International Chemistry Olympiad

IChO 1981

Burgas, Bulgaria · 6 theory problems with complete solutions

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  1. Problem 1: Reaction scheme of a substance A and its compounds (substances A-L)

    The sample A participates in the transformations in scheme 1. Only the products containing A are shown in the scheme 1.

    Figure 1

    Scheme 1

    a) Substance A is a solid and is insoluble in water.

    b) Substances B and I are gases soluble in water.

    c) Substances E, F, J and K are solid and soluble in water.

    d) Aqueous solutions of B, G, H, I, J and K react with F, the products in all cases being E and D.

    e) The following transformations occur during the interaction with an aqueous solution of iodine:

    Figure 2

    Write the chemical equations for the above interactions and balance them.

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  2. Problem 2: Dissociation of maleic acid and the choice of acid-base indicators

    Maleic acid (\(\ce{H2A}\)) is a weak dibasic acid. The correlation between the relative quantities of \(\ce{H2A}\), \(\ce{HA^-}\), \(\ce{A^{2-}}\):

    \[ \alpha_0=\frac{c(\ce{H2A})}{c} \qquad \alpha_1=\frac{c(\ce{HA^-})}{c} \qquad \alpha_2=\frac{c(\ce{A^{2-}})}{c} \]

    and pH values of the solution show that:

    a) \(\alpha_0=\alpha_1\) for pH = 1.92

    b) \(\alpha_1=\alpha_2\) for pH = 6.22

    Find:

    2.1 The values of the dissociation constants of maleic acid for the first (\(K_1\)) and the second (\(K_2\)) degree of dissociation.

    2.2 The values of \(\alpha_0\), \(\alpha_1\), and \(\alpha_2\) for pH = 1.92 and pH = 6.22.

    2.3 What is the value of pH when \(\alpha_1\) attains a maximum value? Find the maximum value of \(\alpha\).

    2.4 Which of the acid-base indicators in the table are suitable for titration of a 0.1 M solution of maleic acid (as a monobasic and as a dibasic acid) with 0.1 M \(\ce{NaOH}\)?

    Fill in the table 1 with the correct answers.

    All the activity coefficients should be considered equal to 1.

    Indicator pH interval
    Methyl green 0.1 – 2.0
    Tropeolin 00 1.4 – 3.2
    β-Dinitrophenol 2.4 – 4.0
    Bromphenol blue 3.0 – 4.6
    Congo red 3.0 – 5.2
    Methyl red 4.4 – 6.2
    Bromphenol red 5.0 – 6.8
    Bromthymol blue 6.0 – 7.6
    Phenol red 6.8 – 8.0
    Cresol red 7.2 – 8.8
    Thymol blue 8.0 – 9.6
    Phenolphthalein 8.2 – 10.0
    Alizarine yellow 10.1 – 12.1
    Tropeolin 0 11.0 – 13.0
    1,3,5-Trinitrobenzene 12.2 – 14.0

    Table 1

    2.1 \(K_1=\)
    \(K_2=\)
    2.2 pH = 1.92 \(\alpha_0=\)
    \(\alpha_1=\)
    \(\alpha_2=\)
    pH = 6.22 \(\alpha_0=\)
    \(\alpha_1=\)
    \(\alpha_2=\)
    2.3 pH =
    \(\alpha_1=\)
    2.4 First indicator equivalence point pH =
    1.
    2.
    3.
    4.
    Second indicator equivalence point pH =
    1.
    2.
    3.
    4.

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  3. Problem 3: Structure of a natural compound X from its reactions

    Compound X has been isolated from a neutral product. Different reagents have been used to establish the structure of X. The following results were obtained:

    Figure 1

    3.1 What conclusions can be drawn on the composition and the structure of X on the basis of the data obtained from each of the above interactions. The conclusions should be formulated in the most concise and clear way. Fill in the table without describing how you reached your conclusions.

    I. ........................................
    II. ........................................
    III. ........................................
    IV. ........................................
    V. ........................................

    3.2 Write the formula of substance X on the basis of the data about the composition and structure obtained in point 1.

    3.3 Write the formulae of substances A, B, C, D, and E and the formula for heptane acid.

    3.4 To what natural substances could this structure correspond? Write the name of the substance and draw the structural formula which best describes its structure properties.

