Chapter 17: Kinetics (TEST #4)
1.) _____ energy is the minimum amount of energy that colliding molecules must possess in order for a chemical reaction to occur. A. activation B. bond C. collision
A. activation
5.) What is the rate equation of a reaction A + BC which has a first-rate order for [A] and zero-rate order for [B]? A. rate = k[A] B. rate = k[A][B] C. rate = k[B]
A. rate = k[A]
RATE LAWS: How must the rate law for a reaction be determined by?
-must be determined by JUST experiments -can't just use a balanced equation
LINEAR FORM OF THE FIRST-ORDER RATE LAW: Explain the linear form of the first-order rate law
(see pic, the top equation is the linear form of the first-order rate law, and the bottom equation is the regular first-order rate law equation) ~General linear equation: y = mx + b ~ln[A]t plotted on the y axis ~time (t) plotted on the x axis ~The slope of the line is -k ~The y-intercept is ln[A]0
What is the equation for the two-point form of the Arrhenius equation? Explain this equation
- see pic for equation EXPLANATION: -This equation can be used with two k and T to calculate Ea. -You do not need to know the value of A to use this equation. -Make sure that the energy units on Ea and R are identical.
FIRST-ORDER INTEGRATED RATE LAW: Explain the first order integrated rate law
-For 1st order reactions of the type A → products Rate = k[A] Integration with respect to time gives us this equation: (see pic, bottom equation) ~k is the rate constant ~t is time ~[A]0 is the initial reactant concentration. ~[A]t is the reactant concentration at time t.
INTEGRATED RATE LAWS: So far we have only talked about the concentration-rate relationship. A → products Rate = k[A]m Explain the integrated rate law
-The rate law can be integrated with respect to time to produce a concentration-time relationship known as an integrated rate law. -This relationship depends on the order of the reaction: Zero order, 1st order, 2nd order, etc. -A new term, the half-life, will also be introduced.
RATE LAWS: EX of the rate law of the equation: N2O5 (g) → 2NO2 (g) + ½O2 (g)
-The rate law for this reaction is: Rate = k[N2O5] -Only reactants appear in a rate law. -k is the rate constant for the reaction.
What is the rate of a reaction?
-The rate of a reaction is the change in the amount of a reactant or product (often in terms of concentration) per unit time. -rate is always positive
The Rate Constant (k) Equation
-see pic for equation A : frequency factor. It is related to the frequency of collision and orientation. e^-Ea/RT: fraction factor. the fractions of collisions which have enough energy to overcome the activation energy Ea. -The rate constant (k) is dependent on Ea and temperature.
What are the three important questions when planning a chemical reaction?
1)"Will the reaction produce the desired products in useful quantities?" ------- Thermodynamics 2)"How rapidly will the reaction occur?" ------- Kinetics 3)"What specific molecular-level processes take place as the reaction occurs?" -------- Kinetics
What are the 5 factors affecting reaction rates?
1.) Chemical nature of the reactants 2.) Surface area of the reactants 3.) Temperature of the reactants 4.) Concentration of the reactants 5.) The presence of a catalyst
ZERO ORDER INTEGRATED RATE LAW: 1.) Does the half life of a zero order reaction depend on or does not depend in the initial concentration of reactant? 2.) What is the equation for the half life of a zero order reaction?
1.) Half life of a zero order reaction DOES DEPEND on the initial concentration of reactant. 2.) see pic
What are the three catalysts described in this chapter?
1.) Homogeneous Catalyst 2.) Heterogeneous Catalyst 3.) Enzyme Biocatalyst
1.) What is the Instantaneous Reaction Rate 2.) How do you calculate the instantaneous reaction rate?
1.) The rate at a specific time and/or concentration 2.) -Calculated using a graph (conc. vs. time) and a slope of a straight line tangent to the curve at that specific time. -Or by determining the avg. rate of reaction over a very short time period.
1.) What is the Average Reaction Rate? 2.) How do you calculate the average reaction rate?
1.) The rate over a time period 2.) Calculated using concentrations at the beginning and end of a time period
1.) What is the Reaction Mechanism and provide an example 2.) What is an elementary reaction in the reaction mechanism? 3.) What are intermediates in a reaction mechanism?
