Chem 105 Mon /19/2011 1

Transcription

1 Chem 105 Mon ) New TurningPoint format 2) Calculations: a. molar mass b. mass to moles c. percent composition d. molecular & elemental formulas 1) Balancing chemical equations 2) Soluble/Insoluble ionic compounds 3) Precipitation reactions. 9/19/2011 1

2 Looking ahead Practice exam posted Wed. or Thurs Monday Wednesday Friday Chap 3, Chem Reactions Monday Review Wednesday First Exam Chap 1-3 Friday Monday Graded exams returned 9/19/2011 2

3 (Not live.) New version of TurningPoint plug-in for PowerPoint. It does not show box numbers, but the actual clicker code. These are arranged in SAME ORDER as last week. You will be able to locate your box using the row and column labels, and the toggling function. This version toggles between THREE colors: 9/19/2011 3

4 1 mole of sodium chloride contains 2 moles of ions. How many moles of ions are in 3 moles of aluminum nitrate? Is a metal or NH 4+ present? Yes (Al). Therefore it s an ionic compound. Group? IIIA. Therefore charge = +3 Nitrate ion? NO 3 - Formula of aluminum nitrate? Al(NO 3 ) 3 Per 1 mole of this compound? there are 4 moles of ions Per 3 moles? there are 3 x 4 = 12 moles of ions 4

5 Molar mass & mole calculations 9/19/2011 5

6 Molar mass (also called molecular weight) Molar mass = Sum of atomic weights of all atoms in formula = mass of 1.00 mole of the compound Units: grams/mole (abbreviated g/mol) Molar mass of aluminum nitrate =? Al(NO 3 ) 3 M(Al(NO 3 ) 3 ) = 1( g/mol) + 3(14.007g/mol) + 9 ( g/mol) M(Al(NO 3 ) 3 ) = g/mol 9/19/2011 6

7 Practice problem using mole and molar mass How many moles of acetonitrile are in 2.50 kg of acetonitrile? H H C H C N 9/19/2011 7

8 How many moles of acetonitrile ( AN ) are in 2.50 kg AN? H On board initial data... = final H C C H g/mol N 2.50 kg AN =? moles AN moles AN 2.50 kg AN =? moles AN kg AN 1mole AN 2.50kg AN =? moles AN kg AN g AN The molar mass 9/19/2011 8

9 1mole AN 2.50kg AN =? moles kg AN g AN AN g AN 1mole AN 2.50kg AN =? moles kg AN g AN AN The metric conversion factor g AN 1mole AN 2.50kg AN = moles kg AN g AN AN 60.9 moles 9/19/ AN

10 Finding the empirical formula and the molecular formula 9/19/

11 What elements does it contain? We can use combustion analysis: X + O 2? What IS this compound? CO 2 - it has carbon H 2 O - It has hydrogen NO 2 - has nitrogen By carefully measuring the amounts of products, we can work backward to a formula. 9/19/

12 First, some definitions Molecular formula = # atoms of each element Benzene C 6 H 6 Empirical formula = smallest integer ratio of elements CH 9/19/

13 This is determined w a mass spectrometer. Azulene, a beautiful blue hydrocarbon, is 93.71% C and has a molar mass of g/mol. What are the empirical and molecular formulas of azulene? This % can be determined by combustion analysis, but we ll skip that for this problem. 1. Start by assuming you have g of sample. Thus, it would contain g C and the remainder, 6.29 g, is H. 2. Calculate # moles of each element: 1molC 93.71g C = mol C g C 1mol H 6.29 g H = mol g H H 9/19/

14 4. Find the smallest whole-number ratio. Start by finding mole ratio to the smallest # of moles = mol C 93.71g C = molC g C 1mol H 6.29 g H = mol H g H = If necessary, multiple by an integer to get whole-number subscripts. ( 4 in this case) x 4 = x 4 = C 5 H 4 The correct Empirical formula for azulene 9/19/

15 Molecular formula The sum of the atomic weights of elements = the molar mass (or molecular weight), which, recall for azulene, = g/mol. So far we have This is the weight of the empirical formula g g 5 4 = 1mol + mol g / mol Therefore, the molecular formula must be TWICE the elemental formula, or = 2.00 Correct molecular formula for azulene is: C 10 H 8 9/19/

16 Now, does this tells us the exact molecular structure of azulene? No because there are many C 10 H 8 compounds. However, there are millions of know compounds containing Carbon and Hydrogen, so this analysis has narrowed the search down to several hundred 9/19/

