Spontaneous and nonspontaneous reactions

Refer to the "Causes of Change" chapter for more information about spontaneous and nonspontaneous reactions. 

Spontaneous and Nonspontaneous Reactions Entropy Eree Energy A Human Perspective Triboluminescence Sparks in the Dark with Candy... 

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. 

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. 

In the same way, the temperature at which the reaction of carbon with water changes between spontaneous and nonspontaneous is 978 K, or 705°C ... 

The spontaneity of the reaction at a given temperature can be found by determining whether AG is positive or negative at that temperature. The changeover point between spontaneous and nonspontaneous can be found by setting AG = 0 and solving for T. 

Electrochemistry is the study of the relationships between electricity and chemical reactions. It includes the study of both spontaneous and nonspontaneous processes. 

As discussed in Chapter 4, oxidation is the loss of electrons in a chemical reaction, and reduction is the gain of electrons, ooo (Section 4.4) Thus, oxidation-reduction (redox) reactions occur when electrons are transferred from an atom that is oxidized to an atom that is reduced. Redox reactions are involved not only in the operation of batteries but also in a wide variety of important natural processes, including the rusting of iron, the browning of foods, and the respiration of animals. Electrochemistry is the study of the relationships between electricity and chemical reactions. It includes the study of both spontaneous and nonspontaneous processes. 

FIGURE 7.28 The effect of an increase in temperature on the spontaneity of a reaction under standard conditions. In each case, "spontaneous" is taken to mean AC0 < 0 and "nonspontaneous" is taken to mean AC° > 0. (a) An exothermic reaction with negative reaction entropy becomes spontaneous below the temperature marked by the dotted line, (b) An endothermic reaction with a positive reaction entropy becomes spontaneous above the temperature marked by the dotted line, (c) An endothermic reaction with negative reaction entropy is not spontaneous at any temperature, (d) An exothermic reaction with positive reaction entropy is spontaneous at all temperatures. 

By the same reasoning, a negative AS ° and a positive AH ° oppose spontaneity, so a reaction in which the system becomes constrained and energy is absorbed is nonspontaneous regardless of temperature. The system and its surroundings both would experience decreases in entropy if such a process were to occur, and this would violate the second law of thermod3mamics. 

AS <0 for the reaction. We already know that AH < 0. Thus, the reaction falls into case 2, spontaneous at low temperatures and nonspontaneous at high temperatures. 

A positive standard cell potential tells you that the cathode is at a higher potential than the anode, and the reaction is therefore spontaneous. What do you do with a cell that has a negative " gii Electrochemical cells that rely on such nonspontaneous reactions cire called electrolytic cells. The redox reactions in electroljdic cells rely on a process called electrolysis. These reactions require that a current be passed through the solution, forcing it to split into components that then fuel the redox reaction. Such cells are created by applying a current source, such as a battery, to electrodes placed in a solution of molten salt, or salt heated until it melts. This splits the ions that make up the salt. 

Spontaneous reactions have values greater than 1. Nonspontaneous reactions have values between 0 and 1. The inverse of a value is the for the reverse reaction. 

Spontaneous reactions release energy and occur with a negative AG. Nonspontaneous reactions require an input of energy to proceed and occur with a positive AG. 

If AH° is positive and the reaction entropy is negative, then both terms are positive, AG° is positive and the process is nonspontaneous at all temperatures. The reverse is true for a reaction for which AH° is negative and the reaction entropy positive AGr° for such a process is negative and the process is spontaneous at all temperatures. 

Electrochemistry is the branch of chemistry that deals with the use of spontaneous chemical reactions to produce electricity and the use of electricity to drive nonspontaneous reactions forward. Electrochemical techniques—procedures based on electrochemistry—allow us to use electronic equipment to monitor concentrations of ions in solution. We can use them to monitor the composition and pH of solutions and to determine the pKa of acids. Electrochemistry even allows us to monitor the activity of our brain and heart (perhaps while we are trying to master chemistry), the pH of our blood, and the presence of pollutants in our water supply. 

Tell whether reactions with the following values of AH and AS are spontaneous or nonspontaneous and whether they are exothermic or endothermic ... 

A mixture of gaseous N2, H2, and NH3, each at a partial pressure of 1 atm, reacts spontaneously at 300 K to convert some of the N2 and H2 to NH3. We can predict the direction of spontaneous reaction from the relative values of the equilibrium constant K and the reaction quotient Q (Section 13.5). Since Kp = 4.4 X 105 at 300 K and Qp = 1 for partial pressures of 1 atm, the reaction will proceed in the forward direction because Qp is less than Kp. Under these conditions, the reverse reaction is nonspontaneous. At 700 K, however, Kp = 8.8 X 10 5, and the reverse reaction is spontaneous because Qp is greater than Kp. 

All reactions proceed spontaneously in the direction that increases the entropy of the system plus surroundings. A reaction that is nonspontaneous in the forward direction is spontaneous in the reverse direction because AStotai for the reverse reaction equals — AStotai for the forward reaction. If AStotai is zero, the reaction doesn t go spontaneously in either direction, and the reaction mixture is at equilibrium. 

Free energy, G = H — TS, is a state function that indicates whether a reaction is spontaneous or nonspontaneous. A reaction at constant temperature and pressure is spontaneous if AG < 0, nonspontaneous if AG > 0, and at equilibrium if AG = 0. In the equation AG = AH — TAS, temperature is a weighting factor that determines the relative importance of the enthalpy and entropy contributions to AG. 

Describe how the signs of AH and AS determine whether a reaction is spontaneous or nonspontaneous at constant temperature and pressure. 

Are these reactions spontaneous or nonspontaneous at 25°C and 1 atm pressure How does AG° change when the temperature is raised ... 

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