Kinetics of Homogeneous Processes. Individual assignments

5.Compare the rate constants of two first-order reactions for the case when the half-time of the first reaction exceeds by two times the half-life of the second reaction.

6.For the reaction,

С6Н5-С С-СООNa + I2 С6Н5-IC=СI-COОNa, the following experimental kinetic data were obtained:

Determine the order of the reaction and calculate its kinetic constant.

Assignment 15

1.Write down the mathematical expression for the actual rate of a chemical reaction.

2.What methods for determination of the reaction order are related to the group of differential methods?

3.Show schematically the kinetic curves for reactants and reaction products in a zero-order reaction.

4.The rate of the homogeneous reaction, 2 NO (g.) + О2 (g.) 2NO2 (g.) is described by the equation, w = k pNO pO2. What is the order of this reaction.

5.Using the values of the initial (pо) and total (p) pressure of gases in the system, find the expression for the kinetic constant, k, for the gas-phase

reaction, A B + C.

6.For the reaction, С2Н4Сl2 (g.) С2Н3Сl (g.) + НСl (g.), it is determined that k = 3.1 × 10 s–l at 450°C. Calculate the time required for one half of the initial quantity of ethylene dichloride to react. What is the amount of time required for the completion of 90% of the reaction?

Assignment 16

1.Which of the most common technological problems can be solved using the kinetic methods?

2.What experimental data are necessary for determination of the reaction order?

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3.What is the order of a reaction with a kinetic constant of k = 0.1 L/(mol min)?

4.Show graphically the dependence of the concentration of reaction products on time for a first-order reaction.

5.Get the expression for the half-life of a third-order reaction under the condition that the concentrations of reactants are equal.

6.The rate constant of the reaction,

СН3СООС2Н5 + NaOH СН3СООNa + С2Н5OH,

is 5.4 L/(mol s). What amount of ester is consumed in 10 min after the reaction start under the condition that a = b = 0.02 mol/L?

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SECTION II. KINETICS OF COMPLEX CHEMICAL REACTIONS

Assignment 1

1.What are the main kinetic relationships governing simple and complex reactions?

2.The quantum yield of a photochemical reaction is not equal to one. How can it be explained?

3.What chemical reactions are considered as reversible? What is a general formula for the rate of a reversible reaction?

4.Draw the kinetic curves for a first-order parallel reaction (k1 = k2).

5.In the reaction, 2Н2 + О2 2Н2О, the following kinetic equation is determined: w = k [H2] [O2]4/3. What is the total order of this reaction? Explain your answer. Suggest a mechanism for this reaction.

6.In the reaction of the conversion of γ-oxybutyric acid into lactone

the concentration of the latter in different moments of time has been changed as follows:

The initial concentration of acid was 0.1823 mol/L, the equilibrium concentration of lacton, x = 0.1328 mol/L. Determine the kinetic constants of the direct and reverse reactions.

Assignment 2

1.Can it be decided whether the reaction is complex or simple based on variation of the reaction order?

2.What are free radicals?

3.What equations are used for calculation of the rate constants of reversible first-order reactions?

4.Draw the curves of the dependence of the concentration of the substances, A and B, on time for parallel reactions given below in the

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case when k1 > k2.

5.The reaction of the photochemical chlorination of formic acid in a gas phase can be described by the following scheme:

Cl2 + hν 2Cl∙

Cl∙ + НСООН НCl + НООС∙ Cl2 + НООС∙ НCl + СО2 + Сl∙ Cl∙ + wall Cl2 (ads.)

Show the stages of chain initiation, chain propagation, and chain termination.

6.The following experimental data were obtained for the reaction of isomerization of β-oxycrotonic ester into acetoacetic ester at 25°C:

The equilibrium concentration of acetoacetic ester (x ) is 0.288 mol/L. Calculate the rate constants of the direct and reverse reactions.

Assignment 3

1.What postulates are used upon deducing the kinetic equations for reversible and parallel reactions?

2.What is the main quantitative characteristic of photochemical reactions?

3.Write down the kinetic equation for the following reaction:

4.Illustrate graphically the dependence of the concentration of reagents on time for the following consecutive reaction:

kk

А1 В 2 С .

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5.The reaction of the chlorination of tetrachloroethylene in СCl4 can be described by the following scheme:

Cl2 + hν 2Cl∙

Cl∙ + С2Cl4 С2Cl5∙ С2Cl5∙ + Cl2 С2Cl6 + Сl∙

С2Cl5∙ + С2Cl5∙ С2Cl6 + С2Cl4.

Show the stages of chain initiation, chain propagation, and chain termination.

6.Calculate the rate constant of the dissociation reaction of acetic acid considering that the dissociation constant of acetic acid is 1.75 × 10–5 and the kinetic constant of the reverse reaction equals 4.5 × 1010 m–1 s–1.

Assignment 4

1.What is called the rate-limiting (rate-determining) step in complex chemical reactions?

2.What are typical signs evidencing the occurrence of a chain reaction mechanism?

3.What equation is used for the calculation of the rate constants of parallel first-order reactions?

4.Give a schematic representation of the consecutive reaction for the cases when (a) it is comprised of two irreversible first-order reactions and (b) it includes a reversible step or few reversible steps.

