Textbook Question
Which of the following processes are spontaneous and which are nonspontaneous: (e) formation of CH4 and O2 molecules from CO2 and H2O at room temperature and 1 atm of pressure?
Ch.19 - Chemical Thermodynamics
Chapter 19, Problem 14
(a) Can endothermic chemical reactions be spontaneous? (b) Can a process be spontaneous at one temperature and nonspontaneous at a different temperature? (c) Water can be decomposed to form hydrogen and oxygen, and the hydrogen and oxygen can be recombined to form water. Does this mean that the processes are thermodynamically reversible?
Verified step by step guidance1
To address part (a), understand that spontaneity of a reaction is determined by the Gibbs free energy change (ΔG). A reaction is spontaneous if ΔG < 0. The Gibbs free energy is given by the equation: , where ΔH is the enthalpy change, T is the temperature in Kelvin, and ΔS is the entropy change. An endothermic reaction has ΔH > 0, but it can still be spontaneous if the TΔS term is large enough to make ΔG negative.
For part (b), consider the temperature dependence of spontaneity. The equation shows that temperature (T) can influence the sign of ΔG. A process can be spontaneous at one temperature and nonspontaneous at another if the TΔS term changes significantly with temperature, affecting the overall sign of ΔG.
In part (c), examine the concept of thermodynamic reversibility. A process is thermodynamically reversible if it can proceed in both directions without a net change in entropy of the universe. While water can be decomposed into hydrogen and oxygen and then recombined, these processes are not thermodynamically reversible because they involve changes in entropy and energy that are not perfectly balanced in real-world conditions.
To further understand part (a), consider an example of an endothermic reaction that is spontaneous. The melting of ice at temperatures above 0°C is endothermic (absorbs heat) but spontaneous because the increase in entropy (ΔS) is significant enough to make ΔG negative.
For part (b), think about the melting of ice again. At temperatures below 0°C, the process is nonspontaneous because the TΔS term is not large enough to overcome the positive ΔH, making ΔG positive. This illustrates how a process can be spontaneous at one temperature and nonspontaneous at another.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Spontaneity of Reactions
A reaction is considered spontaneous if it occurs without external intervention. Spontaneity is determined by the change in Gibbs free energy (ΔG); if ΔG is negative, the reaction is spontaneous. Endothermic reactions, which absorb heat, can still be spontaneous if the overall change in entropy (ΔS) is positive enough to make ΔG negative.
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Spontaneity of Processes
Temperature Dependence of Spontaneity
The spontaneity of a process can depend on temperature, as described by the Gibbs free energy equation: ΔG = ΔH - TΔS. Here, ΔH is the enthalpy change, T is the temperature in Kelvin, and ΔS is the entropy change. A process that is spontaneous at one temperature may become nonspontaneous at another if the balance between enthalpy and entropy changes.
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Thermodynamic Reversibility
A process is thermodynamically reversible if it can proceed in both directions without a net change in the system and surroundings. For the decomposition of water into hydrogen and oxygen and their recombination, if both processes can occur under the same conditions without energy loss, they are considered reversible. However, real processes often involve irreversibilities due to factors like energy dissipation.
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Related Practice
Textbook Question
Which of the following processes are spontaneous?
a. the melting of ice cubes at −10 °C and 1 atm pressure
b. separating a mixture of N2 and O2 into two separate samples, one that is pure N2 and one that is pure O2
c. alignment of iron filings in a magnetic field
d. the reaction of hydrogen gas with oxygen gas to form water vapor at room temperature
e. the dissolution of HCl(g) in water to form concentrated hydrochloric acid
Textbook Question
Indicate whether each statement is true or false. (a) A reaction that is spontaneous in one direction will be nonspontaneous in the reverse direction under the same reaction conditions. (b) All spontaneous processes are fast. (c) Most spontaneous processes are reversible. (d) An isothermal process is one in which the system loses no heat. (e) The maximum amount of work can be accomplished by an irreversible process rather than a reversible one.
Textbook Question
(d) Does the amount of work that a system can do on its surroundings depend on the path of the process?
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Textbook Question
Consider the vaporization of liquid water to steam at a pressure of 1 atm. (a) Is this process endothermic or exothermic?
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Textbook Question
Consider the vaporization of liquid water to steam at a pressure of 1 atm. (b) In what temperature range is it a spontaneous process?