Fall 2006 Studio_3_Percent_composition_FINAL

Fall 2006 Studio_3_Percent_composition_FINAL - Chem. 25:...

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Chem. 25: Studio #3 Conservation of Mass: Metal Carbonates NAME_____________________________ STUDIO:___________________________ The Composition of Metal Carbonates Chemical properties may be used to establish the composition and hence identity of substances. When a chemical reaction occurs, by definition, matter is converted from one substance into another. Bonds in the original material break and new ones form; atoms are conserved in this process and they do not change, but their arrangement at the microscopic, molecular level changes. This is the very soul of chemistry: to define matter in terms of composition (what atoms are present) and structure (how those atoms are arranged), and to be able to intentionally and predictably change the identity and properties of matter by putting the same atoms together in different arrangements. At times chemistry is a lot like accounting: what goes in must come out, and you have to account for every penny in chemistry just like you do in banking. Early chemists were the first to establish the principles of chemical accounting that we still use routinely today. These principles are inviolate: pure substances have a constant composition, which means in the modern sense that the molecules or ions that compose a pure material are all arranged the same way and have exactly the same proportion of atoms by mass. Pure substances are composed of subunits (atoms, molecules or ions), all of which have the same structure and the same composition. This gives rise to the ability to collect certain kinds of information and to assess matter quantitatively in a very meaningful fashion. We can use chemical properties to define the identity and purity of materials the same way we used physical properties like density and melting point. In this experiment we’ll use quantitative aspects of reactions of metal carbonates to distinguish among several compounds and then apply one of the techniques to determine the formula of a compound. The general reaction in this experiment is the decomposition of a metal carbonate. This reaction can be carried out two different ways: thermally and chemically. Many metal carbonates decompose when heated. For example: CaCO 3 (s) ===> CaO (s) + CO 2 (g) Rxn. 1 Most metal carbonates also decompose when treated with strong acid. The general reaction in this experiment is shown below for strontium carbonate and sulfuric acid: SrCO 3 (s) + H 2 SO 4 (aq) ===> SrSO 4 (aq) + CO 2 (g) + H 2 O (l) Rxn. 2 1
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Any strong mineral acid causes the same reaction, and in the lab you will actually use hydrochloric acid. The notations after the substances in these reactions indicate their physical state under the conditions of the reaction: (s) is solid, (g) is gas, (l) is liquid, and (aq) means the material is in aqueous solution. The ‘delta’ under the reaction arrow in the thermal decomposition means the reaction is heated. The first reaction showing the thermal decomposition is commercially significant because
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Fall 2006 Studio_3_Percent_composition_FINAL - Chem. 25:...

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