The Law of multiple proportions is attributed to the English Chemist John Dalton, who discovered it in 1803 (Jaffe, 2012). The Law states that: When element combines with another to form more than one compound the masses of the second element combining with a fixed mass of the first element bear a simple ratio (Chang, 2008). The law of multiple proportions forms one of the basic laws of Stoichiochemistry together with the law of conservation of mass and the law of definite proportions. This essay seeks to explore the law of multiple proportions by looking at the hypothesis, strengths and limitations and Gay-Lussac interpretation of the law.
Dalton at the observation of how elements associate to form compounds was grounded on the idea that repulsive and attractive forces. Moreover, he made use of Proust study evidencing the law of definite proportions (Jaffe, 2012). Dalton assigned Hydrogen which was the lightest known element a relative mass of 1. This followed from Proust data which showed that hydrogen and oxygen bonded in a mass ratio of 1:8. Hence, oxygen had a relative mass of 8. Dalton chose to use 7. Dalton further hypothesizes that it was probably for elements to form a diverse number of compounds if their atoms were attracted to one another in different quantities. For this hypothesis, he evidenced using substances like carbon monoxide, carbon dioxide and the various oxides of nitrogen. Additionally, he held the important idea that atoms are indivisible and only rearrange in a chemical reaction. He maintained on the integer mass ratios linking the changing elements in such compounds.
The elements of constant mass are carbon with a relative mass of 12. In carbon monoxide, the mass is 16 while in carbon dioxide it is 32. These two compounds are in simple whole number ratio, 16:32 which translate into 1:2.
The law can be applied differently but in a complicated manner - for example, N2 O3 and NO. If nitrogen is selected as the constant mass in this pair of compounds, two NO are compared to one N2 O3. The masses of the variable elements in the ration N2 O3:2NO will have a ratio of 48:32 which reduces to 3:2.
Gay-Lussac interpretation of the law.Gay-Lussac's experimental work mentioned the law of multiple proportions in a bid to define the absolute formulas for compounds (Rocke, 1984). Gay-Lussac established that two volumes of hydrogen react with 1 volume of oxygen to form 2 volumes of gaseous water and that 1 volume of hydrogen reacts with 1 volume of chlorine to form 2 molecules of hydrogen chloride (Chang, 2008). Gay-Lussac indicates that the combining weights of several compounds of the same element are modestly related. That is, the weights of one element that can associate with a specified amount of another element are simple multiples. This proves the law of multiple proportions. On this note, Gay-Lussac turned to products as well as reactants. He asserts that in a reaction where both the products and reactants are gaseous, then the volumes are closely related. In his experiment, the volume of carbon dioxide produced in the reaction of carbon monoxide and oxygen is the same as the volume of carbon monoxide used up, not the combined volume of carbon monoxide and oxygen. Rocke determined that Gay-Lussac switch from combining ratios in mass to combining in volume was key to determining the density of gases. Though Dalton's theory did not admit compounds in intermediate composition, Gay-Lussac's experiments demonstrate the law of multiple proportions, positing that, an association of a molecule of one reagent with one or two molecules of the other.
Strengths and Limitations
The law alongside Proust's law of definite proportion was successful in the prediction of unknown chemical compounds (Wakeham, 1945). Dalton evidenced that the chemical formula of these pure substances such as ionic and covalent solids gives those relative ratios of the numbers of their constituent atoms. This is true from the determination of chemical formulas and the accompanying relative atomic masses. This evidence became one of the building blocks for Dalton theory.
The law of multiple proportions holds in experiments that require simple compounds. For example, in an experiment involving hydrocarbon decane (chemical formula C10 H22) and undecane (C 10H 24), one observes that 100 grams of carbon react with 18.46 grams of hydrogen to produce decane or with 18.31 grams of hydrogen to produce undecane, for ratio of hydrogen masses of 121: 120 Is achieved (Chang, 2008). This ratio is hardly a ratio of small whole numbers as hypothesized by the law.
The law fails in inorganic compounds having an elemental composition whose proportion cannot be represented by integers (non-stoichiometric compounds) as well as in polymers and oligomers. Moreover, the existence of isotopes of hydrogen-like H1 or H2 causes discrepancies similar to that observed in the law of constant proportions (Bhushan, & Rosenfeld, 2000). Hence the same isotope or mixture of isotopes should be used throughout the preparation of a series of compounds.
Bhushan, N., & Rosenfeld, S. (2000). Of Minds and Molecules: New Philosophical Perspectives on Chemistry. New York, NY: Oxford University Press.
Chang, R. (2008). General Chemistry: The Essential Concepts. New York, NY: McGraw-Hill College.
Jaffe, B. (2012). Crucibles: The Story of Chemistry from Ancient Alchemy to Nuclear Fission. North Chelmsford, MA: Courier Corporation.
Rocke, A. J. (1984). Chemical Atomism in the Nineteenth Century: From Dalton to Cannizzaro. Columbus, US: Ohio State University Press.
Wakeham, G. (1945). The law of multiple proportions. Journal of Chemical Education, 22(6), 300. doi:10.1021/ed022p300
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