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  • Laboratory Manual Physical Chemistry Year 3

    PHYSICAL CHEMISTRY YEAR 3 LABORATORY MANUAL Table of Contents General Information Report Writing

    Experiment

    1. Bomb Calorimetry

    2. Molecular Spectroscopy

    a. IR Spectroscopy

    b. NMR Spectroscopy

    c. UV Spectroscopy

    3. The Critical Point

    4. Determination of Acid Dissociation Constant For Methyl Red

    5. Phase Diagram of A Three Component Partially Immuscible Liquid

    System

    6. The Hydrolysis of Tert Butyl Chloride

    7. Influence of Elongation on Surface Activity For Normal Aliphatic

    Alcohol Chain

    8. Determination of Vapour Viscocity

  • Laboratory Manual Physical Chemistry Year 3

    EXPERIMENT 1: BOMB CALORIMETRY

    Principles of Calorimetry

    Calorimetry is concerned with determining experimentally the enthalpy change H or

    the energy change E accompanying a given isothermal change in state of a system,

    normally one in which a chemical reaction occurs. The reaction at temperature T can

    be written schematically in the form

    state final state initial

    )()( )()(TDTCTBTA

    (1)

    In practice it is not necessary to actually carry out the change in state isothermally

    because H and E are independent of the path. In caloritmetry it is usually

    convenient to use a path composed of two steps:

    Step I. A change of state is carried out adiabatically in the calorimeter vessel to yield

    the desired products but in general at another temperature :1T

    )1()1()1()()()( TSTDTCTSTBTA (2)

    where S represents those parts of the system (e.g., inside wall of the calorimeter vessel,

    stirrer, thermometer, solvent) that are always at the same temperature as the reactants

    or products because of the experimental arrangement; these parts, plus the reactants or

    products constitute the system under discussion.

    Step II. The products of Step I are brought to the initial temperature T by adding heat

    to (or taking it from) the system:

    )()()()()()( 111 TSTDTCTSTDTC . (3)

    If heat capacity data are available, it is not necessary to carry out this step in actuality.

    By adding Eqs (2) and (3) to obtain Eq (1), it is seen that these two steps describe a

    complete path connecting the desired initial and final steps. Accordingly, H or E for

    the change in state (1) is the sum of values of this quantity pertaining to the two steps:

    IIHHH 1 (4a)

    IIEEE 1 (4b)

  • Laboratory Manual Physical Chemistry Year 3

    Because there is no change in volume of the system, and hence no work is done, and

    heat q for Step I being zero,

    01 pqH (constant pressure) (5a)

    01 vqE (constant volume) (5b)

    Thus, if both steps are carried out at constant pressure,

    IIHH (6a)

    and if both steps are carried out at constant volume

    IIEE (6b)

    Whether the process is carried out at constant pressure or at constant volume is a

    matter of convenience. Combustion reactions are conveniently carried out at constant

    volume in a bomb. H and E differ only in the pressure-volume term ),(PV viz

    )(PVEH (7)

    When all reactants and products are condensed phases, the )(PV term is negligible,

    but when gases are involved, as in the case of combustion, it is likely to be significant in

    magnitude. Eq. (7) may be rewritten in the form

    )( nRTEH (8)

    Where n is the increase in the number of moles of gas in the system.

    Step II can be carried out by adding heat to the system or taking heat away from the

    system and measuring q for this process. However, if the heat capacity of the system is

    known or can be determined, IIH or IIE can be calculated, making use of the

    temperature change )( 1 TT resulting from Step I.

    dTSDCCHT

    TpII )(

    1

    (9a)

    dTSDCCET

    TvII )(

    1

    (9b)

    An indirect method of determining the heat capacity is to carry out another reaction

    altogether, for which the heat of reaction is known, in the same calorimeter under the

    same conditions. This method depends on the fact that in most calorimetric

    measurements on chemical reactions the heat capacity contributions of the actual

    product species (C and D) are very small or negligible, in comparison with the

  • Laboratory Manual Physical Chemistry Year 3

    contribution due to the parts of the system denoted by the symbol S. In a bomb

    calorimeter experiment the reactants or products amount to a gram or two, while the rest

    of the system is equivalent to about 3000 g of water. Thus the value of C(S) can be

    calculated from the heat of the known reaction and the temperature change T

    produced by it, as follows:

    T

    )( knownH

    SC p (constant pressure) (10a)

    T

    )( knownE

    SCv (constant volume) (10b)

    Experiment The detailed information for performing this bomb calorimeter experiment is provided

    with the apparatus.

    (a) Determine the water equivalent of the bomb calorimeter with benzoic acid.

    (b) Determine the heats of combustion of anthracene OR phenanthrene. Repeat

    the experiment if necessary. Hence calculate the standard enthalpy of

    formation and compare with the literature values.

    References

    (1) Finlay, Practical Physical Chemistry, 8th Ed.

    (2) Mahler-Cook Bomb Calorimeter Manual

    Shoemaker and Garland, Experiments in Physical Chemistry, 2nd Edition, McGraw-Hill, 1962.

  • Laboratory Manual Physical Chemistry Year 3

    EXPERIMENT 2: MOLECULAR SPECTROSOPY

    (A) Infra-Red Spectroscopy

    This experiment is designed to familiarize you with certain techniques, features and

    applications of infrared spectroscopy. Specifically, this experiment deals with

    intermolecular hydrogen-bonding of phenol in solution.

    Intermolecular Hydrogen-bonding of Phenol

    General

    An O-H bond normally gives rise to a sharp absorption band in the region 3500-3700 cm-

    1. If, however, the hydroxyl group takes part in a hydrogen bond, the O-H resonance is

    shifted to a lower frequency, considerably intensified in terms of band area, and

    considerably broadened. In cases of intermolecular hydrogen bonding in suitable

    solutions an equilibrium may exist between molecules with free hydroxyl groups and

    those with bonded hydroxyl groups. The spectra of the two (or more) types of molecule

    will be superimposed, giving both sharp O-H absorptions and broader O-H absorptions

    at lower frequency. Dilution breaks up the hydrogen bonds, and, relatively speaking the

    sharp peak will increase in intensity while the hydrogen bonding.

    The intensity of an absorption may be defined by a molar extinction coefficient, , based

    on measurements of peak heights (or, more strictly, on integrated band areas). If I is

    the transmitted intensity of radiation in the absence of the absorption band, but I the

    intensity transmitted at the band absorption maximum, then is given by

    )/(log10 IIlC .. (1)

    Where C is the concentration of the solution in moles per litre and l is the path length in

    cm.

    Suppose an equilibrium exists for phenol in solution in CC14 of the type (N.B. this is but

    one of several possibilities):-

  • Laboratory Manual Physical Chemistry Year 3

    2 2 (PhOH)PhOH

    We will refer to the left-hand side species as free phenol and the right-hand side species

    as bonded phenol. The sharp O-H band at ca. 3620 cm-1 could be assumed to be due

    solely to free phenol. For a given total concentration of phenol only a proportion of the

    molecules will be free. Dilution increases this proportion and in theory at infinite dilution

    all the molecules will be free. If we knew the true extinction coefficient, of the O-H

    absorption of free phenol we could calculate the concentration, fC , of free phenol at

    any total phenol concentration, C. To obtain, we could measure apparent extinction

    coefficients, a , at several concentrations by means of equation (2).

    )/(log1

    10 IIlC

    a .. (2)

    And then extrapolate to C = 0. It is then possible to obtain values of fC in any solution

    by means of (3)

    010 /)/(log1

    af CIIl

    C

    .. (3)

    It can be seen that the path-length, l , does not affect the final resu

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