大學(xué)化學(xué)梁永曄03lecturepreview

1、 2012 pearson education, inc.chapter 3 stoichiometry:calculations with chemical formulas and equationsjohn d. bookstaverst. charles community collegecottleville, molecture presentationstoichiometry 2012 pearson education, inc.law of conservation of mass“we may lay it down as an incontestable axiom t

2、hat, in all the operations of art and nature, nothing is created; an equal amount of matter exists both before and after the experiment. upon this principle, the whole art of performing chemical experiments depends.”-antoine lavoisier, 1789stoichiometry 2012 pearson education, inc.chemical equations

3、chemical equations are concise representations of chemical reactions.stoichiometry 2012 pearson education, inc.anatomy of a chemical equationch4(g) + 2o2(g) co2(g) + 2h2o(g)stoichiometry 2012 pearson education, inc.anatomy of a chemical equationch4(g) + 2o2(g) co2(g) + 2h2o(g)reactants appear on the

4、 left side of the equation.stoichiometry 2012 pearson education, inc.anatomy of a chemical equationch4(g) + 2o2(g) co2(g) + 2h2o(g)products appear on the right side of the equation.stoichiometry 2012 pearson education, inc.anatomy of a chemical equationch4(g) + 2o2(g) co2(g) + 2h2o(g)the states of t

5、he reactants and products are written in parentheses to the right of each compound.stoichiometry 2012 pearson education, inc.anatomy of a chemical equationch4(g) + 2o2(g) co2(g) + 2h2o(g)coefficients are inserted to balance the equation.stoichiometry 2012 pearson education, inc.subscripts and coeffi

6、cients give different information subscripts tell the number of atoms of each element in a molecule. coefficients tell the number of molecules.stoichiometry 2012 pearson education, inc.reaction typesstoichiometry 2012 pearson education, inc.combination reactions examples: 2mg(s) + o2(g) 2mgo(s) n2(g

7、) + 3h2(g) 2nh3(g) c3h6(g) + br2(l) c3h6br2(l) in combination reactions two or more substances react to form one product.stoichiometry 2012 pearson education, inc. in a decomposition reaction one substance breaks down into two or more substances.decomposition reactions examples: caco3(s) cao(s) + co

8、2(g) 2kclo3(s) 2kcl(s) + o2(g) 2nan3(s) 2na(s) + 3n2(g) stoichiometry 2012 pearson education, inc.combustion reactions examples: ch4(g) + 2o2(g) co2(g) + 2h2o(g) c3h8(g) + 5o2(g) 3co2(g) + 4h2o(g) combustion reactions are generally rapid reactions that produce a flame. combustion reactions most ofte

9、n involve hydrocarbons reacting with oxygen in the air.stoichiometry 2012 pearson education, inc.formula weightsstoichiometry 2012 pearson education, inc.formula weight (fw) a formula weight is the sum of the atomic weights for the atoms in a chemical formula. so, the formula weight of calcium chlor

10、ide, cacl2, would be ca: 1(40.08 amu) + cl: 2(35.453 amu) 110.99 amu formula weights are generally reported for ionic compounds.stoichiometry 2012 pearson education, inc.molecular weight (mw) a molecular weight is the sum of the atomic weights of the atoms in a molecule. for the molecule ethane, c2h

11、6, the molecular weight would bec: 2(12.011 amu)30.070 amu+ h: 6(1.00794 amu)stoichiometry 2012 pearson education, inc.percent compositionone can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation:% element =(number of atoms)(ato

12、mic weight)(fw of the compound)x 100stoichiometry 2012 pearson education, inc.percent compositionso the percentage of carbon in ethane is%c =(2)(12.011 amu)(30.070 amu)24.022 amu30.070 amu=x 100= 79.887%stoichiometry 2012 pearson education, inc.molesstoichiometry 2012 pearson education, inc.avogadro

13、s number 6.02 x 1023 1 mole of 12c has a mass of 12.000 g.stoichiometry 2012 pearson education, inc.molar mass by definition, a molar mass is the mass of 1 mol of a substance (i.e., g/mol). the molar mass of an element is the mass number for the element that we find on the periodic table. the formul

14、a weight (in amus) will be the same number as the molar mass (in g/mol).stoichiometry 2012 pearson education, inc.using molesmoles provide a bridge from the molecular scale to the real-world scale.stoichiometry 2012 pearson education, inc.mole relationships one mole of atoms, ions, or molecules cont

15、ains avogadros number of those particles. one mole of molecules or formula units contains avogadros number times the number of atoms or ions of each element in the compound.stoichiometry 2012 pearson education, inc.finding empirical formulasstoichiometry 2012 pearson education, inc.calculating empir

16、ical formulasone can calculate the empirical formula from the percent composition.stoichiometry 2012 pearson education, inc.calculating empirical formulasthe compound para-aminobenzoic acid (you may have seen it listed as paba on your bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.

17、14%), nitrogen (10.21%), and oxygen (23.33%). find the empirical formula of paba.stoichiometry 2012 pearson education, inc.calculating empirical formulasassuming 100.00 g of para-aminobenzoic acid,c:61.31 g x = 5.105 mol ch: 5.14 g x= 5.09 mol hn:10.21 g x= 0.7288 mol no:23.33 g x = 1.456 mol o1 mol

18、12.01 g1 mol14.01 g1 mol1.01 g1 mol16.00 gstoichiometry 2012 pearson education, inc.calculating empirical formulascalculate the mole ratio by dividing by the smallest number of moles:c:= 7.005 7h:= 6.984 7n:= 1.000o:= 2.001 25.105 mol0.7288 mol5.09 mol0.7288 mol0.7288 mol0.7288 mol1.458 mol0.7288 mo

19、lstoichiometry 2012 pearson education, inc.calculating empirical formulasthese are the subscripts for the empirical formula:c7h7no2 stoichiometry 2012 pearson education, inc.combustion analysis compounds containing c, h, and o are routinely analyzed through combustion in a chamber like the one shown

20、 in figure 3.14. c is determined from the mass of co2 produced. h is determined from the mass of h2o produced. o is determined by difference after the c and h have been determined.stoichiometry 2012 pearson education, inc.stoichiometric calculationsthe coefficients in the balanced equation give the

21、ratio of moles of reactants and products.stoichiometry 2012 pearson education, inc.stoichiometric calculationsstarting with the mass of substance a, you can use the ratio of the coefficients of a and b to calculate the mass of substance b formed (if its a product) or used (if its a reactant).stoichi

22、ometry 2012 pearson education, inc.stoichiometric calculationsstarting with 1.00 g of c6h12o6 we calculate the moles of c6h12o6use the coefficients to find the moles of h2oand then turn the moles of water to grams.c6h12o6 + 6 o2 6 co2 + 6 h2ostoichiometry 2012 pearson education, inc.limiting reactantsstoichiometry 2012 pearson education, inc.limiting reactants the limiting reactant is the reactant present in the smallest stoichiometric amount. in other words, its