Chemistry Basics: Conservation of Mass, Elements, and Proportions (Lecture Notes)

Conservation of Mass

  • Demonstration: weigh a candle with air and all contents inside a closed container (e.g., under a glass bell) and then burn the candle. After burning, appearances may show that mass is gone, but the gas released still weighs the same as the original contents. This illustrates the conservation of mass: matter is neither created nor destroyed in chemical changes.
  • Reframing: “conservation of matter” and “conservation of mass” are the same concept; mass is conserved through chemical reactions.
  • Balancing equations is tied to this law: the number of each type of atom on the reactant side must equal the number on the product side.
  • Recurrent reminder: balancing equations is a practical way to enforce the law of conservation of mass.

Elements, Atoms, and Compounds

  • Levasseur/Lavoisier’s idea: matter can be reduced to a small set of fundamental building blocks called elements.
  • Lavoisier’s limitation: he inferred the existence of simple substances but did not know about atoms or their structure.
  • Then, the notion of atoms arises: smallest piece of an element that retains its properties; atoms combine to form compounds.
  • Elements are the items on the periodic table; there are currently 118 elements known.
  • Of these, 94 occur naturally on Earth; the other 24 are synthetic (man-made) and often decay quickly (e.g., technetium, plutonium).
  • Early chemists identified several elements and compounds, and Lavoisier named and defined some of them; he published the first chemistry textbook and introduced the metric system for measurement.
  • Common examples of elements: Gold (Au), Oxygen (O), Nitrogen (N).

The Structure of Matter and the Periodic Table

  • Elements are organized in the periodic table; today there are 118 known elements.
  • The table is arranged primarily by atomic number (the number of protons in the nucleus): the order is 1, 2, 3, …, corresponding to Hydrogen, Helium, Lithium, …
  • Atomic number Z is the number of protons; protons do not change (they are invariant for a given element), while electrons can be added/removed to form ions.
  • Hydrogen (element #1) has 1 proton in its nucleus; the atomic number is the same as the number of protons.
  • The mass of an atom is related to the number of protons (and neutrons), not just the number of protons alone.
  • The modern periodic table has 118 known elements; 94 are naturally occurring; 24 are synthetic.

Real-World Context and Historical Notes

  • Common elements essential for life: Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), and Phosphorus (P).
  • These elements are central to biology and ecology: they appear in biological molecules, water, and soil nutrients (e.g., N and P for fertilizers).
  • Water is a molecule made of hydrogen and oxygen: ext{H}_2 ext{O}; life depends on water quality.
  • Carbon backbone is central to organic chemistry; carbon is abundant in many materials (carbon compounds, silicon-based devices, etc.).
  • Everyday metals: noble metals like Gold (Au) and Silver (Ag) resist oxidation; they are used for coins because they stay lustrous and do not rust easily.
  • Aluminum oxide and aluminum metal: historically, aluminum metal was expensive; a breakthrough in electrolysis allowed extraction of pure aluminum from aluminum oxide, making aluminum valuable and widely used.
    • A famous anecdote: the aluminum pyramid atop the Washington Monument symbolized the metal’s value after the development of the electrolytic process.
    • A reference to a college student at Oberlin who improved the process via electrolysis; the public display emphasized aluminum’s newfound accessibility.
  • The periodic table also groups metals by properties; coinage metals (like nickel, copper, silver) cluster together in the table.
  • In technology, smartphones rely on a range of elements, including rare earth elements (REEs) such as dysprosium, europium, and others; REEs are essential for many modern electronics and are relatively scarce, raising geopolitical and economic considerations.
  • Geopolitical context mentioned: access to rare earth elements and how nations seek to secure supplies for electronics and defense.

Elements, Compounds, and Molecules

  • An element is a pure substance that cannot be broken down into simpler substances by chemical means; it is defined by its atoms.
  • An atom is the smallest unit of an element that retains its properties.
  • A molecule is a group of atoms bonded together.
  • A compound is a pure substance composed of two or more different elements chemically bonded together. Examples:
    • Water: ext{H_2O} (two hydrogens and one oxygen)
    • Sodium chloride: ext{NaCl} (sodium and chloride ions in a compound)
  • Not all molecules are compounds: diatomic nitrogen is N₂, which is a molecule but not a compound because it contains only one element.

Writing Formulas and the Standard Convention

  • Standard convention for writing formulas uses subscripts to show how many atoms of each element are present in a molecule.
  • If no subscript is shown, it implies a count of one for that element (an “invisible one”).
  • Example interpretations:
    • ext{CO} means one carbon and one oxygen (one each) – here the carbon has an invisible subscript of 1:

    • C: 1, O: 1
    • ext{CO_2} means one carbon and two oxygens: C: 1, O: 2
  • Phosphoric acid example: formula ext{H3PO4}
    • Hydrogens: 3
    • Phosphorus: 1
    • Oxygens: 4
  • Quick exercise from the lecture: for phosphoric acid, count atoms as described above and balance as needed in reactions.

