Chemistry
Reversible Reaction
A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions. This means that the products of the reaction can react with each other to reform the original reactants. In a reversible reaction, the system reaches a state of dynamic equilibrium where the rates of the forward and reverse reactions are equal.
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7 Key excerpts on "Reversible Reaction"
eBook - PDF- Ageetha Vanamudan(Author)
- 2023(Publication Date)
- Delve Publishing(Publisher)
This guarantees that the reaction can be carried out in the opposite direction from what was originally intended. Since there is neither an increase nor a decrease in the total amount of chemicals that are involved when two chemical processes operate at the same rate or velocity, there is no change in the total amount of chemicals that are involved (Chaplin, 2006). At this stage, the reaction is complete since all of the reactants have been transformed into products to the greatest extent that is feasible under the conditions that are now present in the reaction. This means that the process has reached its conclusion. Figure 12.1: Chemical Equilibrium. Source: By Krishnavedala - Own work, CC0, https://commons.wikimedia. org/w/index.php?curid=34342616 Some chemical reactions don’t ever seem to reach their full potential, even if they can. The great majority of chemical reactions are reversible, which means that the sequence in which the reactants and products are produced may be reversed whenever the experimenter so chooses. An equation representing a Reversible Reaction will have two arrows, one of which will go forward while the other will point backwards. This is because a Chemical Equilibrium 201 Reversible Reaction contains both forward and reverse processes. At the most fundamental level, the two processes that are opposed to one another are competing against one another to achieve their respective goals. Chemical equilibrium is a type of dynamic equilibrium that allows for the potential of chemical reactions to take place inside its limits. This makes a chemical equilibrium different from other types of equilibrium. There are many distinct chemical processes, and each one may be manipulated to go in either the forward or the backward direction, depending on the context in which it is being carried out.
- James N. Jensen(Author)
- 2022(Publication Date)
- Wiley(Publisher)
A reaction is said to be reversible if the reverse reaction (i.e., conversion of products into reac- tants) can occur spontaneously with an infinitesimal increase in the product concen- tration. Consider the reaction: H + + Cl – → HCl. The reaction is reversible if the reverse reaction (HCl → H + + Cl – ) also occurs spontaneously when the HCl concentration is increased very slightly. In general, a process is said to be reversible if it returns to its initial state when the mass, heat, and energy flows are reversed. What is the relationship between equilibrium and reversibility? Reversible systems are said to be near equilibrium, whereas irreversible systems can be far from the equi- librium state. For a chemical reaction to be at equilibrium, the reaction must be reversible. 3.3.4 Summary The connection between spontaneity, equilibrium, and reversibility casts a new light on equilibrium. For a reaction to be at equilibrium, the reaction must be both sponta- neous and reversible. In other words, both the reaction and its reverse reaction must be spontaneous. For example, it is known that hydrochloric acid equilibrates quickly with H + and Cl – . Thus, both the reactions HCl → H + + Cl – and H + + Cl – → HCl must be spontaneous. This leads to the expression of equilibrium as reactions that proceed in both directions, denoted HCl ⇄ H + + Cl – , or more commonly in aquatic chemistry: HCl = H + + Cl – . At equilibrium, the concentrations of all chemical species do not change with time. This fact sometimes conjures up the image that no reactions are occurring at equilib- rium. However, you now know that all chemical reactions and their reverse reactions proceed spontaneously at equilibrium. Overall, the species concentrations are not changing with time. However, reactants are converted to products and products to reactants continuously at equilibrium in such a way that the reactant and product con- centrations do not change over time.
