Chapter 9 Kinetics

A catalyst would change which of the following? A. ΔH B.Ea C.Σ(ΔHproducts) D.Σ(ΔHreactants)

B

What is the rate of reaction?

How fast the reactants are changed into products

The reaction rate is determined by what three things?

1. How frequently the reactant molecules collide 2. The orientation of the colliding molecule 3. Their energy (activation energy)

Step 1: CO2(g) + H2O(l) ? H2CO3(aq) Step 2: H2CO3(aq) + NH4OH(aq) → NH4HCO3(aq) + H2O(l) Step 3: NH4HCO3(aq) + NaCl(aq) → NaHCO3(s) + NH4Cl(aq) In the mechanism above, ammonium bicarbonate acts as which of the following? A. An intermediate B. A catalyst C. A transition state D. An insoluble salt

A

Which of the following is true about the hypothetical two step reaction shown below? 1. A + 2B —> C + 2D (fast) 2. C —> A + E (slow) A. A is a catalyst, and C is an intermediate B. A is a catalyst, and D is an intermediate C. B is a catalyst, and A is an intermediate D. B is a catalyst, and C is an intermediate

A

hen table sugar is exposed to air it undergoes the following reaction: C12H22O11 + 12O2 —> 12 CO2 + 11 H2O Delta G = -5693 kj/mol When this reaction is observed at the macroscopic level, it appears as though nothing is happening, yet one can detect trace amounts of CO2 and H2O being formed. These observations are best explained by the fact the reaction is: A. Thermodynamically favorable but not kinetically favorable B. Kinetically favorable but not thermodynamically favorable C. Neither kinetically nor thermodynamically favorable D. Both kinetically and thermodynamically favorable

A

Increasing reactant concentration increases reaction rate by doing which of the following? A. Increasing the probability of a collision B. Increasing the average kinetic energy of the reactants C. Increasing the probability of reaction upon collision D. Stabilizing the transition state

A; Addition of more reactant will increase collision probability, which increases the probability of the reaction occurring (choice A is correct). An increase in the average kinetic energy of molecules in the system would be indicative of an increase in temperature (choice B is wrong) and increasing reactant concentration does not increase the chance of the reaction once a collision occurs, though increasing the temperature would do this too (choice C is wrong). Catalysts stabilize the transition state (something additional reactants are unlikely to accomplish - choice D is wrong).

A certain chemical reaction has the following rate law: Rate = k[NO2][Br2]. Which of the following statements necessarily describes the kinetics of this reactions? I. The reaction is second order II. The amount of NO2 consumed is equal to the amount of Br2 consumed III. The rate will not be affected by the addition of a compound other than NO2 and Br2 A. I only B. I and II C. II and III only D. I, II, and III

A; I is a correct statement II is wrong because we do not know the stoichiometric coefficients III is wrong because a catalyst can affect the rate.

Step 1: NO + Br2NOBr2 (Fast) Step 2: NOBr2 + NO → 2 NOBr (Slow) A chemist determines the rate law of the overall process above to be rate = k [NO][Br2], which seems inconsistent with the given mechanism. What best explains her findings? A. An inhibitor was present which affected the first step in the mechanism. B. Catalysis of the first step in the mechanism occurred. C. An increased quantity of Br2 was added to the system. D. An increased quantity of NO was added to the system.

A; The chemist finds the first step is now dictating the overall reaction rate. One possible explanation would be the inhibition of the first step, causing it to become the rate determining step (choice A is correct). Catalysis of the first step and increasing reactant concentrations would only further increase the rate of the first reaction, leaving the second step as the rate determining step (choices B-D do not explain the observation and are wrong).

Which of the following is the best example of a rate? A. Rate = Delta [A] / Delta t B. Rate = Delta [A] / Delta B C. Rate = Delta [A] Delta B D. Rate = Delta [A]^2

A; regardless of topic, rate is always defined as a change in something over a change in time

What are the kinetic quantities on a reaction diagram?

