Chemical equations reversing

To get the proper chemical equation, reverse the zinc half-reaction ... 

This chemical equation is simply the reverse of that written originally N204 and N02 switch sides of the equation. Notice that K" is the reciprocal of 1C the numerator and denominator have been inverted. This illustrates the reciprocal rale ... 

In each step, we may need to reverse the equation or multiply it by a factor. Recall from Eq. 16 that, if wc want to reverse a chemical equation, wc have to change the sign of the reaction enthalpy. If we multiply the stoichiometric coefficients by a factor, we must multiply the reaction enthalpy by the same factor. 

A catalyst speeds up both the forward and the reverse reactions by the same amount. Therefore, the dynamic equilibrium is unaffected. The thermodynamic justification of this observation is based on the fact that the equilibrium constant depends only on the temperature and the value of AGr°. A standard Gibbs free energy of reaction depends only on the identities of the reactants and products and is independent of the rate of the reaction or the presence of any substances that do not appear in the overall chemical equation for the reaction. 

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

This is a quantitative problem, so we follow the standard strategy. The problem asks about an actual potential under nonstandard conditions. Before we determine the potential, we must visualize the electrochemical cell and determine the balanced chemical reaction. The half-reactions are given in the problem. To obtain the balanced equation, reverse the direction of the reduction half-reaction with the... 

The double arrow in the chemical equation above indicates that the reaction is reversible. This means that while some hydrochloric acid molecules are breaking down into hydrogen and chlorine ions, some ions are also combining to produce hydrochloric acid. The same ongoing, continuous process also occurs to the ammonia molecules. Some ammonia molecules accept a hydrogen ion to become an ammonium ion while some ammonium ions give up a hydrogen ion to become an ammonia molecule. 

We used the data in Table 14-2. We doubled the species in the first chemical equation, but did not change the potential. We reversed the second equation, and changed the sign of the potential. We then added both the chemical equations and the potentials to get the answer. 

A given chemical equation is tripled and then reversed. What effect, if any, will there be on the enthalpy change of the reaction ... 

Write the chemical equation for the reversible reaction that has the following equilibrium... 

Conversely, lower pressures favor formation of styrene. So the logic is that steam mixed with the EB permits cracking the hydrogen off at lower pressure and favors the styrene staying cracked. (You may have noticed the chemical equation in Figure 8—6 has arrows going both directions. Thats the chemists notation for this reversibility.)... 

In the special case, where the process supplying the chemical, C, is the mechanistic reverse of the process removing it, chemical equilibrium is said to exist. This condition is illustrated in Figure 5.7 for two reservoirs, Cj and C2, and by the following chemical equation ... 

Electrolysis of water, mentioned above, had been described by the British chemists WilUam Nicholson (1753-1815) and Sir Anthony Carlisle (1768-1842) in 1800. But Grove s experiment seemed to go in the opposite direction. This reverse eleoctrolysis is the basic operation of the fuel cell—the combination of hydrogen gas (H ) and oxygen gas (O ) to produce water and energy, as described in the following chemical equation ... 

Equilibrium state of chemical reaction where the rates of forward and reverse reactions are equal, causing concentrations of reactants and products to remain constant Equilibrium Constant a number equal to the ratio of the concentration of products at equilibrium over the concentration of reactants at equilibrium all raised to a power equal to the stoichiometric coefficient in the chemical equation... 

Any reaction in Eqs. (1) may be written as a conventional chemical equation by setting it equal to zero and transposing the negative terms to the other side of the equation. This notation has been discussed by Aris (14). Chemical equality, denoted by the symbol has been shown by Sellers (15) to be a group equivalence, thus satisfying ordinary rules of mathematical equality. Except when specific reservations are stated, every reaction is assumed to be reversible, that is, to be capable of any real rate of advancement, positive or negative. 

Now let us revert to the chemical equation (306) above. The rate for the forward and reverse processes leads to the establishment of an equilibrium where the concentration of A, B and C are n Q, nBq and. Prior to that time, the concentrations are nA, nB and nc. The usual rate equation is... 

To indicate that the reaction can proceed in both forward and reverse directions, we write the balanced equation with two arrows, one pointing from reactants to products and the other pointing from products to reactants. (The terms "reactants" and "products" could be confusing in this context because the products of the forward reaction are reactants in the reverse reaction. To avoid confusion, we ll restrict the term reactants to the substances on the left side of the chemical equation and the term products to the substances on the right side of the equation.)... 

