# lattice energies v.2

```Chem 150 Activity on Ionic Compounds and Lattice Energy
v. 2 1/07
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:
ClBr3
NCl3
CaSO4
OF2
NaI
Al2S3
PO43-
C7H16
XeF4
Pb(NO3)2
Fe2O3
NH4+
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).
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Chem 150 Activity on Ionic Compounds and Lattice Energy
v. 2 1/07
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
q1q2
d2
where F is the attractive (Coulombic) force between the ions (in units of Newtons), q1 and q2 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.
Q5. Use the relationships in Coulomb’s Law to check your answers to Q3 and Q4. Did your
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?).
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Chem 150 Activity on Ionic Compounds and Lattice Energy
v. 2 1/07
Q10. To check your answer to Q9, consider the following two ions, Li+ and F-. The ionic radius
of Li+ is 76 pm (picometer, 10-12 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.
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Chem 150 Activity on Ionic Compounds and Lattice Energy
v. 2 1/07
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)
∆Hlattice = ?
You are probably familiar instead with the enthalpy of formation, ∆Hf&ordm;. 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)
∆Hf&ordm; = -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.
∆Hsubl =
D) The diatomic gaseous fluorine molecules must be dissociated into gaseous atoms. Find this
enthalpy value in Appendix J of your textbook.
∆Hdissociation =
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.
IElithium =
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.
EAfluorine =
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:
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Chem 150 Activity on Ionic Compounds and Lattice Energy
v. 2 1/07
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.
M (s)
+
&frac12; X2 (g)
∆Hf&ordm;
→
MX (s)
∆Hdissociation
∆Hsublimation
M (g)
X (g)
Ionization energy, IE
Lattice energy
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 7.13. It is a good
idea to read these sections. You should also review Sections 7.8-7.11.
This website is recommended for further reading: http://chemistry.bd.psu.edu/jircitano/BH.html
These Questions for Review and Thought are recommended (Chapter 7): p. 325-327 #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)
∆Hlattice = -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)
∆Hlattice = -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
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Chem 150 Activity on Ionic Compounds and Lattice Energy
v. 2 1/07
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, Ba2+=135pm, O2=275pm; The lattice energy for NaCl is -786 kJ/mol. (Hint: Use proportions).
b) The actual lattice energy for BaO is -3054 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; ∆Hf&ordm; 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 email or call your instructor. In addition, you should do
the reading and the recommended problems from the text.
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