Reversing a reaction

We will use two useful relationships when working with equilibrium constants. First, if we reverse a reaction s direction, the equilibrium constant for the new reaction is simply the inverse of that for the original reaction. For example, the equilibrium constant for the reaction... 

Calculate the effect on K of reversing a reaction or multiplying the chemical equation by a factor (Section 9.7). 

Reversing a Reaction Since equilibrating reactions are, by definition, reversible, what happens to K when the chemical equation is reversed Let s consider a model reaction ... 

Solids and Liquids 221 Reversing a Reaction 221 Acids and Bases 222... 

Recently, the impact of the metal alkyls TIBA, DIBAH and ZnEt2 on molar mass was comparatively studied. In these studies ternary NdV-based catalyst systems were used [178-182]. The first two of these studies focus on molar mass control by DIBAH and by TIBA. Linear dependencies of Mn on monomer conversion were obtained. In addition, PDIs decreased with increasing monomer conversion. On the basis of these observations it was concluded that chain transfer of living poly(butadien)yl chains between Nd and Al is fully reversible. A reaction mechanism which accounts for these features is outlined in Scheme 32. 

Problem 5.8 According to the principle of microscopic reversibility, a reaction and its reverse follow exactly the same path but in opposite directions. On this basis wfite a detailed mechanism for the hydration of alkenes, a reaction that is the exact reverse of the dehydration of alcohols. (Check your answer in Sec. 6.10.)... 

In the desired equation, H2S is a reactant rather than a product. Therefore, Equation c must be reversed. Recall that reversing a reaction changes the sign of AH, so the sign of AH for Equation c becomes positive. 

Like H, G, and S, E° is a thermodynamic quantity. Therefore, its sign changes but its magnitude remains the same when we reverse a reaction. 

FIGURE 5.16 AH for a reverse reaction. Reversing a reaction changes the sign but not the magnitude of the enthalpy change AH2 = -AHi. 

When we reverse a reaction, we reverse the roles of the products and the reactants. From Equation 5.16, we see that reversing the products and reactants leads to the same magnitude of AH but a change in sign ( FIGURE 5.16). 

In a chemical process, the enthalpy of reaction is the enthalpy of the products minus the enthalpy of the reactants AH, = H (products) — H (reactants). Enthalpies of reaction follow some simple rules (1) The enthalpy of reaction is proportional to the amount of reactant that reacts. (2) Reversing a reaction changes the sign of AH. (3) The enthalpy of reaction depends on the physical states of the reactants and products. 

Typically, we speak of the potential for a half-reaction as that half-reaction written as a reduction reaction. When combining two (or more) reactions, at least one must be reversed to express it as an oxidation reaction. When reversing a reaction, the standard potential changes sign. 

Since reversing a reaction means changing the sign of AH but keeping the magnitude the same, AH for reaction (a) is —AHf, or 75 kJ. Thus = 75 kJ. 

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