Processes spontaneous

Just because a process is spontaneous does not necessarily mean that it will occur at an observable rate. A chemical reaction is spontaneous if it occurs on its own accord, regardless of its speed. A spontaneous reaction can be very fast, as in the case of acid-base neutralization, or very slow, as in the rusting of iron. Thermodynamics tells us the direction and extent of a reaction but nothing about the rate rate is the domain of kinetics rather than of thermodynamics. 

If a process is nonspontaneous, does that mean the process cannot occur under any circumstances  

If flask B were smaller than flask A, would the final pressure after the stopcock is opened be greater than, equal to, or less than 0.5 atm  

When stopcock opens, gas expands to occupy both flasks 

The reverse process—gas molecules initially distributed evenly in two flasks all moving into one flask—is not spontaneous. 

We know other spontaneous and nonspontaneous processes that relate more directly to our study of chemistry. For example, a gas spontaneously expands into a vacuum ( FIGU RE 19.2), but the reverse process, in which the gas moves back entirely into one of the flasks, does not happen. In other words, expansion of the gas is spontaneous, but the reverse process is nonspontaneous. In general, processes that are spontaneous in one direction are nonspontaneous in the opposite direction. 

Experimental conditions, such as temperature and pressure, are often important in determining whether a process is spontaneous. We are all familiar with situations in which a forward process is spontaneous at one temperature but the reverse process is 

Predict whether each process is spontaneous as described, spontaneous in the reverse direction, or in equilibrium (a) Water at 40 °C gets hotter when a piece of metal heated to 150 °C is added, (b) Water at room temperature decomposes into H2(g) and 02(g). (c) Benzene vapor, C5Hg(g), at a pressure of 1 atm condenses to liquid benzene at the normal boiling point of benzene, 80.1 °C. 

These examples show that processes that occur spontaneously in one direction cannot, under the same conditions, also take place spontaneously in the opposite direction. 

If we assume that spontaneous processes occur so as to decrease the energy of a system, we can explain why a ball rolls downhill and why springs in a clock unwind. Similarly, a large number of exothermic reactions are spontaneous. An example is the combustion of methane  

Another example is the acid-base neutrahzation reaction  

All of us are familiar with certain spontaneous processes. For example— 

In other words, these three reactions are spontaneous at 25°C and 1 atm.  

The word spontaneous does not imply anything about how rapidly a reaction occurs. A spark is OK, but a continuous input of 

Some spontaneous reactions, notably the rusting of iron, are quite slow. Often a reaction that energy isn t, 

If a reaction is spontaneous under a given set of conditions, the reverse reaction must be nonspontaneous. For example, water does not spontaneously decompose to the elements by the reverse of the reaction referred to above. 

The word spontaneous does not imply anything about how rapidly a reaction occurs. Some spontaneous reactions, notably the rusting of iron, are quite slow. Often a reaction that is spontaneous does not occur without some sort of stimulus to get the reaction started. A mixture of hydrogen and oxygen shows no sign of reaction in the absence of a spark or match. Once started, though, a spontaneous reaction continues by itself without further input of energy from the outside. 

However, it is often possible to bring about a nonspontaneous reaction by supplying energy in the form of work. Electrolysis can be used to decompose water to the elements. Electrical energy must be furnished for the decomposition, perhaps Irom a storage battery. 

A process is said to be spontaneous if it occurs without outside intervention. Spontaneous processes may be fast or slow. As we will see in this chapter, thermodynamics can tell us the direction in which a process will occur but can say nothing about the speed (rate) of the process. As we will explore in detail in Chapter 15, the rate of a reaction depends on many factors, including temperature and concentration. In describing a chemical reaction, the discipline of chemical kinetics (the study of reaction rates) focuses on the pathway between reactants and products in contrast, thermodynamics considers only the initial and final states and does not require knowledge of the pathway between the reactants and products (see Fig. 10.2). 

In summary, thermodynamics lets us predict whether a process will occur but gives no information about the amount of time required. For example, according to the principles of thermodynamics, a diamond should change spontaneously to graphite at 25°C and 1 atm pressure. The fact that we do not observe this process does not mean the prediction is wrong it simply means the process is too slow to observe. Thus we need both thermodynamics and kinetics to describe reactions fully. 

