Unit 3: Chemical Bonds. IB Chemistry SL Ms. Kiely Coral Gables Senior High

Transcription

1 Unit 3: Chemical Bonds IB Chemistry SL Ms. Kiely Coral Gables Senior High

2 Bell Ringer What is the name of Ag₂SO₃? Quiz next class!

3 Answer Silver(I) sulfite

4 Physical Properties of Ionic Compounds 1. Ionic compounds have lattice structures The forces of electrostatic attraction between ions in a compound cause them to surround themselves with ions of opposite charge. As a result, the ionic compound takes on a predictable 3-d crystalline structure known as an ionic lattice. The layout of the lattice depends largely on the size of the ions in the compound, but it always involves this fixed arrangement of ions based on a repeating unit.

5 Note that an ionic lattice consists of a very large number of ions and it can grow indefinitely! However, we always report the simplest ratio of ions in the formula of an ionic compound. Ionic compounds do not exist as units with a fixed number of ions, their formulas are simply an expression of the ratio of ions present. We call this simplest ratio formula a formula unit.

6 VIDEO: How do atoms bond with each other?

7 Physical Properties of Ionic Compounds 2. Ionic Compounds have high melting points and high boiling points due to the strength of ionic bonds. The force of the electrostatic attraction between ions in an ionic compound are very strong and therefore require large amounts of heat to break. The melting and boiling points are generally higher when the charge on the ions is greater, due to increased attraction between the ions. The high melting points of ionic compounds become an economic consideration for many industrial processes since such high temperatures are needed to melt ionic substances in order to extract them.

8 Physical Properties of Ionic Compounds 3. Due to their strong bonds, ionic compounds are solids at room temperature.

9 Physical Properties of Ionic Compounds 4. Ionic compounds have low volatility, and typically have low odor. Volatility is a term used to describe the tendency of a substance to vaporize. Since ionic compounds are held together with strong attractive forces, and thus require high temperatures to boil, they in effect have a low tendency to vaporize.

10 Physical Properties of Ionic Compounds 5. Ionic compounds are generally soluble in ionic or polar solvents, but not soluble in nonpolar solvents. Solubility refers to the ease with a solid (the solute) dissolves in a liquid (the solvent) to form a solution. As the Na+ and Cl ions separate from the ionic lattice, they become surrounded by water molecules and are said to be hydrated. When this happens, the solid is dissolved*. *If a liquid other than water is able to dissolve the solid, the ions are said to be solvated.

11 Physical Properties of Ionic Compounds 6. Ionic compounds do not conduct electricity in the solid state, but conduct when molten or in aqueous solution (dissolved/solvated). The ability of a compound to conduct electricity depends on whether it contains ions that are able to move and carry a charge. Ionic compounds cannot conduct electricity in solid form where the ions are held within the lattice and limited in movement. However, if molten or dissolved, the ions are let loose, allowing for the flow of the ions. Electricity = flow of electrons Anions flowing = electricity

12 Physical Properties of Ionic Compounds 7. Ionic compounds are usually brittle, which means the crystal tends to shatter when force is applied. Ionic compounds are NOT malleable like metals. Applying too much pressure on an ionic solid will cause movement of the ions within the lattice towards ions of the same charge. The repulsive forces will cause the ions to split! Video: Ionic Properties

13 Metallic Bonds and Properties Gold was first discovered in its natural state, in streams all over the world! No doubt it was the first metal known to early hominids over 4 million years ago. Fig.1 Ancient Afghan Crown It has been used for centuries as a sign of wealth and power. Why is gold so treasured? Fig.2 Ancient Egyptian Pharaoh Tomb Of course, the reason why is CHEMISTRY!

14 Metallic Bonds and Properties As you can see with the crown and tomb in figures 1 and 2, gold, like all metals, is tough, malleable, and ductile. This is due to the metallic bond that holds metal atoms together. The way that metal atoms chemically bond allows for the many useful and beautiful physical properties of metals. Fig.2 Ancient Egyptian Pharaoh Tomb Fig.1 Ancient Afghan Crown Note: Gold in particular is also corrosion-resistant, (not all metals are corrosion-resistant), making gold even more valuable.

15 Metals are held together with Metallic Bonds Metallic bonds are strong forces of attraction between positively charged metal ions and delocalized valence electrons. The valence electrons originally belonged to the neutral metal atoms.

16 Metallic Bonds The valence electrons of atoms in a pure metal can be modeled as a sea of electrons. Remember, metals can only give away their electrons and so they always become positive ions, cations. So when metal atoms come together to form a compound, they throw out their valence electrons, meaning the valence electrons are mobile and can drift freely from one part of the metal to another. This is why metals are such good conductors of electricity! Electricity is a flow of electrons.

17 Physical Properties of Metals The sea-of-electrons model of metallic bonds explains physical properties of metals: -Good conductors of electricity: electrons can flow freely in the metal. Electricity is the flow of electrons. -Metals are ductile and malleable: since the electrons are mobile and can flow freely, they insulate the positive cations from contacting and repelling each other. If you were to try to bend a metal and there were no electrons to keep the cations from repelling from each other, the metal would break in half! Instead, when a metal is subjected to pressure, the metal cations easily slide past one another, allowing for metals to bend easily without breaking. Video : Metallic Bonding

18 Physical Properties of Metals The strength of a metallic bond is determined by: 1) The number of delocalized electrons 2) The charge of the cation 3) The radius of the cation The greater the number of delocalized electrons and the smaller the cation, the greater the binding force between them (due to the strength of the nuclear charge). This means that smaller radii in general have higher melting points, which is why melting points decrease down group 1 alkali metals. It is easier to break the bond of a larger metal cation than a smaller one. The smaller one s electrons are closer to the positive pull of the nucleus.

19 Alloys What are alloys and why are alloys important? Alloys are solutions (homogeneous mixtures) of two or more elements, at least one of which is a metal. For example, brass is a mixture of copper and zinc. Alloys are important because their properties are often superior to those of their component elements. For example, sterling silver, which is part silver and part copper, is harder and more durable than pure silver, yet is still soft enough to be made into jewelry and tableware.

20 PRACTICE What are responsible for the high electrical conductivity of metals? A. Delocalized positive ions B. Delocalized valence electrons C. Delocalized atoms D. Delocalized negative ions

21 ANSWER What are responsible for the high electrical conductivity of metals? A. Delocalized positive ions B. Delocalized valence electrons C. Delocalized atoms D. Delocalized negative ions

22 REFLECTION Draw an example of metallic bonding. Label the metal cations and the valence electrons. Underneath your drawing, explain the force of attraction in a metallic bond, and how it is different from that of an ionic bond.