Chemistry lecture 1

• Measurement


  Comparison of an object to some known standards
• Nature of Science


  1. Observation

  2. Hypothesis
• Observation


  Allows scientists to collect empirical evidence that can be used to test hypotheses and theories
• Hypothesis


  A proposed explanation or prediction for a phenomenon that is based on limited evidence or preliminary data
• How does a 'New Theory' get accepted?

  1. Provide clear evidence and testable explanation

  2. Predictions must be verified

  3. Consistent with established principles and existing knowledge

  4. Endorsement of experts, peer review, and publication
• Model


  Creates mental pictures; care must be taken to understand the limits and not take it too seriously
• Theory


  Detailed and can give testable predictions
• Law


  Brief description of how nature behaves in a broad set of circumstances
• Principle


  Similar to a law, but applies to a narrower range of phenomena
• Experimental Error


  Unavoidable deviations from the true value
• Reducing Random Error


  1. Don't rush, be careful

  2. Take as many trials as possible
• Systematic Error


  Can sometimes be accounted for, impacts absolute uncertainty
• Absolute Error


  Difference between measured and true value
• Relative Error


  Absolute error divided by true value
• Percent Error


  Relative error multiplied by 100
• Accuracy


  How close a measurement is to the true value
• Precision


  How close two or more measurements are to each other
• Uncertainty


  Quantitative measure of how much values deviate from a standard or accepted value
• Unit Conversion


  Knowing the conversion factors between units
• Order of Magnitude


  Rounding off all numbers to one significant figure and then calculating
• Significant Figures


  Digits that are known with certainty plus one estimated digit
• Rules for Significant Figures

  1. When adding/subtracting, round to least precise decimal place

  2. When multiplying/dividing, round to fewest significant figures
• Dimensions


  The base units that make up a quantity
• Dimensional Analysis


  Checking the dimensions of all quantities in an equation to ensure they have the same dimensions
• Chemical bond


  An effect that causes certain atoms to join together to form enduring structures that have unique physical and chemical properties
• C.A. Coulson (1910-1974): '"No one has ever seen one[bond]. No one ever can. It is a figment of our own imagination."'
• Chemical bonding


  Occurs when one or more electrons are simultaneously attracted to two nuclei
• Classical


  Models that do not take into account the quantum behavior of small particles, notably the electron
• Ionic bond


  • The bonding electron is not shared at all, but transferred
  • One atom assumes a net positive charge, and the other a net negative charge
  • The bond results from electrostatic attraction between the atoms
• Ionic bond example


  • NaCl: Na → Na+ (loss of an electron), Cl → Cl- (gain of an electron)
• Limitation of ionic model

  • It applies well only to a class of solids composed of Group 1 and 2 elements with highly electronegative elements such as the halogens
• Covalent bond


  A pair of electrons shared between two atoms can create a mutual attraction, and thus a chemical bond
• Polar covalent bond


  Bonds with ionicity percentage less than 35%, indicating unequal sharing of electrons
• Polar covalent bond example


  • SO2: Ionicity % = 15%, hence SO2 is polar covalent
• Limitation of covalent model

  • It raises a question about the nature of the orbitals in which the shared electrons are contained
• Coordinate covalent bond


  An attraction effect between two nucleus/atoms, one of which is electron rich while the other is an electron deficient species, with unequal sharing of 2 electrons
• Coordinate covalent bond example

  • NH3 + H+ → NH4+
• Metallic bond


  An attraction effect between two nucleus/atoms of a metal, arising from electrostatic pull between positive centres embedded in a sea of electrons
• Types of intermolecular forces

  • Strong ionic attraction
  • Temporary dipole-dipole forces
  • Permanent dipole-dipole forces
  • Weak London dispersion forces or van der Waal's force
  • Hydrogen bond
• Ionic attraction


  • Exists between ionic molecules