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Chapter 5: Problem 68

(a) Why does the standard enthalpy of formation of both the very reactive fluorine \(\left(\mathrm{F}_{2}\right)\) and the almost inert gas nitrogen \(\left(\mathrm{N}_{2}\right)\) both read zero? (b) Write the chemical equation for the reaction whose enthalpy change is the standard enthalpy of formation of naphthalene \(\left(\mathrm{C}_{10} \mathrm{H}_{8}\right)\).

Short Answer

Expert verified
(a) The standard enthalpy of formation for both fluorine (F₂) and nitrogen (N₂) gas is zero because they are already in their most stable forms under standard conditions (pressure of 1 atmosphere and temperature of 298 K), and no heat is absorbed or released in their formation. (b) The chemical equation for the reaction representing the standard enthalpy of formation of naphthalene (C₁₀H₈) is: \[ 10 \mathrm{C_{(graphite)}} + 4 \mathrm{H_{2(g)}} \rightarrow \mathrm{C_{10}H_{8(s)}}\]

Step by step solution

01

Define Standard Enthalpy of Formation

Standard enthalpy of formation refers to the heat absorbed or released when one mole of a compound is formed from its constituent elements in their most stable forms, under standard conditions. Standard conditions are defined as a pressure of 1 atmosphere and a temperature of 298 K.
02

Apply the Definition to F₂ and N₂

In the given problem, we are considering the standard enthalpy of formation for fluorine (F₂) and nitrogen (N₂) gas. Both of these are diatomic molecules and represent the most stable forms of their respective elements under standard conditions. Since they are already in their most stable forms, there is no actual formation reaction occurring for these species. This means, there is no heat absorbed or released in their formation (as they are already formed). Therefore, the standard enthalpy of formation for both F₂ and N₂ is zero.
03

Write the Chemical Equation for the Formation of Naphthalene

The chemical formula of naphthalene is C₁₀H₈. To write the equation for its formation from its constituent elements, we need to consider the most stable forms of carbon and hydrogen under standard conditions. Carbon is most stable as solid graphite and hydrogen is most stable as diatomic gas (H₂). The formation reaction of naphthalene is: \[ 10 \mathrm{C_{(graphite)}} + 4 \mathrm{H_{2(g)}} \rightarrow \mathrm{C_{10}H_{8(s)}}\] This is the chemical equation for the reaction whose enthalpy change is the standard enthalpy of formation of naphthalene.

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Most popular questions from this chapter

Atomic hydrogen (H) is used in welding (AHW). The atoms recombine to hydrogen molecules with a large release of heat according to the following reaction: $$ 2 \mathrm{H}(g) \longrightarrow \mathrm{H}_{2}(g) $$ (a) Using the thermodynamic data in Appendix C, calculate the enthalpy change for this reaction per mole of \(\mathrm{H}_{2}\). (b) Which has the higher enthalpy under these conditions, \(2 \mathrm{H}(g)\) or \(\mathrm{H}_{2}(g) ?\)

Burning acetylene 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 acetylene 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 acetylene?

Consider a system consisting of the following apparatus, in which gas is confined in one flask and there is a vacuum in the other flask. The flasks are separated by a valve. Assume that the flasks are perfectly insulated and will not allow the flow of heat into or out of the flasks to the surroundings. When the valve is opened, gas flows from the filled flask to the evacuated one. (a) Is work performed during the expansion of the gas? (b) Why or why not? (c) Can you determine the value of \(\Delta E\) for the process?

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 \(\mathrm{CaC}_{2}(s);\) $$ \begin{aligned} \mathrm{CaC}_{2}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(s)+\mathrm{C}_{2} \mathrm{H}_{2}(g) & \\ \Delta H^{\circ}=&-127.2 \mathrm{~kJ} \end{aligned} $$

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