STOICHIOMETRY. STOICHIOMETRY Chemists use balanced chemical equations to calculate how much reactant is needed or how much product is formed.

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1 STOICHIOMETRY Stoikheion = element; metron = to measure STOICHIOMETRY Chemists use balanced chemical equations to calculate how much reactant is needed or how much product is formed. provides the same kind of quantitative information that a recipe does. STOICHIOMETRY is. Calculation of the quantities of reactants and products in a chemical reaction. The study of the quantitative relationships between the amounts of reactants and products in a chemical reaction. Based on the Law of Conservation of Mass MASS and the NUMBER OF ATOMS are conserved in every chemical reaction. 1

2 A balanced chemical equation can be interpreted in terms of different quantities, including numbers of atoms, molecules, or moles; mass; and volume. Number of Atoms Number of Molecules 1 molecule N molecules H 2 2 molecules NH 3 2

3 Number of Molecules 1 molecule N molecules H 2 2 molecules NH 3 Moles 1 molecule N molecules H 2 2 molecules NH 3 1 mole N moles H 2 2 moles NH 3 Mass 1 molecule N molecules H 2 2 molecules NH 3 1 mole N moles H 2 2 moles NH 3 28 g N x 2 g H 2 2 x 17 g NH 3 3

4 Mass 1 molecule N molecules H 2 2 molecules NH 3 1 mole N moles H 2 2 moles NH 3 28 g N x 2 g H 2 2 x 17 g NH 3 34 g reactants 34 g products Volume 1 molecule N molecules H 2 2 molecules NH 3 1 mole N moles H 2 2 moles NH 3 28 g N x 2 g H 2 2 x 17 g NH 3 34 g reactants 34 g products Mole Ratios in Chemical Reactions The ratio between the coefficients of any two compounds in a balanced chemical reaction. EXAMPLE: 4 Fe (s) + 3 O 2 (g) 2 Fe 2 O 3 (s) 4

5 Stoichiometry Vocabulary Stoikheion = element; metron = to measure Study of the quantitative relationships in chemical formulas & equations Review: a mole (mol) is a certain number of things (atoms, ions or molecules) = to 6.02 x Coefficients in the balanced equation tell you how many moles of each reactant and product are needed Stoichiometry Vocabulary Review: formula or molar mass is found by adding the atomic masses on the periodic table (round to the tenths place) Molar volume (measured at STP standard temperature & pressure ) of any gas is equal to 22.4 L Chapter 11 v. Chapter 12 In Chapter 11, we converted amounts of the SAME substance to different units (moles, particles, L or g). In Chapter 12, we will be looking at chemical equations and calculating amounts of a substance using a given amount of a DIFFERENT substance. Mole ratios are the HEART of every stoichiometry problem. 5

6 Mole Map Volume Volume Wanted Particles Moles Moles Particles Mass Mass Wanted Mole Map Mole-Mole Conversions Mole ratios are the HEART of every stoichiometry problem. (moles of the # moles # moles known moles of CHEM. The mole ratio you use depends on what substance your given is ( known ) and which substance you want to calculate ( ) 6

7 Mole-Mole Conversions - Example 1 N H 2 2 NH 3 If I have 2 moles of N 2 and excess H 2, how many moles of NH 3 can I make? KNOWN (given): UNKNOWN : MOLE RATIO: 2 moles N 2 moles NH 3 Mole-Mole Conversions - Example N H 2 2 NH 3 (moles of the # moles # moles known moles of CHEM. 2 moles N 2 2 moles NH 3 = 4 moles NH3 1 mole N 2 Mole-Mole Conversions - Example 2 H O 2 2 H 2 O If I have 8 moles of O 2 and excess H 2, how many moles of H 2 O can I make? KNOWN (given): UNKNOWN : MOLE RATIO: 8 moles O 2 moles H 2 O 7

