Mass Relationships in Chemical Reactions

Transcription

1 Mass Relationships in Chemical Reactions Chapter 3 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Micro World atoms & molecules Macro World grams Atomic mass is the mass of an atom in atomic mass units (amu) By definition: 1 atom 12 C weighs 12 amu On this scale 1 H = amu 16 O = amu 3.1

3 Natural lithium is: 7.42% 6 Li (6.015 amu) 92.58% 7 Li (7.016 amu) Average atomic mass of lithium:.0742 x x = amu 3.1

4 Average atomic mass (6.941)

5 The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly grams of 12 C. 1 mol = N A = x Avogadro s number (N A ) 3.2

6 eggs Molar mass is the mass of 1 mole of shoes in grams marbles atoms 1 mole 12 C atoms = x atoms = g 1 12 C atom = amu 1 mole 12 C atoms = g 12 C 1 mole lithium atoms = g of Li For any element atomic mass (amu) = molar mass (grams) 3.2

7 One Mole of: C S Hg Cu Fe 3.2

8 Do You Understand Molar Mass? How many atoms are in g of potassium (K)? 1 mol of K = g of K 1 mol of K = x atoms of K g K x 1 mol K g K x x 1023 atoms K 1 mol K = 8.49 x atoms of K 3.2

9 Molecular mass (or molecular weight) is the sum of the atomic masses (in amu) in a molecule. SO 2 1S 2O SO amu + 2 x amu amu For any molecule molecular mass in amu = molar mass in grams 1 molecule of SO 2 weighs amu 1 mole of SO 2 weighs g 3.3

10 Do You Understand Molecular Mass? How many H atoms are in 72.5 g of C 3 H 8 O? moles of C 3 H 8 O = 72.5 g / g/mol = 1.21 mol 1 mol C 3 H 8 O molecules contains 8 mol H atoms 1 mol of H atoms is x H atoms 72.5 g C 3 H 8 O 1 mol C 3 H 8 x O 60 g C 3 H 8 O x 8 mol H atoms 1 mol C 3 H 8 O 5.82 x H atoms x H atoms x = 1 mol H atoms Steps: 1. Convert grams of C 3 H 8 O to moles of C 3 H 8 O. 2. Convert moles of C 3 H 8 O to moles of H atoms. 3. Convert moles of H atoms to number of H atoms. 3.3

11 Formula mass is the sum of the atomic masses (in amu) in a formula unit of an ionic compound. NaCl 1Na 1Cl NaCl amu amu amu For any ionic compound formula mass (amu) = molar mass (grams) 1 formula unit of NaCl = amu 1 mole of NaCl = g of NaCl 3.3

12 Do You Understand Formula Mass? What is the formula mass of Ca 3 (PO 4 ) 2? 1 formula unit of Ca 3 (PO 4 ) 2 3 Ca 3 x g/mol 2 P 2 x g/mol 8 O + 8 x g/mol g/mol Units of grams per mole are the most practical for chemical calculations! 3.3

13 Percent composition of an element in a compound = n x molar mass of element molar mass of compound x 100% n is the number of moles of the element in 1 mole of the compound (assume you have 1 mole!). C 2 H 6 O 2 x (12.01 g) %C = g 6 x (1.008 g) %H = g 1 x (16.00 g) %O = g x 100% = 52.14% x 100% = 13.13% x 100% = 34.73% 52.14% % % = 100.0% 3.5

14 Percent Composition and Empirical Formulas Determine the empirical formula of a compound that has the following percent composition by mass: K 24.75%, Mn 34.77%, O 40.51% percent. n K = g K x n Mn = g Mn x n O = g O x 1 mol K g K = mol K 1 mol Mn = mol Mn g Mn 1 mol O g O = mol O To begin, assume for simplicity that you have 100 g of compound! 3.5

15 Percent Composition and Empirical Formulas n K = , n Mn = , n O = K : ~ Mn : = O : ~ KMnO 4 3.5

16 A process in which one or more substances is changed into one or more new substances is a chemical reaction. A chemical equation uses chemical symbols to show what happens during a chemical reaction. 3 ways of representing the reaction of H 2 with O 2 to form H 2 O reactants products 3.7

