Principles of Chemistry CHEM 101 Lecture 2 Fall 2023 PDF

Principles of Chemistry CHEM 101 Lecture 2 1  Evolution of Understanding of Matter (Atom) 3  Atom? Greeks (400 BC): Matter composed of four fundamental substances Air, Water, Earth, and...

Principles of Chemistry CHEM 101 Lecture 2 1  Evolution of Understanding of Matter (Atom) 3  Atom? Greeks (400 BC): Matter composed of four fundamental substances Air, Water, Earth, and Fire. Matter: continuous – infinitely divisible into smaller pieces vs composed of small invisible particles (Democritus, atomos). Atomos is an ancient Greek adjective "uncuttable" or "indivisible. 4  Foundation of modern Atom? Chemistry 16th century Substance was an element unless it could be broken down into two or more simpler substances. (by Robert Boyle (Metallurgy). 1627 to 1691). Boyle believed that conversion of one metal to Quantitative experiments another is a possibility. (17th century) Combustion - extensively studied by the late 18th century Pressure Antoine Lavoisier (1743 to 1794) – Explained the nature of Combustion Robert Boyle Tremendous progress near the end of 18th century early 19th century – Boyle’s law Industrial revolution through mechanized factories. CxHy + O2 CO2 + H2O + Energy Volume C6H12O6 + 6O2 6CO2 + 6H2O + Energy Antoine Lavoisier – Carefully Glucose Oxygen Carbon Water weighed reactants and products dioxide of various reactions. Law of conservation of mass Mass is neither created nor destroyed in a chemical reaction (Antoine Lavoisier).  Atom? Study the course of chemical reactions and the composition of chemical compounds. Law of definite proportion – A given compound always contains exactly the same proportion of elements by mass (Joseph Proust (1754 to 1826) – Proust’s Law). Principle of constant composition of compounds. For example, copper carbonate is always: ~5.3 parts copper : ~1 part carbon : ~4 parts oxygen (by mass). 5.3 : 1 : 4 (ratio by mass) CuCO3 (Cu: 63.55 g/mol, C: 12 g/mol , O: 16 g/mol x 3 = 48) If elements were composed of tiny individual particles (atoms), a given compound should always contain the same combination of these atoms (John Dalton 1808). 6  Atom? If elements were composed of tiny individual particles (atoms), a given compound should always contain the same combination of these atoms (John Dalton). Ratio between masses of oxygen in compounds I and II 1.33 g : 2.66 g 1:2 Law of multiple proportions – When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers (John Dalton 1808). 7  Atom? Dalton’s Atomic Theory (1808) 1. Each element is made up of tiny particles called atoms. 2. The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways. 3. Chemical compounds are formed when atoms of different elements combine with each other. A given compound always has the same relative numbers and types of atoms. 4. Chemical reactions involve reorganization of the atoms – changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction. Water Hydrogen : Oxygen Atomic mass of H = 1 Water Water 1g : 8g Atomic mass of O = 8 HO H2O 8  Atom? Measurement of volumes of gases that reacted with each other (under same conditions of temperature and pressure) 1809. H H + Cl Cl H Cl H Cl 1 volume of hydrogen 1 volume of chlorine 2 volumes of hydrochloric acid Avogadro’s hypothesis (1811): equal volumes of different gases (at same temperature and pressure) contain the same number of particles. H + Cl HCl H–H = H2 + Cl–Cl = Cl2 2 HCl  Atom? H H H H O O O O H H H H 2 volumes of hydrogen 1 volume of oxygen 2 volumes of water 2 H2 + O2 2 H 2O 10  Relative molecular Atom masses of other Cannizzaro’s interpretation: Dalton + Avogadro's hypothesis gaseous substances. Comparing equal H–H = H2 = 2 relative molecular