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AP Chemistry: Christmas Vacation Homework

AP Problem 1

The postulates of the Bohr model of the hydrogen atom can be stated as follows:

(I) The electron can exist only in discrete states each with a definite energy.

(II) The electron can exist only in certain circular orbits.

(III) The angular momentum of the electron is nh/2where nis any positive integer.

(IV) Radiation is emitted by the atom only when an electron makes a transition from astate of higher energy to one of lower energy.

(a) State whether each of these postulates is currently considered to be correct, according to the

wave mechanical description of the hydrogen atom.

(b) Give the wave mechanical description that has replaced one of the postulates nowconsidered to be incorrect.

AP Problem 2

Answer the following questions regarding light and its interactions with molecules, atoms, andions.

(a) The longest wavelength of light with enough energy to break the ClCl boned in Cl2(g)is

495 nm.

(i) Calculate the frequency, in s1, of the light.

(ii) Calculate the energy, in J, of a photon of the light.

(iii) Calculate the minimum energy, in kJ mol1, of the ClCl bond.

(b) A certain line in the spectrum of atomic hydrogen is associated with the electronic transitionof the H atom from the sixth energy level (n= 6) to the second energy level (n= 2).

(i) Indicate whether the H atom emits energy or whether it absorbs energy during the

transition. Justify your answer.

(ii) Calculate the wavelength, in nm, of the radiation associated with the spectral line.

(iii) Account for the observation that the amount of energy associated with the same

electronic transition (n= 6 to n= 2) in the He+

ion is greater than that associated withthe corresponding transition in the H atom.

AP Problem 3A mixture of H2(g), O2(g), and 2 milliliters of H2O(l)is present in a 0.500 liter rigid container at

25C. The number of moles of H2and the number of moles of O2are equal. The total pressure is

1,146 millimeters mercury. (The equilibrium vapor pressure of pure water at 25C is 24millimeters mercury.)

The mixture is sparked, and H2and O2react until one reactant is completely consumed.

(a) Identify the reactant remaining and calculate the number of moles of the reactant remaining.

(b) Calculate the total pressure in the container at the conclusion of the reaction if the final

temperature is 90C. (The equilibrium vapor pressure of water at 90C is 526 millimeters

mercury.)

(c) Calculate the number of moles of water present as vapor in the container at 90C.

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AP Problem 4

Consider the hydrocarbon pentane, C5H12(molar mass 72.15 g).

(a) Write the balanced equation for the combustion of pentane to yield carbon dioxide and

water.

(b) What volume dry carbon dioxide, measured at 25C and 785 mm Hg, will result from thecomplete combustion of 2.50 g of pentane?

(c) The complete combustion of 5.00 g of pentane releases 243 kJ of heat. On the basis ofthis information, calculate the value of H for the complete combustion of one mole of

pentane.

(d) Under identical conditions, a sample of an unknown gas effuses into a vacuum at twice

the rate that a sample of pentane gas effuses. Calculate the molar mass of the unknowngas.

(e) The structural formula of one isomer of pentane is shown below. Draw the structuralformulas for the other two isomer of pentane. Be sure to include all atoms of hydrogenand carbon in you structures.

AP Problem 5Standard Entropy

Substance J/K mole

N2(g) 45.8H2(g) 31.2

NH3(g) 46.0Ammonia can be produced by the following reaction:

N2(g)+ 3 H2(g)2 NH3(g)

The Gibbs free energy of formation Gfof NH3(g)is -3.94 kilocalories per mole.

(a) Calculate the value for Hfor the reaction above 298K.

(b) Can the yield of ammonia be increased by raising the temperature? Explain.

(c) What is the equilibrium constant for the reaction above at 298K?

(d) If 235 milliliters of H2 gas measured at 25C and 570 millimeters Hg were completelyconverted to ammonia and the ammonia were dissolved in sufficient water to make 0.5000

liter of solution, what would be the molarity of the resulting solution?

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AP Problem 6

a) Discuss briefly the relationship between the dipole moment of a molecule and the polarcharacter of the bonds within it. With this as the basis, account for the difference betweenthe dipole moments of CH2F2and CF4.

b) The boiling points of the following compounds increase in the order in which they are

listed below: CH4< H2S < NH3Discuss the theoretical considerations involved and use them to account for this order.

c) Draw the Lewis electron-dot structures for CO32-, CO2, and CO, including resonance

structures where appropriate.

d) Which of the three species has the shortest C-O bond length? Explain the reason for youranswer.

e) Predict the molecular shapes for the three species. Explain how you arrived at yourpredictions.

AP Problem 7Give a scientific explanation for the following observations. Use equations or diagrams if they are

relevant.

(a) It takes longer to cook an egg until it is hard-boiled in Denver (altitude 1 mile above sealevel) than it does in New York City (near sea level).

(b) Burn coal containing a significant amount of sulfur leads to acid rain.

(c) Perspiring is a mechanism for cooling the body.

(d) The addition of antifreeze to water in a radiator decreases the likelihood that the liquid in the

radiator will either freeze or boil.

AP Problem 8

(a) Solid copper(II) sulfate pentahydrate is gently heated.(i) Balanced equation:

(ii) How many grams of water are present in 1.00 mol of copper(II) sulfate pentahydrate?

(b) Excess concentrated aqueous ammonia is added to a solution of nickel(II) nitrate, leading tothe formation of a complex ion.

(i) Balanced equation:

(ii) Which of the reactants acts as a Lewis acid?

(c) Methylamine (CH3NH2) is added to a solution of hydrochloric acid.

(i) Balanced equation:

(ii) Methylamine dissolves in water to form a solution. Indicate whether this solution isacidic, basic, or neutral.