exercises

Chapter : Organic Chemistry

Sun, 24 Jun 2012 06:57:05 +0100

(i)   A compound D has the molecular formula C2H4O2 and is obtained
      from a reaction between methanoic acid and methanol. Write an
      equation for this reaction and state the name of D.
     …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
(ii)    A second compound, E, has the same molecular formula as D and
       has acidic properties.
       State the name of compound E.
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
Ans: (i)    CH3OH + HCOOH ----> HCOOCH3 + H2O
              for both reactants and [1] for both products
            (accept C2H4O2) methyl methanoate;
       (ii)    ethanoic acid;

_____________________________________________________________________________________________

A student prepared a sample of compound Y from benzene as follows:
    C6H6 → C6H5CH2CH3 → C6H5CHClCH3
                           X                        Y
(a)    (i)    The first step was the conversion of benzene to compound X,
               using chloroethane as the reagent and aluminium chloride as
               a catalyst.Write the equation for the reaction and give equations
               for the mechanism.
            ............................................................................................................................
            ............................................................................................................................

        (ii)    Name the type of mechanism that occurs in the second step when
               X is converted to Y.
              ............................................................................................................................

(b)    (i)    Draw two structures for compound Y, showing the relationship
               between them.

       (ii)    Explain the term plane-polarized light and describe how the optical
            isomers of Y could be distinguished using a polarimeter.
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................

    (iii)    Explain why the sample of Y produced by the student did not show
            optical activity.
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................
            ............................................................................................................................

(c)    Samples of compound Y and chlorobenzene are warmed separately with
        aqueous sodium hydroxide. State, with a reason, whether compound Y
        or chlorobenzene would react more slowly.
            ......................................................................................................................................
            ......................................................................................................................................
            ......................................................................................................................................
            ......................................................................................................................................
(Total 16 marks)

Ans : (a)    (i)    C6H6 + CH3CH2Cl ---> C6H5CH2CH3 + HCl;
                        CH3CH2Cl + AlCl3 ---> CH3CH2+ + AlCl4–;
                        [1] for arrow
                (ii)    free radical substitution;    1
        (b)    (i)    one correct structure;
                       second structure clearly a mirror image;
                (ii)    light in which all waves/vibrations are in one plane;
                        plane-polarized light passed through each isomer (solution);
                        plane of polarization rotates;
                        in opposite directions for each isomer;    4
                (iii)    equal amounts of each isomer were formed/racemic mixture formed;
                        optical activities/rotations cancelled out;    2
        (c)    chlorobenzene;
                nucleophile/OH– repelled by delocalized electrons/carbon atom being
                attacked is less electron-deficient/carbon-chlorine bond is stronger;    2

Chapter : Acids and Bases

Mon, 25 Jun 2012 01:26:34 +0100

(i) Define the terms strong acid and weak acid. Using hydrochloric
    and ethanoic acid as examples, write equations to show the dissociation
    of each acid in aqueous solution.
……………………………………………………………………………………………
    ……………………………………………………………………………………………
    ……………………………………………………………………………………………
Ans :    strong acid completely dissociated/ionized;
            weak acid only partially dissociated/ionized;
            HCl(aq) <----> H+(aq) + Cl+(aq);
            CH3COOH(aq)<---> CH3COO–(aq) + H+(aq);
            Insist on both arrows as shown, state symbols not needed.
            Also accept H2O(1) and H3O+(aq) in equations

(ii) Propanoic acid, CH3CH2COOH is a weak acid.
    (a)    Give the equation for the ionization of propanoic acid in water
            and deduce the expression for the ionization constant, Ka, of
            propanoic acid.
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
    (b)    Calculate the Ka value of propanoic acid using the pKa value in the
            Data Booklet.
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
    (c)    Use your answer from (b) to calculate the [H+] in an aqueous
            solution of propanoic acid of concentration 0.0500 mol dm–3,
            and hence the pH of this solution.
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
           ……………………………………………………………………………………….
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….

Ans : (a)    CH3CH2COOH   CH3CH2COO– + H+ (no penalty for );
                Accept equation with H2O and H3O+.
                Ka = ;
        (b)    1.35×10–5 (mol dm–3)    1
                Units not needed. No penalty for too many sig figs.
        (c)    [H+] =
                       = 8.21 or 8.22×10–4 (mol dm–3);
                Units not needed, ECF from (b). No penalty for too many sig figs.
                pH (= –log[H+] = –log (8.22×10–4) = 3.1 or 3.09 (ECF) penalize
                 more than 4 sig figs;
                If expression for [H+] missing but both answers correct,
                award [3],
                if one answer correct, award [2].