1.) The reaction mechanism provides details regarding the precise, step-by-step process by which a reaction occurs. EX: For example: the decomposition of ozone, appears to follow a mechanism with two steps: O3(g) ⟶ O2(g) + O O + O3(g) ⟶ 2O2(g) 2.) Each step occurs precisely as represented in the step equations, is called an elementary reaction. 3.) Species that are produced in one step and consumed in a subsequent step are called intermediates.
What are the three molecularity categories of an elementary reaction?
1.) Unimolecular 2.) Bimolecular Reaction 3.) Thermolecular Reaction
THE HALF LIFE OF FIRST ORDER REACTION: How to solve } for a first order reaction, at the half life, t1/2
1.) [A]t = ½[A]0 2.) then solve for t in ln[A]t = -kt + ln[A]0
Explain the heterogenous catalyst
A Heterogeneous catalyst is in a different phase from that of the reactants. Most commonly the catalyst is a solid and the reactants are in the gas or liquid phase Example is the catalytic converter in automobiles which consists of platinum and rhodium metals. -see slide #41 of equation example of a heterogeneous catalyst
Explain the homogeneous catalyst
A Homogeneous catalyst is in the same phase as the reactants. It speeds up the reaction by forming a reactive intermediate. •The reactive intermediate is formed but immediately consumed to form product and regenerate the catalyst. . Example: Chlorine radicals (Cl) that catalyze the decomposition of ozone (O3). Cl(g) + O3(g) ⟶ ClO(g) + O2(g) ClO(g) + O(g) ⟶ Cl(g) + O2(g)
12.) SEE HOMEWORK A. B. C.
A.
16.) SEE HOMEWORK A. B. C.
A.
27.) SEE HOMEWORK A. B. C.
A.
28.) Which of the following elementary reactions is unimolecular? (SEE HW) A. B. C.
A.
30.) SEE HOMEWORK A. B. C.
A.
6.) SEE HOMEWORK A. B. C.
A.
7.) SEE HOMEWORK A. B. C.
A.
17.) SEE HOMEWORK A. 14.3 d B. 13.6 d C. 12.5 d
A. 14.3 d
9.) Tripling the concentration of a reactant increases the rate of a reaction three times. What is the order of the reaction with respect to that reactant? A. 1st B. 2nd C. 3rd
A. 1st
PRACTICE QUESTION: Slide #13 Tripling the concentration of a reactant increases the rate of a reaction three times. What is the order of the reaction with respect to that reactant? A. 1st B. 2nd C. 3rd
A. 1st
3.) What is the overall rate order of a reaction A + BC which has the following rate equation: rate = k[A][B]? A. 2 B. 1 C. 0
A. 2
20.) SEE HOMEWORK A. 252 days B. 210 days C.168 days
A. 252 days
PRACTICE QUESTION WITH FIRST-ORDER INTEGRATED RATE LAW (slide #20): Suppose that the rate constant k of steroids decomposition in human body is 1.65× 10-2/day. Assuming that the steroids biodegrade by a first-order process, how long would it take for 1/64 of the initial dose to remain in the athlete's body? A. 252 days B. 210 days C. 168 days
A. 252 days
18.) SEE HOMEWORK A. 8.44 x 10^3h B. 4.23 x 10^2h C. 4.87 x 10^4h
A. 8.44 x 10^3h
SECOND-ORDER INTEGRATED RATE LAW: PRACTICE QUESTION (slide #22) What is the half-life for the decomposition of O3 when the concentration of O3 is 2.35 10-6 M? The rate constant for this second-order reaction is 50.4 L mol-1 h-1. A. 8.44 × 10^3 h B. 4.23 × 10^2 h C. 4.87 × 10^4 h
A. 8.44 × 10^3 h
EXAMPLE PROBLEM FROM SLIDE #35: What is the rate law for the elementary termolecular reaction ? A+ 2B -----> products A. Rate = k[A][B]^2 B. Rate = k[A][B] C. Rate = k[B]^2
A. Rate = k[A][B]^2
PRACTICE QUESTION (slide #24, have to see slide to do question): From the given graph below, determine the order of the following reaction: A. The plot shows that the reaction data are constant with second-order kinetics B. The plot shows that the reaction data are constant with first-order kinetics C. The plot shows that the reaction data are constant with zero-order kinetics
A. The plot shows that the reaction data are constant with second-order kinetics
RATE LAWS: Example question from slide #12 What is the rate equation of a reaction A + B à C which has a first-rate order for [A] and zero-rate order for [B]? A.rate = k[A] B.rate = k[A][B] C.rate = k[B]
A.rate = k[A]
13.) SEE HOMEWORK A. B. C.
