Chemical Bonds & Molecular Attractions

Transcription

1 Chemical Bonds & Molecular Attractions Chapter 12 Student Learning Objectives Determine the type of chemical bond and the chemical formula Describe molecular attractions. Student Learning Objectives Determine the type of chemical bond and the chemical formula Describe molecular attractions. 1

2 What is a chemical bond? A chemical bond is an attraction between atoms. When two substances chemically react, electrons are transferred or shared. The chemical bond puts each atom in a lower energy state. Water H 2 O Ibuprofen C 13 H 18 O 2 What is conserved in chemical reactions? Mass and charge are conserved in chemical reactions. Formula mass is the sum of atomic masses. The relative mass of one type of atom compared to another is always the same for a particular substance. (law of definite proportions) H 2 O Calculating Formula Masses Find the formula mass (FM) of lead chromate, PbCrO 4 used for yellow lines on streets Using the Periodic Table, look up the atomic masses of Pb, Cr, and O Pb (207.2 u), Cr (52.0 u), O (16.0 u) Formula Mass = u u + (4 16 u) FM of PbCrO 4 = u 2

3 Law of Definite Proportions Different samples of a pure compound always contain the same elements in the same proportion by mass. For Example: 9 g H 2 O = 8 g Oxygen + 1 g Hydrogen 18 g H 2 O = 16 g Oxygen + 2 g Hydrogen 36 g H 2 O = 32 g Oxygen + 4 g Hydrogen In each case the ratio (or proportion) by mass of Oxygen to Hydrogen is 8 to 1 Calculating Percentage by Mass of an Element mass of component X Section %X 12.2 by mass = 100% formula mass of compound H 2 O for example: 16 u %O by mass = 100% 88.9% 18 u %H by mass = 11.1% (the remainder) Calculating Percentage by Mass for CO 2 Dry Ice is solid CO 2 AM (atomic mass) of C = 12.0 u & O = 16.0 u FM (formula mass) of CO 2 = 12.0 u + ( u) = 44.0 u % mass of C = (M c /FM CO ) 100 =???% 2 % mass of C = (12.0 u/44.0 u) 100 = 27.3% Since the % mass of C = 27.3% the % mass of O = 72.7% 3

4 Calculating Percentage by Mass for Al 2 O 3 Mineral corundum (ruby & sapphire) is Al 2 O 3 AM (atomic mass) of Al = 27.0 u & O = 16.0 u FM (formula mass) of Al 2 O 3 = ( u) + ( u) = u Total mass of O = = 48.0 u Total mass of Al = = 54.0 u % mass of O = (M O /FM Al ) 100 =???% 2O3 % mass of O = (48.0 u/102.0 u) 100 = 47.1% Since the % mass of O = 47.1% the % mass of Al = 52.9% Definite Proportions When a compound is broken down, its elements are found in a definite proportion by mass Also, when the same compound is formed, the elements will combine in that same proportion by mass Limiting & Excess Reactants If constituent elements are not mixed in the correct proportions then One of the elements will be used completely up and is called the limiting reactant And one of the elements will only partially be used up and is called the excess reactant Let s look at an example 4

5 Practice What is the formula mass for each of these compounds? What is the relative mass? 1. CO 2 2. HCl 3. NH 3 4. CH 4 How does an electron dot structure compare to the shell model? The Lewis electron dot symbols represent the valence electrons of the main group elements. The electron dot structure is based on the octet rule. Practice Draw the electron dot structure. How does it compare to the group number? 1. H 2. C 3. Ar 4. Na 5

6 What is an ionic bond? Ionic bonds form between positive and negative ions. Outer Shell Electrons Ion Mostly Full Gained Negative Mostly Empty Lost Positive Electric force establishes bond Strongest of all chemical bonds Charges must balance to give a net charge of zero when atoms bond. Example: Na + Cl Practice 1) Ionic bonds are usually formed between metals from the left side of the periodic table and nonmetals from the far right side of the periodic table. Why? 2) What is the chemical formula? What is the name of the compound? 1. Mg + S 2. Ca + Br 3. Rb + Se What is a metallic bond? Metallic bonds form in metals as all electrons are shared among all the nuclei in the metal. e e e e e e e e Cu Cu Ni e e e Ni e e e e e Sharing of all electrons establishes bond Results in conductivity and malleability of metals Weakest of all chemical bonds Metal ions are homogeneously distributed in an alloy. 6

7 What is a covalent bond? Covalent bonds are formed between atoms that share pairs of electrons in the inter-nuclear region (between the atoms). Sharing of electron pairs establishes the covalent bond There can be 1, 2, or 3 pairs of electrons shared. H H 2e single bond C O 4e double bond N N 6e triple bond Polar Molecules exhibit uneven sharing of electron pairs. In general, the farther apart (left and right) two atoms are on the periodic table, the more polar the bond will be. Practice 1) Which are polar molecules? Which bond would be the most polar? a) CO b) CO 2 c) H 2 O d) NH 3 2) Polar molecules tend to have high boiling points. Why? 3) Polar molecules and non-polar molecules do not mix. Why? 7

8 Molecular Attractions Molecular attractions occur as a result of polarity. Ion-dipole Dipole-dipole Dipole-induced dipole Induced dipole-induced dipole Molecular attractions result in surface tension. Hydrogen bonding is a special form of a dipoledipole molecular attraction. H is part of the bond pair Very uneven sharing of e Results in high L f and L v for water A hydrogen bond is the attraction between the positively charged hydrogen and the negatively charged lone pair electrons. 8