(iii) Predict whether each of the following solutions would be acidic,
    alkaline or neutral. In each case explain your reasoning.
    (a)    0.1 mol dm–3 FeCl3(aq)
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    (b)    0.1 mol dm–3 NaNO3(aq)
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    (c)    0.1 mol dm–3 Na2CO3(aq)
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
Ans : (i)    acidic;
                Fe(H2O)63+ is a weak acid/Fe3+ reacts with OH–/equation
                to show formation of HCl or H+;
                “FeCl3 is acidic” is not acceptable.
        (ii)    neutral;
                NaNO3 / sodium nitrate is formed from strong base and strong
                acid/ions do not hydrolyse;
        (iii)    alkaline;
                As CO32– is weak base/combines with H+/equation showing
                formation of OH–;    2
                Acidic, neutral, alkali mark in each case is independent of reason.

Chapter : Chemical Equilibrium

Sun, 24 Jun 2012 05:47:58 +0100

(i) Consider the following equilibrium reaction.
    2SO2(g) + O2(g) <-----> 2SO3(g)     ∆H = –198 kJ
    Using Le Chatelier’s Principle, state and explain what will happen to
    the position of equilibrium if
    (a)    the temperature increases.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
    (b)    the pressure increases.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Ans : (a)    (position of) equilibrium shifts to the left/towards reactants;
                (forward) reaction is exothermic/∆H is negative/the reverse
                reaction is endothermic/OWTTE;
                Do not accept “Le Chatelier’s Principle” without some additional
                explanation.
        (b)    (position of) equilibrium shifts to the right/towards products;
                fewer gas molecules on the right hand side/volume decreases in
                forward reaction/OWTTE;
                Do not accept “Le Chatelier’s Principle” without some additional
                explanation.

(ii) The equilibrium between nitrogen dioxide (dark brown) and dinitrogen
        tetroxide (colourless) is represented by the following equation.
        2NO2(g) <----> N2O4(g)        ∆H = negative Kc = 1 at 328K
        (a)    Write the equilibrium constant expression, Kc.
           .....................................................................................................................................
                  .....................................................................................................................................
        (b)    State and explain the effect of an increase in temperature on
                the value of Kc.
                  .....................................................................................................................................
                 .....................................................................................................................................
                 .....................................................................................................................................
                 .....................................................................................................................................
        (c)    State and explain the visible change that takes place as a
                result of a decrease in pressure, after equilibrium is
                re-established.
                .....................................................................................................................................
                .....................................................................................................................................
                .....................................................................................................................................
                .....................................................................................................................................
        (d)    Two moles of NO2(g) and two moles of N2O4(g) were placed in an
                empty 1 dm3 container and allowed to come to equilibrium at
                328 K.
                Predict, with reference to the value of Kc, whether the
                equilibrium mixture would contain more or less than two moles of
                NO2(g).
                .....................................................................................................................................
                .....................................................................................................................................
                .....................................................................................................................................
                .....................................................................................................................................
Ans : No ECF throughout this question.
        (a)     1
        (b)     Kc decreases;
                forward reaction is exothermic/∆H is negative/equilibrium
                moves to left/OWTTE;    2
        (c)    (mixture will get) darker/darker than expected;
                equilibrium position moves to the left/towards reactants as
                there is an increase in the number of moles of gas from
                right to left;
        (d)    (equilibrium mixture contains) less (than 2 moles NO2);
                given values make   i.e. too much NO2/OWTTE;

Chapter : Chemical Bonding

Sun, 24 Jun 2012 05:26:22 +0100

(i)    State the shape of the electron distribution around the oxygen atom
       in the water molecule and state the shape of the molecule.
………………………………………………………………………………………….
………………………………………………………………………………………….

(ii)    State and explain the value of the HOH bond angle.
…………………………………………………………………………………………..
…………………………………………………………………………………………..
…………………………………………………………………………………………..
…………………………………………………………………………………………..
Ans : (i)    tetrahedral (accept correct 3-D diagram);
                bent/V-shape/angular (accept suitable diagram);
        (ii)    105° (accept 103 – 106°);
                lone pairs repel each other more than bonding pairs;
                Do not accept repulsion of atoms.

(iii) Explain why the bonds in silicon tetrachloride, SiCl4, are polar,
       but the molecule is not.
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
Ans : bonds are polar as Cl more electronegative than Si;
         Allow “electronegativities are different”
         molecule is symmetrical, hence polar effects cancel out/OWTTE;

(iv)   Which of the following increase(s) for the bonding between carbon atoms
        in the sequence of molecules C2H6, C2H4 and C2H2?
        I.    Number of bonds
        II.    Length of bonds
        III.    Strength of bonding
        A.    I only
        B.    I and III only
        C.    III only
        D.    I, II and III
Ans: B

Chapter : Periodic Table

Sun, 24 Jun 2012 05:18:27 +0100

Explain the following statements.