17 Balancing chemical equations: A chemical reaction is just the REARRANGEMENT of atoms. None gained; none lost. 9/19/

18 2H 2 balanced equation + O 2 2 H 2 O 9/19/

19 What does this equation really say? 2H 2 + O 2 2 H 2 O 2 molecules of H 2 react with 1 molecule of O 2 to give 2 molecules of water Or you can think of the balanced chemical equation as a representation of ratios in larger-scale reactions: 4 molecules H molecules O 2 4 molecules H 2 O 2000 molecules H molecules O molecules H 2 O 2 moles H mole O 2 2 moles H 2 O Think of the mole as the chemist s dozen 9/19/

20 Balancing by inspection: combustion 1. Write formulas of reactants & products Unbalanced equation C 3 H 6 (g) + O 2 (g) CO 2 (g) + H 2 O(g) 2. Balance non-o, non-h element first. Unbalanced equation C 3 H 6 (g) + O 2 (g) 3 CO 2 (g) + H 2 O(g) Unbalanced equation 3. Balance H s C 3 H 6 (g) + O 2 (g) 3 CO 2 (g) + 3 H 2 O(g) 4. Balance O s [ C 3 H 6 (g) + 9/2 O 2 (g) 3 CO 2 (g) + 3 H 2 O(g) ]x 2 In general we balance equations with whole number coefficients. 2 C 3 H 6 (g) + 9 O 2 (g) 6 CO 2 (g) + 6 H 2 O(g) 9/19/

21 Hints: 1)Every element must be balanced. 2)Cannot add (or remove) reactants as it suits you to balance the equation manipulate coefficients only. 3)Do not change the subscripts! 9/19/

22 In the 1940 s cyclopropane was used as an anesthetic gas. This practice was discontinued due to its tendency to form explosive mixtures with oxygen (O 2 )! TIME Magazine 9/19/

23 Is the following equation balanced? 2 N 2 H 4 (l) + N 2 O 4 (l) 3 N 2 (g) + 4 H 2 O (g) Yes 2. No 3. Don t know 18 2 Yes No Don t know 23

24 Is the following equation balanced? 2 N 2 H 4 (l) + N 2 O 4 (l) 3 N 2 (g) + 4 H 2 O (g) 1. Yes 2. No H? Yes: 8 on left; 8 on right O? Yes: 4 on left; 4 on right N? Yes: 6 on left; 6 on right 9/19/

25 Chapter 3 in Kotz discusses several kinds of basic chemical reactions 1. Precipitation (formation of insoluble product) 2. Acid-base neutralization 3. Gas-forming reactions (CO2, SO2) 4. Redox reactions These all involve the reactions of IONIC COMPOUNDS. 9/19/

26 Reactions of Ionic Compounds in Aqueous Solution These are variations on ion exchange reactions where the IONS CHANGE PARTNERS A + B - + C + D - A + D - + C + B - 9/19/

27 Snapshot of a soluble ionic compound dissolving K + (aq) MnO 4- (aq) KMnO 4 (s) 9/19/ Potassium permanganate KMnO 4

28 Ions in solution (aq) sometimes combine to form a solid (insoluble) product (s). Ag + (aq) + Cl - (aq) AgCl (s) Silver (I) ions chloride ions Silver (I) chloride 9/19/

29 Others ionic compounds readily dissolve in water to form a solution (aq). This is a product-favored reaction in the direction away from the solid. KMnO 4 (s) K + (aq) + MnO 4- (aq) Potassium permanganate Why do different combinations of cations and anions behave differently in water? 9/19/

30 H 2 O H 2 O H 2 O H 2 O H 2O H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O - H 2 O H 2 O H 2 O H 2O H2 O - H 2 O H 2 O H 2 O H2 O H 2 O + H H 2 O 2 O Mutual attractions of ions in SOLID Solvation force on ions in SOLUTION In some compounds the solvation forces are stronger, which leads to high solubility. (e.g. KMnO 4 ) In others, the solvation forces are weak compared to forces within the crystal, which leads to low solubility. (e. AgCl) 9/19/

31 Solubility. Defined as the tendency to form a solution when the solid is place in solvent (water). 9/19/

32 Will a precipitate form if two solutions of ionic compounds are mixed? 1. Interchange the cation-anion pairs. 2. Form the (correct) new ionic compounds. 3. Check the known solubilities of the new compounds. 4. (Learn and use the Solubility Rules ) 9/19/

33 The End 9/19/