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Transcript of Practice Exemption Chem 120 - Wellesley 2007-05-18¢  Microsoft Word - Practice Exemption...

  • Practice Questions

    Placement Exam for Exemption from Chemistry 120

  • Potentially Useful Information Avogadro's number = 6.0221420×1023 h = 6.6260688×10−34 J s c = 2.9979246×108 m/s 1amu = 1.6605387×10−27 kg 1MeV = 1.60217646 ×10−13 J 1Å=10−10 m

    c λν= E hν=

    2E mc=

    v h h p m

    λ = =

    ( )( ) 4 hx p π

    ∆ ∆ ≥

    1 2

    2m hv = −ν φ

    18

    2

    2.179 10 J nE n

    −− × =

    ∆E q w= +

    VPw ∆−=

    H E PV= + q H= ∆

    ( )q mc T= ∆

    ( )q C T= ∆

    no

    p c o

    c RTK K P

    ∆ ⎛ ⎞

    = ⎜ ⎟ ⎝ ⎠

    ∆ ∆

    S H Tsurr

    sys= −

    G H TS= −

    ∆ ∆ ∆G H T S= − G G RT P= + ln ∆ ∆G G RT Q= + ln

    ∆G RT K= − ln

    ln K H RT

    S R

    = − + ∆ ∆

    K = 273.15 + °C R = 8.314 J mol−1K−1 = 0.0821 liter atm mol−1K−1 = 1.987 cal mol−1K−1 1 atm = 760 Torr PV nRT= aA +bB cC +dD→

    rate a

    d A dt c

    d C dt

    = −

    = 1 1[ ] [ ]

    [ ] [ ]x yRate k A B=

    1 1

    0[ ] [ ]A A kt− =

    0ln[ ] ln[ ]A kt A= − +

    0[ ] [ ]A kt A= − +

    t k A1 2 0

    1 / [ ] =

    t A k1 2

    0

    2/ [ ]

    =

    t k1 2

    0 693 /

    . =

    k A e E

    RT a

    = −

  • E IR=

    QI t

    = 1=96, 485.34 C mole−ℑ

    ∆G RT K= − ln

    o o o cell cathode anodeE E E= −

    elecw QE= −

    elecw nFE= −

    rev, elecw G= ∆

    cellG nFE∆ = − o cell ln

    RTE K nF

    =

    o cell cell ln

    RTE E Q nF

    = −

    o

    cell cell 0.0592V logE E Q

    n = −

    p logK K= −

    +pH log[H ]= − pOH log[OH ]−= −

    wp pH + pOH 14K = =

    w h

    a

    = KK K

    0 a

    0

    [anion]pH = p +log [acid]

    K ⎛ ⎞ ⎜ ⎟ ⎝ ⎠

    u RT Mrms

    = 3

    E RT= 3 2

    A A TP X P=

  • Question 1 (a) The gas in interstellar space consists primarily of hydrogen atoms at such low densities that

    extremely high quantum states can be attained. In particular, transitions from the n=110 to n=109 for the hydrogen atom have been detected. Calculate the wavelength of the light emitted when an electron undergoes such a transition.

    (b) What information is provided by a diagram such as the one below?

  • Question 2 Use Lewis structures and VSEPR to predict the shapes and bond angles of the following chemical species. In addition, specify whether the molecule or ion will possess a dipole moment. Indicate the formal charges, if any, on both Lewis structures. The central atom is underlined in each case.

    (i) CS2

    (ii) SeF4

  • Question 3 The study of carbon-containing compounds and their properties is called organic chemistry. Besides carbon atoms, organic compounds also contain hydrogen, oxygen, and nitrogen atoms (as well as other types of atoms). A common trait of simple organic compounds is to have Lewis structures where all atoms have a formal charge of zero. Consider the following incomplete Lewis structure of an organic compound called histadine (one of the amino acids, which are the building blocks of proteins found in human bodies):

    NC

    C N C

    H

    H

    C C

    H

    H N

    H

    H C O

    H H

    O H

    (a) Complete the Lewis structure for histidine in which all atoms have a formal charge of zero.

    (b) What is the hybridization of the carbon atom connected to two oxygen atoms?

  • Question 4 When one electron is added to an oxygen molecule, a superoxide ion (O2−) is formed. The addition of two electrons gives a peroxide ion (O22−). Removal of an electron from O2 leads to O2+.

    (a) Construct a molecular orbital energy level diagram for O2.

    (b) Give the molecular electron configuration for each of the following species: O2+, O2, O2−, and O22−.

