Estos son ejercicios de tarea para acompañar el Mapa de texto creado para “Química: la ciencia central” por Brown et al. Se pueden encontrar bancos de preguntas de química general complementaria para otros mapas de texto y se puede acceder aquí . Además de estas preguntas disponibles públicamente, el acceso al banco privado de problemas para su uso en exámenes y tareas está disponible para los profesores solo de manera individual; comuníquese con Delmar Larsen para obtener una cuenta con permiso de acceso.
Problemas conceptuales
- Un vehículo espacial ruso desarrolló una fuga, que resultó en una caída de presión interna de 1 atm a 0,85 atm. ¿Es este un ejemplo de una expansión reversible? ¿Se ha hecho el trabajo?
- ¿Qué miembro de cada par espera tener una entropía más alta? ¿Por qué?
- fenol sólido o fenol líquido
- 1-butanol o butano
- ciclohexano o ciclohexanol
- 1 mol de N 2 mezclado con 2 mol de O 2 o 2 mol de NO 2
- 1 mol de O 2 o 1 mol de O 3
- 1 mol de propano a 1 atm o 1 mol de propano a 2 atm
- Determine si cada proceso es reversible o irreversible.
- hielo derritiéndose a 0 ° C
- sal que cristaliza en una solución salina
- evaporación de un líquido en equilibrio con su vapor en un matraz sellado
- una reacción de neutralización
- Determine si cada proceso es reversible o irreversible.
- espagueti para cocinar
- la reacción entre sodio metal y agua
- consumo de oxígeno por la hemoglobina
- evaporación de agua en su punto de ebullición
- Explica por qué aumentar la temperatura de un gas aumenta su entropía. ¿Qué efecto tiene esto en la energía interna del gas?
- Para una serie de compuestos relacionados, ¿aumenta o disminuye ΔS vap con un aumento en la intensidad de las interacciones intermoleculares en estado líquido? ¿Por qué?
- ¿El cambio en la entalpía de reacción o el cambio en la entropía de reacción es más sensible a los cambios de temperatura? Explica tu razonamiento.
- El cloruro de potasio sólido tiene una estructura reticular altamente ordenada. ¿Espera que ΔS soln sea mayor o menor que cero? ¿Por qué? ¿Qué factores opuestos deben considerarse al hacer su predicción?
- La anilina (C 6 H 5 NH 2 ) es un líquido oleoso a 25 ° C que se oscurece con la exposición al aire y a la luz. Se utiliza para teñir telas y para teñir madera en negro. Un gramo de anilina se disuelve en 28,6 ml de agua, pero la anilina es completamente miscible con etanol. ¿Espera que ΔS solución en H 2 O sea mayor, menor o igual que ΔS solución en CH 3 CH 2 ¿OH? ¿Por qué?
Respuestas conceptuales
- No, es irreversible; no se realiza ningún trabajo porque la presión externa es efectivamente cero.
- reversible
- irreversible
- reversible
- irreversible
- El agua tiene una estructura altamente ordenada, unida al hidrógeno que debe reorganizarse para acomodar solutos hidrófobos como la anilina. Por el contrario, esperamos que la anilina pueda dispersarse aleatoriamente en todo el etanol, que tiene una estructura significativamente menos ordenada. Por lo tanto, predecimos que la solución ΔS en etanol será más positiva que la solución ΔS en agua.
Problemas numéricos
- El nitrógeno líquido, que tiene un punto de ebullición de -195.79 ° C, se usa como refrigerante y como conservante para los tejidos biológicos. ¿Es la entropía del nitrógeno mayor o menor a −200 ° C que a −190 ° C? Explica tu respuesta. El nitrógeno líquido se congela a un sólido blanco a −210.00 ° C, con una entalpía de fusión de 0.71 kJ / mol. ¿Cuál es su entropía de fusión? ¿La congelación de tejido biológico en nitrógeno líquido es un ejemplo de un proceso reversible o un proceso irreversible?
- Utilizando la segunda ley de la termodinámica, explique por qué el calor fluye de un cuerpo caliente a un cuerpo frío pero no de un cuerpo frío a un cuerpo caliente.
- Una prueba de la espontaneidad de una reacción es si la entropía del universo aumenta: ΔS univ > 0. Usando un argumento entrópico, demuestre que la siguiente reacción es espontánea a 25 ° C: [19459009 ]
4Fe (s) + 3O 2 (g) → 2Fe 2 O 3 (s)
¿Por qué aumenta la entropía del universo en esta reacción a pesar de que se consumen moléculas gaseosas, que tienen una alta entropía?