Law and Proportions in Chemistry (Key Concepts)

  • Law of Constant Composition (Law of Definite Proportions): any given pure substance has a fixed, definite composition of its constituent elements, regardless of source or method of preparation. Example: all water molecules are the same: ext{H_2O}.
  • Law of Definite Proportions: for a given compound, the elements are present in fixed mass ratios independent of the sample source.
  • Law of Multiple Proportions (John Dalton): when two elements form more than one compound, the ratios of the amounts of one element that combine with a fixed amount of the other element are simple whole-number ratios.
    • Example involving carbon and oxygen:
    • Carbon monoxide: ext{CO} (1 carbon:1 oxygen)
    • Carbon dioxide: ext{CO_2} (1 carbon:2 oxygens)
    • The essential idea: atoms combine in whole-number ratios to form compounds; there are multiple possible compounds with different ratios.
  • Distinction:
    • Definite proportions means a given compound has a fixed ratio (e.g., for ext{CO} vs ext{CO_2}, the ratios are 1:1 and 1:2, respectively).
    • Multiple proportions means there can be several different compounds formed from the same elements in different whole-number ratios.
  • Connection to atoms: these laws arise because matter is composed of atoms that combine in whole-number ratios to form compounds.

Notation Details and Practice with Formulas

  • When writing chemical formulas, subscripts indicate the number of atoms; if a subscript is absent, it implies 1.
  • Example from the lecture: the mass relationship in ext{CO} and ext{CO_2} is consistent with the law of definite proportions and multiple proportions:
    • In ext{CO_2}, the oxygen mass is twice that in ext{CO} (per molecule):
    • rac{m(O ext{ in } CO_2)}{m(O ext{ in } CO)} = 2
  • Practice writing formulas from an element list:
    • Phosphoric acid: ext{H3PO4} (H: 3, P: 1, O: 4)
  • Balance and interpret chemical formulas and their meaning in terms of atoms and molecules.

Common Elements and Life Dependence

  • Very common elements highlighted: Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), and Phosphorus (P) – essential for life and biological systems.
  • These elements are central to agriculture, soil chemistry, and biology:
    • Nitrogen and phosphorus are key nutrients in fertilizers; they support plant growth.
    • Plants need these elements to build proteins, nucleic acids, and energy molecules; humans obtain them by eating plants/animals.
  • Water (as above) and carbon-containing compounds are pervasive in biology and daily life.

Noble Metals, Coinage, and Historical Context

  • Noble metals (e.g., Gold, Platinum, Silver) resist corrosion and oxidation; they are commonly used for coins and jewelry due to their durability and appearance.
  • Aluminum story: aluminum was once hard to isolate; initial oxides were common, and extracting pure aluminum required significant energy and invention (electrolysis).
  • The Washington Monument anecdote: a display of aluminum as a symbol of technological progress and value at the time.
  • The periodic table and neighboring elements: Coinage metals (nickel, copper, silver) cluster together; platinum is a precious metal near these in the table.

Elements in Modern Technology and the Earth

  • The smartphone contains many elements beyond carbon, hydrogen, oxygen, and silicon; includes metals like aluminum, copper, nickel, and battery materials with cobalt, rare earth elements (REEs) such as dysprosium, europium, neodymium, and others.
  • Rare earth elements are called “rare” not because they are scarce, but because they are chemically similar and dispersed; high demand for electronics drives geopolitical considerations.
  • The transcript references geopolitics around access to rare earth elements (e.g., discussions about supply chains and international deals).

Safety, Show-and-Tell, and Lab Etiquette

  • Lab safety gear emphasized: wraparound goggles (side protection) for liquids under pressure, to prevent splashes and leaks.
  • Lab rules: students must wear protective equipment if enrolled in the lab; the instructor will enforce posture and behavior; non-compliance may lead to removal from lab.
  • The show-and-tell element: students bring materials to show practical lab gear and chemical interactions; a reminder that proper gear is essential for safe experimentation.

Recap: Distinguishing Elements, Compounds, and Molecules

  • Elements: pure substances consisting of only one type of atom; examples include H, C, N, O, noble gases, metals.
  • Atoms: the smallest unit of an element; cannot be broken into smaller units that retain the element’s properties.
  • Molecules: two or more atoms bonded together; may be of the same element (e.g., N2) or different elements (e.g., H2O).
  • Compounds: a pure substance composed of two or more different elements bonded together; the composition is fixed (e.g., H_2O, NaCl).
  • A quick check: is N_2 a compound? No, because it contains only one element (though it is a molecule).

Looking Ahead

  • The next topic will cover the scientific method more formally.
  • Homework: obtain the textbook, log in to the course materials, and prepare for an assignment due before the weekend; a formal homework will be given after a short period of access to the textbook.