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Chapter 5: Problem 67

(a) What is meant by the term standard conditions with reference to enthalpy changes? (b) What is meant by the term enthalpy of formation? (c) What is meant by the term standard enthalpy of formation?

Short Answer

Expert verified
(a) Standard conditions refer to the specific conditions, typically 298 K (25掳C) and 1 atm (101.3 kPa), under which enthalpy changes are measured, ensuring consistency and comparability across experiments. (b) Enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable forms at constant pressure. (c) Standard enthalpy of formation, represented as 螖Hf掳, is the change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable forms under standard conditions (298 K and 1 atm of pressure).

Step by step solution

01

(a) Definition of Standard Conditions

Standard conditions, with reference to enthalpy changes, refer to the specific conditions under which the measurements (like enthalpy changes) are taken. The standard conditions are set to ensure consistency and comparability in results across various experiments. Some common standard conditions are temperature at 298 K (25掳C) and pressure at 1 atm (101.3 kPa).
02

(b) Definition of Enthalpy of Formation

Enthalpy of formation is the change in enthalpy (heat content) when one mole of a compound is formed from its constituent elements in their most stable forms (reference states) at constant pressure.
03

(c) Definition of Standard Enthalpy of Formation

Standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable forms under standard conditions (298 K and 1 atm of pressure). It is represented as 螖Hf掳, where the superscript "掳" denotes the standard state. Standard enthalpy of formation can be either positive or negative depending on whether the formation process is endothermic (absorbing heat) or exothermic (releasing heat), respectively.

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Most popular questions from this chapter

Burning methane in oxygen can produce three different carbon-containing products: soot (very fine particles of graphite), \(\mathrm{CO}(g)\), and \(\mathrm{CO}_{2}(g)\). (a) Write three balanced equations for the reaction of methane gas with oxygen to produce these three products. In each case assume that \(\mathrm{H}_{2} \mathrm{O}(l)\) is the only other product. (b) Determine the standard enthalpies for the reactions in part (a). (c) Why, when the oxygen supply is adequate, is \(\mathrm{CO}_{2}(g)\) the predominant carbon- containing product of the combustion of methane?

Calcium carbide \(\left(\mathrm{CaC}_{2}\right)\) reacts with water to form acetylene \(\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)\) and \(\mathrm{Ca}(\mathrm{OH})_{2}\). From the following enthalpy of reaction data and data in Appendix C, calculate \(\Delta H_{f}^{\circ}\) for \(\mathrm{CaC}_{2}(s)\) : $$ \begin{array}{r} \mathrm{CaC}_{2}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(s)+\mathrm{C}_{2} \mathrm{H}_{2}(g) \\ \Delta H^{\circ}=-127.2 \mathrm{~kJ} \end{array} $$

Gasoline is composed primarily of hydrocarbons, including many with eight carbon atoms, called octanes. One of the cleanestburning octanes is a compound called 2,3,4-trimethylpentane, which has the following structural formula: CC(C)C(C)C(C)C The complete combustion of one mole of this compound to \(\mathrm{CO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(g)\) leads to \(\Delta H^{\circ}=-5064.9 \mathrm{~kJ} / \mathrm{mol}\). (a) Write a balanced equation for the combustion of \(1 \mathrm{~mol}\) of \(\mathrm{C}_{8} \mathrm{H}_{18}(l)\). (b) By using the information in this problem and data in Table 5.3, calculate \(\Delta H_{f}^{\circ}\) for \(2,3,4\)-trimethylpentane.

(a) Why is the change in enthalpy usually easier to measure than the change in internal energy? (b) \(H\) is a state function, but \(q\) is not a state function. Explain. (c) For a given process at constant pressure, \(\Delta H\) is positive. Is the process endothermic or exothermic?

(a) Calculate the kinetic energy, in joules, of a \(1200-\mathrm{kg}\) automobile moving at \(18 \mathrm{~m} / \mathrm{s}\). (b) Convert this energy to calories. (c) What happens to this energy when the automobile brakes to a stop?
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