Ionic Compounds and Lattice Energy

Transcription

1 Ionic Compounds and Lattice Energy Directions for Class Activities: 1. A reader will be assigned for each group. This person will read each question aloud. 2. The group should discuss what the answer should be. If no consensus is reached, raise your hand for the instructor s guidance. 3. When a consensus is reached, everyone should write down the answer. Make sure everyone is writing very similar words. 4. This packet will be given to the person on your right. You will receive a packet from your left. This rotation occurs after every question (Q) to ensure your group is on the same problem. This is why it is important to follow Step 3, since you don t know which packet you will end up with at the end of today s lesson. You don t want to be left with a partially blank packet because someone in your group didn t do their job! As its name suggests, an ionic compound (or salt) is composed of oppositely charged ions. Q1. In the following list of chemical formulas, circle those that are ionic compounds: ClBr 3 NCl 3 CaSO 4 OF 2 NaI Al 2 S 3 PO 4 3- C 7 H 16 XeF 4 Pb(NO 3 ) 2 Fe 2 O 3 NH 4 + Q2. You should have circled five compounds in Q1. What are the features that allow you to identify an ionic compound based on its chemical formula? Imagine two charged particles (called ions). One is positive (called the cation, pronounced cat-eye-on ) and the other is negative (called the anion, pronounced ann-eye-on ). They will feel an attraction to each other because they are oppositely charged. For the next two questions give an answer that is based on intuition and common sense. If you are stuck, then take the best guess you can for Q3 and Q4 and then move on to Q5. Q3. As the ions move apart, do you think the attractive force between them increases or decreases? Q4. If the positive ion has a +2 charge and the negative ion has a -2 charge, do you think the attraction between them is bigger or smaller than if their charges were +1 and -1? (Assume the only difference between the ions is the charge). Page 1 of 6

2 Model 1. The attraction between oppositely charged ions increases with charge and decreases with the distance between the ions, and is given by Coulomb s Law: F = k where F is the attractive (Coulombic) force between the ions (in units of Newtons), q 1 and q 2 are the charges of the two ions (each given by units of Coulombs), d is the distance between the ions (in meters), and k is a constant. q q d Q5. Use the relationships in Coulomb s Law to check your answers to Q3 and Q4. Did your intuition lead you to the correct answers in both cases? Q6. In Q4 you were asked to compare the attractive force between a pair of +2/-2 charged ions and +1/-1 charge ions. Using Coulomb s Law, did the attractive force of the +2/-2 pair double that of the other pair? Explain. Information. The size of atoms and ions is related to the number of electrons and occupied shells. The more electrons there are, the larger the atom or ion becomes due to the filling of more shells. In addition, the repulsions between the electrons cause the electron clouds to swell. The less electrons there are, the greater effect the nucleus has on pulling in the electrons and making the atom or ion smaller. Q7. Based on the text above, which is larger, a Li atom or a Li + ion? A Na atom or a Na + ion? Q8. Based on the text above, which is larger, a F atom or a F - ion? A Cl atom or a Cl - ion? Q9. Which pair of ions would you expect to have a stronger attraction: Li + /F -, or Na + /Cl -? (Hint: Consider Coulomb s Law. What factor would be different in these two pairs?). Page 2 of 6

3 Q10. To check your answer to Q9, consider the following two ions, Li + and F -. The ionic radius of Li + is 76 pm (picometer, m). The ionic radius of F - is 133 pm. Sketch these values on the drawing below. What is the distance between the centers of these ions? The ionic radius of Na + is 102 pm and Cl - is 181 pm. What is the distance between the centers of these ions? Based on Coulomb s Law, which pair of ions has the greater attraction? Q11. Due to the attractive forces between oppositely-charged ions, do you believe the decomposition (breaking apart) of an ionic compound would be exothermic (release energy) or endothermic (absorb energy)? Would the change in enthalpy for the decomposition be positive or negative? Then write an equation for the decomposition of an ionic compound into its ions. Q12. Based on your response above, do you believe the formation of an ionic compound would be exothermic or endothermic? Would the change in enthalpy for the formation be positive or negative? Then write an equation for the formation of an ionic compound from its ions. Page 3 of 6