Activation energy and activation complexes (transition states)

Take this reaction for example 1. 2 NO2 --> N2O2 2. N2O2 + O2 --> 2 NO What is N2O2 in this reaction?

An intermediate

What substances are includes in rate law?

Aqueous solutions and gases (no solids or solvents)

The addition of a catalyst to a chemical reaction will bring about: A. an increase in activation energy and an increase in the rate of the reverse reaction. B. a decrease in activation energy and an increase in the rate of the forward reaction. C. an increase in activation energy and a decrease in the rate of the forward reaction. D. a decrease in activation energy and a decrease in the rate of the reverse reaction.

B

A chemist is working with a well-characterized reaction in her laboratory. Addition of an unknown compound to the reaction results in an increase in initial reaction rate. However, upon addition of larger quantities of the same compound, no further increase is detected. What is the most likely identity of the unknown compound? A. An intermediate B. A catalyst C. An inhibitor D. A product

B; Addition of a catalyst will increase the rate of a reaction by decreasing the activation energy of the step in question but will not continue to do so with increasing concentrations of catalyst (choice B is correct). While addition of an intermediate may increase the rate of a reaction, the initial rate will not plateau at higher concentrations (choice A is wrong). An inhibitor will likely decrease the reaction rate (choice C is wrong) and addition of a product is unlikely to speed the reaction (choice D is wrong).

Given the average rate for the following reaction is 2.25 × 10-2 M/min, how long will it take to generate 0.45 moles of NO2(g) in one liter of solution? 2 N2O5(g) → 4 NO2(g) + O2(g) A. 15 minutes B. 5 minutes C. 10 minutes D. 20 minutes

B; Given the average rate for the following reaction is 2.25 × 10-2 M/min, it would take 5 minutes to generate 0.45 moles of NO2(g) in one liter of solution. The average rate expression for this reaction is as follows: -1 [N2O5 (g)] -1 [NO2(g) 1 [O2(g) -------------- = ------------- = ------------- = 2.25 x 10^-2 M/min 2 delta T 4 delta T delta T Thus the change in nitrogen dioxide concentration over time is 0.09 M/min (four times that of the average rate) and 0.45 moles would form in 5 minutes (0.45 moles / 0.09 M/min).

Which of the following changes to an equilibrated system is most likely to change the proportion of reactants and products at equilibrium? A. Increasing activation energy B. Increasing product stability C. Increasing reactant concentration D. Increasing reaction rate

B; Increasing product stability will increase the relative proportion of products at equilibrium (choice B is correct). Changes in reaction rate (and other kinetic changes) are not likely to impact equilibrium (or other thermodynamic properties - choices A and D are wrong). Increasing reactant concentration will temporarily decrease the reaction quotient, Q, but once equilibrium has been re-established, the same proportions of reactants and products will exist (choice C is wrong).

Given the following experimental data, which of the following would allow for the determination of the rate law for the reaction in question? Initial [CO] (mm Hg) Initial [NO3] (mm Hg) Initial Rate (mm Hg/s) Trial 1: 278 180 212 Trial 2: 282 355 401 A. The reaction order for nitrogen trioxide B. The rate constant C. The activation energy for the reaction D. Additional trials with constant carbon monoxide levels

B; The isolation method can be used to determine the reaction order for reagents in question to help formulate a rate law. The data provided here can be used to determine the reaction order of nitrogen trioxide (choices A and D are wrong) but is insufficient to ascertain the reaction order of carbon monoxide. If we were provided with the rate constant, the reaction order for carbon monoxide could be calculated and a complete rate law could be determined (choice B is correct). While the activation energy is a component of the rate constant and provides valuable information about the reaction, it would not aid us in determining the rate law (choice C is wrong).

