Chemistry VSI

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Bonds: ionic, covalent, metallic; reactions: proton transfer, electron transfer, radical, Lewis acids
Author

Stephen J. Mildenhall

Published

2024-06-21

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Chemistry

  • Divisions of chemistry
    • Physical = physics \(\cap\) chemistry; quantum mechanics (for structure of atoms and molecules) and thermodynamics for role and deployment of energy; rates of reactions
    • Organic = compounds of carbon(!), midpoint in first row of periodic table
    • Inorganic = everything else; includes solid-state chemistry
    • Analytic = what is there incl. forensic
    • Biochemistry = reactions related to living things
    • Industrial = scale it up
  • Reactant and product
  • Solar system: earth is 100 sun radii from the sun; atom’s electrons are 10,000 times the nucleus radius away—a fly in a football stadium
  • A grain of salt contains more ions than there are stars in the visible universe
  • Anions are negative, gained an electron
  • Cations are positive, lost an electron
  • Four types of bonds
    1. ionic bonds, NaCl, swap or transfer an electron; make aggregates “chunks of solids”; all rigid, brittle solids at room temperature; bones
    2. covalent bonds, H2O, NH3, share an electron, make molecules; “just about all” substances that are liquids at room temperature are molecules with covalent bonds; sharing allows an electron shell to be completed (H2 2 protons plus O 8 gives 10 protons filling the second shell, like Neon; same with ammonia 3 + 7 = 10 and methane CH4 6 + 4); sucros C12H22O11 is a solid covalent compound
    3. metallic bonds: cations in a sea of electrons, hence conduct electricity
  • Four types of reactions
    1. proton transfer: acids = donor and bases acceptor; H written first indicates donor; alkali = base dissolved in water; salt = reaction of acid and base; water is an acid and a base
    2. electron transfer: oxidation (originally reaction with O, now loss of an electron) and reduction (originally extraction of metal from ore, now electron gain); batteries
    3. radical reactions: radical = molecule with odd number of electrons, mostly “aggressively reactive”; one unpaired electron, flames, polymerization
    4. Lewis acids: base reactions; colors; breathing
  • Techniques
    1. classical laboratory equipment
    2. analysis and synthesis
    3. filtration and distillation
    4. spectroscopy emission and absorption lines; UV light with enough energy to move electrons; IR light stimulates vibrations of molecules
    5. chromatography
    6. nuclear magnetic resonance = MRI, of H “magnets”,
    7. mass spectroscopy
    8. X-ray diffraction, crystallography; X-ray wavelengths comparable to atom-atom spacing in molecules
    9. STM, AFM: scanning tunneling microscopy, atomic force microscopy
    10. computational
  • H and O can be stored together at room temperature
  • you need 275 molecules of water before it can show ice-like properties and about 475 before it becomes truly ice

GPT on methane-oxygen reaction

The reaction between methane (CH₄) and oxygen (O₂) is a combustion reaction, which produces carbon dioxide (CO₂) and water (H₂O) as products. The balanced chemical equation for this reaction is:

\[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}. \]

Steps of the Reaction:

  1. Reactants:
    • Methane (CH₄) is a hydrocarbon with one carbon atom and four hydrogen atoms.
    • Oxygen (O₂) is a diatomic molecule.
  2. Combustion Process:
    • Combustion involves breaking the bonds in the reactants (CH₄ and O₂) and forming new bonds to create the products (CO₂ and H₂O).
    • This reaction requires an initial input of energy to overcome the activation energy barrier, usually provided by a spark or flame.
  3. Bond Breaking and Formation:
    • Methane has four C-H bonds.
    • Oxygen has a double bond between the two oxygen atoms (O=O).
  4. Intermediate Steps:
    • Methane reacts with oxygen in a series of steps involving free radicals and various intermediate species, though these intermediates are not shown in the overall balanced equation.
  5. Products:
    • Carbon dioxide (CO₂) is produced, with one carbon atom double-bonded to two oxygen atoms.
    • Water (H₂O) is produced, with two hydrogen atoms each bonded to an oxygen atom.

Energy Considerations:

  • Combustion of methane is an exothermic reaction, meaning it releases energy in the form of heat and light.
  • The energy released can be calculated from the bond energies of the reactants and products.

Applications:

  • This reaction is commonly used in natural gas combustion for heating, cooking, and electricity generation.

Example Calculation:

To show the energy aspect, consider the bond energies (approximate values in kJ/mol):

  • C-H bond: 413
  • O=O bond: 498
  • C=O bond (in CO₂): 799
  • O-H bond: 467

For methane combustion:

  1. Breaking bonds:
    • 4 C-H bonds in CH₄: ( 4 = 1652 ) kJ/mol
    • 2 O=O bonds in O₂: ( 2 = 996 ) kJ/mol
    • Total energy to break bonds: ( 1652 + 996 = 2648 ) kJ/mol
  2. Forming bonds:
    • 2 C=O bonds in CO₂: ( 2 = 1598 ) kJ/mol
    • 4 O-H bonds in 2 H₂O: ( 4 = 1868 ) kJ/mol
    • Total energy released in forming bonds: ( 1598 + 1868 = 3466 ) kJ/mol
  3. Net energy change:
    • Energy released (exothermic reaction): ( 3466 - 2648 = 818 ) kJ/mol

This net release of energy explains why methane combustion is an efficient and widely used process for producing heat and power.

Deets

  • Peter Atkins
  • Volume 417
  • Published 2015