Chemistry

• A scientist in the broadest term means a person who studies the art of science
• Your training in the university does not just involve memorizing your notes and passing your exams, it involves something more
• As young scientists it is our obligation to inculcate into our faculties the scientific spirit and methodology
• Facts


  Observed phenomena which occur naturally or are induced by experimentation. Facts are reality and are always the same under the same conditions
• Concepts


  Ideas in the form of hypotheses, theories or laws. Valid concepts are retained based on quantitative experimentation, otherwise they are rejected as fallacious ideas
• Hypothesis


  An assumption which needs further experimental investigation for its proof
• Theory


  A logical interpretation of a body of facts. A theory is retained as long as it can predict new facts and provide rational explanation of the facts. It is rejected as soon as it fails to fit the facts
• Law or Principle


  A statement of generalisation that relates an established fact
• The scientific method


  1. Observation of facts

  2. A hypothesis is put forward based on these facts

  3. The hypothesis is tested via quantitative experimentation to acquire more facts

  4. If the results of the experiments agree with the hypothesis, then the hypothesis becomes a theory

  5. The theory is continually used to predict new facts which are tested by experimentation and the results rationalised on the basis of the theory

  6. As new experimental facts are being discovered, the theory could be modified and retained until it fails to explain new facts

  7. When this happens it is rejected as a fallacy and replaced by a new theory
• The rejection of a theory should not be considered as retrogression. All theories provoke search for new facts and in this respect all theories, rejected or valid are successful concepts
• Matter


  Anything that occupies space and has mass and inertia. It may be classified as solid, liquid or gas. Matter is also defined in terms of radiant energy as any mass-energy with velocity less than that of light (3 x 10^8 m/s)
• Chemistry


  The study of the nature and composition of matter and the changes associated with all ordinary forms of matter
• At about 400BC, the first idea that matter is composed of particles was put forward by the Great philosopher Democritus and his teacher Leucippus
• Plato and Aristotle did not accept the idea that matter is made up of particles and as a result the concept lay dormant for about 20 centuries
• In about 1660AD, the idea of the particulate nature of matter resurfaced during the time of Robert Boyle and Isaac Newton
• At about 1800AD experimental evidence in support of atomic hypothesis was provided and the results were summed up in the Laws of Chemical Combinations by Mass, or the Gravimetric Laws
• Law of Conservation of Mass


  Matter is neither created nor destroyed during a chemical reaction
• Law of Constant Composition (Definite Proportions)


  Pure chemical compounds, no matter how prepared, have fixed composition by mass
• Law of Multiple Proportions


  The masses of an element A which combine with a fixed mass of an element B are in simple whole number ratio
• Law of Reciprocal Proportions


  The masses of elements A, B and C that separately combine with a fixed mass of element D are the masses in which they will combine with themselves
• Experiments have shown that these laws are not strictly valid in all cases because of interconvertibility of mass and energy, discovery of isotopes, non-stochiometric compounds, and isotopes
• Before Dalton, William Higgins in 1789 had presented a similar theory to Dalton's in a book he published that year
• Dalton's Atomic Theory


  1. Atoms of the same element are identical in mass
  2. Matter is made up of tiny particles called atoms
  3. Atoms do not subdivide into smaller particles during a chemical reaction
  4. When atoms react they do so in simple whole numbers
• Dalton's theory is no longer strictly valid due to the discovery of isotopes, subatomic particles, nuclear chemistry, and non-stochiometric/carbon compounds
• Dalton's theory stimulated many new ideas and helped a great deal in the development of chemistry more than any other theory
• Gay-Lussac's Law


  When gases combine they do so in volumes which bear a simple ratio to one another, and to the product if gaseous, temperature and pressure remaining constant
• Avogadro's Hypothesis


  Equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules, not atoms
• Cannizzaro's Principle


  The masses of equal volumes of gases at the same temperature and pressure must be in the ratio of their relative molecular masses
• Van Helmont (1577-1644AD)


  Discovered CO₂, used the term gases. He believed all substances arose from H₂O.
• Robert Boyle (1627-1691 AD)


  Boyle's law: P x 1/v; T = constant. In defining elements he implied that matter was discrete, made up of particles.
• Isaac Newton (1642-1727)


  Newton's laws of motion in explaining Boyle's observations implied a discrete nature of matter.
• Lemery (1675) & Boerhaave (1724)


  Implied discrete nature of matter.
• William Higgins (1789AD)


  In his suggestions on chemical combinations implied a particulate nature of matter.
• 1800AD


  1st experimental evidence of particulate nature of matter surfaced and was summed up into the Gravimetric Laws.
• John Dalton (1766-1844)


  First Atomic theory in his articles "A New system of chemical philosophy" explaining Laws of chemical combination by mass.
• Consequences of Dalton's Atomic theory


  • Determination of Atomic mass of Elements
  • Use of Atomic symbols
  • Concept of molecules, compounds, compound atoms
  • Use of relative instead of actual atomic masses
  • Use of equivalent masses
• Shortcomings: No numerical ratio in which atoms react.
• Gay Lussac's Law (1808AD)


  Law of combination by volume (ratio of combination). When gases react they do so in volumes which bear a simple ratio to one another and to the volume of the product if gaseous T and P constant.
• Consequences: Allowed for reacting vols: to represent, reacting atoms
• Formula for H,O Bérzelius


  • 2 vol H + 1 vol O = 2 vols water
  • 2H + O = 2H₂O
  • H + O = H₂O