17 How to Read Chemical Equations 2 Mg + O 2 2 MgO 2 atoms Mg + 1 molecule O 2 makes 2 formula units MgO 2 moles Mg + 1 mole O 2 makes 2 moles MgO 48.6 grams Mg grams O 2 makes 80.6 g MgO IS NOT 2 grams Mg + 1 gram O 2 makes 2 g MgO 3.7

18 Balancing Chemical Equations 1. Write the correct formula(s) for the reactants on the left side and the correct formula(s) for the product(s) on the right side of the equation. Ethane reacts with oxygen to form carbon dioxide and water C 2 H 6 + O 2 CO 2 + H 2 O 2. Change the numbers in front of the formulas (coefficients) to make the number of atoms of each element the same on both sides of the equation. Do not change the subscripts. 2 C 2 H 6 NOT C 4 H

19 Balancing Chemical Equations 3. Start by balancing those elements that appear in only one reactant and one product. C 2 H 6 + O 2 CO 2 + H 2 O start with C or H but not O 2 carbon on left C 2 H 6 + O 2 1 carbon on right 2 CO 2 + H 2 O multiply CO 2 by 2 6 hydrogen on left 2 hydrogen on right multiply H 2 O by 3 C 2 H 6 + O 2 2 CO H 2 O 3.7

20 Balancing Chemical Equations 4. Balance those elements that appear in two or more reactants or products. C 2 H 6 + O 2 2 CO H 2 O multiply O 2 by oxygen on left 4 oxygen (2x2) + 3 oxygen (3x1) = 7 oxygen on right C 2 H O CO H 2 O 2 C 2 H O 2 4 CO H 2 O remove fraction multiply both sides by 2 3.7

21 Balancing Chemical Equations 5. Check to make sure that you have the same number of each type of atom on both sides of the equation. 2 C 2 H O 2 4 CO H 2 O 4 C (2 x 2) 4 C 12 H (2 x 6) 12 H (6 x 2) 14 O (7 x 2) 14 O (4 x 2 + 6) Reactants 4 C 12 H 14 O Products 4 C 12 H 14 O 3.7

22 Stoichiometry Calculations: Amounts of Reactants and Products Use the fabulous four steps! 1. Write the balanced chemical equation. 2. Convert quantities of known substances into moles. 3. Use coefficients in balanced equation to calculate the number of moles of the sought quantity. 4. Convert moles of sought quantity into the desired units. 3.8

23 Methanol burns in air according to the equation 2 CH 3 OH + 3 O 2 2 CO H 2 O If 209 g of methanol are used up in the combustion, what mass of water is produced? grams CH 3 OH moles CH 3 OH moles H 2 O grams H 2 O 209 g CH 3 OH 1 mol CH 3 OH x 32.0 g CH 3 OH x 4 mol H 2 O 2 mol CH 3 OH 18.0 g H 2 O x = 1 mol H 2 O 235 g of H 2 O 3.8

24 Do You Understand Limiting Reactants? In a reaction, 124 g of Al are reacted with 601 g of Fe 2 O 3. 2 Al + Fe 2 O 3 Al 2 O Fe Calculate the mass of Al 2 O 3 formed in grams. Fabulous Four Steps 1. Balanced reaction: Done. 2. Moles of given reactants. Moles of Al = 124 g / g/mol = 4.60 mol Moles of Fe 2 O 3 = 601 g / g/mol = 3.76 mol 3.9

25 3. Moles of desired product, Al 2 O 3. 2 Al + Fe 2 O 3 Al 2 O Fe Moles of Al 2 O 3 = 4.60 mol Al X 1 mol Al2O3 = 2.30 mol Al 2 O 3 based on Al 1 2 mol Al Moles of Al 2 O 3 = 3.76 mol Fe2O3 X 1 mol Al2O3 = 3.76 mole Al 2 O 3 based on Fe 2 O mol Fe2O3 Keep the smaller answer! Al is the limiting reactant. 4. Grams of Al 2 O 3. Grams of Al 2 O 3 = 2.30 mol X g/mol = 235 g 3.9

26 Thermite Reaction 2 Al (s) + Fe 2 O 3 (s) 2 Fe (l) + Al 2 O 3 (s)

27 Reaction Yield Theoretical Yield is the amount of product that would result if all the limiting reagent reacted. Actual Yield is the amount of product actually obtained from a reaction. % Yield = Actual Yield Theoretical Yield x

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