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Chapter 6: Problem 41

What is meant by the standard enthalpy of a reaction?

Short Answer

Expert verified
The standard enthalpy of a reaction, \( \Delta H^0 \), refers to the change in enthalpy (total energy) that comes with a reaction taking place under standard conditions. These conditions usually are 25°C (298 K), 1 bar or 1 atm pressure, and concentrations of 1 mol/L for all substances in solution.

Step by step solution

01

Define Enthalpy

The term 'enthalpy' refers to the total energy of a thermodynamic system. It includes the internal energy of the system as well as the energy involved in doing work against the system's surrounding atmosphere.
02

Introduce Concept of Standard Enthalpy

The 'standard' in standard enthalpy refers to 'Standard conditions', usually defined as 25°C (298 K), 1 bar or 1 atm pressure, and solutions at 1 mol/L. Standard states are important for calculating or comparing thermodynamic quantities consistently.
03

Explain Standard Enthalpy of a Reaction

The standard enthalpy of a reaction (\( \Delta H^0 \)) is the change in enthalpy that comes along with a chemical reaction happening at standard conditions. In other words, it is the heat absorbed or released in a reaction under standard conditions.

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Most popular questions from this chapter

Calculate the standard enthalpy change for the reaction $$ 2 \mathrm{Al}(s)+\mathrm{Fe}_{2} \mathrm{O}_{3}(s) \longrightarrow 2 \mathrm{Fe}(s)+\mathrm{Al}_{2} \mathrm{O}_{3}(s) $$ given that $$ \begin{aligned} 2 \mathrm{Al}(s)+\frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow & \mathrm{Al}_{2} \mathrm{O}_{3}(s) \\ \Delta H_{\mathrm{rxn}}^{\circ} &=-1669.8 \mathrm{~kJ} / \mathrm{mol} \\ 2 \mathrm{Fe}(s)+\frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow & \mathrm{Fe}_{2} \mathrm{O}_{3}(s) \\ \Delta H_{\mathrm{rxn}}^{\circ} &=-822.2 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$

The standard enthalpy change for the following reaction is \(436.4 \mathrm{~kJ} / \mathrm{mol}\) : $$ \mathrm{H}_{2}(g) \longrightarrow \mathrm{H}(g)+\mathrm{H}(g) $$ Calculate the standard enthalpy of formation of atomic hydrogen (H).

(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.

Define these terms: thermochemistry, exothermic process, endothermic process.

A 1.00 -mole sample of ammonia at 14.0 atm and \(25^{\circ} \mathrm{C}\) in a cylinder fitted with a movable piston \(\mathrm{ex}\) pands against a constant external pressure of 1.00 atm. At equilibrium, the pressure and volume of the gas are 1.00 atm and 23.5 L, respectively. (a) Calculate the final temperature of the sample. (b) Calculate \(q, w,\) and \(\Delta E\) for the process. The specific heat of ammonia is \(0.0258 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\)
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