2 SO2(g) + O2(g) 2 SO3(g) A chemist adds a reagent to the following equilibrated reaction and immediately observes the forward and reverse reaction rates to increase by 2.14 and 2.29, respectively. Which of the following was most likely added to the reaction? A. SO3(g) B. V2O5(s) C. SO2(g) D. O2(g)

B; The situation here describes the addition of a catalyst to an equilibrated reaction. Catalysts increase the rates of both the forward and reverse reaction by decreasing the activation energy. Addition of a reactant (SO2(g) and O2(g)) would initially increase the rate of the forward reaction alone while adding a product (SO3(g)) would initially increase only the rate of the reverse reaction. Thus V2O5(s) must serve as a catalyst in this reaction and results in the observed rate changes.

A catalyst is discovered that doubles the rate for the second step in the following mechanism. What impact does this have on the overall reaction? Step 1: 2 NO2 → NO + NO3 (slow) Step 2: NO3 + CO → NO2 + CO2 (fast) A. Doubles the reaction rate B. Halves the reaction rate C. No change to the reaction rate D. Insufficient information to determine the change in rate

C

A chemist attempts to determine the rate law of a poorly characterized reaction. Propose the best rate law consistent with the following data. Initial [H2O2] (M) Initial [I-] (M) Initial Rate (M/s) Trial 1: 3.0 x 10^-2 5.6 x 10^-5 1.4 x 10^2 Trial 2: 3.0 x 10^-2 1.1 x 10^-4 2.8 x 10^2 Trial 3: 9.0 x 10^-2 1.1 x 10^-4 8.4 x 10^2 Trial 4: 6.0 x 10^-2 5.6 x 10^-5 2.8 x 10^2 A. rate = k [H2O2] B. rate = k [I-] C. rate = k [H2O2] [I-] D. rate = k [H2O2]2 [I-]

C

A reaction between two species is experimentally observed to be second order overall. If the concentration of one of the species doubles, what happens to the reaction rate? A. The rate doubles B. The rate is halved C. Cannot be determined D. The rate quadruples

C

Which of the following statements is always true about the kinetics of a chemical reaction? A. The rate law includes all reactants in the balances equation B. The overall order equal the sum of the reactant coefficients in the overall reaction C. The overall order equals the sum of the reactant coefficients in the slow step of a reaction D. The structure of the catalyst remains unchanged throughout the reaction progress.

C

Which of the following statements is true? A. Catalysts decrease activation energy for the forward reaction only B. Catalysts decrease the activation energy of the reverse reaction only C. Catalysts decrease the activation energy of both the forward and reverse reactions D. Catalysts decrease the activation energy of the forward reaction and increase the activation energy of the reverse reaction.

C;

Why will increasing the temperature of a reaction increase reaction rate? I. It increases collision frequency. II. It decreases activation energy. III. It increases the probability of reaction upon collision. A. I only B. II only C. I and III only D. I, II, and III

C; Increasing temperature increases reaction rate by increasing the average kinetic energy of molecules in solution, which increases their probability of collision (greater velocity results in more chances for collision) as well as provides them with additional energy to help overcome the activation energy necessary to achieve the transition state (statements I and III are true; choices A and B are wrong). While decreasing activation energy will increase reaction rate, an increase in temperature is unlikely to have a favorable impact on the stability of the transition state (statement II is not true, choice C is correct).

Why will increasing the temperature of a reaction increase reaction rate? It increases collision frequency. It decreases activation energy. It increases the probability of reaction upon collision. A. I only Your Answer B. II only C. I and III only D. I, II, and III

C; Increasing temperature increases reaction rate by increasing the average kinetic energy of molecules in solution, which increases their probability of collision (greater velocity results in more chances for collision) as well as provides them with additional energy to help overcome the activation energy necessary to achieve the transition state (statements I and III are true; choices A and B are wrong). While decreasing activation energy will increase reaction rate, an increase in temperature is unlikely to have a favorable impact on the stability of the transition state (statement II is not true, choice C is correct).

