Ch. 14 & 21 nuclear chem. & kinetics              test                 [80 points]                               AP Chem

 

For problems involving calculations, show your work in an organized manner, include any relevant equation (or formula and conversion factor(s)), put the proper units in your calculations / answer, and have the proper number of significant figures in your answer.  Clearly identify the problems done and do them in sequence and your answer must begin and end on the same page of paper; otherwise, you will lose points.  attach this sheet to the front of your exam.

 

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1.   The half-life for the process:  238U à  206Pb is 4.5 * 109 years.  A mineral sample contains 50.0 mg of 238U and 24.0 mg 206Pb.  There was 10.0 mg 206Pb prior to any 238U decay.  What is the age of the mineral ?  [10 points; based on weekly quiz & HW 10.39]

 

2.  Based on the following hypothetical experimental data, determine the rate law (include the value & units of the rate constant) for the reaction:  N2 +  H2  à  NH[15 points;  based on weekly quiz]

 

Experiment

[N2]  (M)

[H2]  (M)

Δ [NH3 ] / Δt  (mM / s)

1

1.0

1.0

 2.0

2

4.0

1.0

 4.0

3

9.0

2.0

 12.0

 

3.  For the reaction:  A  à  B, derive the expression for the half-life in a __ order reaction, then sketch / label a graph of A versus time to illustrate if the half-life is a constant, decreases, or increases as a function of time (show / label the half-life in your graph).  [30 points]

 

a.  1st 

b.  2nd

c. zeroth

 

4.  Describe / rationalize the experimental data needed and its subsequent analysis, including the graphical analysis, to determine the activation energy of a reaction,.  [15 points]

 

5.  old topic:  [10 points; hints:  recent email & thermo lab]

 

The Born-Haber cycle [figure 8.4 in your textbook; ch. 8.2 ionic bonding] can be used to determine the lattice energy; it’s the energy, where a solid ionic compound dissociates into its ions (in the gas phase), e.g.

 

NaCl (s)  à 

 

Determine the lattice energy of NaCl based on the following information [hints:  definition of Δ Hf , ionization energy, electron affinity, and pay attention to the phases:  g, l, s, aq]

 

Δ Hf (NaCl(s)) = - 411 kJ

 

Δ Hf (Na(g)) = 108 kJ

 

Δ Hf (Cl(g)) = 122 kJ

 

1st ionization energy of (gaseous) Na = 496 kJ

 

Electron affinity of (gaseous) Cl (atom) = - 349 kJ