Unit 2 Atoms and Bonding 2.81 Ionic Bonding Textbook ch 8.2

Big Idea 2: Chemical and physical properties of materials can be explained by the structure and the arrangement of atoms, ions, or molecules and the forces between them. Students will be able to demonstrate understanding by laboratory investigation, analysis of data and creation of models. SWBAT: Define lattice energy and be able to arrange compounds in order of increasing lattice energy based on charges and sizes of ions involved. Learning Objectives :

What is a Bond? A force that holds atoms together. Why? We will look at it in terms of energy. Bond energy = the energy required to break a bond. Why are compounds formed? Because it gives the system the lowest energy.

Ionic Bonding: Ionic Bonding: Electrons are transferred Ionic bonds occur Between: metal and nonmetal Ex. NaF Or metal +polyatomic ion Ex. CaCO 3 Or ammonium + nonmetal Ex. NH 4 Cl An atom with a low ionization energy reacts with an atom with high electron affinity and the electron moves. Opposite charges hold the atoms together. Electronegativity differences are generally greater than 1.7 The formation of ionic bonds is always exothermic!

Review: Naming Ionic Compounds First element (metal) “as is” Second element (nonmetal) add “ide” ending –Ex. NaF ExceptionsExceptions –Polyatomics name “as is” Ex. CaCO 3 –Transition metals with multiple charges need Roman numeral Ex. Fe 3+ and Cl - sodium fluoride calcium carbonate iron (III) chloride

Properties of Ionic Compounds

Sodium Chloride Crystal Lattice crystals Ionic compounds form solid crystals at ordinary temperatures. Ionic compounds organize in a characteristic crystal lattice of alternating positive and negative ions. All salts are ionic compounds and form crystal lattices.

The reason ionic compounds are so stable is the attraction between ions of opposite charges. The attraction draws ions together releasing energy Causing ions to form solid array or lattice Lattice energy - Energy required to completely separate one mole of solid ionic compound into its gaseous ions

Estimate  H f for Sodium Chloride Na(s) + ½ Cl 2 (g)  NaCl(s) Na(s)  Na(g) kJ Na(g)  Na + (g) + e kJ ½ Cl 2 (g)  Cl(g) + ½(239 kJ) Cl(g) + e -  Cl - (g) kJ Na + (g) + Cl - (g)  NaCl(s) -786 kJ Na(s) + ½ Cl 2 (g)  NaCl(s) -412 kJ/mol Very exothermic

Coulomb's Law and Lattice energy E = k Q 1 Q 2 /d E = electrostatic potential energy or lattice energy k = a constant Q is the charge on particles in coulombs d is the distance between the centers.

Coulomb's Law and Lattice energy E = k Q 1 Q 2 /d Look at equation without doing math… what patterns can we estimate? Lattice Energy increases- as charges (Q) increase Lattice Energy increases- as radii (d) decreases If charges are opposite, E is negative exothermic Same charge, E is positive endothermic, requires energy to bring them together.

Check for understanding Arrange NaF, CsI and KF in order of increasing lattice energy. Arrange NaF, AlCl 3, and MgBr 2 in order of increasing lattice energy.

References I modified his original PPT to fit our needs in AP Chemistry needs. Our textbook: Brown, Lemay et all. AP edition chemistry, 13 th edition, 2015