Given the average rate for an unknown reaction is described by -Δ[H2]/Δt, -Δ[I2]/Δt, and (1/2) Δ[HI]/Δt, what is the most likely reaction? A. HI(g) → 1/2 H2(g) + 1/2 I2(g) B. H2(g) + I2(g) → HI(g) C. H2(g) + I2(g) → 2 HI(g) D. H2(g) + I2(g) → 1/2 HI(g)

C; Reaction rate can be characterized by changes in reactants or products over time. In the generic reaction aA → bB, the reaction rate = -(1/a) Δ[A]/Δt = (1/b) Δ[B]/Δt. Thus we can reconstruct our reaction from the above rates and find the reaction rate to be H2(g) + I2(g) → 2 HI(g) (choice C is correct). Note that simply utilizing process of elimination for unbalanced reactions would eliminate answer choices B and D.

CO(g) + NO3(g) → NO2(g) + CO2(g) The overall reaction order for the elementary step above is: A. zeroth order. B. first order. C. second order. D. third order

C; The rate law for this elementary step is rate = k [CO][NO3] which, due to each reactant being first order, is a second order process overall (choice C is correct).

A reaction is run without a catalyst and is found to have an activation energy of 140 kJ/mol and a heat of reaction, delta H, of 30 kj/mol. In the presence of a catalyst, however, the activation energy is reduced to 120 kj/mol. What will be the heat of reaction in the presence of a catalyst? A. -10 kj/mol B. 10 kj/mol C. 30 kj/mol D. 50 kj/mol

C; catalysts do not affect thermodynamics

What are homogenous catalysts?

Catalysts that are in the same state as the reactants.

As the temperature at which a reaction takes place is increased: A. the reaction rate will remain constant, but the rate constant will increase. B. the reaction rate and the rate constant will remain constant. C. the reaction rate will increase, but the rate constant will remain constant. D. the reaction rate and the rate constant will increase

D

Based on the reaction mechanism shown below, which of the following statements is correct? 2 NO + O2 —> 2 NO2 1. 2 NO —> N2O (fast) 2. N2O2 + O2 —> 2 NO2 (slow) A. Step 1 is the rate determining step and the rate of the overall reaction is k[N2O2] B. Step 1 is the rate determining step and the rate of the overall reaction is k[NO2]^2 C. Step 2 is the rate determining step and the rate of the overall reaction is k[NO2]^2 D. Step 1 is the rate determining step and the rate of the overall reaction is k[N2O2][O2]

D

Catalysts increase reaction rate by stabilizing the transition state. What impact does this have on the reverse reaction? A. No impact: a separate catalyst would be required for the reverse reaction B. No impact: the reverse reaction is catalytically resistant C. Increases reverse reaction rate to the same degree as the forward reaction Correct Answer D. Increases reverse reaction rate by changing the reverse reaction intermediates

D

In a third order reaction involving two reactants and two products, doubling the concentration of the first reactants causes the rate to increase by a factor of 2. What will happen to the rate of this reaction if the concentration of the second reactant is cut in half. A. It will increase by a factor of 2 B. It will increase by a factor of 4 C. It will decrease by a factor of 2 D. It will decrease by a factor of 4

D

The rate-determining step of a chemical reaction is the step: A. that is the fastest. B. involving the most molecular mass. C. involving the least molecular mass. D. that is the slowest.

D

When a catalyst is added to a chemical reaction, what will be its effect on the energy of the activated complex and on the rate of the reaction? A. The energy of the activated complex will increase, and the reaction rate will decrease. B. The energy of the activated complex will decrease, and the reaction rate will increase. C. The energy of the activated complex and the reaction rate will decrease. D. The energy of the activated complex and the reaction rate will increase.

