Nonspontaneous processes

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. 

Self-Test 7.18A Can a nonspontaneous process with a negative A5 become spontaneous if the temperature is increased ... 

Adenosine triphosphate (ATP) is an extremely important molecule in biological systems. Consult standard reference sources in your library to describe how this molecule is used in energy transfer to facilitate nonspontaneous processes necessary for life. 

C14-0080. Two children on opposite ends of a seesaw can be used as an analogy for a coupled reaction. Describe the coupling of spontaneous and nonspontaneous processes during the actions of a seesaw. 

C14-0117. The notion of thermodynamic coupling of a nonspontaneous process with a spontaneous process is not restricted to chemical reactions. Identify the spontaneous and nonspontaneous portions of the following coupled processes (a) Water behind a dam passes through a turbine and generates electricity, (b) A gasoline engine pumps water from a valley to the top of a hill. 

We must narrow the options. There will be only one cathode reaction and only one anode reaction. How do we pick the correct half-reactions If one of the half-reactions were spontaneous (positive), we would pick it for that electrode. (If more than were spontaneous, we would pick the largest positive value.) All four half-reactions in this case are nonspontaneous (negative). This is typical for electrolysis, because you are using electrical energy to force a nonspontaneous process to take place. 

The second law of thermodynamics, as formulated by Kelvin, states that no process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work in other words, in any natural process involving a transfer of energy, some energy is converted irreversibly into heat that cannot be involved in further exchange. The second law of thermodynamics, therefore, is a recognition of spontaneous and nonspontaneous processes and the fact that natural processes have a sense of direction. 

Sledding downhill is a spontaneous process that, once started, continues on its own. Hauling the sled back uphill is a nonspontaneous process that requires a continuous input of energy. 

Knowing the values of AH and AS for a phase transition makes it possible to calculate the temperature at which the change occurs. Recall from Section 8.14 that AG is negative for a spontaneous process, positive for a nonspontaneous process, and zero for a process at equilibrium. Thus, by setting AG = 0 and solving for T in the free-energy equation, we can calculate the temperature at which two phases are in equilibrium. For the solid — liquid phase change in water, for instance, we have... 

Why is it that when an object is dropped it continues to fall without any further intervention Do objects need to be coached in how to fall Of course not gravity takes over once you let go of an object and no further intervention is needed to make the object hit the floor. Processes that occur without added external energy or without additional intervention are called spontaneous processes. Now imagine that the very same object that was dropped to the floor suddenly leaves the floor and comes back to your hand (assume no springs, rockets, or legs to jump with). That would be quite a surprise An object that has fallen does not come back to your hand because that would be a nonspontaneous process. There are many spontaneous processes in our universe for example, the spontaneous decay of the nucleus of an atom. It should be noted that if a reaction is spontaneous, the reverse reaction will not be spontaneous. If we are going to make a nonspontaneous reaction occur, then energy and outside intervention will be needed. 

The synthesis of glucose directly from C02 and H20 and the synthesis of proteins directly from amino acids are both nonspontaneous processes under standard conditions. Yet these processes must occur for life to exist. In light of the second law of thermodynamics, how can life exist ... 

Answer nonspontaneous process or endergonic process In a/an reaction, a substance loses electrons. 

Can a nonspontaneous reaction ever occur A nonspontaneous reaction cannot occur unless some form of energy is added to the system. Figure 15 shows that the nonspontaneous reaction of photosynthesis occurs with outside assistance. During photosynthesis, light energy from the sun is used to drive the nonspontaneous process. This reaction is described by the equation below. 

The entropy of a system can decrease during a spontaneous process or increase during a nonspontaneous process, depending on the accompanying AS j.j.. If AS yg is negative... 

How is it possible, then, for organisms to overcome the tendency to disorder and maintain their complex structures and life functions The secret of their success is their ability to absorb energy from outside sources, such as light and food. These outside sources continue pumping in the energy needed to keep the otherwise-nonspontaneous processes going. 

Under conditions of constant temperature and pressure, the free-energy change AG is less than zero for a spontaneous process and greater than zero for a nonspontaneous process. For an equilibrium process, AG = 0. 

Explain what is meant by a spontaneous process. Give two examples each of spontaneous and nonspontaneous processes. 

Active transport is the process in which a substance is transferred from a region of lower concentration to one of higher concentration. This is a nonspontaneous process and must be coupled to a spontaneous process, such as the hydrolysis of ATP. The concentrations of ions in the blood plasma and in nerve cells are 15 mM and 400 mM, respectively (1 mM = 1 X 10 M). Use Equation (18.9) to calculate AG for the process at the physiological temperature of 37°C ... 

Theoretical estimates show that the E° value for the overall process is about 4 V, which means that this is a nonspontaneous process. Therefore, a minimum of 4 V must be supplied by the battery to carry out the reaction. In practice, a higher voltage is necessary becanse of inefficiencies in the electrolytic process and because of overvoltage, to be discussed shortly. 

A process, such as a rock falling or a fuel burning, is called spontaneous because, once started, it continues by itself. A nonspontaneous process requires a continuous input of energy to occur. A reaction proceeding toward equilibrium is a spontaneous process. 

For a nonspontaneous process (AG > 0) at constant T and P, AG is the minimum work that must be done to the system to make the process take place. 

Distinguish between the terms spontaneous and nonsponta-neous. Can a nonspontaneous process occur Explain. 

A spontaneous process occurs by itself, whereas a nonspon-taneous process requires a continuous input of energy to make it happen. It is possible to cause a nonspontaneous process to occur, but the process stops once the energy source is removed. A reaction that is nonspontaneous under one set of conditions may be spontaneous under a different set of conditions. 20.5 The transition from liquid to gas involves a greater increase in dispersal of energy and freedom of motion than does the transition from solid to liquid. 20.6 In an exothermic reaction, A5 r,. > 0. In an endothermic reaction, A5, rr < 0. A chemical cold pack for injuries is an example of an application using a spontaneous endothermic process. 20.8(a), (b), and (c) 20.10(a) positive... 

An example of a nonspontaneous process is the reverse of the reaction that we saw in Section 1.9—namely, the phosphorylation of ADP (adenosine diphosphate) to give ATP (adenosine triphosphate). This reaction takes place in living organisms because metabolic processes supply energy. 

How can we predict what reactions will happen in living cells In a spontaneous process, the free energy decreases (AGis negative). In a nonspontaneous process, the free energy increases. 

Processes (a) and (b) are spontaneous, whereas processes (c) and (d) are not The spontaneous processes represent an increase in disorder (increase in the entropy of the Universe) and have a negative ACP at constant temperature and pressure, while the opposite is true of the nonspontaneous processes. 

For a spontaneous process AG < 0, for a nonspontaneous process AG >0, and for a system at equilibrium AG = 0. Therefore, AG becomes an essential parameter that determines if a polymer will be spontaneously dissolved in a solvent or not. 

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. 

Because real processes can at best only approximate the infinitely slow change associated with reversible processes, all real processes are irreversible. Further, as noted earlier in this discussion, the reverse of any spontaneous process is a nonspontaneous process. A nonspontaneous process can occur only if the surroundings do work on the s) stem. Thus, any spontaneous process is irreversible. Even if we return the system to the original condition, the surroundings will have changed. 

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