8 Mole-Mole Conversions - Example (moles of the # moles # moles known moles of CHEM. 8 moles O 2 2 moles H 2 O = 16 moles H2 O 1 mole O 2 Mole-Mole Calculations You try 4 Al + 3 O 2 2 Al 2 O 3 If I have 12 moles of Al 2 O 3 how many moles of O 2 do I need? 1. KNOWN: UNKNOWN: 12 moles Al 2 O 3 moles O 2 2. Mole ratio: moles moles known 3 moles O2 2 moles Al 2O3 3. Calculate : 12 moles Al2O3 3 moles O2 18 moles 2 moles Al2O3 O2 Mole Map 8

9 Mole-Mass Conversions Conversion if you know the number of moles of a reactant or product in a reaction and you want to calculate the mass of another product or reactant. Mole-Mass Calculations - Example N H 2 2 NH 3 If I have 2 moles of N 2 and excess H 2, how many grams of NH 3 can I make? (moles of the # moles g # moles known 1 mole CHEM. UNKNOWN 2 moles N 2 2 moles NH g NH 3 = 68.0 g NH 3 1 mole N 2 1 mole NH 3 Mole-Mass Calculations - Example 2 H 2 + O 2 2 H 2 O If I have 8 moles of O 2 and excess H 2, how many grams of H 2 O can I make? (moles of the # moles g # moles known 1 mole CHEM. UNKNOWN 8 moles O 2 2 moles H 2 O 18.0 g H 2 O 1 mole O 2 1 mole H 2 O = g H 2 O 9

10 Mass-Mole Calculation - Example 2 H 2 + O 2 2 H 2 O If I have 8 grams of O 2 and excess H 2, how many moles of H 2 O can I make? (grams of the 1 mole known # moles moles of grams known # moles known KNOWN CHEM. 8 g O 2 1 mole O 2 2 moles H 2 O 32.0 g O 2 1 mole O 2 = 0.5 mole H 2 O Mole Map Mass-Mass Conversions Conversion if you know the mass of a reactant or product in a reaction and you want to calculate the mass of another product or reactant. (grams 1 mole known # moles grams grams known # moles known 1 mole KNOWN UNKNOWN 10

11 Mass-Mass Calculations - Example N H 2 2 NH 3 If I have 2.0 grams of N 2 and excess H 2, how many grams of NH 3 can I make? (grams 1 mole known # moles grams grams known # moles known 1 mole KNOWN UNKNOWN 2.0 g N 2 1 mole N 2 2 moles NH grams NH 3 = 2.4 g NH grams N 2 1 moles N 2 1 mole NH 3 Mass-Mass Calculations - Example If I have 37.0 grams of H 2, how many grams of O 2 will react? (grams 1 mole known # moles grams grams known # moles known 1 mole KNOWN UNKNOWN 37.0 g H 2 1 mole H 2 1 moles O grams O 2 = 296 g O grams H 2 2 moles H 2 1 mole O 2 Mass-Mass Calculations - Example If I have 37.0 grams of H 2, how many grams of O 2 will react? (grams 1 mole known # moles grams grams known # moles known 1 mole KNOWN UNKNOWN 37.0 g H 2 1 mole H 2 1 moles O grams O 2 = 296 g O grams H 2 2 moles H 2 1 mole O 2 11

12 Mole Map Mixed Stoichiometry Calculations If I have 37.0 grams of H 2, how many moles of O 2 will react? (grams 1 mole known # moles grams known # moles known moles of KNOWN Mixed Stoichiometry Calculations If I have 2.0 moles of H 2, how many grams of O 2 will react? (moles # moles # moles known grams 1 mole UNKNOWN 12

13 Mixed Stoichiometry Calculations If I have 2.0 moles of H 2, how many moles of O 2 will react? (moles # moles # moles known CHEM. moles of Mixed Stoichiometry Calculations If I have 37.0 grams of H 2, how many grams of O 2 will react? (grams 1 mole known # moles grams grams known # moles known 1 mole KNOWN UNKNOWN 13