Practice term test #2

1. [5]  When heated, diaminoselenium, (NH₂)₂Se, decomposes  according to the equation

                (NH₂)₂Se(s)  -->  1/8Se₈(s)    +    N₂(g)    +   2H₂(g)

A sample of solid diaminoselenium, (NH₂)₂Se(s), was placed in an evacuated cylinder and heated to 300 degrees C, resulting in the formation of a mixture of molecular nitrogen, elemental selenium and molecular hydrogen gas. After twenty minutes of heating at 300 degrees C, the reaction reached equilibrium and the total pressure climbed to a maximum value of 8.44 atmospheres. Some solid diaminoselenium was still present in the cylinder at equilibrium. Use the information given to calculate K_p for the diaminoselenium decomposition reaction at 300 degrees C.

p(total) = 8.44 atm = p(N2) + p(H2)

Let x = p(N2). Then, from the balanced reaction, since two moles of H2 form for every mole of N2 (which means that p(H2) = 2p(N2) ), it must be that

i) p(N2) = x (already said this!)

ii) p(H2) = 2x

iii) since p(total) = p(H2) + p(N2), then p(total) = 3x

p(total) = 8.44 atm = p(N2) + p(H2) = 3x

x = 2.813 atm

K_p =4 * (2.813)³ = 89.0

2. [6]   At elevated temperatures, potassium perchlorate decomposes to produce potassium hypochlorite and molecular oxygen gas. A 0.8988 g sample of  impure potassium perchlorate was heated until the production of molecular oxygen gas ceased.  A total 48.8 mL of molecular oxygen gas were collected in a buret over water at a temperature of 22.0 ^oC and a barometric pressure of 734 mm Hg. A water column 2.23 cm high was observed inside the buret.

a)      Write a balanced equation for the decomposition reaction.

KClO_{4 (s)} --> KClO (s) + 3/2 O₂ (g)

b)      Calculate the mass percent of potassium perchlorate in the original sample.

p (total) = p(O2) + p(wc) + p(H2O)

734 mmHg = p(O2) + 1.64 mmHg + 19.8 mmHg  

 (since p(wc) = pressure due to water column = height in millimetres divided by specific gravity of Hg)

so p(O2) = 712.6 mmHg

Using PV=nRT, where P= 712.6 mmHg / 760 mmHg per atm, T = 295 K, R = 0.0206 L atm/K mol, and V = 0.0488 L we get that the moles of dioxygen, n= 1.89 X 10^-3

Multiply by 2/3 to get the moles of potassium perchlorate this came from, and then multiply by the formula mass (138.547 g/mol) to get the mass of potassium perchlorate, 0.174 g.

Mass % = ( 0.174 g / 0.8988 g ) X 100% = 19.4 %

3. [4]   A compressed gas cylinder contains 1.00 x 10³ grams of Ar and 8.20 x 10² grams of Ne. The pressure inside the cylinder is 148.8 atm. The internal volume for the cylinder is 17.8 L.

 a)    Calculate the partial pressure for the Ar gas in the cylinder.

Using the atomic mass for the monatomic gas, argon, 1000 g of Ar is 25.03 mol. Similarily there are 40.63 mol of Ne in 820 g of the gas.

p(Ar) = (mole fraction of Ar) * total pressure

= ( 25.03 mol / (25.03 mol + 40.63 mol) ) (148.8 atm) = 56.7 atm

b)  What is the temperature of the gaseous mixture in the cylinder?

T = PV / nR = (148.8 atm)(17.8 L) / [ (25.03 mol + 40.63 mol) (0.08206 L atm/K mol) ] = 492 K

4. [3]  Which of the following three p-block atoms, P, S or Cl , has

             a)  the smallest first ionization energy?

S, because the fourth p-electron is in a doubly occupied orbital, so it is repelled by the oother electron in that orbital (phosphorus has three p-orbitals, all in singly occupied orbitals)

             b)  the smallest radius?

Cl, because it has the highest atomic number, or most protons, within the period

            c)  the largest effective nuclear charge. (Be sure to justify your answers)

Cl, for the same reason as above

5. [3]  Give the four quantum numbers for each electron in the valence shell

       of the N²⁺ ion.

N2+ : 2s²2p^{1 so n=2 l=0 ml = 0 and m_s = +/- 1/2}

For the p electron there are several allowable combinations. Here's one:

n=2 l=1 m_l=-1 m_s=+1/2

6. [3]  Which transition-metal tri-cation has the electronic configuration [Xe]₅₄4f¹⁴5d⁸ ?  How many electrons are left unpaired in this ion?

Au(III), 2 unpaired

7. [6]   Write down the complete electron configurations for the following neutral atoms.  Predict the three most stable ions/oxidation states for each atom (apart from the zerovalent state) and classify each ion/oxidation state according to its type. Lastly, give the abbreviated electronic configuration for each ion/oxidation state.

i)  Selenium

ii)  Antimony

8. [13]  Fill in each blank with the appropriate answer. 

i)  Which element has the higher first ionization energy:  N or O? N

ii)  The anion with the smallest radius is F-

iii)  The salt MnCl₂ is  _____________________ (diamagnetic/paramagnetic)

iv)  The Tc³⁺ ion has  4 unpaired electrons in its valence shell.

v)  Apart from Group 2 metals and transition metals, Group 14 (or 12) main-group metals can form stable 2+ ions. 

vi )  Hund’s rule states that the lowest energy electron configuration for an atom is the

         one that has the maximum multiplicity, or number of unpaired electrons.

vii)  As we go down a particular vertical group in the periodic table, the effective

         nuclear charge stays approximately constant.

viii)  What is the generalized valence shell electron configuration common to all of the

          halogens?  ns²np⁵

  ix)  Identify the neutral element that is isoelectronic with the anion 

         As^3- Kr

x)      Provide the appropriate value for the missing quantum number :

n = 3, l = 0, m_l = _____?, m_s = -1/2.  Zero! It's an s orbital since l=0 so ther's no other choice.

xi)   Max Planck found that energy can only be gained or lost in whole number multiples of hv (read v as the Greek 'nu')

 xii)  The n = 3 shell of the hydrogen atom contains zero f-orbitals.

xiii)  Which of the following isoelectronic ions has the largest radius?: 

            La³⁺, I^- or Te^2-

9. [3]   An unknown p-block element has the general valence shell electron configuration

            ns²np³.

a)      Would this element have a greater or smaller ionization energy than fluorine?

smaller since, being in group 15 it's effective nuclear charge is less

b)      What is the formula of the compound this element would form with Mg?

(represent the p-block element with the symbol A)

A^3- + Mg²⁺ --> Mg₃A₂

c)      Would this element have a larger or smaller radius than Ba?

smaller - the biggest group 15 element is Bi, and it's smaller than Ba because it has a higher effective nuclear charge

Bonus Question (2 marks):

A beam of electrons and a beam of protons are moving with the same speed.  Which has the longer de Broglie wavelength?  Why?

Electrons are less massive than protons, so electrons have a lower frequency. Lower frequency means longer wavelength. The answer is electrons.