- Calcule los datos que faltan en la siguiente tabla.
| Compuesto | ΔH fus (kJ / mol) | ΔS fus [J / (mol · K)] | Punto de fusión (° C) |
|---|---|---|---|
| ácido acético | 11,7 | 16,6 | |
| CH 3 CN | 8,2 | 35,9 | |
| CH 4 | 0,94 | −182,5 | |
| CH 3 OH | 18,2 | −97,7 | |
| ácido fórmico | 12,7 | 45,1 |
Based on this table, can you conclude that entropy is related to the nature of functional groups? Explain your reasoning.
- Calculate the missing data in the following table.
| Compound | ΔH vap (kJ/mol) | ΔS vap [J/(mol·K)] | Boiling Point (°C) |
|---|---|---|---|
| hexanoic acid | 71.1 | 105.7 | |
| hexane | 28.9 | 85.5 | |
| formic acid | 60.7 | 100.8 | |
| 1-hexanol | 44.5 | 157.5 |
The text states that the magnitude of ΔS vap tends to be similar for a wide variety of compounds. Based on the values in the table, do you agree?
Conceptual Problems
- How does each example illustrate the fact that no process is 100% efficient?
- burning a log to stay warm
- the respiration of glucose to provide energy
- burning a candle to provide light
- Neither the change in enthalpy nor the change in entropy is, by itself, sufficient to determine whether a reaction will occur spontaneously. ¿Por qué?
- If a system is at equilibrium, what must be the relationship between ΔH and ΔS?
- The equilibrium
2AB⇌A 2 B 2 is exothermic in the forward direction. Which has the higher entropy—the products or the reactants? ¿Por qué? Which is favored at high temperatures?
- Is ΔG a state function that describes a system or its surroundings? Do its components—ΔH and ΔS—describe a system or its surroundings?
- How can you use ΔG to determine the temperature of a phase transition, such as the boiling point of a liquid or the melting point of a solid?
- Occasionally, an inventor claims to have invented a “perpetual motion” machine, which requires no additional input of energy once the machine has been put into motion. Using your knowledge of thermodynamics, how would you respond to such a claim? Justify your arguments.
- Must the entropy of the universe increase in a spontaneous process? If not, why is no process 100% efficient?
- The reaction of methyl chloride with water produces methanol and hydrogen chloride gas at room temperature, despite the fact that
ΔH ∘ rxn = 7.3 kcal/mol. Using thermodynamic arguments, propose an explanation as to why methanol forms.
Conceptual Answers
- In order for the reaction to occur spontaneously, ΔG for the reaction must be less than zero. In this case, ΔS must be positive, and the TΔS term outweighs the positive value of ΔH.
Numerical Problems
- Use the tables in the text to determine whether each reaction is spontaneous under standard conditions. If a reaction is not spontaneous, write the corresponding spontaneous reaction.
- (mathrm{H_2(g)}+frac{1}{2}mathrm{O_2(g)}rightarrowmathrm{H_2O(l)})
- 2H 2 (g) + C 2 H 2 (g) → C 2 H 6 (g)
- (CH 3 ) 2 O(g) + H 2 O(g) → 2CH 3 OH(l)
- CH 4 (g) + H 2 O(g) → CO(g) + 3H 2 (g)
- Use the tables in the text to determine whether each reaction is spontaneous under standard conditions. If a reaction is not spontaneous, write the corresponding spontaneous reaction.
- K 2 O 2 (s) → 2K(s) + O 2 (g)
- PbCO 3 (s) → PbO(s) + CO 2 (g)
- P 4 (s) + 6H 2 (g) → 4PH 3 (g)
- 2AgCl(s) + H 2 S(g) → Ag 2 S(s) + 2HCl(g)
- Nitrogen fixation is the process by which nitrogen in the atmosphere is reduced to NH 3 for use by organisms. Several reactions are associated with this process; three are listed in the following table. Which of these are spontaneous at 25°C? If a reaction is not spontaneous, at what temperature does it become spontaneous?
| Reaction | |
|
|---|---|---|
| (a) (frac{1}{2}mathrm{N_2}+mathrm{O_2}rightarrowmathrm{NO_2}) | 8.0 | −14.4 |
| (b) (frac{1}{2}mathrm{N_2}+frac{1}{2}mathrm{O_2}rightarrowmathrm{NO}) | 21.6 | 2.9 |
| (c) (frac{1}{2}mathrm{N_2}+frac{3}{2}mathrm{H_2}rightarrowmathrm{NH_3}) | −11.0 | −23.7 |
- A student was asked to propose three reactions for the oxidation of carbon or a carbon compound to CO or CO 2 . The reactions are listed in the following table. Are any of these reactions spontaneous at 25°C? If a reaction does not occur spontaneously at 25°C, at what temperature does it become spontaneous?
| Reaction | |
|
|---|---|---|
| C(s) + H 2 O(g) → CO(g) + H 2 (g) | 42 | 32 |
| CO(g) + H 2 O(g) → CO 2 (g) + H 2 (g) | −9.8 | −10.1 |
| CH 4 (g) + H 2 O(g) → CO(g) + 3H 2 (g) | 49.3 | 51.3 |
- Tungsten trioxide (WO 3 ) is a dense yellow powder that, because of its bright color, is used as a pigment in oil paints and water colors (although cadmium yellow is more commonly used in artists’ paints). Tungsten metal can be isolated by the reaction of WO 3 with H 2 at 1100°C according to the equation WO 3 (s) + 3H 2 (g) → W(s) + 3H 2 O(g). What is the lowest temperature at which the reaction occurs spontaneously? ΔH° = 27.4 kJ/mol and ΔS° = 29.8 J/K.
