Data Response question on ionic bonding.

The purpose of this exercise is to get students to think critically about what they read.

The following extract is taken from a very well-respected British chemistry textbook for 16–19 year olds (specifically: G. Hill and J. Holman, Chemistry in Context, 5th edn, Nelson, 2000). It introduces the topic of ionic bonding using lithium oxide as the example. Many other books use sodium chloride as the example, set out in a similar way.

Read the extract carefully then consider the four questions that appeared after the extract in the book:

1. What is the electron structure of: (a) the lithium ion; (b) the oxide ion?

2. Which noble gas has an electron structure like that of Li⁺?

3. Which noble gas has an electron structure like that of O^2–?

4. Why is it that two lithium atoms react with only one oxygen atom?

(You will need the IB data booklet to answer some of the questions.)

1. Why are some of the electrons shown singly and others in pairs?

2. Comment on the size of the lithium atom compared with that of the oxygen atom. (Support your answer by finding out the values for their atomic radii.)

3. Is the process of a lithium atom losing an electron exothermic or endothermic? Find its value.

4. The removal of an electron from lithium involves the first ionization energy of lithium. What must be done to the lithium atom before the value for the first ionization energy can be used? Is this process exothermic or endothermic? Find its value.

5. What name is given to the enthalpy change for adding two electrons to an oxygen atom? Is it an exothermic or endothermic process? Find its value.

6. Comment on the size of a lithium ion compared with that of a lithium atom.

7. Comment on the size of an oxide ion compared with that of an oxygen atom.

8. Write the equation for the complete combustion of lithium in oxygen. Include the physical states in your balanced equation.

9. Does the diagram represent the equation you have written? If not, what extra enthalpy change is necessary for oxygen? Is this change exothermic or endothermic? Find its value (remember this is for the formation of one atom of gaseous oxygen).

10. Look at all the values for the changes taking place that have been discussed so far and add them up (remember there are two atoms of lithium reacting with one atom of oxygen).

11. The total enthalpy change for the reaction is −596 kJ mol⁻¹ Suggest which energy factor you have not yet taken into account, and calculate its value.

12. If you were asked to write a chemistry textbook, could you now come up with a better diagram to illustrate ionic bonding using lithium oxide?

13. The enthalpy term arrived at in question 11 is known as the lattice enthalpy change. Would you expect the lattice enthalpy change for sodium chloride (NaCl) to be larger or smaller in absolute terms than that for lithium oxide (Li₂O)? Explain your answer.

14. Finally, a tough question! The lattice enthalpy change can be calculated theoretically and obtained experimentally. For sodium chloride there is excellent agreement between the two values obtained. However, the experimental value for lithium oxide (Li₂O) is –2906 kJ mol⁻¹, whereas the theoretical value is –2799 kJ mol⁻¹. Suggest a reason why the two values are not the same.