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Laws of ChemIstry. Law of Conservation of Mass. Lavoisier , the father of modern chemistry did the following:
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Lavoisier, the father of modern chemistry did the following: Step I: He put a strip of magnesium ribbon in a crucible which was open to air. When he heated the crucible , a white powder was formed.The mass of the powder was greater than that of magnesium strip used. Step II: When he heated the same strip of magnesium in a sealed container,a white powder formed again. But this time , the mass of the closed system was the same before and after reaction. Step III: the sealed container of step II was opened and closed very quickly. The container was reweighed. It showed an increase in mass. What is your conclusion on Lavoisier’s experiment?
LAW OF CONSERVATION OF MASS The mass of a closed system will remain constant, whatever happens inside the system. Matter cannot be created or destroyed, although it may be rearranged. This implies that for any chemical process in a closed system, the mass of the reactants must equal to the mass of the products.
total mass = total mass calcium oxide + carbon dioxide calcium carbonate CaO + CO2 CaCO3 Law of Conservation of Mass: The total mass of substances does not change during a chemical reaction. reactant 1 + reactant 2 product 56.08g + 44.00g 100.08g
THE LAW OF CONSTANTPROPORTIONS “Whenever two elements combine to form a compound, they combine in a constant proportion by mass.” Joseph Proust
EXPERIMENTAL RESULTS OFFeS (Iron sulfide formation) Let’s find the ratio of mass of iron to mass of sulfurin each trial. What is your conclusion?
Thecombiningratio of theelementsto form a specificcompound is alwaysconstant
First, chemists recognized that the same two elements may combine to give more than one kind of compound. Can you give an example? • Then, they have observed that there is a simple relationship between the masses of the one element that combines with a constant mass of the other element in the compounds
In 1803 Dalton noted that oxygen and carbon combined to make two compounds. The Atomic Basis of the Law of Multiple Proportions
For compounds NO2 and N2O5 Give a ratio between nitrogen in the first and second compounds.
THE LAW OF MULTIPLE PROPORTIONS (John Dalton 1766-1844) • “When more than one compound are produced by the same two elements, the ratio of the masses of one of these elements combined with a fixed mass of the other element can be expressed by small and whole numbers. “
Dalton’s proposal was an important step towards atomic theory which was the basis for chemical formulas for compounds...
John Dalton's first paper on the atomic theory appeared in 1803... Dalton’s Postulates 1)All atoms of any one element are exactly alike in all aspects; in particular, same mass. 2)Atoms of different elements have different properties; in particular, different mass. 3)Atomscombine in small, wholenumberratios. 4)In the chemical reactions that may occur, none of the atoms are destroyed and no new ones are created.
An image from John Dalton’sA New System of Chemical Philosophy, the first modern explanation of atomic theory...
Some of the symbols Dalton used for atoms of elements and molecules of compounds. He probably used a circle for each because, like the ancient Greeks, he thought of atoms as tiny, round hard spheres.
Dalton’s Laws • The Law of Constant Composition: • “Any given compound always consists of the same atoms and the same ratio of atoms. For example, water always consists of oxygen and hydrogen atoms, and it is always 89 percent oxygen by mass and 11 percent hydrogen by mass” • 2. The Law of Conservation of Mass: • “The total mass of materials before and after a chemical • reaction must be the same. For example, if we combine • 89 grams of oxygen with 11 grams of hydrogen under • the appropriate conditions, 100 grams of water will be • produced—no more and no less.”
Dalton’s Laws 3. The Law of Multiple Proportions: “If two elements combine to form more than one compound,the masses of one of the elements that can combine with a given mass of the other element are related by factors of small wholenumbers” For example, water has an oxygen-to-hydrogen mass ratio of 7.9:1. Hydrogen peroxide, another compound consisting of oxygen and hydrogen, has an oxygen-to-hydrogen mass ratio of 15.8:1. The ratio of these two ratios gives a small whole number.
Shortcomings of Dalton’sAtomicTheory: • Not allatoms of thesame element arealike in mass (isotopes) • Atoms can be broken apart
Law of CombiningVolumes 1 volume of nitrogen and 3 volumes of hydrogen react to form 2 volumesammonia
Gay-Lussac (1778-1850) He found the oxygen could combine with exactly twice its own volume of hydrogen.
Avogadro’s Law: “Equal volume of different gases (under identical conditions of temperature and pressure)contain equal number of particles.” “
Amadeo AVOGADRO In the early 1800s, the chemical formulas of even simple substances like hydrogen, chlorine, and water were not known with certainty. In 1811 that Amedeo Avogadrousedthe term moleculeforthefirst time, by which he meant a group of atoms Hishypothesiswas that gas volumes musthave something to do with molecules, because molecules also reacted and are formed in ratios of small whole numbers. For example, Hydrogen gas + Chlorine gas →Hydrogen Chloride gas1 dm3 + 1 dm3→ 2 dm3
Amadeo AVOGADRO Avogadro's Principle Avogadro’s Law: Equal volumes of gases contain equal numbers of molecules (Basically, if there are10 liters of oxygen gas and 10 liters of helium gas in the same room, each gas will have the exact same number of particles in it.)
