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Chapter 16: Q16.55 P (page 731)

a) For a reaction with a given Ea, how does an increase in T affect the rate?

(b) For a reaction at a given T, how does a decrease in Ea affect the rate?

Short Answer

Expert verified
  1. As the temperature rises, the fraction of molecules with energy greater than the threshold energy increases. As a result, more molecules have the activation energy to collide. As a result, the number of collisions rises, as does the rate of reaction.
  2. When the activation energy drops, more molecules have the activation energy required for collisions. As a result, the number of collisions rises, as does the rate of reaction.

Step by step solution

01

Step 1: How does an increase in T affect the rate? 

At any given temperature, the kinetic energy of the reactant molecules is distributed. At any given temperature, collisions between molecules have a range of energies.

The percentage of molecules with energy greater than the threshold energy grows as the temperature rises. As a result, more molecules have the activation energy needed to collide. This increases the number of collisions and, as a result, the rate of reaction.

02

Step 2: How does a decrease affect the rate?

The kinetic energy of the reactant molecules is dispersed at every given temperature. Collisions between molecules have a variety of energies at any given temperature.

More molecules have the activation energy necessary for collisions when the activation energy decreases. This increases the number of collisions and, as a result, the rate of reaction.

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Most popular questions from this chapter

The rate constant of a reaction is 4.50×10-5L/mol.sat 1950cand 3.20×10-3L/mol.s at 2580C . What is the activation energy of the reaction?

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.

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?

To determine its rate law. Assuming that you have a valid experimental procedure for obtaining [A2] and [B2] at various times, explain how you determine

A2(g)+B2(g)→2AB(g)

(a) the initial rate,

(b) the reaction orders, and

(c) the rate constant.

Question:Many drugs decompose in blood by a first-order process.

(a) Two tablets of aspirin supply 0.60 g of the active compound. After 30 min, this compound reaches a maximum concentration of 2 mg/100 mL of blood. If the half-life for its breakdown is 90 min, what is its concentration (in mg/100 mL) 2.5 h after it reaches its maximum concentration?

(b) For the decomposition of an antibiotic in a person with a normal temperature (98.6°F),k=3.1×10-5s-1; for a person with a fever at 101.9°F, k=3.9×10-5s-1. If the person with the fever must take another pill when of the first pill has decomposed, how many hours should she wait to take a second pill? A third pill? (Assume the pill is effective immediately.)

(c) Calculate Ea for decomposition of the antibiotic in part (b).
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