Ch.9 - Thermochemistry: Chemical Energy

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McMurry 8th Edition

Ch.9 - Thermochemistry: Chemical Energy

Problem 59

Chapter 9, Problem 59

Under what circumstances are ΔE and ΔH essentially equal?

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Understand that ΔE represents the change in internal energy of a system, while ΔH represents the change in enthalpy.

Recall the relationship between ΔE and ΔH: ΔH = ΔE + PΔV, where P is pressure and ΔV is the change in volume.

Recognize that ΔE and ΔH are essentially equal when the term PΔV is negligible.

Identify the conditions under which PΔV is negligible: this typically occurs when the reaction involves no gases or when the volume change is very small, such as in reactions occurring in the liquid or solid phase.

Conclude that ΔE and ΔH are essentially equal under conditions of constant volume or when the work done by the system is minimal, such as in reactions involving only solids and liquids.

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Key Concepts

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

Internal Energy (ΔE)

Internal energy (ΔE) is the total energy contained within a system, encompassing kinetic and potential energy at the molecular level. It reflects the energy changes that occur during chemical reactions or physical processes. Understanding ΔE is crucial for analyzing energy transfers and transformations in thermodynamics.

Enthalpy (ΔH)

Enthalpy (ΔH) is a thermodynamic quantity that represents the total heat content of a system at constant pressure. It accounts for internal energy and the work done by the system due to volume changes. ΔH is particularly important in chemical reactions, as it indicates whether a reaction is exothermic or endothermic.

Constant Pressure Conditions

Under constant pressure conditions, the change in enthalpy (ΔH) is equal to the heat exchanged (q) during a process. When a reaction occurs in an open system at atmospheric pressure, the internal energy change (ΔE) and enthalpy change (ΔH) can be considered equal, especially when the volume change is negligible, allowing for simplified calculations in thermodynamics.

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Related Practice

Textbook Question

What is the difference between the internal energy change ∆E and the enthalpy change ∆H? Which of the two is mea- sured at constant pressure and which at constant volume?

Textbook Question

The enthalpy change for the reaction of 50.0 mL of ethylene with 50.0 mL of H2 at 1.5 atm pressure (Problem 9.51) is∆H = -0.31 kJ. What is the value of ∆E?

Textbook Question

What is the enthalpy change (ΔH) for a reaction at a constant pressure of 1.00 atm fi the internal energy chagne (ΔE) is 44.0 kJ and the volume increase is 14.0 L? (1 L-atm = 101.325 J)

Textbook Question

When a sample of a hydrocarbon fuel is ignited and burned in oxygen, the internal energy decreases by 7.20 kJ. If 5670 J ofheat were transferred to the surroundings, what is the sign and magnitude of work? If the reaction took place in an environ- ment with a pressure of 1 atm, what was the volume change?

Textbook Question

At a constant pressure of 0.905 atm, a chemical reaction takes place in a cylindrical container with a movable piston having a diameter of 40.0 cm. During the reaction, the height of the piston drops by 65.0 cm. (The volume of a cylinder is V=pr2h,wherehistheheight;1Latm=101.3J.) (b) What is the value in joules of the work w done during the reaction?

Textbook Question

Assume that a particular reaction evolves 244 kJ of heat and that 35 kJ of PV work is gained by the system. What are the values of ∆E and ∆H for the system? For the surroundings?