D

Which of the following gives the form of the rate law from the balanced reaction: 4A + 2B —> C + 3D A. Rate = k[A]^4[B]^2 B. Rate = k[A]^2[B] C. Rate = k [C][D]^3/[A]^4[B]^2 D. Cannot be determined from the information given

D

A scientist adds reactants and an enzyme to a beaker placed on the benchtop. As the reaction proceeds, which of the following events would have the smallest effect on the rate of the forward reaction? A. More reactants are added to the beaker. B. The temperature of the beaker is changed. C. The solution is stirred. D. Products are removed from the beaker

D; A scientist adds reactants and an enzyme to a beaker placed on the benchtop. Removing products from the beaker would have the smallest effect on the rate of the forward reaction. Temperature, reactant concentrations, and activation energy are the three major factors that play a role in a reaction's kinetics. If the temperature of the beaker changes, the rate of the forward reaction will be affected. When more reactants are added or the mixture stirred, the probability of collisions among the molecules increases. Thus, the forward reaction rate will increase. While removing products will shift the equation to the right (favoring the production of more products) by Le Chatelier's Principle, this is an effect on the equilibrium, or thermodynamics, of the reaction and does not affect the rate of the forward reaction.

Which of the following characterizes the rate for the reaction above? 2 NO + O2 → 2 NO2 A. rate = -Δ[NO]/Δt B. rate = -2 Δ[O]/Δt C. rate = -Δ[NO2]/Δt D. rate = -(1/2) Δ[NO]/Δt

D; Reaction rate can be characterized by changes in reactants or products over time. In the generic reaction aA → bB, the reaction rate = -(1/a) Δ[A]/Δt = (1/b) Δ[B]/Δt. Thus the rate expression for the given reaction would be = -(1/2) Δ[NO]/Δt = -Δ[O2]/Δt = Δ(1/2) [NO2]/Δt (choice D is correct).

Which of the following experimental methods should NEVER affect the rate of a reaction? A. Placing an exothermic reaction in an ice bath B. Increasing the pressure of a reactant in a closed container C. Putting the reactants into an aqueous solution D. Removing the product of an irreversible reaction.

D; Temperature always effect rate, so A is eliminated Changing the partial pressures of a gas will affect the number of effective collisions per time, making B incorrect. Solvents affect the order of the reaction depending on how the reactants interact with the solvent Removing the product should not affect rate because rate law is independent of the concentration of products.

Which of the following mechanisms best describes the overall reaction CH4 + Cl2 → CH3Cl + HCl with an overall rate expression of rate = k [CH4][Cl2]? A. CH4 + Cl2 → •CH3 + •Cl + HCl (fast) •CH3 + •Cl → CH3Cl (slow) B. CH4 + 2 Cl2 → •CH3 + HCl + 3Cl• (slow)CH3 + Cl• → CH3Cl (fast) C. CH4 + 2 Cl2 → •CH3 + HCl + 3Cl• (fast)CH3 + Cl• → CH3Cl (slow) D. CH4 + Cl2 → •CH3 + •Cl + HCl (slow) •CH3 + •Cl → CH3Cl (fast)

D; The overall reaction CH4 + Cl2 → CH3Cl + HCl with an overall rate expression of rate = k [CH4][Cl2] is best described by the mechanism: CH4 + Cl2 → •CH3 + •Cl + HCl (slow) •CH3 + •Cl → CH3Cl (fast) A mechanism describes the elementary steps that compose an overall reaction. The steps in a mechanism must add to give the overall reaction and the overall rate expression should be that of the slow step. The only answer choice which satisfies both these requirements is CH4 + Cl2 → •CH3 + •Cl + HCl (slow) •CH3 + •Cl → CH3Cl (fast)

Which of the following elementary steps is best described by the following rate law: rate = k [NO][ClNO2]? A. 2 ClNO(l) + NO(g) NO2(g) + 2 ClNO(g) B. 2 ClNO(g) + NO(g)NO2(g) + 2 ClNO(g) C. ClNO2(l) + NO(g)NO2(g) + ClNO(g) D. ClNO2(g) + NO(g) NO2(g) + ClNO(g)

D; The rate law for each elementary step can be derived by taking each non-liquid/solid reactant and raising it to the power of its coefficient. The rate laws for the answer choice A-D are k [NO], k [NO][ClNO]2, k [NO], and k [NO][ClNO2], respectively (choice D is correct).

What are thermodynamic quantities of a reaction diagram?

Delta G and intermediates, as well as reactant and products

What is the transition state of a reaction?