           (a)    The first ionization energy of sodium is
                (i)    less than that of magnesium.
                    …………………………………………………………………………………
                    …………………………………………………………………………………
                    …………………………………………………………………………………
                    …………………………………………………………………………………
               (ii)    greater than that of potassium.
                    …………………………………………………………………………………
                    …………………………………………………………………………………
        (b)     The electronegativity of chlorine is higher than that
                    of sulfur.
                    ……………………………………………………………………………………….
                    ……………………………………………………………………………………….
                    ……………………………………………………………………………………….
                    ……………………………………………………………………………………….
Ans :
(a)
(i)    Na has lower nuclear charge/number of protons;
electrons being removed are from same energy level/shell;
or Na has larger radius/electron further from nucleus;
Award this mark if both electron arrangements are given.

(ii)    Na electron closer to nucleus/in lower energy level/
Na has less shielding effect;
Allow counter arguments for Mg in (i) and K in (ii).

(b)    chlorine has a higher nuclear charge;
attracts the electron pair/electrons in bond more strongly;

The compounds Na2O, Al2O3 and SO2 respectively are

        A.    acidic, amphoteric and basic.
        B.    amphoteric, basic and acidic.
        C.    basic, acidic and amphoteric.
        D.    basic, amphoteric and acidic.
        (Ans : D )

Chapter: Atomic Structure

Sun, 24 Jun 2012 05:19:49 +0100

The element vanadium has two isotopes, 51V and 50V and a relative atomic mass of 50.94.

            (a)    Define the term isotope.
                    ……………………………………………………………………………………….
                    ……………………………………………………………………………………….
            (b)    State and explain which is the more abundant isotope.
                    ……………………………………………………………………………………….
                    ……………………………………………………………………………………….
            (c)    State the name and the mass number of the isotope relative
                    to which all atomic masses are measured.
                    ……………………………………………………………………………………….

The diagram below (not to scale) represents some of the electron energy levels in the hydrogen atom.

              ________________________________   n =
              ________________________________   n = 6
            _________________________________   n = 5

            _________________________________   n = 4

            ________________________________   n = 3

              ________________________________   n = 2

              ________________________________   n = l

            (i)       Draw an arrow on the diagram to represent the electron
                    transition for the ionization of hydrogen.
                      Label this arrow A.
            (ii)      Draw an arrow on the diagram to represent the lowest
                      energy transition in the visible emission spectrum.
                      Label this arrow B.
                      (Total 4 marks)

Answer : (i)    line starting at n = 1;
                    line finishing at n = ∞ (not above ∞);
                    upward arrow;    2 max
                    3 correct [2], 2 correct [1].
              (ii)    Line from n = 3 to n = 2;
                    Arrow pointing downward (in any transition);    2

Which statement is correct for the emission spectrum of the hydrogen atom?

        A.    The lines converge at lower energies.
        B.    The lines are produced when electrons move from
                lower to higher energy levels.
        C.    The lines in the visible region involve electron transitions into
                the energy level closest to the nucleus.
        D.    The line corresponding to the greatest emission of energy
                 is in the ultraviolet region.
        (Ans : D )

Which types of bonding are present in CH3CHO in the liquid state?

        I.    Single covalent bonding
        II.    Double covalent bonding
        III.    Hydrogen bonding
        A.    I and II only
        B.    I and III only
        C.    II and III only
        D.    I, II and III
        (Ans: A )

(a) State a physical property that is different for isotopes of an element.

                    .......................................................................................................
        (b)    Chlorine exists as two isotopes, 35Cl and 37Cl. The
                relative atomic mass of chlorine is 35.45.
                Calculate the percentage abundance of each isotope.
                    .....................................................................................................................................
                    .....................................................................................................................................
                    .....................................................................................................................................
                    .....................................................................................................................................
Ans : (a)    mass/density/for gases: rate of effusion or diffusion/
                melting point/boiling point
                Do not accept mass number.
        (b)    if 35Cl = x, then (x = 35.00) + (1 – x) 37.00 = 35.45
                Award [1] for set up.
                therefore, x = 0.775;    2
                35Cl = 77.5% and 37Cl = 22.5%;
                (need both for mark);

Chapter : Matter

Mon, 25 Jun 2012 01:27:04 +0100

Naturally occurring iridium, Ir is composed of 2 isotopes 191IR and 193IR in the ratio 5:8. The relative mass of 191IR and 193IR are 191.02 and 193.025 respectively. Calculate the relative atomic mass or iridium.

(Ans : 192.254)

In a certain experiment, 14.6g of SbF3 was allowed to react with CCl4 in excess. After the reaction was finished, 8.62g of CCl2F2 was obtained.
           3 CCl4     +     2 SbF3 ----> 3 CCl2F2    +    2 SbCl3

     [ Ar Sb = 122, F = 19, C= 12, Cl = 35.5 ]

a) What was the theoretical yield of CCl2F2 in grams ?
b) What was the percentage yield of CCl2F2 ?

      Ans : a) 11.6 g                      b) 74.31 %