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Chapter 16: Q16.66P (page 731)

Explain why the coefficients of an elementary step equal the reaction orders of its rate law but those of an overall reaction do not.

Short Answer

Expert verified

When two molecules collide, an elementary step is created in a single step. The rate of an elementary step is proportional to the product of the concentrations of the reactants with the same exponents (order) as the stoichiometric coefficients in the equation for this step. The complete reaction may have a number of such stages with varying speeds. On the other hand, the speed is computed using the slowest step. As a result, the reaction orders of its rate law do not necessarily match the stoichiometric coefficients of the overall equation.

Step by step solution

01

What is the reaction of an elementary?

A chemical reaction in which one or more chemical species react directly to create products in a single reaction step and with a single transition state is known as an elementary reaction.

02

Why do the coefficients of an elementary step match the reaction orders of its rate law

The collision of two molecules creates an elementary step in a single step. The product of the concentrations of the reactants with the same exponents (order) as the stoichiometric coefficients in the equation for this step is proportional to the rate of an elementary step. A number of such phases with varying speeds may be included in the total reaction. However, the pace is calculated using the slowest step. As a result, the reaction orders of its rate law do not always match the overall equation's stoichiometric coefficients.

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Most popular questions from this chapter

Like any catalyst, palladium, platinum, and nickel catalyze both directions of a reaction: the addition of hydrogen to (hydrogenation) and its elimination from (dehydrogenation) carbon double bonds.

(a) Which variable determines whether an alkene will be hydrogenated or dehydrogenated?

(b) Which reaction requires a higher temperature?

(c) How can all-trans fats arise during the hydrogenation of fats that contain some cis-double bonds?

By what factor does the rate change in each of the following cases (assuming constant temperature)?

(a) A reaction is first order in reactant A, and [A] is doubled.

(b) A reaction is second order in reactant B, and [B] is halved.

(c) A reaction is second order in reactant C, and [C] is tripled.

Question: Reaction rate is expressed in terms of changes in concentration of reactants and products. Write a balanced equation for

Rate=-∆[CH4]∆t=12∆[O2]∆t=12∆[H2O]∆t=-∆[CO2]∆t

Even when a mechanism is consistent with the rate law, later work may show it to be incorrect. For example, the reaction between hydrogen and iodine has this rate law: rate=k[H2][I2] . The long-accepted mechanism had a single bimolecular step; that is, the overall reaction was thought to be elementary:

H2(g)+I2(g)→2HI(g)

In the 1960s, however, spectroscopic evidence showed the presence of free I atoms during the reaction. Kineticists have since proposed a three-step mechanism:

(1)I2(g)⇌2I(g)[fast](2)H2(g)+I(g)⇌H2I(g)[fast](3)H2I(g)+I(g)⇌2HI(g)[slow]

Show that this mechanism is consistent with the rate law.

Reaction rate is expressed in terms of changes in the concentration of reactants and products. Write a balanced equation for

Rate=−12Δ[N2O5]Δt=14Δ[NO2]Δt=Δ[O2]Δt
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