Equations Equilibrium reactions, arrows

In addition to defined standard conditions and a reference potential, tabulated half-reactions have a defined reference direction. As the double arrow in the previous equation indicates, E ° values for half-reactions refer to electrode equilibria. Just as the value of an equilibrium constant depends on the direction in which the equilibrium reaction is written, the values of S ° depend on whether electrons are reactants or products. For half-reactions, the conventional reference direction is reduction, with electrons always appearing as reactants. Thus, each tabulated E ° value for a half-reaction is a standard reduction potential. 

The reactants appear on which side of the reaction arrow in an equilibrium equation ... 

When you see two arrows that point in opposite directions in a chemical equation, the reaction can proceed in both directions. This type of reaction is called an equilibrium reaction. You will learn more about equilibrium reactions in Unit 4. 

The same idea applies to a chemical reaction at equilibrium. While reactants are becoming products, products are returning to reactants. The reaction in which reactants become products is called the forward reaction, and the reaction in which products become reactants is called the reverse reaction. At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction. When a reaction is an equilibrium reaction, there are two arrows between the reactant side of the equation and the product side of the equation. One arrow points to the products, and one arrow points to the reactants. All chemical equilibrium reactions are written with a two-way arrow. For example ... 

If a reaction is reversible, both products and reactants will be present during that reaction. If the reaction of A -l- B to give C -i- D is reversible, all four components—A, B, C, and D—will be present. In principle, the reaction will reach an equilibrium condition and at that point the amounts of A-D will be relatively constant. If the reaction comes to equilibrium (designated by the stacked forward and reverse reaction arrows), the position of the equilibrium is defined by the equilibrium constant, K (K q). The value of K is determined by the concentrations of all species, K = products/reactants, where products are on the right side of the reaction and reactants are on the left side of the equation. In this particular reaction, K = [C][D]/[A][B]. 

Reactions are in equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction. Equilibrium is emphasized in equations for reactions by writing double arrows pointing in both directions between reactants and products. 

As indicated by the double arrow in the equation, the reaction is an equilibrium. The equilibrium expression for the autoionization of water is... 

In each of these equations, the double arrows indicate that the reactions proceed in both directions and that equilibrium is maintained among the various carbon-containing species. The important result is that even clean rainwater is slightly acidic due to the formation of hydronium ions, H3O+. Rainwater tends, therefore, to have a pH of about 5.5. 

The reactions introduced in Equations (4.6)-(4.10) are equilibrium reactions, as indicated by the double arrow. This means that reactants and products approach a stable mixture that is defined by the thermodynamic equilibrium. This occurs theoretically at infinite residence time. Because of high temperatures, equilibrium may be also approached in technical gasification systems at finite residence time. However, some deviations remain. 

As far as equations like Equation (8.2) are concerned, we tend to think of a chemical reaction occurring in a forward direction, so the product in Equation (8.2) is the chemical at the head of the arrow in Equation (8.1). Consequently, the concentration of product will always increase with time until the reaction reaches its position of equilibrium (when the rate will equal zero). This explains why the rate of reaction always has a positive value. The rate is generally cited with the units of mol dm-3s-1, i.e. concentration change per second. 

This reaction quotient is a fraction. The numerator is the product of the chemical species on the right hand side of the equilibrium arrow, each one raised to the power of that species coefficient in the balanced chemical equation. The denominator is the product of the chemical species on the left hand side of the equilibrium arrow, each one raised to the power of that species coefficient in the balanced chemical equation. It is called Qc, in this case, since molar concentrations are being used. If this was a gas phase reaction, gas pressures could be used and it would become a Qp. 

The double arrows indicate that the reaction proceeds either way. This condition of reciprocal reaction is called chemical equilibrium, and its importance to chemistry cannot be overemphasiTed. An equilibrium state is a stable, balanced condition, and it can be reproduced by many laboratory researchers. It also can be modeled well by simple mathematical equations. 

Enzymes are no exception to the rule that catalysts do not affect reaction equilibria. The bidirectional arrows in Equation 6-1 make this point any enzyme that catalyzes the reaction S —> P also catalyzes the reaction P —> S. The role of enzymes is to accelerate the interconversion of S and P. The enzyme is not used up in the process, and the equilibrium point is unaffected. However, the reaction reaches equilibrium much faster when the appropriate enzyme is present, because the rate of the reaction is increased. 

Write the equations for each of the reactions shown below. Using the E° values below, calculate approximate Gibbs energies for each reaction, and show by the relative length of the arrows on which side of the reaction the equilibrium lies. 

