Chapter 2. Law of Definite Proportions. Law of Definite Proportions. Law of Conservation of Mass 10/7/2011 WATER H 2 O

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1 Chapter 2 Fundamental Chemical Laws (2.2) Dalton s Atomic Theory (2.3) Defining the Atom (2.5) Atomic Structure (2.6) Molecules and Ions (2.7) The Periodic Table (2.8) Nomenclature (2.9) Law of Conservation of Mass The total mass of substances does not change during a chemical reaction mass is neither created nor destroyed. This is the outline for the content we will cover in lecture. Please read the entire chapter. 2 Law of Definite Proportions No matter what its source, a particular chemical compound is composed of the same elements in the same parts (fractions) by mass. WATER H 2 O No matter what the source water is ALWAYS 1 part hydrogen to 8 parts oxygen (by mass: one molecule is 2 amu H and 16 amu O) 3 Law of Definite Proportions Chemical analysis of a 9.07 g sample of calcium phosphate shows that it contains 3.52 g of Ca. How much Ca could be obtained from a kg sample? Mass fraction of Ca = 3.52 g Ca = * 100 = 38.8% 9.07 g sample (i.e., ANY sample of calcium phosphate is 38.8% Ca by mass ) Mass of Ca in kg of sample = kg sample x 38.8 kg Ca = kg Ca 100 kg sample or 388 g Ca 4 1

2 Law of Multiple Proportions In a nutshell, two (or more) compounds can contain different relative amounts of the same elements: If elements A and B react to form two compounds, the different masses of B that combine with a fixed mass of A can be expressed as a ratio of small whole numbers. (Evidence of the existence tiny individual particles ) Law of Multiple Proportions Mass of oxygen that combines with 1.00 g of carbon: Compound #1 Compound # g O per g C 2.66 g O per g C mass of O in compound #2 = 2.66 g = 2 mass of O in compound # g 1 EXACT 2:1 RATIO Dalton s Atomic Theory 1. All matter consists of tiny particles called atoms. 2. Atoms of an element are identical in mass and other properties and are different from atoms of any other element. 3. Compounds result from the chemical combination of a specific ratio of atoms of different elements. 4. Chemical reactions involve reorganization of the atoms changes in the way they are bonded. Atoms of one element do not change and cannot be converted into atoms of another element during chemical reactions. John Dalton 8 2

3 Thomson and Cathode Rays Thomson used partially evacuated glass tubes to discover the existence of negatively charged particles called electrons. Millikan and Oil Drops Millikan used oil droplets and X-rays to determine the magnitude of charge on an electron. Concluded that atoms must also possess positively charged particles, which led to the plum pudding model. With Thomson s cathode ray experiment, determined the mass of an electron: 9.11x10-31 kg Plum Pudding Model 9 10 Rutherford Experiment Aim: To study the internal structure of the atom. Investigate the mass distribution in the atom. Procedure: Use a radioactive source to bombard a thin piece of gold foil with a particles. If Plum Pudding is correct, a particles (high energy and positively charged) would go through the foil

4 Nuclear Atom Model Original Theory: Plum pudding model Revised Theory: Nucleus (dense positive charge) at the center of the atom. Large amount of space between nucleus and electrons. Atomic Structure The Angstrom (Å) is a unit of atomic distance. 1. Every atom contains small, dense nucleus. 2. All of the positive charge and most of the mass are concentrated in the nucleus. 3. The nucleus is surrounded by a large volume of nearly empty space that makes up the rest of the atom. 4. The rest of the atom is thinly populated by electrons, the total charge of which exactly balances the positive charge of the nucleus. If an atom was the size of a baseball stadium, the nucleus would be the size of a fly on home plate. This is a femtometer (fm). 1 amu = 1.66 x g What roles do the different particles play? # Protons = chemical identity of the atom (which element is it?) in an electrically-neutral atom, the number of protons in the nucleus is exactly balanced by the number of electrons around it # Electrons = ionic character of the atom An ion has either more or fewer electrons than the electricallyneutral atom. anion = more electrons, so ion is negatively-charged cation = fewer electrons, so ion is positively-charged # Neutrons = isotopic character of the atom isotopes have the same number of protons, but different number of neutrons (chemically indistinguishable) an atom of an element usually comes in at least 2 or 3 different isotopes (sometimes more) usually there will be one isotope that is far more abundant than the others 15 Atomic Structure Definitions Atomic number (Z): the number of protons in the nucleus of an atom Mass number (A): the sum of the numbers of protons and neutrons in the nucleus of an atom Atomic Mass: an average of the atomic masses of the most common isotopes Nuclear Symbol A X Z In the periodic table... Atomic symbol (X) 8 O For example: 16 O 8 Atomic number (Z) Atomic Mass (related to A) or Atomic symbol (X) 16 O 16 4

