kinetics - modify te 07 acj h1 ans

Upload: ying-liang

Post on 30-May-2018

217 views

Category:

Documents


0 download

TRANSCRIPT

  • 8/14/2019 Kinetics - Modify TE 07 ACJ H1 Ans

    1/2

    (Chemical Kinetics and Chemical Bonding)

    Suggested Answers1 (a) (i) Comparing Expt 1 and 2:

    Increasing [propanone] by four times causes rate to increase by four times.Reaction is first order with respect to propanone.

    Comparing Expt 1 and 4:

    Increasing [H+] by two times causes the rate to increase by two times.Reaction is first order with respect to H+.

    Comparing Expt 3 and 4:Increasing [iodine] by 8 times did not cause the rate to increase.

    Reaction is zero order with respect to I2.

    Since the concentration of the intermediate is directly affected by theconcentration of propanone and the concentration of intermediate is directlyaffected by the concentration of H+. Thus, the rate equation of the slow stepwhich is first order with respective to the intermediate, will also be first orderwith H+ and propanone.

    This is shown in the slow step of the reaction. Thus, the mechanism isplausible.

    (ii) Rate = k[H+][propanone]

    (iii) Using experiment 1:0.001 = k(0.01)(0.01)k = 10 mol-1 dm3 min-1

    (b) (i)

    Ea (step 3)

    Hr

    CCH3 CH3

    O

    + H+

    CCH3 CH3

    OH+

    CCH2 CH3

    OH

    CIH2C CH3

    O

    + IH

    Ea (slow step)

    Energy

    Reaction Coordinate

    Ea

    (step

    1)

    Note: Only need to indicate Ea of the slow step and the Hr. I have includedthe other Ea so that you can see that step 2 has the highest Ea.Hr= +22 kJ mol

    -1 (step taken: bond broken bond formed)

    (ii) H+ acts as a homogenous catalyst.

    Kwok YL

  • 8/14/2019 Kinetics - Modify TE 07 ACJ H1 Ans

    2/2

    (Chemical Kinetics and Chemical Bonding)

    Kwok YL

    Number of molecules

    Energy

    At temperature, T

    Ea(uncatalysed)Ea(catalysed)0

    H+ provides an alternative reaction pathway with a lower energy barrier. Therefore, there will be more reactant molecules which possess the

    minimum energy for the reaction to occur. Hence, the number of effective collision increase and hence rate of

    reaction increases.

    (c) (i) From the Data Booklet: BrBr, ClCl, and II

    The bond strength is in the order of ClCl being the strongest and IIbeing the weakest.

    This is because as we move from Cl to I, the atomic radius increase.Since formation of covalent bond is due to overlapping atomic orbital.

    Hence, the overlapping of atomic orbitals between two I atoms is less

    effective than the overlapping of atomic orbital between two Cl atoms,since the former is larger than the latter.

    (ii) Melting point trend: Cl2 < Br2,< I2

    Are all simple discrete molecules. Melting these substances wouldrequire the breaking of van der waals forces of interaction betweenthem.

    Since, I2 has the largest electron cloud size, it is the most polarisableand hence its induced dipole moment is the largest. Therefore, its id-idinteraction is the strongest, thus it has the highest melting point.

    Cl2 has the smallest electron cloud size and it is thus the leastpolarisable and hence its induced dipole moment is the smallest.Therefore, its id-id interaction is the weakest, thus the lowest meltingpoint.

    (d) Rate of iodination of propanone occurs as fast as the rate of chlorination of

    propanone. As the reaction occurs in a multiple steps reaction, the rate of the reaction is

    dependent on the activation energy of the slow step. This is because that the breaking of the halogen-halogen bond does not happen

    in the rate determination step. The activation energy for the slow step foriodination of propanone and the chlorination of propanone is the same.

    Thus rate of reaction remains the same despite II being weaker than the Cl

    Cl.