General Physical Science. The Law of Conservation of Mass. Example. Chapter 12 Chemical Bonding

Transcription

1 General Physical Science Chapter 12 Chemical Bonding The Law of Conservation of Mass No detectable change in the total mass occurs during a chemical reaction Example In a chemical reaction involving carbon and oxygen, laboratory measurements showed that the complete burning in oxygen of 4.09 g of carbon produced g of carbon dioxide as the only product. How many grams of oxygen reacted? g = 4.09 g + x g x = = g oxygen 1

2 The Law of Conservation of Mass Learning Goal: State and use the Law of Conservation of Mass. Questions: 1-31 Exercise: 1, 3 Law of Definite Proportions Different samples of a pure compound always contain the same elements in the same proportion by mass. Atomic Mass (AM) The number below the elemental symbol. Formula Mass (FM) Sum of the atomic masses AKA formula weight/molecular weight Example Calculate the formula mass of PbCrO 4, the bright yellow compound used in paint for the yellow lines on streets. Atomic Masses Pb = 207.2; Cr = 52.0; O = (4 x 16.0)

3 Percent by Mass Since all compounds have the same elemental makeup, we should be able to calculate how much of each element is present (by mass) (Mass element / FM) * 100% Example Calculate the percent by mass for carbon and oxygen in dry ice (CO 2 ) Formula Mass = *2 = 44.0 Formula Mass Percent Carbon (12.0 / 44.0) * 100% = 27.3% Percent Oxygen (16.0*2 / 44.0) * 100% = 72.7% Note that = Limiting Reactant 3

4 Law of Definite Proportions Learning Goals Calculate the formula masses of compounds. State and use the Law of Definite Proportions. Questions: 4-6 Exercises: odd Dalton s s Atomic Theory 3 postulates Each element is composed of small indivisible particles called atoms which are identical for that element but different from atoms of other elements Chemical combination is the bonding of a definite, small number of atoms of each of the combining elements to make one molecule of the formed compound. No atoms are gained, lost, or changed in identity during a chemical reaction: they are just rearranged. Dalton s s Atomic Theory Explains Law of Conservation of Mass If we rearrange atoms we still have the same amount of stuff Explains Law of Definite Proportions Test with similar compounds CO and CO 2 For a fixed amount of C, the ratio of the masses of oxygen in CO and CO 2 is a small whole number. 4

5 Dalton s s Atomic Theory Learning Goal State the postulates of Dalton s s atomic theory. Questions: 7-8 Ionic Bonding Valence electrons control chemical behavior. Noble gases do not react. Noble gas configuration must be stable! Octet Rule Atoms try to surround themselves with 8 electrons Octet Rule 2 methods Ionic Bonding Transfer of electrons Sharing of electrons Ionic compounds are formed by the transfer of electrons from one species (metal) to another species (nonmetal) 5

6 Formation of NaCl Lewis Symbols G. N. Lewis Uses dots to show valence electrons Dots on each of the four sides of the symbols All single dots before any dots are paired Ions Cations positively charged ions For the representative elements, the group number will equal the charge on the ion. Anions negatively charged ions charge = group number - 8 6

7 Ionic Compounds Ions are held together by electrostatic attraction between the ions. Formula for an ionic compound represents the smallest ratio of cations and anions to give a neutral species in reality there are no molecules of ionic compounds exist in a large matrix Ionic Compounds Example Write the formula for calcium phosphate, the major component in bones. Calcium; Group 2 (metal) charge will be +2 Phosphate polyatomic ion; charge is -3 Ca 3 (PO 4 ) 2. 7

8 Electrical Conduction Electricity can be conducted by ions. Ionic solid - no electrical conduction Ions are locked into position Ionic liquid - will conduct electricity Most ionic compounds melt at temps well in excess of 300 O C Soluble ionic compound conducts elec. Ions free to move in solution Stock System Nomenclature Applies to metals that form ions with more than one charge Typically transition elements Some heavier post-transition transition representative metals also Name of metal (charge of ion) anion iron(ii) chloride copper(i) nitrate Ionic Bonding Learning Goals Write formulas for ionic compounds Describe the characteristics of ionic compounds Name compounds containing a metal that forms more than one ion. Questions: Exercises: odd 8

9 Covalent Bonding Sharing of electrons Stable covalent bonds will give all atoms a noble gas configuration Shared electrons are counted twice! H 2, Cl 2, H 2 O, NH 3, CH 4. Multiple bonding between 2 atoms Double bonds Triple Bonds Covalent Bond in Hydrogen Polyatomic Ions Contain both ionic and covalent characteristics Atoms in a polyatomic ion are held together with covalent bonds To get enough electrons to share and complete all octets the polyatomic anions need extra electrons - gives negative charge This then forms ionic bond with metals. 9

10 Prediction of Bond Type Only nonmetals - covalent Metals/nonmetals (generally) ionic Metals/polyatomic ions ionic Gases, liquids, low mp solids covalent Conduct electric current when molten (but not when solid) ionic Polar Covalent Bonding Unequal sharing of electrons Leads to partial negative and partial positive on covalent molecule. Dipole Electronegativity Linus Pauling Tendency of an element to attract an electron in a bond. Electronegativity Increases for representative elements from bottom left to top right. Excludes noble gases Noble gases don t t form bonds! Fr lease electronegative F most electronegative 10

11 Polar Molecules Must have two things Polar bond Occurs when elements of different electronegativities are bonded. Essentially, different nonmetals Non symmetric arrangement Symmetry - tug-of of-war with no winner Polarity drives solubility Likes dissolve likes Covalent Bonding Learning Goals Write molecular formulas and Lewis structures for covalent compounds Describe the characteristics of covalent compounds Questions: Exercises: odd Hydrogen Bonding Solubility of table salt ion-dipole interaction molecules have dipole-dipole interaction energy that holds the molecule together higher the energy, higher the boiling point Hydrogen bond Special type of dipole-dipole interaction. Large value for dipole small size of hydrogen 11

12 Hydrogen Bonding Occurs when H bonded to F, O, or N Hydrogen bonding about 5-10% 5 of strength of covalent bond Typical molecular interaction less than 1% of covalent bond! Causes ice to be less dense than water Leads to life on Earth! Holds DNA together Leads to life on Earth! Hydrogen Bonding Learning Goals Predict when hydrogen bonding will occur Describe how hydrogen bonding affects a compound s s properties. Questions: 32, 33 Key Terms; Matching, Multiple Choice, and Fill-in in-the-blank Questions; Visual Connection and Applying your Knowledge General Physical Science Chapter 12 Chemical Bonding 12