To explore the idea of spontaneity, consider the following physical and chemical processes  

A ball rolls down a hill but never spontaneously rolls back up the hi . 

If exposed to air and moisture, steel rusts spontaneously. However, the iron oxide in rust does not spontaneously change back to iron metal and oxygen gas. 

The process is imderstood as any change in the system state, even if only one of its parameters changes (temperature, volume, pressure, composition, etc.). Hydrogeochemistry studies mostly changes in the composition of natural waters, i.e., chemical processes, which are tied with the action of inter-atomic and inter-molecular forces. A special role in these processes is played by those which cause transformation of one substance into another and are called chemical reaction. All processes, including also chemical reactions, are subdivided first of all into unspontaneous and spontaneous. 

Unspontaneous processes are associated with the acquisition or loss by a limited system of energy or substances as result of outside interference. They are caused by forces which are positioned outside of the system. If the 

Spontaneous processes are associated with the actions of unbalanced forces within the system and are directed to levelling off of intensive parameters within its boundaries (heat exchange, mass exchange, chemical transformations within the system). They are always directed to restoring of disrupted equilibrium. In the final result, in the absence of unspontaneous processes spontaneous processes lead the system to an equilibrium state. 

Spontaneous processes, in their turn, are subdivided into reversible processes (equilibrium, quasi-static) and irreversible processes or unequilibrium. Real processes, as a rule, are irreversible as they occm with loss of energy to the surrounding medium, and that is why the system cannot spontaneously return to its previous state. 

Thermodynamic equilibrium is such a state of the system which does not change in time and has only reversible processes. What is required to achieve it are stable conditions and the absence of outer influence. From the viewpoint of statistical mechanics, such an equilibrium state is considered 

A more formal way of expressing the directionality of nature is to note that our intuition is predicated on the fact that some things just happen, but others do not. 

Some processes occur without any outside intervention, and we say that such a process is spontaneous. From a thermodynamic perspective, then, a spontaneous process is one that takes place without continuous intervention. The distinction between spontaneous and nonspontaneous reactions may seem obvious, but we ll see that it is not always so. 

Students often misinterpret the word spontaneous as indicating that a process or reactions will take place quickly. But note that our actual definition does not refer to the speed of the process at all. Some spontaneous processes are very fast, but for example. 

Some spontaneous processes take place over geological time scales—the formation of petroleum used for plastics feedstocks, 

The reactions used to produce many polymers behave much like the combustion of gasoline. Once initiated, the reaction is usually spontaneous and can proceed without intervention. The production of poly(methyl methacrylate)— Plexiglas , or PMMA—is a good example. 

In Chapter 6 we encountered the first of three laws of thermodynamics, which says that eneigy can be converted from one form to another, but it cannot be created or destroyed. One measure of these changes is the amount of heat given off or absorbed by a system during a constant-pressure process, which chemists define as a change in enthalpy (AH). 

The second law of thermodynamics explains why chemical processes tend to favor one direction. The third law is an extension of the second law and will be examined briefly in Section 18.4. 

A spontaneous reaction does not necessarily mean an instantaneous reaction. 

Because of activation energy barrier, an input of energy is needed to get this reaction started. 

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Thus, for spontaneous processes at constant temperature and volume a new quantity, the Helmholtz free energy A, decreases. At equilibrium under such restrictions cL4 = 0. 

For spontaneous processes at constant temperature and pressure it is the Gibbs free energy G that decreases, while at equilibrium under such conditions dG = 0. 

It should be noted that the differential expressions on the right-hand side of equation (A2.1.33). equation (A2.1.34), equation (A2.1.35), equation (A2.1.36), equation (A2.1.37), equation (A2.1.38), equation (A2.1.39) and equation (A2.1.40) express for each fiinction the appropriate independent variables for that fiinction, i.e. the variables—read constraints—drat are kept constant during a spontaneous process. 

In equation (Cl.4.14) the saturation parameter essentially defines a criterion to compare the time required for stimulated and spontaneous processes. If I then spontaneous coupling of the atom to the vacuum modes of the field is fast compared to the stimulated Rabi coupling and the field is considered weak. If s" 1 then the Rabi oscillation is fast compared to spontaneous emission and the field is said to be strong. Setting s equal to unity defines the saturation condition... 