- Sulfur trioxide (SO 3 ) is produced in large quantities in the industrial synthesis of sulfuric acid. Sulfur dioxide is converted to sulfur trioxide by reaction with oxygen gas.
- Write a balanced chemical equation for the reaction of SO 2 with O 2 (g) and determine its ΔG°.
- What is the value of the equilibrium constant at 600°C?
- If you had to rely on the equilibrium concentrations alone, would you obtain a higher yield of product at 400°C or at 600°C?
- Calculate ΔG° for the general reaction MCO 3 (s) → MO(s) + CO 2 (g) at 25°C, where M is Mg or Ba. At what temperature does each of these reactions become spontaneous?
| Compound | |
S° [J/(mol·K)] |
|---|---|---|
| MCO 3 | ||
| Mg | −1111 | 65.85 |
| Ba | −1213.0 | 112.1 |
| MO | ||
| Mg | −601.6 | 27.0 |
| Ba | −548.0 | 72.1 |
| CO 2 | −393.5 | 213.8 |
- The reaction of aqueous solutions of barium nitrate with sodium iodide is described by the following equation:
Ba(NO 3 ) 2 (aq) + 2NaI(aq) → BaI 2 (aq) + 2NaNO 3 (aq)
You want to determine the absolute entropy of BaI 2 , but that information is not listed in your tables. However, you have been able to obtain the following information:
| Ba(NO 3 ) 2 | NaI | BaI 2 | NaNO 3 | |
|---|---|---|---|---|
| |
−952.36 | −295.31 | −605.4 | −447.5 |
| S° [J/(mol·K)] | 302.5 | 170.3 | 205.4 |
You know that ΔG° for the reaction at 25°C is 22.64 kJ/mol. What is ΔH° for this reaction? What is S° for BaI 2 ?
Numerical Answers
- −237.1 kJ/mol; spontaneous as written
- −241.9 kJ/mol; spontaneous as written
- 8.0 kJ/mol; spontaneous in reverse direction.
- 141.9 kJ/mol; spontaneous in reverse direction.
- Not spontaneous at any T
- Not spontaneous at 25°C; spontaneous above 7400 K
- Spontaneous at 25°C
- MgCO 3 : ΔG° = 63 kJ/mol, spontaneous above 663 K; BaCO 3 : ΔG° = 220 kJ/mol, spontaneous above 1562 K
Conceptual Problems
- Do you expect products or reactants to dominate at equilibrium in a reaction for which ΔG° is equal to
- 1.4 kJ/mol?
- 105 kJ/mol?
- −34 kJ/mol?
- The change in free energy enables us to determine whether a reaction will proceed spontaneously. How is this related to the extent to which a reaction proceeds?
- What happens to the change in free energy of the reaction N 2 (g) + 3F 2 (g) → 2NF 3 (g) if the pressure is increased while the temperature remains constant? if the temperature is increased at constant pressure? Why are these effects not so important for reactions that involve liquids and solids?
- Compare the expressions for the relationship between the change in free energy of a reaction and its equilibrium constant where the reactants are gases versus liquids. What are the differences between these expressions?
Numerical Problems
- Carbon monoxide, a toxic product from the incomplete combustion of fossil fuels, reacts with water to form CO 2 and H 2 , as shown in the equation
CO(g)+H 2 O(g)⇌CO 2 (g)+H 2 (g), for which ΔH° = −41.0 kJ/mol and ΔS° = −42.3 J cal/(mol·K) at 25°C and 1 atm.
- What is ΔG° for this reaction?
- What is ΔG if the gases have the following partial pressures: P CO = 1.3 atm, (P_{mathrm{H_2O}}) = 0.8 atm, (P_{mathrm{CO_2}})
= 1.3 atm? - What is ΔG if the temperature is increased to 150°C assuming no change in pressure?
- Methane and water react to form carbon monoxide and hydrogen according to the equation
CH 4 (g) + H 2 O(g) ⇌ CO(g) + 3H 2 (g).
- What is the standard free energy change for this reaction?
- What is K p for this reaction?
- What is the carbon monoxide pressure if 1.3 atm of methane reacts with 0.8 atm of water, producing 1.8 atm of hydrogen gas?
