Chemistry
Law of Definite Proportions
The Law of Definite Proportions states that a given chemical compound always contains the same elements in the same proportion by mass. This means that the ratio of the masses of the elements in a compound is always constant, regardless of the amount of the compound that is present. This law is a fundamental principle in chemistry.
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eBook - ePub- Dov M. Gabbay, Paul Thagard, John Woods(Authors)
- 2011(Publication Date)
- North Holland(Publisher)
5. The Law of Definite Proportions and the Problem of Chemical Combination
Lavoisier understood water to be composed of definite proportions of hydrogen and oxygen as a result of his analyses conducted in the 1770s. What was to become formally named as the Law of Definite Proportions in the first decade of the 19th century after Proust was generally acknowledged to have won a protracted dispute with Berthollet was thus already widely accepted by leading chemists. Perhaps it was Berthollet's championing the contrary view, that the strength of chemical affinity between given elements varies according to circumstances, that forced chemists to explicitly acknowledge and defend the principle of constant proportions. At all events, the law was taught as part of chemical theory from the early 19th century, and provided a criterion on the basis of which a definition of compounds could be formulated. It decided the issue between Davy, who thought air to be a compound, and Dalton, who took it to be a homogeneous mixture with variable composition (see the article “John Dalton” in this Volume). It was supposed that whilst the component elements of compounds were chemically combined, the component substances of homogeneous mixtures (called solutions) were merely mechanically mixed. In both cases, the properties of the original components are changed—the compound sodium chloride is neither a soft grey metal like sodium nor a greenish-yellow gas like chlorine (at room temperature and pressure), and brine is neither a white solid like sodium chloride nor a tasteless liquid like water. But the law of constant proportions only gave a criterion of chemical combination; it provided no explanation of what chemical combination amounts to.When the law was first formulated, the caloric theory of heat still reigned. But by mid century the basic tenet of the theory, that caloric is conserved in all processes, was disproved with the realisation, embodied in the first law of thermodynamics, that heat is not preserved but is interconvertible into mechanical work. Caloric gave way to the new concept of energy, in terms of which an equivalence between heating and working is measured determining the amounts of heating in principle obtainable from a given amount of working. The Black-Lavoisier account of phase change was no longer tenable and the distinction between, for example, base of oxygen and oxygen was undermined. Lavoisier's insight that a compound like water can't be said to be composed of elements associated with particular phases still held good: water is solid or liquid under conditions that oxygen and hydrogen are gases. The statement of the Law of Definite Proportions requires a concept of substance which is independent of phase. Accordingly, a substance, be it an element or a compound, is not in general bound to a particular phase, but the phase changes requiring input of a certain latent heat are changes in one and the same substance from one phase to another. (Compound substances might not be stable under phase change; ammonium chloride, for example, decomposes on heating under normal pressure rather than changing from solid to liquid or gaseous ammonium chloride.)
eBook - PDF- James Shipman, Jerry Wilson, Charles Higgins, Bo Lou, James Shipman(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
Did You Learn? ● ● The formula mass of a compound or element is the sum of the atomic masses given in the formula of a substance. ● ● The Law of Definite Proportions states that different samples of a given compound always contain the same elements in the same proportion by mass. 12.3 Dalton’s Atomic Theory Key Questions ● ● According to Dalton’s theory, are atoms lost or gained during chemical reactions? ● ● Which two laws did Dalton’s hypotheses explain? In 1803, atomic scientist John Dalton (●● Fig 12.4; see also Chapter 9.1) proposed the fol- lowing hypotheses to explain the laws of conservation of mass and definite proportions. 1. Each element is composed of tiny, indivisible particles called atoms, which are identi- cal for that element but are different (particularly in their masses and chemical prop- erties) from atoms of other elements. (c) (b) (a) Copper(II) sulfide Sulfur Copper + + + + + 10.00 g Cu 5.06 g S 15.06 g CuS 10.00 g Cu 7.06 g S 2.00 g S 15.06 g CuS 20.00 g Cu 5.06 g S 10.00 g Cu 15.06 g CuS Figure 12.3 The Law of Definite Proportions (a) The Law of Definite Proportions indicates that when Cu and S react to form a specific com- pound, they will always react in the same ratio by mass. (b) and (c) If the ratio in which they are mixed is dif- ferent, then part of one reactant will be left over. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 12.4 Ionic Bonding 343 2. Chemical combination is simply the bonding of a definite, small whole number of atoms of each of the combining elements to make one molecule of the formed com- pound.
eBook - PDF- Amos Turk(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
Second, the Law of Definite Proportions: since the mass of the atoms of any given element is constant and since the atoms, by assumption, combine in fixed whole number ratios for a given combination, the weight ratio of the elements in a given compound must be constant. For example, 20 χ 10 10 C atoms combine with 20 χ 10 10 0 atoms form-ing 20 χ 10 10 CO molecules. Each CO molecule contains 1 C atom and 1 Ο atom. Since each molecule contains the same number of C and Ο atoms and since the masses of these atoms are also constant, the mass ratio of carbon to oxygen must be constant in any given quantity of carbon monoxide. * Present knowledge recognizes that the atoms of a given element do not possess uni-form masses. Silver, for example, occurs as two isotopes (p. 12), but as found in nature, these silver atoms are always mixed in the same number ratio so that the average mass of the silver atoms is always the same. 65 • 4.4 LAW OF COMBINING VOLUMES 4.4 • THE LAW OF COMBINING VOLUMES; THE AVOGADRO HYPOTHESIS As a result of his experimental studies of the volumes of gases involved in chemical changes, Joseph Gay-Lussac concluded (1808) that the volumes of gases consumed and produced in a chemical reaction, mea-sured at the same temperature and pressure, are in ratios of small whole numbers. For example, at the same temperature and pressure, 10.6 ml hydrogen gas combines with 10.6 ml chlorine gas to produce 21.2 ml hydrogen chloride gas (1:1:2). 20.4 ml hydrogen gas combines with 10.2 ml oxygen gas to produce 20.4 ml water vapor (2:1:2). 12 ml hydrogen gas combines with 4.0 ml nitrogen gas to produce 8.0 ml ammonia gas (3:1:2). 6.3 ml water vapor combines with solid carbon to produce 6.3 ml hydrogen and 6.3 ml carbon monoxide gas (1:1:1).
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