5 Isotopes of sodium Available on the course website under Exam Info and Lecture Notes Cobalt-60 Uranium-238 Chlorine-37 anion Copper-63 cation Copper-65 cation Let s count some particles 60 Co 238 U 37 Cl 63 Cu Cu 2+ # protons # electrons # neutrons = = = = = 36 Isotopes and Atomic Mass Atoms of the same element that differ in mass (e.g. 12 C, 13 C, 14 C) isotopes are the same element isotopes have the same number of protons isotopes differ in the number of neutrons, and therefore they differ in mass (more on amu in Ch 3). Many isotopes occur in nature. Most natural isotopes are not radioactive, nor are they necessarily harmful. A sample of an element will contain some percentage of all its isotopes. 20 5

6 Modern Reassessment of the Atomic Theory 1. All matter is composed of atoms. Although atoms are composed of smaller particles (electrons, protons, and neutrons), the atom is the smallest body that retains the unique identity of the element. 2. Atoms of one element cannot be converted into atoms of another element in a chemical reaction. Elements can only be converted into other elements in nuclear reactions in which protons are changed. 3. All atoms of an element have the same number of protons and electrons, which determine the chemical behavior of the element. Isotopes of an element differ in the number of neutrons, and thus in mass number, but not in chemical behavior (much). A sample of the element is treated as though its atoms have an average mass. 4. Compounds are formed by the chemical combination of two or more elements in specific ratios, as originally stated by Dalton. 21 Molecules and Ions Molecules are formed when atoms bind together. The type and number of bonds an atom forms depends on both the number of electrons and protons the atom possesses. Two limits to the type of bond that can be formed: Covalent: the sharing of electrons between atoms Ionic: the transfer of electrons from one atom to another We ll discuss bonding in much more detail in Chem 152 & 162 The goal in 142 is to give you a feeling for the role of electrons in bonding. 22 A chemical bond forms when the electrons on two atoms are able to interact. Energy is released when a chemical bond is formed. The driving force for bond formation is the attainment of a complete octet in the valence shell of both atoms. We will discuss two main types of chemical bonds: ionic and covalent. Chemical Bonds Ionic bonds form when valence electrons are transferred from a metal to a non-metal. Covalent bonds form when two nonmetals share their valence electrons. 23 Ionic Compounds The transfer of an electron from sodium to chlorine results in two oppositelycharged ions that each have a complete octet in their valence shells. The ionic bond is due to the electrostatic attraction between Na + and Cl. An ionic solid consists of an extended network of anions and cations. 24 6

7 When ionic compounds form, the overall charge of the compound must be zero. This requirement determines the relative number of cations to anions in the formula unit. Ionic Compounds Ionic Compounds The result of the electron transfer is the formation of an ion: Positive ion: fewer electrons than protons Negative ion: more electrons than protons Important!! Ions are NOT formed by the transfer of protons Ion Formation Trends Metals tend to lose electrons to form cations. Non-metals tend to gain electrons to form anions. Many transition metals are capable of forming more than one kind of cation. A: No interaction, atoms too far apart. B: Atoms move closer, electron clouds distort. C: Covalent bond forms. D: The protons from each nucleus share the overlapping electron cloud

8 Diatomic Molecules The Periodic Table of Elements Copyright Cengage Learning. All rights reserved 29 Reading the Periodic Table A groups 1 1A B groups (transition metals) A 4A 5A 6A 7A 8A B 2B 3B 4B 5B 6B 7B 8B 9B 10B These elements are also part of the transition metal block 8

9 Periodic Trends At temperatures near room temperature, most elements are in the solid phase. Groups 1 and 2 tend to lose electrons and in forming compounds. Periodic Trends Group 7 tends to gain electrons in forming compounds. Exceptions: Gases: H 2, N 2, O 2, F 2, Cl 2, He, Ne, Ar, Kr, Xe Liquids: Hg, Br 2 Chemical Nomenclature Chemical formulas are derived from symbols for the elements and their relative amounts in the compounds. We will start by naming binary compounds (two elements). Different classes: a metal with a nonmetal: ionic compounds Type I: main group metals (not TM) Type II: transition metals (TM) two nonmetals: Type III: nonmetals Formulas of Elements For many elements, the chemical formula is just the atomic symbol, e.g. Li, Cs, Au, Ne, Pt Some elements form diatomic (2-atom) molecules, for which the chemical formula is the atomic symbol with a subscript 2 to indicate two of the same type of atom. H 2, N 2, O 2, F 2, Cl 2, Br 2, and I 2 are ones that you should know Other exceptions that we will encounter: P 4, S 8, C 60 Rules for naming compounds containing polyatomic ions are similar to those for binary ionic compounds

