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Chemistry – Ionic Compounds

Unit V Problem Solving

In the ionic compounds LiF, NaCl, KBr, and RbI, the measured cation–anion distances are 2.01 Å (Li–F), 2.82 Å(Na-Cl), 3.30 Å (K–Br), and 3.67 Å (Rb–I), respectively.

1. Predict the cation-anion distance of each ionic compound above using the values of ionic radii given in Figure 7.7 on page 257 in the textbook. To receive full credit, you must show how you predicted the distance for each molecule by providing a step-by-step solution for each molecule. The solution for each molecule is worth 8 points; thus, a total of 32 total points is possible for this question.

2. Calculate the difference between the experimentally measured ion-ion distances provided at the beginning of this assignment and the ones you have predicted in problem 1. You must show how you calculated the difference for each molecule by providing a step-by-step solution for each molecule to get full credit (6 points for each molecule; total possible points for this section is 24 points). Assuming that we have an accuracy of 0.04Å in the measurement, please indicate for each molecule if your predicted ion-ion distances are accurate or not accurate with respect to their experimental ion-ion distances (2 points for each molecule; total possible points for this section is 12 points). A total of 32 total points is possible for this question.

3. What estimates of the cation-anion distance would you obtain for these four compounds using their bonding atomic radii? (Note: these values are also in Figure 7.7 but are different from the ionic radii.) For each of the four compounds, calculate the difference between the predicted ionic bond distance, which you calculated in problem 1, and predicted atomic bond distances you calculated in this problem. To receive full credit, you must show how you calculated the difference for each molecule by providing a step-by step solution for each molecule (6 points for each molecule; total possible points for this section is 24 points). Again, assuming that we have an accuracy of
0.04Å in the measurement, indicate for each molecule if your predictions are less or more accurate using the
atomic radii prediction method; explain your answer (2 points for each molecule; total points possible for this
section is 8 points). Using your knowledge of atomic and ionic radii trends in the periodic table, why might one prediction method be more appropriate to use than the other in certain molecules (worth 4 points)? A total of 36 points is possible for this set of questions.

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