5.Prove that the kinetics of the reaction,

6.The rate of the chemical transformation of the substance, A, is characterised by the equation,

d[A] (k1 k2 k3 )[A] . d

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Show that the parity, (k1 + k2 + k3)∙τ = 1, is fulfilled at the time moment, τ, corresponding to a decrease in the concentration of A down to 0.368 from the initial value.

Assignment 5

1.What is meant by the mechanism of a chemical reaction?

2.Which of the chemical reactions are called consecutive? Give examples.

3.The reaction,

CH4 + Cl2 + hν CH3Cl + HCl,

proceeds via a chain mechanism according to the following scheme: Cl2 + hν 2Cl∙

CH4 + Cl∙ СH3∙ + HCl

СH3∙ + Cl2 СH3Cl + Сl∙ СH3∙ + СH3∙ C2H6 СH3∙ + Сl∙ СH3Cl

Сl∙ + Сl∙ Cl2

Show the chain termination steps.

4.Draw the curves of the dependence of the concentrations of A, B, and C on time for the reaction,

5.Write down the kinetic equation for the reaction,

under the stipulation that the initial concentrations of reactants are unequal.

6.The reaction of isopropanol decomposition proceeds in the presence of a catalyst by the following scheme:

.

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If the initial concentration of С3Н7ОН is 44.7 mol/L, and the sum, k1 + k2 + k3, of the rate constants of the parallel reactions equals 0.115 s–1, then what will be the concentration of С3Н7ОН in 4.3 s after the reaction start?

Assignment 6

1.What is understood under the term "fractional reaction order"?

2.What is the core concept of the Semenov-Bodenshtein principle of quasi stationary concentrations? Is the method of quasi stationary concentrations accurate?

3.Illustrate graphically the dependence of the rate of a branched chain reaction on pressure and give its interpretation.

4.The rate of a chemical reaction is described by the equation,

To which type of reactions does this reaction belong? Give its schematic representation.

5.Write down the differential equations of the rate of the change in the concentration of the reactant, A, and the rate of accumulation of the reaction products, B and C, for the following reaction:

6.The quantum yield of the acetone photolysis,

upon the light exposure with a wavelength of 300 nm makes 0.2. If the intensity of the absorbed light equals 10–2 J/s, then how many moles of carbon monoxide is formed per one second?

Assignment 7

1.What values can take the order of a complex reaction?

2.Give the scheme of an unbranched chain reaction.

3.Can the kinetic constant, k1, be determined in the reaction,

,

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by measuring only the current concentrations of A and B?

4.By what kind of an equation is the overall reaction rate described in the case of the following parallel reactions:

k

A 1 B

k

A C 2 B D ?

5. Decomposition of N2O5 proceeds through the following mechanism:

Assuming that the concentration of NO3 is constant, derive the kinetic equation for the decomomposition of N2O5.

6. Fot the reaction,

,

the rate constants, k1 and k2, at 25°С equal 0.00685 and 0.00254 min-1, respectively. If the initial concentration of acid is 0.1823 mol/L, then what fractional amount of the starting material is converted into lactone in 100 min from the reaction start?

Assignment 8

1.What is called "quantum yield of a photochemical reaction"? What range of values can it take?

2.How can complex reactions be classified based on the mutual influence of their separate steps?

3.Illustrate schematically kinetic curves for a first-order reversible reaction.

4.A branched chain reaction of the oxidation of hydrogen includes the following steps:

H∙ + O2 OH∙ + ∙O∙ ∙O∙ + H2 OH∙ + H∙ OH∙ + H2 H2O + H∙

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H∙ + wall 1/2 Н2

Show the steps of chain initiation, chain branching, chain propagation, and chain termination.

5. The rate of a chemical reaction is described by the equation,

d(a x) k1 (a x)(b x) k2 (c x) . d

To which type of chemical reactions does this reaction belongs? Give its schematic representation.

6. For the reaction,

,

the sum of the rate constants is expressed by the formula, k1 + k2 = x/[a (a – x)],

where а is the initial concentration of NO, and (а – х) is its concentration at the time moment, . Under the condition that k1 and k2 equal 25.7 and 18.2 L/(mol s), respectively, determine what amount of nitrogen and nitrous oxide (laughing gas) is formed upon heating nitric oxide (4 mol/L) at 1300 К in 0.1 s after the reaction start.

Assignment 9

1.What is called "the elementary act of a chemical reaction"?

2.What is the most important peculiarity of photochemical reactions?

3.Illustrate schematically the change in the concentrations of the compounds, A, B, and C, along with the lapse of time for the following reaction,

stipulating that k1 > k2.

4.What equation does describe the rate of the reversible reaction,

5.Designating the change in the concentration of phenol via x and the

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initial concentration of phenol and acid via а and b, respectively, write down the rate equation for the nitration reaction of phenol, which proceeds by the following scheme:

.

6.Calculate the rate constants of the direct and reverse reactions for the reversible first-order reaction,

Upon reaching the equilibrium state, 23.2% of ammonium thiocyanate is converted into thiourea.

Assignment 10

1.How can one reveal that a particular chemical reaction has a complex mechanism?

2.Name the reasons, by which chain reactions are initiated.

3.What data are required for the calculation of the k1 value in the reaction,

4.Write down the expression for the rate of the changes in the concentrations of А, В, and С in the reaction,

5.Write down the kinetic rate equation and derive the formula for the calculation of (k1 + k2) for the reaction,

.

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