The above equations illustrate via the double arrows an important facet of polyamides—the equilibrium nature of the polymerization reactions. Achieving and maintaining useful molecular weights (about 10,000 or more) for plastics applications require low moisture contents in order... 

For efficiency, we can write both the forward and reverse reactions of an equilibrium system in one equation, using a double arrow to denote that the reaction goes in both directions. For example ... 

Two exactly opposite processes occurring in the same place at the same time at the same rate constitute a state called equilibrium. Although no reaction appears to be occurring in a mixture at equilibrium because the effects of the opposite processes cancel each other, each process continues. We say that equilibrium is a dynamic state. Both reactions can be represented in one chemical equation, using a double arrow to indicate an eqnilibrium (Section 18.2). 

Even a very few sodium ions in the water might react with the calcium zeolite to cause this reaction to take place. The steady state occurs when the concentrations of calcium ion and sodium ion in the water and bound into the zeolite are such that the rate at which calcium ion is replacing sodium ion is just equal to the rate at which sodium ion is replacing calcium ion this equilibrium of the two rates can be expressed by a single equation, with a double arrow ... 

To show that a reaction can go in either direction the two equations can be put together and the single arrows are replaced by the equilibrium symbol, a double arrow as in the esterification reaction ... 

Table 5.3. 1 Difference fractionation factors, e= product- Sre ctant, for important equilibrium (equations with two-way arrows) and kinetic (one-way arrows) reactions among the elements H, C, 0, and N Equilibrium fractionation factors are for 20 °C. Kinetic fractionation factors are approximate as they vary in the marine environment. ...

Many of the equations in this book are written with the reversible reaction sign (two-way half-arrows e.g. eqn. 2.5). This shows that the reaction can proceed in either direction and this is fundamental to equilibrium-based chemistry (see Box 3.2). Reactions depicting dissolution of substances in water may or may not show the water molecule involved, but dissolution is implied by the (aq) status symbol. Equation 2.7, read from left to right, shows dissolution of rock salt (halite). 

The double arrow ( ) indicates that the reaction is reversible—that is, both the forward and reverse reactions occur simultaneously. In discussions of chemical equilibrium, the substances that appear on the left side of the balanced chemical equation are called the reactants, and those on the right side are called the products. In fact, the reaction can proceed in either direction. When A and B react to form C and D at the same rate at which C and D react to form A and B, the system is at equilibrium. 

In Examples 18-8 and 18-9 the value of an equilibrium concentration was used to determine the change in concentration. You should become proficient at using a variety of data to determine values that are related via a chemical equation. Let s review what we did in Example 18-9. Only the equilibrium expression, initial concentrations, and the equilibrium concentration of H3O+ were known when we started the reaction summary. The following steps show how we filled in the remaining values in the order indicated by the numbered red arrows. 

Note General reaction types or conditions that correspond to the differential rate equations are given parenthetically. Some reactions are irreversible (denoted by — ) and others reversible (denoted by double arrows). Note that the rate constant, k is always positive. In the integrated rate expressions the concentration of A = Ao, at r = 0, and A = AJ2 at half-time (ti/i). A denotes the equilibrium, mineral saturation or steady state concentration of species A. 

Notice that the single arrow in the equation has been replaced with double arrows. Because an arrow shows what direction the reaction is going, the double arrow indicates that the reaction can go in either direction. In this case, CaC03 decomposes into CaO and CO2. But, as those products form, CO2 and CaO combine to form CaC03. The rate, or speed, of each reaction can be determined by how quickly a reactant disappears. Eventually, the rates of the two reactions are equal, and an equilibrium exists. 

With this description, it becomes apparent why exactly 4 Zn atoms per elementary step have to react it is necessary to have a balance of lost and gained electrons of the oxidation and of the reduction step. In this case, in the first step of the reaction, 8 electrons are necessary, 4 Zn atoms have to lose them. Apart from this, one should check, if the charges left and right of the arrow are equal in the oxidation and in the reduction step, and may equate the complete equation. Since the hydrogen sulfide gas escapes from the equilibrium, a shift in favor of the products occurs. 

Enzymes do not change the equilibrium of a chemical reactionist the rate. Many reactions in the body can go in either direction, as indicated by the use of a double-headed arrow ( ) in equations. In some cases, the concentrations of the reactants in the body are such that the reaction always proceeds only in one direction. Such reactions are said to be physiologically irreversible and are written with a single-headed arrow ( ). This is not a property of the enzyme, but depends on the reaction and the reactant concentrations. 

Transition-state representations are shown for two acid-base reactions. Por each one, write the equation for the reaction it represents in the direction for which the equilibrium lies to the right. Label the acid, the base, the conjugate acid, and the conjugate base, and use curved arrows to show the flow of electrons. 

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