14.3.1 The Collision Model

Question 1

The Collision Model

Answer 1

• According to the collision model, the reaction rate is determined by the collision rate and the fraction of collisions that successfully produce products.
• The fraction of collisions that successfully produce products is determined by the activation energy and the orientation of the colliding reactants.
• The collision rate depends on the concentrations of reactants and their molecular speeds.

Question 2

note

Answer 2

• According to the collision model, the reaction rate is
  determined by the collision rate and the fraction of collisions that successfully produce products.
• An analogy for this is a junior high school dance. The rate of production of dancing couples is determined by the frequency of dance invitations times the fraction of invitations that are accepted. If either of these is zero, no dancing couples are formed.
• One factor determining the fraction of collisions that
  successfully produce products is the energy required for the reaction to occur. This factor is the activation energy (E a ). The activation energy is the difference in energy between the reactants (A and BC) and an activated complex (A–B–C).
• Slower moving reactants have lower kinetic energy. They therefore have a smaller probability of having enough energy to form the activated complex. Kinetic energy is temperature dependent.
• These concepts lead to a mathematical representation of the fraction of collisions with enough energy to react (f).
• Another factor determining the fraction of collisions that successfully produce products is the orientation of the
  colliding reactants.
• If the reactants are oriented correctly when a collision occurs, that collision can lead to products.
• However, if the reactants are not oriented correctly when a collision occurs, that collision cannot lead to products.
• The steric factor (P) quantifies the fraction of collisions that occur in the correct orientation for a reaction to take place. P can range from zero (no collisions correctly aligned) to one (all collisions correctly aligned)

Question 3

note 2

Answer 3

• The collision rate depends on the concentrations of reactants and their molecular speeds.
• The collision rate factor (Z) reflects the speeds of the
  molecules. The collision rate factor has a weak temperature dependence—although temperature greatly affects the fraction of collisions with enough energy to react, it only has a small effect on the collision rate.
• The reaction rate is determined by the collision rate and the fraction of collisions that successfully produce products. This allows for the rate of reaction to be expressed as the product of the steric factor (P), the fraction of collisions with enough energy to react (f), the collision rate factor (Z), and the concentrations of reactants.
• Combining P, f, and Z into the rate constant (k) yields the rate law as it was previously described. The steric factor (P) and the collision rate factor (Z) are combined in the frequency factor (A). This leads to an equation relating the rate constant (k) to the frequency factor (A), the activation energy (E a ), the universal gas constant (R), and the temperature (T). This equation can be used to calculate the impact of temperature on reaction rate.

Question 4

In the synthesis reaction A + B → AB, what is the fraction of effective collisions, given the temperature is 64°C and the energy of activation is 84 kJ / mole.

Answer 4

9.40 × 10^−14

Question 5

Which component on this potential energy diagram represents an enthalpy change?

Answer 5

ΔE

Question 6

Which part of collision theory is the last step in a successful reaction?

Answer 6

Molecules will separate after reaction occurs.

Question 7

Consider the relationship among activation energy, temperature, and the specific rate constant of a reaction:
e^-Ea/RT k = A

If Ea decreases, the effect will be all of the following except:

Answer 7

The reaction slows down

Question 8

Examine the potential energy diagram below.

The activated complex A–B–B can be described as:

Answer 8

endothermic with respect to both reactants and products

Question 9

The fundamental notion of collision theory is that in order for atoms, ions, or molecules to react what must be true?

Answer 9

They must first collide.

Question 10

Which is not a valid way to change a reaction rate?

Answer 10

Changing the ΔE of the reaction

Question 11

Which of the following factors directly influences the number of effective collisions in a reaction?

Answer 11

both Ea and P

Question 12

Refer to the potential energy diagram. Which of the following statements regarding the reaction is false?

Answer 12

Collisions with energy less than or equal to the activation energy result in reactions.

Question 13

The Arrhenius equation may be rearranged by taking the natural logarithm of each side to give:

ln (k) = −Ea / R(1 / T) + ln (A)

this is a linear equation: y = mx + b

Another equation can be derived from the logarithmic form of the Arrhenius equation; it is
ln (k2 / k1 ) = (Ea / R) [(1 / T1 ) − (1 / T2 )]

Using this equation, find the rate constant for the following reaction at 190°C:
H2(g) + I2(g) → 2HI(g)

The rate constant at 302°C is 2.45 × 10−4 L / mol s, and the activation energy is 167 kJ / mol.

Answer 13

5.22 × 10^−8 L / mol s

Question 14

If the temperature of the reactants is increased by 30° C, and all other factors remain constant, how will this affect the reaction rate?

Answer 14

It will increase 8 fold.