Chapter 12 Stoichiometry. Composition Stoichiometry – mass relationships of elements in compounds Reaction Stoichiometry – mass relationships between

Chapter 12 Stoichiometry

Composition Stoichiometry – mass relationships of elements in compounds

Reaction Stoichiometry – mass relationships between reactants and products in a chemical reaction

Copyright © by Holt, Rinehart and Winston. All rights reserved.

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Chapter 9 Visual Concepts

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Stoichiometry

Visual Concept

Mole Ratio – conversion factor (unit factor) that relates one chemical to another – comes from the coefficients in a chemical equation

2 Al2O3 4 Al + 3 O2

2 mol Al2O3 yields 4 mol Al

2 mol Al2O3 yields 3 mol O2



Chapter 9

Mole Ratio

• A mole ratio is a conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction

Example: 2Al2O3(l) 4Al(s) + 3O2(g)

Mole Ratios: 2 mol Al2O3 2 mol Al2O3 4 mol Al

4 mol Al 3 mol O2 3 mol O2

, ,

Section 1 Introduction to Stoichiometry

In the standard stoichiometry calculations you should know, ALL ROADS LEAD TO MOLS. You can change any amount of any measurement of any material in the same equation with any other material in any measurement in the same equation. That is powerful. The setup is similar to Dimensional Analysis.

1. Start with what you know (GIVEN), expressing it as a fraction. 2. Use definitions or other information to change what you know to mols of that material. 3. Use the mol ratio to exchange mols of the material given to the mols of material you want to find. 4. Change the mols of material you are finding to whatever other measurement you need.





Visual Concept

Molar Mass as a Conversion Factor





Visual Concept

Conversion of Quantities in Moles



Chapter 9

Solving Stoichiometry Problems with Moles or

Grams

Section 2 Ideal Stoichiometric Calculations

How many grams of ammonia can you make with 25 grams of hydrogen?

N2 + 3H2 2NH3

You are given the mass of 25 grams of hydrogen. Start there.

141.67 g NH3





Visual Concept

Mass and Number of Moles of an Unknown

What mass of oxygen will react with 96.1 grams of propane?

First, write the balanced reaction:

C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)

Second, convert the mass to moles. Note that the molecular weight of propane is 44.1 g/mole.



Chapter 9

Limiting Reactants• The limiting reactant is the reactant that limits the

amount of the other reactant that can combine and the amount of product that can form in a chemical reaction.

• The excess reactant is the substance that is not used up completely in a reaction.

Section 3 Limiting Reactants and Percentage Yield





Visual Concept

Limiting Reactants and Excess Reactants

Limiting Reactant – reactant that controls the amount of product that can be produced

For example, nitrogen gas is prepared by passing ammonia gas over solid copper(II) oxide at high temperatures. The other products are solid copper and water vapor.

2 NH3(g) + 3 CuO(s) → N2(g) + 3 Cu(s) + 3 H2O(g)

If 18.1 g of NH3 are reacted with 90.4 g of CuO, which is the limiting reagent? How many grams of N2 will be formed?

22

22

2

22

23

2

3

33

N g 06.1N molN g 28N mol 379.0

N less makesit b/creactant limiting is CuO

N mol 379.0CuO mol 3N mol 1

CuO g 79.5CuO molCuO g 4.90

N mol 532.0NH mol 2N mol 1

NH g 17NH molNH g 1.18



Chapter 9

Percentage Yield

• The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant.

• The actual yield of a product is the measured amount of that product obtained from a reaction.

• The percentage yield is the ratio of the actual yield to the theoretical yield, multiplied by 100.

percentage yield

actual yieldtheorectical yield

100

Section 3 Limiting Reactants and Percentage Yield





Visual Concept

Comparing Actual and Theoretical Yield

Actual Yield% Yield 100Theoretical Yield

What is the % yield of N2 in the previous problem if only 0.95 g were actually obtained from the reaction?

89.62% Yield %

100N g 1.06N g 0.95 Yield %

2

2

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Chapter 12 Stoichiometry. Composition Stoichiometry – mass relationships of elements in compounds Reaction Stoichiometry – mass relationships between