B.
29.) SEE HOMEWORK A. B. C.
B.
26.) SEE HOMEWORK A. (a) B. (b)
B. (b)
4.) What is the rate order with respect to [B] in the reaction A + BC which has following rate equation: rate = k[B]? A. 2 B. 1 C. 0
B. 1
23.) SEE HOMEWORK A. 214 kJ/mol B. 191 kJ/mol C. 191 J/mol D. 214 J/mol
B. 191 kJ/mol
Practice question from slide #32: The rate constant for the decomposition of ethanal, CH3CHO is 1.1 × 10-2 L mol-1 s-1 at 703 K and 4.95 L mol-1 s-1 at 865 K. Determine the activation energy for this decomposition. A. 214 kJ/mol B. 191 kJ/mol C. 191 J/mol
B. 191 kJ/mol
24.) SEE HOMEWORK A. 30 kJ B. 25 kJ C. 20 kJ
B. 25 kJ
8.) Doubling the concentration of a reactant increases the rate of a reaction four times. What is the order of the reaction with respect to that reactant? A. 1st B. 2nd C. 3rd
B. 2nd
21.) Account for the relationship between the rate of a reaction and its activation energy. A.The rate of reaction will increase as the activation energy increases. B.The rate of reaction will increase as the activation energy decreases. C.The rate of reaction will remain the same as the activation energy decreases.
B. The rate of reaction will increase as the activation energy decreases.
2.) What affect does temperature have on the rate of a reaction? A. the lower the temperature, the lower the kinetic energy which results in more effective collisions therefore increasing the rate of a reaction B. the higher the temperature, the greater the kinetic energy which results in more effective collisions therefore increasing the rate of a reaction C. the higher the temperature, the greater the kinetic energy which results in more effective collisions therefore decreasing the rate of a reaction
B. the higher the temperature, the greater the kinetic energy which results in more
What is a bimolecular reaction?
Bimolecular reaction involves two reactant species: A + B ⟶ product(s) or 2A ⟶ product(s)
14.) SEE HOMEWORK A. B. C.
C.
22.) SEE HOMEWORK A. B. C.
C.
19.) SEE HOMEWORK A. 0.2% B. 4% C. 0.4%
C. 0.4%
THE HALF LIFE OF FIRST ORDER REACTION: PRACTICE QUESTION FROM SLIDE #21 The first-order decay of technetium-99 is used to image heart muscle in patients with suspected heart problems. The half-life is 6 h. What percent of the radioactivity would remain after 2 days? A. 0.2% B. 4% C. 0.4%
C. 0.4%
THE RATE CONSTANT (k): PRACTICE QUESTION (slide #30) The rate constant for the decomposition of C4H8 is 6.1×10-8 s-1 at 598K, Ea is 261 kJ/mol. What is the frequency factor? A. 1.9 × 10^12 /s B. 3.1 × 10^13 /s C. 3.9 × 10^15 /s
C. 3.9 × 10^15 /s
11.) SEE HOMEWORK A. The rate increases by a factor of 1 B. The rate increases by a factor of 2 C. The rate increases by a factor of 3 D. The rate is not affected.
C. The rate increases by a factor of 3
25.) Nitrogen and oxygen react at high temperatures. What will happen to the concentrations of N2, O2, and NO at equilibrium if a catalyst is added? A. increase B. decrease C. no change
C. no change
What is a catalyst?
Catalyst - A substance that increases the rate of a reaction by lowering the activation energy without being consumed. -The catalyst must be regenerated in the process. -Catalysts provide an alternate reaction pathway with lower Ea. -Sometimes the catalyzed path contains multiple steps, but each individual step has lower Ea than that of uncatalyzed path.
5 FACTORS AFFECTING REACTION RATES: 5.) The presence of a catalyst
Catalysts usually speed up reactions without being consumed
Chemical reactions occur as the result of what?
Chemical reactions occur as the result of collisions between reactant molecules.
What is the study of chemical kinetics?
Concerns these two main points: 1.) The rate at which a reaction yields products 2.) The molecular-scale means by which a reaction occurs
10.) Increasing the concentration of CO from 0.01 M to 0.03 M. How much will this change affect the rate of the reaction: (SEE HW) A. The process reduces the rate by a factor of 4 B. The process reduces the rate by a factor of 3 C. The process reduces the rate by a factor of 2 D. Since CO does not appear in the rate law, the rate is not affected.
