Ch.7 - Thermochemistry

Problem 75b

Chapter 7, Problem 75b

Determine whether each process is exothermic or endothermic and indicate the sign of ΔH. a. natural gas burning on a stove b. isopropyl alcohol evaporating from skin c. water condensing from steam Indicate the sign of ΔH for the following processes.

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Identify the type of process for each scenario: exothermic or endothermic.

For an exothermic process, energy is released to the surroundings, and the sign of \( \Delta H \) is negative.

For an endothermic process, energy is absorbed from the surroundings, and the sign of \( \Delta H \) is positive.

a. Natural gas burning on a stove is a combustion reaction, which is exothermic, so \( \Delta H \) is negative.

b. Isopropyl alcohol evaporating from skin is an endothermic process, as it requires energy to change from liquid to gas, so \( \Delta H \) is positive.

c. Water condensing from steam is an exothermic process, as it releases energy when changing from gas to liquid, so \( \Delta H \) is negative.

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Key Concepts

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

Exothermic and Endothermic Processes

Exothermic processes release energy, usually in the form of heat, to the surroundings, resulting in a negative change in enthalpy (ΔH < 0). In contrast, endothermic processes absorb energy from the surroundings, leading to a positive change in enthalpy (ΔH > 0). Understanding these definitions is crucial for determining the nature of the processes described in the question.

Enthalpy (ΔH)

Enthalpy (ΔH) is a thermodynamic quantity that represents the total heat content of a system. It is used to quantify the energy change during a chemical reaction or physical process. The sign of ΔH indicates whether the process is exothermic (negative ΔH) or endothermic (positive ΔH), which is essential for analyzing the processes mentioned in the question.

Phase Changes and Energy Transfer

Phase changes, such as evaporation and condensation, involve energy transfer that affects the temperature and state of a substance. For example, evaporation is an endothermic process as it requires energy to convert liquid to gas, while condensation is exothermic as it releases energy when gas turns into liquid. Recognizing these phase changes helps in determining the sign of ΔH for the processes listed.

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Textbook Question

When 0.514 g of biphenyl (C₁₂H₁₀) undergoes combustion in a bomb calorimeter, the temperature rises from 25.8 °C to 29.4 °C. Find ΔE_rxn for the combustion of biphenyl in kJ/mol biphenyl. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.86 kJ/°C.

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Textbook Question

Zinc metal reacts with hydrochloric acid according to the balanced equation: Zn(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂(g) When 0.103 g of Zn(s) is combined with enough HCl to make 50.0 mL of solution in a coffee-cup calorimeter, all of the zinc reacts, raising the temperature of the solution from 22.5 °C to 23.7 °C. Find ΔH_rxn for this reaction as written. (Use 1.0 g/mL for the density of the solution and 4.18 J/g•°C as the specific heat capacity.)

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Textbook Question

Instant cold packs used to ice athletic injuries on the field contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the endothermic reaction: NH₄NO₃(s) → NH₄⁺(aq) + NO₃^– (aq) In order to measure the enthalpy change for this reaction, 1.25 g of NH₄NO₃ is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 °C and the final temperature (after the solid dissolves) is 21.9 °C. Calculate the change in enthalpy for the reaction in kJ. (Use 1.0 g/mL as the density of the solution and 4.18 J/g•°C as the specific heat capacity.)

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Textbook Question

For each generic reaction, determine the value of ΔH₂ in terms of ΔH₁.

a. A + B → 2 C ΔH₁

2 C→ A + B ΔH₂ = ?

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Textbook Question

For each generic reaction, determine the value of ΔH₂ in terms of ΔH₁.

b. A + 1/2 B → C ΔH₁

2 A + B → 2 C ΔH₂ = ?