mass (arbitrarily assigned) volumes of hydrogen and other gases. O–O = O2 = 32 relative molecular mass Relative atomic mass of carbon dioxide = 44 (relative to 2 for H2) Experiments had shown that carbon dioxide contains 27% carbon by mass (0.27)(44 g) = 12 g of carbon in 44 g of carbon dioxide 44 g – 12 g = 32 g of oxygen Relative atomic mass of oxygen = 16 The formula of carbon dioxide is assumed to be CO2 Relative atomic mass of carbon = 12 12 + 2(16) = 44 11  Relative mass data for several gases containing carbon Compound Relative Percent Carbon Relative mass of Molecular Mass (by mass) carbon present Methane 16 75 % 12 Ethane 30 80 % 24 Propane 44 82 % 36 Butane 58 83 % 48 Carbon dioxide 44 27 % 12 Compound Relative Percent Hydrogen Relative mass of Molecular Mass (by mass) carbon present Methane 16 25 % 4 Ethane 30 20 % 6 Propane 44 18 % 8 Butane 58 17 % 10  Atom Relative molecular masses of many other elements were determined. Concept of atom became quite clear. What is an atom made of? How do the atoms of the various elements differ?  Composition and Structure of Atom J. J. Thomson (1898-1903): Electrical discharges in partially evacuated tubes called cathode-ray tubes - cathode ray when high voltage was applied to the tube. the ray was a stream of negatively charged particles, now called electrons. 14  Composition and Structure of Atom J. J. Thomson (1898-1903): Electrical discharges in partially evacuated tubes called cathode-ray tubes - cathode ray when high voltage was applied to the tube. 15  Composition and Structure of Atom J. J. Thomson (1898-1903): Electrical discharges in partially evacuated tubes called cathode-ray tubes - cathode ray when high voltage was applied to the tube. 16  Composition and Structure of Atom J. J. Thomson (1898-1903): Electrical discharges in partially evacuated tubes called cathode-ray tubes - cathode ray when high voltage was applied to the tube. 𝑒 = −1.76 × 108 C/g 𝑚 Atom consisted of a diffuse cloud of positive charge with the negative electrons embedded randomly in it. The plum pudding model of the atom. 17  Composition and Structure of Atom Robert Millikan (1909) 𝑒 Charge on electron: 1.602 × 10-19 coulombs. = −1.76 × 108 C/g 𝑚 Mass of electron: 9.109 × 10-31 kilogram.  Composition and Structure of Atom Henri Becquerel (1896) found accidentally that a piece of a mineral containing uranium could produce its image on a photographic plate in the absence of light. Spontaneous emission of radiation by the uranium – radioactivity (Marie Curie). Gamma  ray is high-energy “light (photons)”; β particle is a high-speed electron; and an alpha α particle has a 2+ charge (He nucleus: 2 protons + 2 neutrons, 7300 times the mass of electrons). Ernest Rutherford (1911) 0.00004 cm 400 nm 0.4 m  Composition and Structure of Atom J. J. Thomson Model 20 Composition and Structure of Atom Nucleus is at least 10,000 times smaller than the radius of the atom Nucleus Small size High density Mass of nuclear material ~size of chickpea = 250 million tons 21 Composition and Structure of Atom Magnitude Charge on electron or proton 1.602 x 10-19 C (coulomb) 22  Composition and Structure of Atom “If all atoms are composed of these same components, why do different atoms have different chemical properties?” Arrangement of electrons 23  Composition and Structure of Atom Covalent bonding Water, H2O Ammonia, NH3 Methane, CH4 (Sharing of electrons) 24  Composition and Structure of Atom Sodium Chloride : NaCl Neutral sodium atom (Na) Chloride ion Sodium ion (Cl–) Neutral chlorine (Na+) atom (Cl) Minus 1 11+ 11+ Plus 1 electron 17+ 17+ electron 10 electrons 17 electrons 11 electrons 18 electrons Na+ Cl– 25 26 An Introduction to the Periodic Table (Self study)