D. Since CO does not appear in the rate law, the rate is not affected.
PRACTICE QUESTION: Slide #13 Increasing the concentration of CO from 0.01 M to 0.03 M. How much will this change affect the rate of the reaction: if the rate law for the reaction is: Rate = 1.5[H2]3 ? A. The process reduces the rate by a factor of 4 B. The process reduces the rate by a factor of 3 C. The process reduces the rate by a factor of 2 D. Since CO does not appear in the rate law, the rate is not affected.
D. Since CO does not appear in the rate law, the rate is not affected.
15.) SEE HOMEWORK A. B. C. D. E.
E.
5 FACTORS AFFECTING REACTION RATES: 1.) Chemical nature of the reactants
EX: As you move down a group the alkali metals react at a faster rate with water
Explain the Enzyme that is a biocatalyst
Enzymes are biocatalysts -Protein molecules
Explain the rate-limiting step, also known as the rate-determining step, of the reaction.
NO2(g) + CO(g) ⟶ CO2(g) + NO(g) -At temperatures below 225 °C, the overall reaction rate is: Rate = k[NO2]2 -This reaction follows a two-step mechanism: 1.) NO2(g) + NO2(g) ⟶ NO3(g) + NO(g) (slow) 2.) NO3(g) + CO(g) ⟶ NO2(g) + CO2(g) (fast) -The rate-determining (slower) step gives a rate law showing second-order dependence on the NO2 concentration, and the sum of the two equations gives the net overall reaction. -In general, when the rate-determining (slowest) step is the first step in a mechanism, the overall rate law is the same as the rate law for this step.
What is the collision theory?
Reactants (atoms, molecules, or ions) must collide in order to react with each other. 1) Rate of Reaction is proportional to the rate of collisions between reactants. Why does not every collision lead to a reaction? 2) Molecules must be oriented properly when they collide. Why does not every collision lead to a reaction? 3.) Molecules must have adequate kinetic energy to react. -The kinetic energy supplied must be high enough to break the chemical bonds in reactants. -Molecules with kinetic energies too small just bounce off each other and don't react. -This required energy is called the activation energy.
5 FACTORS AFFECTING REACTION RATES: 4.) Concentration of the reactants
Reaction rate usually increases with higher concentration of reactants.
5 FACTORS AFFECTING REACTION RATES: 3.) Temperature of the reactants
Reaction rate usually increases with higher temperature.
What is a termolecular reaction?
Termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. (not common): Example: 2NO + O2 ⟶ 2NO2
What is the rate law of the elementary reaction: TERMOLECULAR REACTION RATE LAW
Termolecular reaction rate law: 2NO + O2 ⟶ 2NO2 rate = k[NO]2[O2]
THE HALF LIFE OF FIRST ORDER REACTION: What is the half life (t1/2) of a reaction?
The half life (t1/2) of a reaction is the time it takes for one half of a given amount of reactant to be consumed.
SECOND-ORDER INTEGRATED RATE LAW: 1.) Does the half-life of a second order reaction depend or not depend on the initial concentration of reactant?
The half-life of a second order reaction DOES depend on the initial concentration of reactant.
What is the Initial Reaction Rate?
The instantaneous reaction rate at "time zero".
What is the molecularity of an elementary reaction?
The molecularity of an elementary reaction is the number of reactant species (atoms, molecules, or ions).
HOW TO DETERMINE A RATE LAW: The order with respect to a particular reactant can be determined by .....
The order with respect to a particular reactant can be determined by varying its initial concentration while holding the initial concentration(s) of the other reactant(s) constant. 1.) measure the initial rate with each of the two different concentrations. 2.) The order with respect to that reactant can then be calculated. 3.) Repeat this process with all reactants.
THE RATE CONSTANT (k): Is the rate constant (k) dependent or independent on Ea and temperature?
The rate constant (k) is dependent on Ea and temperature.
5 FACTORS AFFECTING REACTION RATES: 2.) Surface area of the reactants
The rate of most reactions increase with increasing surface area contact between reactants.
What is the rate-limiting step, also known as the rate-determining step, of the reaction?
The slowest step is called the rate-limiting step (or rate-determining step) of the reaction
What is a unimolecular reaction?
