E1MultiPropsF08_Revised - EXPERIMENT 1 The Law of Multiple...

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1 – 1 EXPERIMENT 1 The Law of Multiple Proportions Objective: The law of multiple proportions will be illustrated by recording and comparing masses for compounds composed of the elements copper and bromine. Introduction: The birth of chemistry as a separate intellectual discipline is usually dated to 1661. It was in this year that the Englishman Robert Boyle (1627 – 1691) published his book The Skeptical Chymist , a quantitative examination of the nature and behavior of gases. In addition, Boyle was the first to clearly define an element as a substance that cannot be chemically broken down further and to suggest that a substantial number of different elements might exist. Progress in chemistry was halting in the 100 years following Boyle. It wasn’t until the work of Antoine Lavoisier (1743 – 1794) that the next great leap forward was made. In 1774, Lavoisier performed an experiment in which he heated a sealed glass vessel containing tin and air. Heating converted the tin into tin oxide. Lavoisier showed by careful measurements that the mass prior to heating (glass vessel + tin + air) was equal to the mass after heating (glass vessel + tin oxide + remaining air). In other experiments in which he weighed the reactants and products of various reactions, Lavoisier also found that the starting and ending masses are the same. These results are summarized in what came to be known as the law of conservation of mass . Law of conservation of mass: mass is neither created nor destroyed in chemical reactions. The decades following Lavoisier were dominated by chemists who, following Lavoisier’s lead, performed careful weighing experiments to study chemical reactions and determine the compositions of chemical compounds. One of these chemists, Joseph Proust (1754 – 1826), demonstrated what is now known as the law of definite proportions . Law of definite proportions: different samples of a pure chemical substance always contain the same proportion of elements by mass. For example, every sample of water contains 1 part hydrogen and 8 parts oxygen by mass; every sample of carbon dioxide contains 1 part carbon and 2.7 parts oxygen by mass; and so on. Proust’s discovery and the prior work of Lavoisier inspired John Dalton (1766 – 1844), an English schoolteacher, to suggest that elements were composed of tiny particles called atoms . In 1808, Dalton published A New System of Chemical Philosophy in which he presented his atomic theory of matter: Elements are made of tiny particles called atoms. Each element is characterized by the mass of its atoms. Atoms of the same element have the same mass, but atoms of different elements have different masses. Chemical compounds are formed when atoms combine in small, whole-number ratios.
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This note was uploaded on 10/06/2008 for the course CHEM 2090 taught by Professor Zax,d during the Fall '07 term at Cornell University (Engineering School).

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E1MultiPropsF08_Revised - EXPERIMENT 1 The Law of Multiple...

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