- What is the hydrogen gas pressure if 2.0 atm of methane is allowed to react with 1.1 atm of water?
- At what temperature does the reaction become spontaneous?
- Calculate the equilibrium constant at 25°C for each equilibrium reaction and comment on the extent of the reaction.
-
CCl 4 (g)+6H 2 O(l)⇌CO 2 (g)+4HCl(aq); ΔG° = −377 kJ/mol -
Xe(g)+2F 2 (g)⇌XeF 4 (s); ΔH° = −66.3 kJ/mol, ΔS° = −102.3 J/(mol·K) -
PCl 3 (g)+S⇌PSCl 3 (l); ΔG ∘ f (PCl 3 ) = −272.4 kJ/mol,ΔG ∘ f (PSCl 3 ) = −363.2 kJ/mol
- Calculate the equilibrium constant at 25°C for each equilibrium reaction and comment on the extent of the reaction.
-
2KClO 3 (s)⇌2KCl(s)+3O 2 (g); ΔG° = −225.8 kJ/mol -
CoCl 2 (s)+6H 2 O(g)⇌CoCl 2 ⋅6H 2 O(s); ΔH ∘ rxn = −352 kJ/mol,ΔS ∘ rxn = −899 J/(mol·K) -
2PCl 3 (g)+O 2 (g)⇌2POCl 3 (g); ΔG ∘ f (PCl 3 ) = −272.4 kJ/mol,ΔG ∘ f (POCl 3 ) = −558.5 kJ/mol
- The gas-phase decomposition of N 2 O 4 to NO 2 is an equilibrium reaction with K p = 4.66 × 10 −3 . Calculate the standard free-energy change for the equilibrium reaction between N 2 O 4 and NO 2 .
- The standard free-energy change for the dissolution
K 4 Fe(CN) 6 ⋅H 2 O(s)⇌4K + (aq)+Fe(CN) 6 4− (aq)+H 2 O(l) is 26.1 kJ/mol. What is the equilibrium constant for this process at 25°C?
- Ammonia reacts with water in liquid ammonia solution (am) according to the equation
NH 3 (g) + H 2 O(am) ⇌ NH 4 + (am) + OH − (am). The change in enthalpy for this reaction is 21 kJ/mol, and ΔS° = −303 J/(mol·K). What is the equilibrium constant for the reaction at the boiling point of liquid ammonia (−31°C)?
- At 25°C, a saturated solution of barium carbonate is found to have a concentration of [Ba 2 + ] = [CO 3 2− ] = 5.08 × 10 −5 M. Determine ΔG° for the dissolution of BaCO 3 .
- Lead phosphates are believed to play a major role in controlling the overall solubility of lead in acidic soils. One of the dissolution reactions is
Pb 3 (PO 4 ) 2 (s)+4H + (aq)⇌3Pb 2 + (aq)+2H 2 PO 4 − (aq), for which log K = −1.80. What is ΔG° for this reaction?
- The conversion of butane to 2-methylpropane is an equilibrium process with ΔH° = −2.05 kcal/mol and ΔG° = −0.89 kcal/mol.
- What is the change in entropy for this conversion?
- Based on structural arguments, are the sign and magnitude of the entropy change what you would expect? ¿Por qué?
- What is the equilibrium constant for this reaction?
- The reaction of CaCO 3 (s) to produce CaO(s) and CO 2 (g) has an equilibrium constant at 25°C of 2 × 10 −23 . Values of
ΔH ∘ f are as follows: CaCO 3 , −1207.6 kJ/mol; CaO, −634.9 kJ/mol; and CO 2 , −393.5 kJ/mol.
- What is ΔG° for this reaction?
- What is the equilibrium constant at 900°C?
- What is the partial pressure of CO 2 (g) in equilibrium with CaO and CaCO 3 at this temperature?
- Are reactants or products favored at the lower temperature? at the higher temperature?
- In acidic soils, dissolved Al 3 + undergoes a complex formation reaction with SO 4 2− to form [AlSO 4 + ]. The equilibrium constant at 25°C for the reaction
Al 3 + (aq)+SO 4 2− (aq)⇌AlSO 4 + (aq) is 1585.
- What is ΔG° for this reaction?
- How does this value compare with ΔG° for the reaction
Al 3 + (aq)+F − (aq)⇌AlF 2 + (aq), for which K = 10 7 at 25°C? - Which is the better ligand to use to trap Al 3 + from the soil?
Numerical Answers
- −28.4 kJ/mol
- −26.1 kJ/mol
- −19.9 kJ/mol
- 1.21 × 10 66 ; equilibrium lies far to the right.
- 1.89 × 10 6 ; equilibrium lies to the right.
- 5.28 × 10 16 ; equilibrium lies far to the right.
- 129.5 kJ/mol
- 6
- 6.0 atm
- Products are favored at high T; reactants are favored at low T.