eBook - PDF- Martinus A.J.S. van Boekel(Author)
- 2008(Publication Date)
- CRC Press(Publisher)
Chemical Thermodynamics in a Nutshell 3 -7 process. A reversible process in thermodynamic terms is brought about by in fi nitesimal changes of a variable and it means that going along the same path in reverse restores the system as well as the surroundings to its original state. With irreversible processes, permanent changes have occurred in the system and = or surroundings. This terminology may cause some confusion because a Reversible Reaction in chemical terms refers to a reaction in which products are formed from reactants but reactants can also be formed from products; an irReversible Reaction means essentially that reactants are completely converted into products. The difference is thus in the words ‘‘ process ’’ and ‘‘ reaction. ’’ A reversible process does not really occur in nature, it is rather an idealized process, a thought experiment, which is nevertheless useful in practice because it helps in determining the limits of real processes. Real, natural processes are irreversible, though some real processes may approximate reversible processes, such as the melting of ice around 0 8 C. An important point is, however, that some thermodynamic functions can only be evaluated when considering reversible processes. We describe fi rst classical thermodynamics for reversible processes; at the end of this chapter we will spend some words on irreversible thermodynamics because of its practical importance. Another important consideration is that of ideality; ideal gases and solutions are characterized by the absence of interactions between molecules. They represent limiting cases of real behavior. For instance, ideal solutions do not show a heat or volume effect upon mixing, whereas real solutions do. This is particularly of importance when trying to describe properties of foods in thermodynamic terms. Foods behave by no means as ideal solutions and gases; they are usually inhomogeneous, concentrated, and show many interactions between components.
eBook - PDF- Leo J. Malone, Theodore O. Dolter(Authors)
- 2012(Publication Date)
- Wiley(Publisher)
To illustrate chemical equilibrium, let’s return to the hypothetical reaction discussed in Section 15-2 and illustrated in Figure 15-1. We will assume that this reaction is reversible. A C C OBJECTIVE FOR SECTION 15-3 Using Le Châtelier’s principle, predict what effect changing certain conditions will have on the equilibrium point. 526 CHAPTER 15 Reaction Rates and Equilibrium Time X (start): Forward reaction begins, reverse reaction zero Time Y: Forward reaction decreasing, reverse reaction increasing Time Z and later (equilibrium): Forward and reverse reactions occurring at equal rates X Z Y Time Increasing rate of reaction B Key A 2 B 2 AB Forward reaction A 2 + B 2 2AB A A A A A A A A B B B B B B B B A A A A A A A A A A B B B B B B B B B B B B B B B Equilibrium Reverse reaction 2AB A 2 + B 2 A A A A A A Reversible Reaction is one where both a forward reaction (form- ing products) and a reverse reaction (re-forming reactants) can occur. Reversible Reactions where both reactions occur simultaneously reach a point of equilibrium. The point of equilibrium in a reversible process is when both the forward and reverse processes proceed at the same rate, so the concentrations of reactants and products remain constant. In our hypothetical reaction, the reaction mixture initially contains only A 2 and B 2 . As the reaction proceeds, the concentrations of these two molecules begin to decrease as the concentration of the product, AB, increases. The rate of buildup of AB begins to decrease until, eventually, the concentration of AB no longer changes despite the presence of excess reactant molecules. To understand this, we turn our attention to the product molecule AB. If we had started with pure AB, we would find that AB slowly decomposes to form A 2 and B 2 , just the reverse of the original reac- tion. This reaction occurs when two AB molecules collide with the proper orientation and sufficient energy to form A 2 and B 2 .
- (Author)
- 2002(Publication Date)
- Academic Press(Publisher)
.......... . . . . . ; ......... Z;IIIII ; ..................................... ; ........................... ; ~. ; ~ ...... ..Z;7..2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . @a,~d 11 3.3 order 0.8 0.6 0.4 0.2 2 4 6 8 10 T 7.4 Reversible Reactions---Chemical Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . ,,,, ,,,,,,=,,,, ,, ,,,,,,,,,, , , ,,,,,, ,, ,,,,,,.,.,,,.,,,,, ,,i , i Ill i I I Ill l llll i lllllllllllllllllllll I I I lllllllk III III I III Chemical reactions do not move in the forward direction only but in either direction and come to a resting point of concentrations known as the position of chemical equilibrium. Our goal in this section is to understand how we analyze such a common situation and at the same time to discover the interrelationships between kinetics and thermodynamics as they apply to chemical systems. Take as a starting point the simplest most, Reversible Reaction: A=B This is simple because the stoichiometry is one mole of reactant goes to one mole of product, and because the conversion of A to B follows first-order kinetics, as does the conversion of B back to A. Thus, when we assemble the two-component mass balance equations in a constant volume batch reactor, we find: d Ca dt d Cb dt = --ra q- rb = ra -- rb 318 Chapter 7 Reacting Systems---Kinetics and Batch Reactors ....... III ............. III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I III I I I .......... I ........ II III I 11 . . . . . . . . . . . J . I ........... These expressions reflect the fact that the overall rate of accumulation of either species will be the difference between their rates of formation and depletion, that is, the net rate.