Unimolecular reaction involves the reaction of a single reactant species to produce one or more molecules of product: A ⟶ product(s)
What is the rate law of the elementary reaction: UNIMOLECULAR REACTION RATE LAW
Unimolecular reaction rate law: A ⟶ product(s) rate = k[A]
THE HALF LIFE OF FIRST ORDER REACTION: For a first order reaction, is the half life independent or dependent of the initial concentration of reactant?
independent
RATE & COLLISION: As reactants get consumed, collisions happen less or more frequently?
less frequently, which results in the reaction rate decreasing
What is Activation Energy (Ea)?
minimum energy necessary to form a product during a collision between reactants
RATE & COLLISION: The higher the concentration of reactant molecules the more or less likely molecules will collide and react, and therefore the faster the reaction rate?
more
Is rate always positive or negative?
positive
RATE LAWS: Rate is directly related to what?
reactant concentration A + B → products Rate = k[A]m[B]n -There are three orders: 1.) m is the order with respect to A 2.) n is the order with respect to B 3.) the overall order of the reaction = m + n Reaction orders are usually positive integers (1, 2...) but can also be fractions, zero, or negative numbers. -The rate constant k is independent of the reactant concentrations, but it does vary with temperature.
Reaction Diagram: Catalyzed vs. Uncatalyzed
see pic
What is the equation for the Arrhenius equation?
see pic
Reaction Mechanism Example
see slide #37 of the old powerpoint
What is the rate law of the elementary reaction: BIMOLECULAR REACTION RATE LAW
}Bimolecular reaction rate law: EX #1: A + B⟶ product(s) rate = k[A][B] EX #2: 2A⟶ product(s) rate = k[A][A] = k[A]2
Chemical Reaction Rates Equations
~ aA + bB ® cC + dD Rate in terms of consumption of A = - "∆[A] " /("∆" t) Rate in terms of consumption of B = - "∆[B] " /("∆" t) Rate in terms of production of C = "∆[C] " /( "∆" t) Rate in terms of production of D = "∆[D] " /( "∆" t) ~All the rates are related by stoichiometry: Rate = - 1/a "∆[A] " /("∆" t) = - 1/b "∆[B] " /("∆" t) = 1/c "∆[C] " /( "∆" t) = 1/d "∆[D] " /( "∆" t) (see slide #4)
SECOND-ORDER INTEGRATED RATE LAW: Explain the second-order integrated rate law
~For A→ products, Rate = k[A]^2 ~Integration with respect to time gives us the equation: (see pic)
ZERO ORDER INTEGRATED RATE LAW: Explain the zero order integrated rate law
~For a zero order reaction: A → Products Rate= k[A]^0=k Integration with respect to time gives us the equation: (see pic)
What is the Kinetic Theory?
~Higher temperatures mean higher kinetic energies. ~The higher the temperature, the larger the fraction of molecules with kinetic energies equal to or greater than the activation energy (Ea). ~With a larger fraction of molecules possessing Ea, a larger fraction of collisions lead to product formation, resulting in a higher reaction rate. -see slide #29 for the pictures of the diagrams (the area under the curve is directly proportional to the number of molecules possessing those energies)
TRANSITION-STATE MODEL: Explain the Reaction Energy Diagram
~Potential energy is plotted on the y-axis ~Reaction path (or extent of reaction) is plotted on the x-axis. ~The reactants changes into an activated complex . ~The state at the activated complex is called a transition state. -see slide #27 for picture of the reaction energy diagram
REACTION RATE AND TEMPERATURE: Thus far we have discussed 1.)The concentration-rate relationship 2.) The concentration-time relationship Now we will look at the temperature-rate relationship
~Reaction rate usually increases with temperature. EX: To cook food more quickly, raise the oven temperature. EX: To slow the reactions that lead to food spoilage, put food in the refrigerator. EX: To really slow the reactions down, put food in the freezer.
FIRST-ORDER INTEGRATED RATE LAW: Note the conditions of when using all of the integrated rate laws (regular and linear)
~The amount of reactant (A) does not always need to be expressed in Molarity (M) as the equations imply. That being said, the amount can also be expressed in: 1.) Mass (g, mg, etc.) or moles. 2.) Number of molecules or atoms 3.) Other conc. units such as g/L 4.) Partial pressure if A is a gas (assuming volume and temperature remain constant).