If we write the chemical equation in the reverse direction, the new equilibrium constant expression is the reciprocal of the original expression, and the new equilibrium constant Kc is the reciprocal of the original equilibrium constant Kc ... 

The sequence of conversions in Figure 18.20 is used to calculate the mass or volume of product produced by passing a known current through a cell for a fixed period of time. The key is to think of the electrons as a "reactant" in a balanced chemical equation and then to proceed as with any other stoichiometry problem. Worked Example 18.10 illustrates the calculations. Alternatively, we can calculate the current (or time) required to produce a given amount of product by working through the sequence in Figure 18.20 in the reverse direction, as shown in Worked Example 18.11. 

The fuel cell was discovered during electrolysis experiments with water. It is the reverse process which produces the electricity. Write a balanced chemical equation to represent the overall reaction taking place in a fuel cell. 

The Japanese physico-chemist Horiuti in his paper "How can a kinetic equation be found for a reverse reaction gives an extremely vivid description of a reaction between the complex and the simple in chemical kinetics. He claims "We can write an arbitrary number of chemical equations for the same reaction, and each will be accounted for by some equilibrium constant. It can be pleasant to write a chemical equation so that it has the least integer coefficients, but we have no grounds (within the confines of our discussion) to give preference to this chemical equation and its corresponding equilibrium constant. 

We reverse the second equation so that the CO is on the right (crossing out the original equation), but that also cancels the 2CO2 and one of the oxygen molecules (cross them out). Also, when we reverse the chemical equation, we reverse the sign associated with the AH of the second equation of +566.0 kJ. 

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 ... 

Equilibrium State reached by a reversible reaction when forward and reverse reaction rates are equal represented in chemical equations by =s instead of — . 

The atomic processes that are occurring (under conditions of equilibrium or non equilibrium) may be described by statistical mechanics. Since we are assuming gaseous- or liquid-phase reactions, collision theory applies. In other words, the molecules must collide for a reaction to occur. Hence, the rate of a reaction is proportional to the number of collisions per second. This number, in turn, is proportional to the concentrations of the species combining. Normally, chemical equations, like the one given above, are stoichiometric statements. The coefficients in the equation give the number of moles of reactants and products. However, if (and only if) the chemical equation is also valid in terms of what the molecules are doing, the reaction is said to be an elementary reaction. In this case we can write the rate laws for the forward and reverse reactions as Vf = kf[A]"[B]6 and vr = kr[C]c, respectively, where kj and kr are rate constants and the exponents are equal to the coefficients in the balanced chemical equation. The net reaction rate, r, for an elementary reaction represented by Eq. 2.32 is thus... 

At the same time that direct reactions are taking place, there will be reverse reactions, dissociations of water molecules to produce hydrogens and oxygons. From the chemical equation (1.1) we see that two water molecules must be present in order to furnish the necessary atoms to break up into hydrogen and oxygen molecules. Thus, by the type of argument we have just used, the rate of the reverse reaction must be proportional to the square of the number of water molecules per unit volume or to the square of the partial pressure of water we may write it as... 

Most acids are weak acids. Whenever you see a reversible chemical equation involving an acid, you can safely assume that the acid is weak. 

NaCl(s). What does it mean when there is a double arrow in a chemical equation Here is an equation with a double arrow A + B <—> C + D. Reactions with a double arrow mean that the reaction is a reversible reaction. As substances A and B react to form the products C and D (the forward reaction), the products C and D can form the original reactants (the reverse reaction). It sounds counterproductive, but it happens. Later you will see how conditions can be manipulated to make a reaction favor the products, but for now the focus is only on what a reversible reaction means for the enthalpy changes during the reaction. 

For a reversible reaction, aA + bB cC + dD the product of the concentrations of the products (species on the right), each raised to the power that corresponds to its coefficient in the balanced chemical equation, divided by the product of the concentrations of reactants (species on the left), each raised to the power that corresponds to its coefficient in the balanced chemical equation. At equilibrium the mass action expression is equal to K at other times it is Q.[C]c[D]d [A]a[B]b = Q, or at equilibrium K Mass Deficiency... 

---