If there are no reactions, the conservation of the total quantity of each species dictates that the time dependence of is given by minus the divergence of the flux ps vs), where (vs) is the drift velocity of the species s. The latter is proportional to the average force acting locally on species s, which is the thermodynamic force, equal to minus the gradient of the thermodynamic potential. In the local coupling approximation the mobility appears as a proportionality constant M. For spontaneous processes near equilibrium it is important that a noise term T] t) is retained [146]. Thus dynamic equations of the form... 

While this is an easy calculation to make, Eq. (3.7) does little to clarify exactly what AS means. Phenomenological proofs that AS as defined by Eq. (3.7) is a state variable often leave us with little more than a lament for the inefficiency of spontaneous processes. 

Laser radiation is emitted entirely by the process of stimulated emission, unlike the more conventional sources of radiation discussed in Chapter 3, which emit through a spontaneous process. 

The more negative the value of AG, the more energy or useful work can be obtained from the reaction. Reversible processes yield the maximum output. In irreversible processes, a portion of the useful work or energy is used to help carry out the reaction. The cell voltage or emf also has a sign and direction. Spontaneous processes have a positive emf the reaction, written in a reversible fashion, goes in the forward direction. 

Thus the formation of an ideal solution from its components is always a spontaneous process. Real solutions are described in terms of the difference in the molar Gibbs free energy of their formation and that of the corresponding ideal solution, thus ... 

The oxidation of hydrogen to water (Hj -t- i Oj -> HjO) is thermodynamically spontaneous and the energy released as a result of the chemical reaction appears as heat energy, but the decomposition of water into its elements is a non-spontaneous process and can be achieved only by supplying energy from an external source, e.g. a source of e.m.f. that decomposes the water electrolytically. Furthermore, although the heat produced by the spontaneous reaction could be converted into electrical energy, the electrical... 

Perhaps the simplest way to define spontaneity is to say that a spontaneous process is one that moves the reaction system toward equilibrium. A nonspontaneous process moves the system away from equilibrium. 

The relationship between entropy change and spontaneity can be expressed through a basic principle of nature known as the second law of thermodynamics. One way to state this law is to say that in a spontaneous process, there is a net increase in entropy, taking into account both system and surroundings. That is,... 

Notice that the second law refers to the total entropy change, involving both system and surroundings. For many spontaneous processes, the entropy change for the system is a negative quantity. Consider, for example, the rusting of iron, a spontaneous process ... 

A spontaneous process is capable of producing useful work. 

Coupled reactions are common in human metabolism. Spontaneous processes, such as the oxidation of glucose,... 

Key Terms enthalpy, H free energy of formation, AG standard entropy change, AS° entropy, S spontaneous process standard free energy change, AG° free energy, G... 

Second law of thermodynamics A basic law of nature, one form of which states that all spontaneous processes occur with an increase in entropy, 457 Second order reaction A reaction whose rate depends on the second power of reactant concentration, 289,317q gas-phase, 300t... 

Thermodynamic, second law The entropy of the universe increases in a spontaneous process and remains unchanged in a reversible process. It can never decrease. 

Since q >0, and l/T > /T2 with T2 >7), we conclude that AS for this allowed, spontaneous process is greater than zero. Having obtained this result for the specific case, we can extend it to the general case, because our earlier conclusion that there is an allowed direction to spontaneous adiabatic processes applies to all thermodynamic systems. 

Equation (5.47) gives the criterion for reversibility or spontaneity within subsystem A of an isolated system. The inequality applies to the spontaneous process, while the equality holds for the reversible process. Only when equilibrium is present can a change in an isolated system be conceived to occur reversibly. Therefore, the criterion for reversibility is a criterion for equilibrium, and equation (5.47) applies to the spontaneous or the equilibrium process, depending upon whether the inequality or equality is used. 

Equation (5.52) is the first of our criteria. The subscripts indicate that equation (5.52) applies to the condition of constant entropy, volume, and total moles, with the equality applying to the equilibrium process and the inequality to the spontaneous process. 

Entropy is a measure of disorder according to the second law of thermodynamics, the entropy of an isolated system increases in any spontaneous process. Entropy is a state function. 

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