eBook - PDFFundamentals of Chemical Reactor Engineering
A Multi-Scale Approach
- Timur Dogu, Gulsen Dogu(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
2 Reversible Reactions and Chemical Equilibrium Most of the chemical reactions are reversible. When the forward rate of a reaction is equal to the backward rate, the net rate of the reaction becomes zero, and the reaction is considered to be at equilibrium. In general, a closed system is said to be at equilibrium when it does not tend to change its properties with time. The fractional conversion of reactants to products reached at equilibrium is the maximum possible conversion that can be achieved at the given reaction temperature and pres-sure. As discussed later in this chapter, equilibrium conversion cannot be exceeded at the operating conditions of the reactor. Achievement of conversion values over the equilibrium value by the use of multifunctional reactors is discussed in Chapter 13. 2.1 Equilibrium and Reaction Rate Relations Equilibrium calculations are critical in reactor design to determine the maximum possible extent of chemical conversion. For some reactions, the rate of reaction may be relatively high. However, if the equilibrium conversion is low, that reaction is controlled by the equilibrium. The equilibrium conversion may be very high in some other reactions, and the reaction rate controls the conversion. As illustrated in Figure 2.1, equilibrium conversion may be very high for some reactions and com-plete conversion of the limiting reactant may be approached. These reactions can be considered irreversible. The reaction rate controls the fractional conversion achieved in these reactions. How-ever, for some other reactions, equilibrium conversion is relatively low, and the increase of reaction rate does not help to increase the final conversion level that can be reached.
- Elias I. Franses(Author)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
16 Chemical reaction equilibria. One reaction 16.1 INTRODUCTION ................................................................................................. A chemical reaction is a process in which at least one chemical bond between two atoms is formed or broken. Often several bonds are broken and several new bonds are formed. In one example, for the reaction N 2 þ 3H 2 ¼ 2NH 3 ð 16 : 1 Þ four bonds are broken, namely N — H and H — H (three times), and six N — H bonds are formed (three for each of the two NH 3 molecules), when the reaction occurs, or proceeds, from left to right. When the reaction occurs from right to left, six N — H bonds must be broken, and four bonds must be formed. A chemical bond, or “ valence bond, ” is the positioning of two atoms close together, such that some electrons are shared by the two atoms. The two atoms usually move as a group, even though they may vibrate around an average distance. The formation of chemical bonds leads to a reduction of energy, Δ E (in J/mol), which is quite high in absolute value. Typically, Δ E can be 10 to 100 kcal/mol (or from 40 to 400 kJ/mol). The overall energy change, or enthalpy change, of a reaction (for all bond changes involved) can be negative (i), positive (ii), or zero (iii). Then the reaction is called exothermic in case (i), endothermic in case (ii), or “ athermal ” in case (iii). When the reaction is exothermic, and the energy of the product molecules is lower than the energy of the reactant molecules, the energy difference is released to the reacting mixture itself or to the surroundings. In the fi rst case, the reacting mixture is heated. In the second case, and if the reacting mixture remains at the same temperature, the surroundings are heated. That is why it is called exothermic. The word “ exo ” means the exterior, or the outside. The opposite happens when the reaction is “ endothermic ” ; “ endo ” means interior or inside.
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