Concept 01 / 06Some Basic Concepts of Chemistry
Laws of Chemical Combination and Dalton's Atomic Theory
Conservation of mass, definite proportions, multiple proportions, reciprocal proportions, Gay-Lussac's law of gaseous volumes and Avogadro's law, how Dalton's atomic theory explains them, and the modern exceptions such as isotopes and non-stoichiometric compounds.
Chemistry became quantitative when a handful of experimental laws about the masses and volumes of reacting substances were discovered. Each law is a direct consequence of matter being made of atoms that combine in whole-number ratios.
Law of conservation of mass (Lavoisier). In a chemical reaction the total mass of the reactants equals the total mass of the products. Atoms are rearranged, not created or destroyed. (In nuclear reactions mass and energy interconvert through ; in chemical reactions the mass change corresponding to reaction energies is far too small to measure.)
Law of definite proportions (Proust). A given compound always contains the same elements combined in the same fixed ratio by mass, whatever its source. Pure water always contains hydrogen and oxygen in the mass ratio . Exceptions: compounds made with different isotopes (for example and differ in mass ratio), and non-stoichiometric solids such as , whose composition varies because of crystal defects.
Law of multiple proportions (Dalton). If two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio. With g of carbon, oxygen combines as g in and g in , a ratio of , as in the figure. This law is the strongest evidence for atoms: a whole-number ratio of masses reflects a whole-number ratio of atoms.
Law of reciprocal proportions (Richter). If elements and each combine with a fixed mass of , then the ratio in which and combine with each other is the same as, or a simple multiple of, the ratio of those masses. This led to the idea of equivalent masses.
Gay-Lussac's law of gaseous volumes. At the same temperature and pressure, the volumes of reacting gases and gaseous products are in simple whole-number ratios. For example, volumes of hydrogen react with volume of oxygen to give volumes of steam.
Avogadro's law. Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. It explains Gay-Lussac's law, and it forced the idea that elementary gases such as hydrogen and oxygen exist as diatomic molecules: gives volumes of steam, which would be impossible if oxygen were monatomic.
Dalton's atomic theory. Matter consists of indivisible atoms; atoms of an element are identical in mass and properties; atoms of different elements differ; compounds form when atoms combine in simple whole-number ratios; reactions rearrange atoms. Modern corrections: atoms are divisible (electrons, protons, neutrons), and atoms of one element can differ in mass (isotopes). The theory still correctly explains all the mass laws above.
Key Takeaways:
- Mass is conserved in chemical reactions.
- Definite proportions: fixed mass ratio for a compound (exceptions: isotopes, non-stoichiometric solids).
- Multiple proportions: masses of one element per fixed mass of the other are in whole-number ratio.
- Gay-Lussac: gas volumes react in simple ratios at the same and .
- Avogadro: equal volumes contain equal numbers of molecules; elementary gases like , , are diatomic.
- Pitfall: applying Gay-Lussac's volume ratios to liquids or solids.
Brain-Teaser 1: Two oxides of a metal contain and oxygen by mass. Show that they obey the law of multiple proportions.
Solution: Per g of oxide: first oxide has g metal and g O, so O per gram of metal . Second: g metal and g O, so . Ratio . A simple whole-number ratio () confirms the law; these are the data for and (oxygen per iron ). The trick is always to fix the mass of one element before comparing.
Brain-Teaser 2: mL of a gaseous hydrocarbon needs mL of oxygen for complete combustion and gives mL of , all at the same and . Find the formula.
Solution: By Avogadro's law, volumes are proportional to moles. . Per mL of hydrocarbon: and , so . The gas is (ethene). Volume data translate directly into mole ratios for gases.
Brain-Teaser 3: When g of magnesium is burnt in a sealed flask containing excess oxygen, the flask's mass is unchanged. When burnt in an open crucible, the crucible gains mass. Explain both observations.
Solution: In the sealed flask, the oxygen consumed was already inside, so the total mass of the closed system is conserved. In the open crucible, oxygen from the air (not originally weighed) combines with the magnesium: , so the solid gains the mass of that oxygen (about g, giving g of ). Conservation of mass applies to the whole system, including gases entering or leaving, which is the usual source of apparent violations.
Practice this concept
Free · no account required · instant solutions