High energy and short lived molecule. The transition state is an an energy maximum

What are the two ways to change the rate of a zero order reaction?

Increase temperature or add a catalyst

How does increasing the concentration of reactants affect a reaction rate?

Increases the number of collisions therefore speeds up the reaction, EXCEPT for zero order kinetics.

How is a catalyst changed in a reaction?

It is not changed or consumed

What is activation energy?

Minimum energy required of reactant molecules during a molecular collision in order for the action to proceed to products.

The rate constant is __________ to the concentration of reactants

NOT affected

What do kinetics tell us about the spontaneity of a reaction?

Nothing!

Rate of collision equation

Rate = Z f Z = number of collisions f = frequency of collisions

Rate for a zero order reaction

Rate = k

What does the rate of a first order reaction look like?

Rate = k[A]^1 or k = [B]^1

What are second order reactions?

Rate that is proportional to either the concentration of two reactants or the square root of a single reactant

How does temperature affect reaction rates?

Reaction rate will increase as the temperature increases.

What is the collision molecular theory?

States that the rate of a reaction is proportional to the number of collisions per second between reacting molecules.

The rate contstant of a zero order reaction is dependent on what?

Temperature

What is a zero order reaction?

Where the concentration of a reactant has no effect on the rate of reaction.

What is a first order reaction?

Where the rate of reaction is directly proportional to the concentration of one reactant.

What is the slope of a zero order reaction?

[A] vs t where the slope is -k

What is an intermediate in a reaction?

a product of one step in a reaction mechanism and a reactant in the next step

The rate constant is _________ proportional to activation energy

activation energy

What are heterogenous catalysts?

catalysts in a different phase from the reactants

The rate constant is __________ proportional to temperature

directly

What do catalysts not affect?

equilibrium or thermodynamics

Rate law can only be determined...

experimentally

The greater the concentration of reactants, the __________ the reaction rate

faster

The higher the temperature of the reaction mixture, the _______ the reaction rate.

faster

The lower the activation energy, the _______ the reaction step

faster

The rate law for a unimolecular elementary step is

first order

How do you determine the rate of a reaction?

it is determined by measuring the concentrations of a reactants and/or products as an actual chemical reaction occurs add ^x and ^y

What is the rate constant?

k, proportionality constant that measures how fast a particular reaction occurs

What is the Arrhenius equation?

k=Ae^(-Ea/RT) - A = frequency factor (constant) - Ea = activation energy - R = gas constant - T = temp - rate increases exponentially with T

Units for a zero order reaction

m/s

The intermediate of a reaction is at an energy local ________

minimum

When writing reaction rates in change over time, products always have a _________ sign and reactants have a ________ sign

negative positive

Give the rate for the equation 2 H2 + O2 --> 2 H2O

r = -Δ [O2] / Δt = + H2O / 2 Δt

What is the rate with respect to oxygen in the following reaction? 2 H2 + O2 --> 2 H2O

r = -Δ [O2] / Δt = + H2O / 2 Δt

Give an example of a first order reaction?

radioactive decay

Rate of a second order reaction

rate = k[A]^2, rate = k[B]^2, or rate = k[A]^1[B]^1

What are rate laws?

relate the rate of a reaction to the concentrations of species involved - Rate = k[A]^x[B]^y[C]^z - k = rate constant - x = order of rxn with respect to A - y = order of rxn with respect to B - z = order of rxn with respect to C - x + y + z = overall order of rxn

What are the units of a first order reaction?

s-1 or 1/s

The rate law is _________ order for a bimolecular elementary step

second

How does the medium affect reaction rate?

some mlcs are more likely to react in an aqueous or non-aqueous solvent. physical state can also have an effect. Typically, polar solvents are preferred because mlcr dipole tends to polarize and weaken bonds of reactants, making reaction go faster

What is a catalyst?

substance that speeds up the rate of a chemical reaction

What is the rate determining step of a reaction?

the slowest step in a reaction

What is kinetics?

the study of reaction rates