CHM 142

Assuming the collision factors are comparable, which reaction will have the smallest rate constant?

- The rate constant depends on the magnitude of Ea → generally, the lower the value of Ea is, the larger the rate constant and the faster the reaction. - The higher the value of Ea is, the smaller the rate constant and the slower the reaction. The Ea for reaction 2 in the reverse direction is 40 kJ/mol, making it the largest Ea of the given reactions. This means it also has the smallest rate constant.

Choose the rate law for each reactant 1. Cl2(g)→2Cl(g) 2. OCl (aq)+H2O(l)→ HOCl(aq)+OH−(aq) 3. NO(g)+Cl2(g)→NOCl2(g)

1. rate=k[Cl2] 2. rate=k[OCl−][H2O] 3. rate=k[NO][Cl2]

How many elementary reactions are in the reaction mechanism? How many intermediates are formed in the reaction?

2 ( Elementary reactions occur in a single step and correlate directly to the number of energy maxima. Since the reaction profile contains two maxima, there are two elementary reactions in the mechanism.) 1 (The step that has the larger activation energy will be the rate-limiting step. A reaction at a given temperature contains molecules with varying kinetic energies. The chance that these particles will have enough energy to react decreases as the energy needed increases.)

difference between the bimolecular and unimolecular

A unimolecular elementary reaction involves only one reactant molecule ; the activated complex is derived from a single molecule A bimolecular elementary reaction involves two reactant molecules; the activated complex is derived from the overall process.

What variables does the rate constant depend on?

Depends on - reaction temp - magnitude of Ea Does not depend on - concentrations of the reactant - concentrations of the product

What is the activation energy for the reverse reaction?

Ea reverse action = Ea - ΔE

What factors determine whether a collision between two molecules will lead to a chemical reaction?

Energy of the collision Orientation of the molecules

Write the chemical equation for each step in the reaction.

First Step: NO2+F2→NO2F+F Second Step: NO2+F→NO2F Overall Reaction: 2NO2+F2→2NO2F intermediate in the mechanism: F Rate Law: k[NO2][F2]

Which factor in the Arrhenius equation represents the frequency of collisions and the probability that the collisions are favorably oriented: k = Ae-Ea/RT

Frequency factor A

Does the rate constant for a reaction generally increase or decrease with an increase in reaction temperature? Which factor is most sensitive to changes in temperature?

Generally, the rate constant increases with increasing temperature, and thus the rate increases with increasing temperature. The factor most sensitive to increasing temperatures is the fraction of molecules with energy greater than the activation energy.

Indicate whether each statement is true or false. (activation energy)

The activation energy, Ea not enthalpy, can be calculated from the measurement of the rate constant of a reaction at varying temperatures. The overall change in energy, ΔE, is independent of the reaction rate. Changing the temperature of a reaction will not affect the activation energy, only the kinetic energy of the molecules involved. All False - If you measure the rate constant for a reaction at different temperatures, you can calculate the overall enthalpy change for the reaction - Exothermic reactions are faster than endothermic reactions - If you double the temperature for a reaction, you cut the activation energy in half.

The activation energy of a first-order reaction is 83.5 kJ/mol. The rate constant is 3.54 x 10−5 s−1 at 45 °C. What is the rate constant at 65°C?

To determine the rate constant, k2, at T2 = 65°C, we can use Equation in picture Ea = 83.5 kJ/mol, k1 = 3.54 x 10−5 s−1 T1 = 45 °C. This gives 2.29 x 10−4 s−1 for k2. Note that the units of k1 are the same as those of k2 and that the rate constant at 65°C is larger than it is at 45°C, as it should be.

Statement true or false

True -Increasing the reaction temperature increases the fraction of successful collisions between reactants. False -A reaction that has a small rate constant must have a small frequency factor. - If you compare two reactions with similar collision factors, the one with the larger activation energy will be faster.

What is the molecularity of each of the following elementary reactions?

Unimolecular Cl2(g)→2Cl(g) Bimolecular OCl (aq)+H2O(l)→ HOCl(aq)+OH−(aq) NO(g)+Cl2(g)→NOCl2(g)

If the barrier were lower than as shown in the figure, would the golfer have to hit the ball as hard? (golf related)

less kinetic energy would be required if the barrier were lower.

Activation energy Forward Reaction Example

ΔH= Ea- Eb