Saturday Study Session 2Theme of the day:

MoleculesSession 4 – Kinetics: How to turn

your hoopty reaction into a Ferrari reaction

Experiment [X]0 [Y]0 Initial Rate of Formation of Z (mol

L-1 sec-1)1 0.40 0.10 R2 0.20 0.20 ?

1. The table above shows the results from a rate study of the reaction X + Y→Z. Starting with known concentrations of X and Y in experiment 1, the rate of formation of Z was measured. If the reactions was first order with respect to X and second order with respect to Y, the initial rate of formation of Z in experiment 2 would be

Question 1 Answer

DClue: [X] cut in half but

[Y] doubled.

Rate = [X]1[Y]2

Since concentration of X is cut in half we’ll put in ½ for X and the concentration of Y is doubled we’ll put in 2 for YRate = [½ ]1[2]2

And ½ x 4 = 2 so the overall rate should double.

A) 15.5 secB) 124 secC) 41.3 secD) 62.0 sec

2. Gaseous cyclobutene undergoes a first-order reaction to form gaseous butadiene. At a particular temperature, the partial pressure of cyclobutene in the reaction vessel drops to one-eighth its original value in 124 seconds. What is the half-life for this reaction at this temperature?

Question 2 Answer

CClue: How many half

lives?

1st order so half life is constant

• In the stem it specifies that 1/8 of the original reactant is left which means the amount of reactant gets cut in half 3 times ½ x ½ x ½ = 1/8

• If 3 half lives occurs in 124 seconds then 1 half life occurs in 41.3 seconds.

A) Kinetic energy of collisions B) Frequency of collisions AND Kinetic

energy of collisionsC) Frequency of collisions AND Orientation of

particles during collisions.D) Kinetic energy of collisions AND

Orientation of particles during collisions AND Frequency of collisions

3. Factors of reactant molecule collisions that affect the rate of a chemical reaction include which of the following?

Question 3 Answer

DClue: basic definitions

For a reaction to occur molecules must collide, collide with enough energy (kinetic) and collide in the right orientation.

A) 5.6 x 10-3 mol L-1 s-1

B) 1.4 x 10-3 mol L-1 s-1

C) 2.8 x 10-3 mol L-1 s-1

D) 7.0 x 10-4 mol L-1 s-1

4. X + 2Y Z + 3QFor the reaction represented above, the initial rate of decrease in [X] was 2.8 x 10-3 mol L-1 s-1. What was the initial rate of decrease in [Y]?

Question 4 Answer

AClue: Not really kinetics,

more stoichiometry.

According to the equation 1X + 2Y Z + 3Q

however much X disappears 2x much Y should disappear.

H2 gas and N2 gas were placed in a rigid vessel and allowed to reach equilibrium in the presence of a catalyst according to the following equation.

A) The concentration of N2 decreased.B) The rate of formation of NH3 molecules was

equal to the rate of disappearance of H2 molecules.

C) The number of effective collisions between H2 and N2 was zero.

D) The rates of the forward and reverse reactions were equal.

5. Which of the following was true for the system between time t1 and time t2 ?

Question 5 Answer

DClue: The graph is the

key.

The graph shows between t1 and t2 the concentrations are constant. However there are still collisions going on. It is just that the reaction is happening at the same rate forward and backward.

B looks promising but the balanced equation has different coefficients for N2 and NH3. That is AP trying to see if you’ll bite at an answer that looks good but isn’t really true. Remember to read all the choices and to think critically.

6. Which of the following statements best explains why an increase in temperature of 5-10 Celsius degrees can substantially increase the rate of a chemical reaction?

A) The activation energy for the reaction is lowered.

B) The number of effective collisions between reactant particles is increased.

C) The rate of the reverse reaction is increased.

D) H for the reaction is lowered.

Question 6 Answer

BClue: What does

temperature describe about molecules?

Increasing the temperature means the molecules have more kinetic energy or are moving faster.

When they move faster they will collide more meaning a better chance of hitting hard enough in the right orientation.

Activation energy and H would remain unchanged.

7. A reaction mechanism for the destruction

of ozone, O3(g), is represented above. In the

overall reaction, NO(g) is best described as ?

A) an inhibitorB) a catalystC) a reactantD) an intermediateE) a product

Question 7 Answer

AClue: Look at NO at the beginning and the end.

Something that occurs at the beginning, changes in the middle and yet at the end is returned to normal is called a catalyst.

Catalysts do not take change in the chemical reaction and are left behind to catalyze more reactions..

8. The rate law for the reaction of nitrogen dioxide and chlorine is found to be rate = k [NO2]2[Cl2]. By what factor does the rate of the reaction change when the concentrations of both NO2 and Cl2 are doubled?

A) 6B) 8C) 4D) 2

Question 8 Answer

CClue: Like the Glade

commerical says, plug it in, plug it in.

rate = k [NO2]2[Cl2]And both reactant concentrations are doubled sorate = k [2]2[2]

So the rate will go up by a factor of 4x2 or 8.

Free Response 1 (8 pts possible)

Short Free Response 1(3-5 points possible)

a.

b.

c.

Short Free Response 2(4 points possible)

a.