Foundations of Atomic Theory Law of Conservation of Mass Mass is neither destroyed nor created during ordinary chemical reactions. Law of Definite Proportions The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. Lavoisier (credited with Law of Conservation of Mass). Proust (credited with Law of Definite Proportions). Dalton (credited with Law of Multiple Proportions). Law of Multiple Proportions If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first elements is always a ratio of small whole numbers.

Conservation of Atoms 2 H2 + O2 2 H2O O H2 H2O O2 + 4 atoms hydrogen John Dalton 2 H2O H O H2 O2 H2O + “Conservation of Atoms”   Description: This slide illustrates conservation of atoms in a chemical reaction. Basic Concepts Atoms are conserved in chemical reactions, but molecules are not. Atoms are neither created nor destroyed in chemical reactions. They are only rearranged. Equation coefficients can be interpreted as the relative numbers of molecules, formula units, or moles of reactants and products. Teaching Suggestions This slide shows that atoms are neither created nor destroyed in a chemical reaction but are merely rearranged. Use this slide and worksheet to help students understand formula equations. You may need to review how gram formula mass is determined. Questions State the law that explains why the number of oxygen and hydrogen atoms is the same on both sides of the equation shown in the diagram. In what ways are the atoms rearranged by the reaction? Write a word equation for the reaction taking place in the diagram. In the balanced equation shown in the diagram, what is the coefficient of H2? Of O2? Give two ways in which the coefficients in the balanced equation can be interpreted. Use the balanced equation to determine how many moles of H2O would be produced by the reaction of 4 moles of H2 with 2 moles of O2. The gram formula mass of a substance is the number of grams of the substance containing a mole of formula units. Write the equation for the reaction in question 5 showing the number of moles of the reactants and the product. Calculate the gram formula mass of H2, O2, and H2O. How many grams of H2 and O2 react in the reaction in part a? How many grams of H2O are produced? Rewrite the equation, giving the masses of reactants and products. How do you know that mass is conserved in this reaction? Do the masses of the reactants in the equation in part d have the same ratio as the coefficients of the equation in part a? Why or why not? Which do you think is most useful to a chemist: the balanced formula equation (at the top of the diagram), the molecular sketch, the word equation, or an equation that gives the masses of reactants and products? Which would be the least useful? Explain your reasoning. 4 atoms hydrogen 2 atoms oxygen 4 atoms hydrogen 2 atoms oxygen Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

Legos are Similar to Atoms H O H2 O2 H2O + Lego's can be taken apart and built into many different things. Atoms can be rearranged into different substances.

Conservation of Mass + + High voltage electrodes Before reaction glass chamber High voltage After reaction 0 g H2 40 g O2 300 g (mass of chamber) + 385 g total O2 H2O H2 5.0 g H2 “Conservation of Mass” (Lavoisier)   Description: This slide illustrates a reaction between hydrogen and oxygen in a nonstoichiometric mixture of these gases. Basic Concepts ·         Mass and atoms are conserved in chemical reactions. ·         When non-stoichiometric quantities of substances are mixed, they react in stoichiometric proportions. Any reactants in excess remain unreacted. Teaching Suggestions Explain that the first diagram shows the amount of oxygen and hydrogen in a closed chamber. A spark passes between the electrodes, causing the O2 and H2 to react rapidly. The second diagram shows what is in the chamber after the reaction. Use this slide to illustrate that reactants combine in the stoichiometric proportions. Stress that is is not sufficient to know the amounts of starting materials present. One must also know the amounts of reactants that will take part in the reaction. Questions What is the ratio of the mass of O2 to H2 before the reaction? What is the ratio of the number of moles of O2 to H2 before the reaction? How do you account for the fact that the mass of the chamber and its contents is the same before and after the reaction. Why is some oxygen left in the chamber after the reaction? What are the masses of H2 and O2 that take part in the reaction? What is the ratio of the mass of O2 to H2 taking part in this reaction? What is the ratio of the number of moles of O2 to H2 taking part in the reaction? Why is this mole ratio different from the mass ratio? If there were twice as much H2 in the chamber (10 g) but the same amount of O2 (80g), what would you expect to find in the chamber after the reaction? Explain your answer. O2 80 g O2 45 g H2O ? g H2O 300 g (mass of chamber) + 385 g total Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

Law of Definite Proportions Joseph Louis Proust (1754 – 1826) Each compound has a specific ratio of elements It is a ratio by mass Water is always 8 grams of oxygen for every one gram of hydrogen Photo pg 100 Ihde text (Edgar Fahs Smith Collection) Joseph Louis Proust (1754- 1826), French chemist given credit for law of definite composition. Whether synthesized in the laboratory or obtained from various natural sources, copper carbonate always has the same composition. Analysis of this compound led Proust to formulate the law of definite proportions.

The Law of Multiple Proportions Dalton could not use his theory to determine the elemental compositions of chemical compounds because he had no reliable scale of atomic masses. Dalton’s data led to a general statement known as the law of multiple proportions. Law states that when two elements form a series of compounds, the ratios of the masses of the second element that are present per gram of the first element can almost always be expressed as the ratios of integers. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Daltons Atomic Theory Dalton stated that elements consisted of tiny particles called atoms He also called the elements pure substances because all atoms of an element were identical and that in particular they had the same mass.

Dalton’s Atomic Theory All matter consists of tiny particles. Dalton, like the Greeks, called these particles “atoms”. Atoms of one element can neither be subdivided nor changed into atoms of any other element. Atoms can neither be created nor destroyed. 4. All atoms of the same element are identical in mass, size, and other properties. Atoms and Molecules “The idea that matter is made up of tiny indivisible particles was first suggested by the Greek philosopher Democtitus (c. 460-370 BC). He called these particles atoms. In the late 18th century, a modern theory about atoms originated. By then new gases, metals, and other substances had been discovered. Many chemical reactions were studies and the weights of substances involved were measured carefully. John Dalton’s atomic theory arose from these observations. He believed that the atoms of an element were all identical and differed from those of a different element. Two or more of these atoms could join together in chemical combinations producing “molecules” of substances called compounds. The molecules in a compound were all identical.” - Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 16 Atoms of one element differ in mass and other properties from atoms of other elements. In compounds, atoms of different elements combine in simple, whole number ratios.

Dalton’s Symbols John Dalton 1808 Jons Jakob Berzelius (1799 -1848) Swedish chemist who invented modern chemical symbols. Berzelius discovered the elements silicon, selenium, cerium, and thorium. John Dalton 1808

Daltons’ Models of Atoms Carbon dioxide, CO2 Water, H2O Dalton would have shown water with a single hydrogen and a single oxygen. Dalton did not know that hydrogen was diatomic and had a mass of 1 amu. Methane, CH4