4 Calculating Lattice Energies by using the Born-Haber Cycle Q13. The energy that is released from the formation of an ionic compound from a gaseous cation and gaseous anion is called the Lattice Energy. For example: A) Li + (g) + F - (g) LiF (s) H lattice =? You are probably familiar instead with the enthalpy of formation, H f º. As a review, complete the thermochemical equation for the formation of one mole of solid lithium fluoride from its elements in their standard states. Don t forget to label physical states! It s very important in this exercise!!! The enthalpy of formation value for this process is provided below. B) H f º = -612 kj/mol Equation B is much different from equation A. To find the lattice energy, let s start with equation B and step through all the processes to transform equation B into equation A. To do so, write out the following thermochemical equations: C) The conversion of solid lithium into gaseous lithium is a sublimation process that requires 161 kj/mol of energy. H subl = D) The diatomic gaseous fluorine molecules must be dissociated into gaseous atoms. Find this enthalpy value in Appendix J of your textbook. H dissociation = E) The gaseous lithium atoms from equation C are ionized by losing an electron. Find the ionization energies from Figure 7.20 in your textbook. IE lithium = F) The gaseous fluorine atoms from equation D are ionized by adding an electron. The energy associated with an atom GAINING an electron is called the electron affinity (EA). Find the EA from Table 7.10 in your textbook. EA fluorine = After you are finished writing Equations B through F, notify your instructor who will check them or send you to another group to check answers. Make sure the energy signs are correct and the balanced equation is correct. After verifying they are correct, use Hess Law and equations B through F to find the lattice energy in equation A. This method for solving multistep sequences is called the Born-Haber cycle. The cycle is visualized in the following diagram: Page 4 of 6

5 The Born-Haber Cycle The energy associated with each change is shown next to the arrow for that process. You should understand each process and know the name for the energy involved in that process. H f º M (s) + ½ X 2 (g) MX (s) H sublimation H dissociation M (g) X (g) Lattice energy Ionization energy, IE Electron Affinity, EA M + (g) + X - (g) Exercises (you do not need to rotate papers) This material is covered in Section 3.5 on ionic compounds, Section 7.12 and It is a good idea to read these sections. You should also review Sections This website is recommended for further reading: These Questions for Review and Thought are recommended (Chapter 7): p #96, 106,108, 110, 118, 119, 142. Your first quiz will be on anything from this activity, exercises, and recommended problems from the text. You will not be asked to submit any of this work. 1. List three examples of ionic compounds that you can think of without referring to any compounds mentioned in this exercise. 2. For practice, provide names of all the compounds in Q1 (page 1 of this activity). You are still expected to recall polyatomic ion names and formulas from memory! 3. Predict which compound will have the greater Coulombic attraction between its ions: NaCl or KCl. 4. The formation of NaCl from its gaseous ions has a lattice energy of -788 kj/mol: Na + (g) + Cl - (g) NaCl (s) H lattice = -788 kj/mol The formation of KCl from its gaseous ions has a lattice energy of -718 kj/mol: K + (g) + Cl - (g) KCl (s) H lattice = -718 kj/mol a) Which is requires more energy to decompose, NaCl or KCl? b) Which is held together by a stronger attraction, NaCl or KCl? c) Were you correct in exercise #3? Continued on page 6 Page 5 of 6

6 5. As the lattice energy increases (gets more positive), the Coulombic attraction between the ions (increases or decreases) 6. What do you think is the relationship between the lattice energy of an ionic compound and its melting point? Try to think it through without looking in the textbook. The more positive the lattice energy, the (higher or lower) the melting point. 7. Is the sign of most lattice energies positive or negative? Explain why. 8. Is the lattice energy proportional to the Coulombic force? To find out, do parts a and b below. a) Without calculating the Coulombic force (you do not have k ), use the following data to predict the lattice energy for BaO: Ionic radii of Na + =102 pm, Cl - =181 pm, Ba 2+ =135pm, O 2- =275pm; The lattice energy for NaCl is -786 kj/mol. (Hint: Use proportions). b) The actual lattice energy for BaO is kj/mol. How was your prediction? Does the lattice energy seem to be proportional to the Coulombic force experience by two ions? 9. a) Write an equation for the sublimation of sodium metal. What is the energy of this process called? b) Write an equation for the ionization of sodium metal. What is the energy of this process called? c) Write an equation for the formation of sodium chloride from its elements. What is the energy of this process called? d) The lattice energy of NaCl is -786 kj/mol. Write a thermochemical equation. 10. Show how the Born-Haber cycle can be used to calculate the lattice energy of KBr (s). Use the following values (in kj/mol): sublimation of K H=90; dissociation of Br2 H=112; IE of K=419; EA of Br = -325; H f º of KBr(s)=-392. To check your answer, go to the recommended website listed at the top of the page (under Exercises ) and click on Lattice Energy U under the diagram for the Born-Haber cycle. Look up the value for KBr. An answer key with worked out solutions for these exercises will be posted online. To prepare for the quiz that will be given at the beginning of the next class period, go through this entire activity and make sure you understand the point of every question. If you do not, please go to office hours. If you cannot, then send an or call your instructor. In addition, you should do the reading and the recommended problems from the text. Page 6 of 6