Thermochemistry

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University of California, Santa Barbara *

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Chemistry

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Apr 3, 2024

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Thermochemistry, covering heat transfer in chemical reactions, enthalpy, calorimetry, and Hess's law: **1. Question:** Define thermochemistry. **Solution:** Thermochemistry is the branch of chemistry that deals with the study of heat transfer in chemical reactions and the associated changes in energy, including enthalpy changes. **2. Question:** What is heat? **Solution:** Heat is a form of energy that is transferred between two objects or systems due to a temperature difference. **3. Question:** Define enthalpy (\(H\)). **Solution:** Enthalpy (\(H\)) is a thermodynamic property of a system that represents the total heat content of the system at constant pressure. **4. Question:** Describe an endothermic reaction and an exothermic reaction. **Solution:** - An endothermic reaction absorbs heat from its surroundings, resulting in a decrease in temperature of the surroundings. - An exothermic reaction releases heat to its surroundings, resulting in an increase in temperature of the surroundings. **5. Question:** Write the equation for calculating the change in enthalpy (\(ΔH\)) for a chemical reaction. **Solution:** The equation is: \(ΔH = H_{\text{products}} - H_{\text{reactants}}\), where \(H_{\text{products}}\) is the enthalpy of the products and \(H_{\text{reactants}}\) is the enthalpy of the reactants. **6. Question:** What is specific heat capacity? **Solution:** Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius (or Kelvin). **7. Question:** Define calorimetry. **Solution:** Calorimetry is the experimental technique used to measure the heat absorbed or released during a chemical or physical process, typically using a calorimeter. **8. Question:** Describe how to calculate the heat gained or lost by a substance using its specific heat capacity. **Solution:** The heat gained or lost (\(q\)) by a substance is calculated using the formula: \(q = mcΔT\), where \(m\) is the mass of the substance, \(c\) is its specific heat capacity, and \(ΔT\) is the change in temperature. **9. Question:** Explain the concept of Hess's law.
**Solution:** Hess's law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps or the pathway taken to achieve the final products, as long as the initial and final conditions are the same. **10. Question:** Define standard enthalpy of formation (\(ΔH_f^\circ\)). **Solution:** The standard enthalpy of formation (\(ΔH_f^\circ\)) is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states at a specified temperature (usually 25°C or 298 K). **11. Question:** Describe how to use enthalpy of formation values to calculate the enthalpy change for a reaction. **Solution:** The enthalpy change for a reaction can be calculated using Hess's law by subtracting the sum of the standard enthalpies of formation of the reactants from the sum of the standard enthalpies of formation of the products. **12. Question:** Explain the difference between heat and temperature. **Solution:** Heat is a form of energy transferred between objects or systems due to a temperature difference, while temperature is a measure of the average kinetic energy of the particles in a substance. **13. Question:** Define standard conditions in thermochemistry. **Solution:** Standard conditions in thermochemistry refer to a set of conditions commonly used for reporting enthalpy changes, including a temperature of 25°C (298 K), a pressure of 1 atm, and solutions with a concentration of 1 M. **14. Question:** Describe an experiment to determine the heat of reaction using a coffee cup calorimeter. **Solution:** In a coffee cup calorimeter experiment, the reactants are mixed in a polystyrene foam cup, and the temperature change of the reaction mixture is measured using a thermometer. The heat of reaction (\(q\)) is calculated using the formula \(q = mcΔT\), where \(m\) is the mass of the solution, \(c\) is the specific heat capacity of the solution, and \(ΔT\) is the change in temperature. **15. Question:** Define the standard enthalpy change of reaction (\(ΔH^\circ_{rxn}\)). **Solution:** The standard enthalpy change of reaction (\(ΔH^\circ_{rxn}\)) is the enthalpy change that occurs when a reaction takes place under standard conditions, with all reactants and products in their standard states.
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