    (c) Give the bond order of each species

    (d) Predict which species are paramagnetic

    (e) Predict the order of increasing bond dissociation energy among the species.

  • Question 5 (a) State Le Chatelier’s principle

    (b) Graphite (a form of solid carbon) is added to a vessel that contains CO2(g) at a pressure of

    0.824 atmosphere at a certain high temperature. The pressure rises due to a reaction that produces CO(g). The total pressure reaches an equilibrium value of 1.366 atm. (i) Write a balanced equation for the process. (ii) Calculate the equilibrium constant.

  • Question 6 A student who has but a superficial knowledge of thermodynamics comes to you with the following question:

    The equilibrium constant Kp for the reaction 2 H2 (g) + O2 (g) 2H2O (g)

    is 5 x 1041 at 25 °C. I learned in class that a very large equilibrium constant indicates that the reaction overwhelmingly favors the formation of product. However, despite this fact, I know that a mixture of hydrogen and oxygen gases can be kept at room temperature, 25 °C, without producing any detectable amount of water. I am confused! Please explain.

  • Question 7 (a) (5 points) Discuss the validity of the following statement: “A bimolecular reaction is often a

    second order reaction while a second order reaction is always bimolecular.” (b) (5 points) One of the very complex set of reactions occurring inside hot internal combustion

    engines is the reaction of NO2 with CO as shown below:

    NO2 + CO → NO + CO2

    The experimental rate law for this reaction is: Rate of disappearance of NO2 = k [NO2]2 Is the above reaction an elementary reaction? Explain

    (c) The first step in a two-step reaction mechanism proposed for the above reaction is shown below:

    NO2 + NO2 → NO3 + NO

    What is the second step? (Hint: consider the overall reaction) Which step will be rate limiting?

  • Question 8 The decay of radioactive nuclides with known half-lives enables geochemists to measure the age of rocks from their isotopic compositions. In your excavations around Paramacium pond, suppose you find a uranium bearing rock that has remained geologically unaltered until the present time. The 238U in the rock decays with a half-life of 4.51 × 109 years to form a series of short-lived intermediates, ending in the stable lead isotope 206Pb. If you find that the ratio of the abundance of 206Pb to 238U is 0.361, determine the time elapsed since the rock was originally formed.

  • Question 9 Determine the equilibrium constant for the reaction HPO42− +HCO3− PO43−+ H2CO3 Identify the stronger Brønsted-Lowry acid and the stronger Brønsted-Lowry base.

  • Question 10

    Consider the reaction between permanganate ion (MnO4−) and Fe2+ to form Mn2+ and Fe3+ in acid solution.

    a. Write balanced half reactions and a balanced overall reaction for this process.

    b. Identify the species being oxidized and the oxidizing agent.

    c. Diagram an electrochemical cell in which you might study this reaction.

    d. Will permanganate spontaneously oxidize Fe2+ under the following conditions, at 298K?

    [Mn2+] = 1×10−6 M

    [MnO4−] = 0.01 M

    [Fe2+] = 1×10−3 M

    [Fe3+] = 1×10−6 M

    pH = 4.0

    Be sure to show all of your calculations and explain your results.

  • Question 11

    A volume of 50.00 mL of a monoprotic weak acid of unknown concentration is titrated with a 0.100 M solution of NaOH. The equivalence point is reached after 39.30 mL of NaOH solution has been added. At the half-equivalence point (19.65 mL), the pH is 4.85. Calculate the original concentration of the acid and its ionization constant Ka.

  • Question 12 Suppose a layer of chromium 3.0 × 10−3 cm thick is to be plated onto an automobile bumper with a surface area of 2.0 × 103 cm2. If a current of 250 A is used, how long must current be passed through the cell to achieve the desired thickness? As current is passed through the cell, chromium in chromic acid (H2CrO4) is reduced to elemental chromium. The density of chromium is 7.2 g cm−3.

  • QUESTION 13 Consider three identical flasks filled with different gases.

    Flask A: CO at 760 torr and 300 K Flask B: N2 at 250 torr and 300 K Flask C: H2 at 100 torr and 300 K

    (a) In which flask will the molecules have the greatest average kinetic energy (per molecule)? Justify your answer

    (b) In which flask will the molecules have the greatest root mean square velocity? Justify your

    answer It is not necessary to do a calculation.

    (c) On a single graph, sketch three curves depicting the Maxwell-Boltzmann distribution for the molecules in flasks A, B, and C. Make sure you properly label the axes.