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Chapter 6: Problem 47

Which is the more negative quantity at \(25^{\circ} \mathrm{C}: \Delta H_{\mathrm{f}}^{\circ}\) for \(\mathrm{H}_{2} \mathrm{O}(l)\) or \(\Delta H_{\mathrm{f}}^{\circ}\) for \(\mathrm{H}_{2} \mathrm{O}(g) ?\)

Short Answer

Expert verified
The enthalpy of formation, \( \Delta H_{\mathrm{f}}^{\circ}\) for \mathrm{H}_{2} \mathrm{O}(l) [liquid water] at 25掳C is the more negative quantity when compared to the \Delta H_{\mathrm{f}}^{\circ} for \mathrm{H}_{2} \mathrm{O}(g) [gaseous water] at the same temperature.

Step by step solution

01

Understanding Enthalpy of Formation

By definition, the enthalpy of formation, \( \Delta H_{\mathrm{f}}^{\circ} \), is the change in enthalpy (heat content) when 1 mole of a compound is formed from its elements under standard conditions (25掳C, 1 atm), all substances in their standard state. Enthalpy of formation measures the energy involved in forming a compound.
02

Liquid Water vs Gaseous Water

Water in its liquid state and its gaseous state at the same temperature will have different enthalpy of formation values because transitioning to different states involves energy changes. It's known that gaseous water has higher enthalpy than liquid water, because energy is added to liquid water to overcome the attractive forces between water molecules and convert it to a gaseous state.
03

Comparing Enthalpy of Formation

So, given that the enthalpy of formation, \( \Delta H_{\mathrm{f}}^{\circ} \), for liquid water is -285.8 kJ/mol at 25掳C and for gaseous water it is -241.8 kJ/mol at the same temperature, it's clear that the \Delta H_{\mathrm{f}}^{\circ} for \mathrm{H}_{2} \mathrm{O}(l) would be the more negative quantity.

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Most popular questions from this chapter

Methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) is an organic solvent and is also used as a fuel in some automobile engines. From the following data, calculate the standard enthalpy of formation of methanol: $$ \begin{aligned} 2 \mathrm{CH}_{3} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CO}_{2}(g)+4 \mathrm{H}_{2} \mathrm{O}(l) \\ \Delta H_{\mathrm{rxn}}^{\circ}=-1452.8 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$

Stoichiometry is based on the law of conservation of mass. On what law is thermochemistry based?

(a) A snowmaking machine contains a mixture of compressed air and water vapor at about 20 atm. When the mixture is sprayed into the atmosphere it expands so rapidly that, as a good approximation, no heat exchange occurs between the system (air and water) and its surroundings. (In thermodynamics, such a process is called an adiabatic process.) Do a first law of thermodynamics analysis to show how snow is formed under these conditions. (b) If you have ever pumped air into a bicycle tire, you probably noticed a warming effect at the valve stem. The action of the pump compresses the air inside the pump and the tire. The process is rapid enough to be treated as an adiabatic process. Apply the first law of thermodynamics to account for the warming effect. (c) A driver's manual states that the stopping distance quadruples as the speed doubles; that is, if it takes \(30 \mathrm{ft}\) to stop a car traveling at \(25 \mathrm{mph}\) then it would take \(120 \mathrm{ft}\) to stop a car moving at 50 mph. Justify this statement by using the first law of thermodynamics. Assume that when a car is stopped, its kinetic energy \(\left(\frac{1}{2} m u^{2}\right)\) is totally converted to heat.

What is the difference between specific heat and heat capacity? What are the units for these two quantities? Which is the intensive property and which is the extensive property?

Consider this reaction: $$ \begin{aligned} 2 \mathrm{CH}_{3} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow & 4 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g) \\ \Delta H &=-1452.8 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$ What is the value of \(\Delta H\) if (a) the equation is multiplied throughout by \(2,\) (b) the direction of the reaction is reversed so that the products become the reactants and vice versa, (c) water vapor instead of liquid water is formed as the product?
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