    3.5 Give three properties of this compound that do not correspond to the structure found in point 2.

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  4. Problem 4: Thermochemical cycle for the decomposition of water at moderate temperature

    The thermal decomposition of water

    Figure 1

    can be traced (\(\alpha = 10^{-3}\)) at temperature above 1700 K. This process can be realized at temperatures 800 – 900 K as well as through subsequent stages carried out in a cycle. Suggest such a process on the basis of the reactions:

    Figure 2

    and

    Figure 3

    satisfying the following requirements:

    a) Only water should be consumed during the process.

    b) Oxygen and hydrogen alone should be the end products of the process.

    c) In addition to the above substances, a 25 % ammonia solution is needed for the cycle.

    d) The temperature for each step in the cycle should not exceed 840 K.

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  5. Problem 5: Chlorination of hydrocarbon A: intermediates for nylon and chloroprene rubber

    Compounds B and C are structural isomers. They can be obtained when hydrocarbon A interacts with chlorine. Hydrocarbon A is a basic product of an industrial organic synthesis. It can react with ozone, yielding an ozonide.

    Isomer B can be used for the technical production of compounds D and E that are the initial compounds in the production of the fibre nylon:

    Figure 1

    Compound D is soluble in bases, E in acids.

    The reaction between isomer C and an alcohol solution of an alkaline base yields monomer F which is used for the production of chloroprene (neoprene) rubber

    Figure 2

    This method has a technical application.

    5.1 Write down the structural formulae of A, B, C, D, E, F and their names in the IUPAC nomenclature.

    5.2 Write down the mechanism of the reaction between hydrocarbon A and chlorine. What type of reaction is it in terms of its mechanism? Which of the two isomers is obtained in larger quantities under ordinary conditions?

    5.3 Write down the equations for:

    – the production of D and E from isomer B,
    – the production of monomer F from isomer C,
    – the ozonolysis of hydrocarbon A and hydrolysis of the ozonide.

    5.4 Write down the chemical scheme for an industrial production of hydrocarbon A from the hydrocarbon which is main component of natural gas.

    5.5 For chloroprene rubber, write down the formulae of the possible steric forms of the elementary unit.

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  6. Problem 6: Kinetics of the catalytic decomposition of isopropanol on V2O5

    The catalytic decomposition of isopropanol on the surface of a \(\ce{V2O5}\) catalyst, leading to the products in the scheme, satisfies a first order kinetic equation.

    Figure 1

    Five seconds after initiation of the reaction at 590 K, the concentrations of the components in the reaction mixture are:

    \[ \begin{aligned} c_{\mathrm A}&=28.2\ \mathrm{mmol\,dm^{-3}}\\ c_{\mathrm B}&=7.8\ \mathrm{mmol\,dm^{-3}}\\ c_{\mathrm C}&=8.3\ \mathrm{mmol\,dm^{-3}}\\ c_{\mathrm D}&=1.8\ \mathrm{mmol\,dm^{-3}} \end{aligned} \]

    6.1 What is the initial concentration \(c_0\) of \(\ce{C3H7OH}\) in the system?

    6.2 What is the value of the rate constant \(k\) for the process:

    Figure 2

    6.3 What is the interval of time \((\tau_{1/2})\) in which the concentration of \(\ce{C3H7OH}\) will reach the value \(c=c_0/2\)?

    6.4 What are the values of rate constants \(k_1\), \(k_2\), and \(k_3\)?

    6.5 What are the values of concentrations \(c_{\mathrm B}\), \(c_{\mathrm C}\), \(c_{\mathrm D}\) at \(t=\tau_{1/2}\)?

    The equation describing the concentration changes of A with time \(t\) for the first order reaction has the form:

    \[ c_{\mathrm A}=c_0\exp(-kt) \]

    or

    \[ \log(c_0/c_{\mathrm A})=0.4343\,kt \]

    or

    \[ \ln(c_0/c_{\mathrm A})=kt \]

    Fill in the table with the answers obtained.

    1 \(c_0=\)
    2 \(k=\)
    3 \(\tau_{1/2}\)
    4 \(k_1=\)
    \(k_2=\)
    \(k_3=\)
    5 \(c_{\mathrm B}=\)
    \(c_{\mathrm C}=\)
    \(c_{\mathrm D}=\)

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