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[Page 1] For More Material Join: @JEEAdvanced_2024 [Page 2] For More Material Join: @JEEAdvanced_2024 [Page 3] For More Material Join: @JEEAdvanced_2024 [Page 4] Laws of Chemical Combination Chemical reactions take place according to certain laws. These laws are called the Laws of Chemical Combination. “These are no longer useful in chemical calculations now but gives an idea of earlier methods of analysing and relating compounds by mass.” v Law of Conservation of Mass [Lavoisier (1774)] During any physical or chemical change, the sum of masses of all substances present in reactions vessel remain conserved. v Law of Constant Composition or Definite Proportions [Proust (1799)] In a given chemical compound, the elements are always combined in the same proportions by mass. v Law of Multiple Proportions [Dalton (1803)] Whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers. v Law of Reciprocal Proportions [Richter (1792)] When two elements combine separately with a fixed mass of a third elements then the ratio of their masses in which they do so is either same or some whole number multiple of the ratio in which they combine with each other. v Gay-Lussac’s Law of combining volumes According to Gay-Lussac’s law of combining volume, when gases react together, they always do so in volumes which bear a simple ratio to one another and to the volumes of the products, if these are also gases, provided all measurements of volumes are done under similar conditions of temperature and pressure. v Avogadro’s Law The volume of a gas (at fixed pressure and temperature) is proportional to the number of moles (or molecules of gas present). Mathematically we can write V n μ Or Equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules. 1 STOICHIOMETRY 1 For More Material Join: @JEEAdvanced_2024 [Page 5] Dalton’s Atomic Theory v Matter consists of tiny particles called atoms. v Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart. v In any sample of a pure element, all the atoms are identical in mass and other properties. v The atoms of different elements differ in mass and other properties. v When atoms of different elements combine to form compounds, new and more complex particles are formed. However, in a given compound the constituent atoms are always present in the same fixed numerical ratio. Modern Atomic Theory v Atom is no longer considered to be indivisible : It has been found that an atom has a complex structure and is composed of sub-atomic particles such as electrons, protons and neutrons. v Atoms of the same element may not be similar in all respects : Ex. Isotopes ( , ) 11 23 11 24 Na Na v Atoms of different elements may be similar in one or more respects : Ex. Isobars. ( , ) 20 40 18