10 Binary Ionic Compounds Zumdahl Nomenclature: Type I: Ionic Cations (M n+ ): name of atom + cation Magnesium: Mg magnesium cation Cesium: Cs +... cesium cation Anions (X m- ): root of atom name + -ide Fluorine: F -... fluoride anion Sulfur: S sulfide anion Selenium: Se selenide anion Compound Name: <cation name> <anion name> Binary Ionic Compounds Table of common ions and nomenclature: Chemical Formula: M m X n Name the following compounds: KCl Naming Examples Write formulas for the following: barium hydride Binary Ionic Compounds Zumdahl Nomenclature: Type II: Ionic Cations (M n+ ): name of atom + (oxidation state) + cation Oxidation state = charge. Use Roman numerals. Example: Chromium: Cr chromium(ii) cation Cr chromium(iii) cation CaBr 2 aluminum sulfide Anions (X m- ): root of atom name + -ide (same as before) Compound Name: <cation name> <anion name> Chemical Formula: M m X n

11 Name the following compounds: Naming Examples Write formulas for the following: Binary Compounds of Transition Metals There are both new school and old school ways to indicate the charge of the cation: CuCl MnO 2 vanadium(v) fluoride tin(iv) bromide ic is the higher oxidation state. ous is the lower oxidation state. You will need to know the common oxidation states for a given metal Type III Binary Compounds (two nonmetals, or a nonmetal and a metalloid) 1. The first element in the formula is named first, and the full element name is used. 2. The second element is named as though it were an anion. 3. Prefixes are used to indicate the number of atoms present. 4. The prefix mono- is never used for naming the first element. i.e., CO is carbon monoxide, not monocarbon monoxide. Prefix # Indicated mono- 1 di- 2 tri- 3 tetra- 4 penta- 5 hexa- 6 hepta- 7 octa- 8 nona- 9 deca- 10 Examples Name the following compound: BF 3 Name the following compound: I 2 O 7 Write the formula for: phosphorus trichloride Write the formula for: dinitrogen trioxide

12 Polyatomic Ions There are many polyatomic ions that are referred to as a single group (unit) when naming a compound. Ionic Compounds with Polyatomic Ions Writing names and formulas is exactly the same as for binary ionic compounds cation is named first, anion is named second multiples of the ions are used to ensure charge neutrality Example: Na 2 SO 4 2Na + + SO 4 2- sodium sulfate manganese(ii) hydroxide Mn OH - Mn(OH) 2 KHSO 4 K + + HSO 4 - potassium hydrogen sulfate ammonium sulfide 2 NH 4+ + S 2 (NH 4 ) 2 S FePO 4... Fe 3+ + PO 4 3- iron(iii) phosphate aluminum acetate Al CH 3 COO Al(CH 3 COO) HYDRATES Compounds containing WATER molecules MgSO 4 7H 2 O magnesium sulfate heptahydrate Acids Many common anions, when combined with H + as the cation(s), make a very reactive compound called an acid, which dissociates in water to give the two separate ions, both dissolved in water. CaSO 4 2H 2 O calcium sulfate dihydrate Examples: Ba(OH) 2 8H 2 O barium hydroxide octahydrate H 3 PO 4 (aq) H + (aq) + H 2 PO 4- (aq) HCl (aq) H + (aq) + Cl - (aq) phosphoric acid hydrochloric acid CuSO 4 5H 2 O copper(ii) sulfate pentahydrate H 2 SO 4 (aq) 2 H + (aq) + SO 4 2- (aq) sulfuric acid Na 2 CO 3 10H 2 O sodium carbonate decahydrate ph = quantitative measure of H + concentration in water

13 Naming Acids Closing Thoughts on Naming 1) Binary acid solutions form when certain gaseous compounds dissolve in water. For example, when gaseous hydrogen chloride (HCl) dissolves in water, it forms a solution called hydrochloric acid. Prefix hydro- + anion nonmetal root + suffix -ic + the word acid hydrochloric acid 2) Oxoacid names are similar to those of the oxoanions, except for two suffix changes: Anion -ate suffix becomes an -ic suffix in the acid. Anion -ite suffix becomes an -ous suffix in the acid. The oxoanion prefixes hypo- and per- are retained. Thus, BrO 4 - is perbromate and HBrO 4 is perbromic acid IO 2 - is iodite and HIO 2 is iodous acid. Zumdahl provides several naming flow charts use them if you would like, but you do NOT need to memorize them. In the end, the only real way to learn naming rules is to practice and get a lot of experience. You will need to become very familiar with the names and formulas of common cations and anions (both monatomic and polyatomic). ALEKS will give you many opportunities to practice, and next week s worksheet in discussion section will, too