Prof Gilbert LECTURE 6 CHEM 1211 Fall 10 Announcements Reminders About the first homework assignment We are skipping Section 2 9 Last time Isotopes The number of protons in the nucleus of an atom defines its atomic number Z and the identity of the element the combined number of protons and neutrons nucleons equals its mass number A The weighted average of the masses of its stable not radioactive isotopes defines the average atomic mass of an element Periodic Tables of the Elements The Rosetta Stone of Chemistry Earliest successful attempt 1860s Dmitri Mendeleev elements arranged in order of increasing atomic mass and by the formulas of the compounds they form with O and H The modern version is based on atomic number Both versions arrange elements in columns that have similar chemical properties CHAPTER 3 Molar Mass M mass in grams of a mole mol of a substance that is Avogadro s number NA 6 022 1023 particles atoms ions or molecules of the substance M of an element its atomic mass in grams M of a compound is the weighted sum of molar masses of the elements in its molecular or empirical formula Be able to convert between convert a chemical formula into composition This time Chemical Formulas from Percent Composition Consider another iron oxide mineral magnetite An assay of a sample of it for iron discloses that is 72 36 Fe Question What is the formula of this compound C O We know Fe of the compound and need to determine its chemical formula The only other element in the compound is O A If the only elements in the compound are Fe and O then the O content of the compound must be 100 00 72 36 27 64 O One way to translate values into moles is to assume that we have 100 grams of the compound then the values are the same as grams and can be converted into moles S The values of Fe and O turn into the same values in grams 72 36 g Fe and 27 64 g O The the number of moles of Fe and O in these masses is Calculate the ratio of these values by dividing both by the smaller one This mole ratio 1 333 1 000 is the same as 4 3 Therefore the chemical formula of the mineral is Fe3O4 This is an empirical formula Chemical reactions Reactants Products Ex formation of H2SO4 in the atmosphere As cartoon suggests the masses of the reactants must equal the masses of the products that is the number of atoms of each element must be same on both sides of the reaction arrow Calculations based on balanced chemical equations 1 Mass of product produced by a given mass of a reactant In Class Inquiry how much excess stomach acid HCl is neutralized by one super strength TUMS tablet active ingredient 1 00 g CaCO3 In Class Inquiry How many tons of SO2 are produced by a power plant that burns 50 tons of coal that is 3 0 carbon In this example we will use a mass ratio shortcut you may find useful in the future Limiting reactants The ham and cheese metaphor How many ham and cheese sandwiches could you make from 8 slices of bread 3 slices of ham and 4 slices of cheese Scenario two you work for a manufacturer of the pesticide malathion To make a batch you first need to make PCl3 Into a gas tight reaction vessel you add 100 g P4 s and 300 g Cl2 g How much PCl3 could you make Yield Reactions don t always yield the quantities of products they re expected to Reasons 1 reaction comes to chemical equilibrium before all the reactants are consumed 2 reaction rates slow down as reactants are consumed and the chemist gets impatient 3 side reactions form different products than those desired To calculate the yield of a reaction take the ratio of the quantity of product that was made to the maximum quantity of product that could have been made Consider the industrial synthesis of ammonia A sealed reaction vessel is charged with 210 pounds of N2 and 55 pounds of H2 If 167 pounds of NH3 is recovered what was the yield N2 g 3H2 g 2 NH3 g
View Full Document
Unlocking...