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Chapter 5: Problem 65

(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

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(a) Standard conditions refer to a set of agreed-upon conditions, specifically 298 Kelvin (25 degrees Celsius) temperature and 1 atmosphere (atm) pressure, for conducting experimental observations and measurements related to enthalpy changes in chemical reactions. (b) Enthalpy of formation (螖H_f) represents the change in enthalpy during the formation of one mole of a substance from its constituent elements in their most stable forms at constant pressure. (c) Standard enthalpy of formation (螖H_f掳) describes the change in enthalpy when one mole of a substance is formed from its constituent elements in their most stable forms under standard conditions (298 K temperature and 1 atm pressure).

Step by step solution

01

a) Standard Conditions

Standard conditions are defined as a set of conditions agreed upon for conducting experimental observations and measurements in the field of chemistry and physics. For enthalpy changes, standard conditions refer to a temperature of 298 Kelvin (25 degrees Celsius) and a pressure of 1 atmosphere (atm). These conditions are used to ensure consistency and comparability when dealing with enthalpy changes in chemical reactions.
02

b) Enthalpy of Formation

Enthalpy of formation, denoted as 螖H_f, is the change in enthalpy during the formation of one mole of a substance from its constituent elements in their most stable forms at a constant pressure. For example, the enthalpy of formation of water would be the change in enthalpy when one mole of water is formed from hydrogen and oxygen molecules. It helps determine whether a particular reaction is exothermic (releases heat) or endothermic (absorbs heat).
03

c) Standard Enthalpy of Formation

The standard enthalpy of formation, denoted as 螖H_f掳, is the change in enthalpy when one mole of a substance is formed from its constituent elements in their most stable forms under standard conditions (298 K temperature and 1 atm pressure). By convention, the standard enthalpy of formation for an element in its most stable and standard state is zero. Knowing the standard enthalpies of formation for various compounds, chemists can easily compare the energy changes that occur during different chemical reactions under standard conditions.

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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), \(\mathrm{CO}(\mathrm{g})\), and \(\mathrm{CO}_{2}(\mathrm{~g}) .\) (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?

Consider the following unbalanced oxidation-reduction reactions in aqueous solution: $$ \begin{aligned} \mathrm{Ag}^{+}(a q)+\mathrm{Li}(s) & \longrightarrow \mathrm{Ag}(s)+\mathrm{Li}^{+}(a q) \\ \mathrm{Fe}(s)+\mathrm{Na}^{+}(a q) & \longrightarrow \mathrm{Fe}^{2+}(a q)+\mathrm{Na}(s) \\ \mathrm{K}(s)+\mathrm{H}_{2} \mathrm{O}(l) & \longrightarrow \mathrm{KOH}(a q)+\mathrm{H}_{2}(g) \end{aligned} $$ (a) Balance each of the reactions. (b) By using data in Appendix C, calculate \(\Delta H^{\circ}\) for each of the reactions. (c) Based on the values you obtain for \(\Delta H^{\circ}\), which of the reactions would you expect to be thermodynamically favored? (That is, which would you expect to be spontaneous?) (d) Use the activity series to predict which of these reactions should occur. ono (Section 4.4) Are these results in accord with your conclusion in part (c) of this problem?

Without referring to tables, predict which of the following has the higher enthalpy in each case: (a) \(1 \mathrm{~mol} \mathrm{CO}_{2}(\mathrm{~s})\) or \(1 \mathrm{~mol} \mathrm{CO}_{2}(\mathrm{~g})\) at the same temperature, (b) \(2 \mathrm{~mol}\) of hydrogen atoms or 1 mol of \(\mathrm{H}_{2}\), (c) \(1 \mathrm{~mol} \mathrm{H}_{2}(\mathrm{~g})\) and \(0.5 \mathrm{~mol}\) \(\mathrm{O}_{2}(g)\) at \(25^{\circ} \mathrm{C}\) or \(1 \mathrm{~mol} \mathrm{H}_{2} \mathrm{O}(g)\) at \(25^{\circ} \mathrm{C}\), (d) \(1 \mathrm{~mol} \mathrm{~N}_{2}(g)\) at \(100^{\circ} \mathrm{C}\) or \(1 \mathrm{~mol} \mathrm{~N}_{2}(g)\) at \(300^{\circ} \mathrm{C}\).

What is the connection between Hess's law and the fact that \(H\) is a state function?

A watt is a measure of power (the rate of energy change) equal to \(1 \mathrm{~J} / \mathrm{s}\). (a) Calculate the number of joules in a kilowatt-hour. (b) An adult person radiates heat to the surroundings at about the same rate as a 100 -watt electric incandescent lightbulb. What is the total amount of energy in kcal radiated to the surroundings by an adult in 24 hours?
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