/(g•°C)(b) 1.53 J/(g•°C)(c) 0.992 J/(g•°C)(d) 0.794 J/(g•°C)

When 12.5 g of NH4NO3 is dissolved in 150.0 g of water of 25.0 °C in a coffee cup calorimeter, the final temperature of the solution of 19.7 °C. Assume that the specific heat of the solution is the same as that of water, 4.18 J/(g•°C). What is the ΔH per mol of NH4NO3? (LO 9.10) NH4NO3 (s) → NH4+ (aq) + NO3−(aq) ΔH = ? (a) +3.60 kJ (b) +23.0 kJ (c) +21.3 kJ (d) −3.60 kJ

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