Pre AP Chemistry Chapter 9 “Stoichiometry”

How much can a reaction produce? To this point we are able to predict reactants and products of chemical reactions to a relatively high degree of accuracy. These are referred to as qualitative values. To this point we are able to predict reactants and products of chemical reactions to a relatively high degree of accuracy. These are referred to as qualitative values. We will now concentrate on predicting amounts of substances involved in chemical reactions. These are referred to as quantitative values. We will now concentrate on predicting amounts of substances involved in chemical reactions. These are referred to as quantitative values. stoichiometry- the branch of chemistry dealing with quantitative relations of reactants and products in chemical reactions. stoichiometry- the branch of chemistry dealing with quantitative relations of reactants and products in chemical reactions.

The coefficients of reactants and products in a chemical equation represent the number of moles of substances. In the equation below, determine the number of moles and the mass of each substance represented. In the equation below, determine the number of moles and the mass of each substance represented. 2HCl  H 2 + Cl 2 2HCl  H 2 + Cl 2 moles hydrochloric acid = _____moles, mass = _____g moles hydrochloric acid = _____moles, mass = _____g moles of hydrogen gas = _____moles, mass = _____g moles of hydrogen gas = _____moles, mass = _____g moles of chlorine gas = _____moles, mass = _____g moles of chlorine gas = _____moles, mass = _____g

Applications of Stoichiometry composition stoichiometry- the mass relationships of elements in chemical compounds. composition stoichiometry- the mass relationships of elements in chemical compounds. reaction stoichiometry- the mass relationships among reactants and products in chemical reactions. reaction stoichiometry- the mass relationships among reactants and products in chemical reactions.

Reaction-Stoichiometry Problems There are four types of reaction-stoichiometry problems. There are four types of reaction-stoichiometry problems. 1. mole-mole problems 1. mole-mole problems given: moles of a substance given: moles of a substance calculate: moles of a substance calculate: moles of a substance 2. mole-mass problems 2. mole-mass problems given: moles of a substance given: moles of a substance calculate: mass of a substance calculate: mass of a substance

Types of reaction-stoichiometry problems(continued) 3. mass-mole problems 3. mass-mole problems given: mass of a substance given: mass of a substance calculate: moles of a substance calculate: moles of a substance 4. mass-mass problems 4. mass-mass problems given: mass of a substance given: mass of a substance calculate: mass of a substance calculate: mass of a substance

Mole-Mole Calculations Ammonia (NH 3 ) is widely used as a fertilizer and in many household cleansers. How many moles of ammonia are produced when 6 moles of hydrogen gas react with an excess of nitrogen gas? Ammonia (NH 3 ) is widely used as a fertilizer and in many household cleansers. How many moles of ammonia are produced when 6 moles of hydrogen gas react with an excess of nitrogen gas? 3H 2 + N 2  2NH 3 3H 2 + N 2  2NH 3 6mol H 2 (2mol NH 3 /3mol H 3 ) = 4mol NH 3 6mol H 2 (2mol NH 3 /3mol H 3 ) = 4mol NH 3

Sample Problem The decomposition of potassium chlorate (KClO 3 ) is used as a source of oxygen in the laboratory. How many moles of potassium chlorate are needed to produce 15 mol of oxygen? The decomposition of potassium chlorate (KClO 3 ) is used as a source of oxygen in the laboratory. How many moles of potassium chlorate are needed to produce 15 mol of oxygen? 2KClO 3  2KCl + 3O 2 2KClO 3  2KCl + 3O 2 Page 301, 1-4 Page 301, 1-4

Mole-Mass Calculations What mass of MgO is produced when 2.00 mol of Mg burns in air? What mass of MgO is produced when 2.00 mol of Mg burns in air? 2Mg + O 2  2MgO 2Mg + O 2  2MgO 2mol Mg x 2mol MgO/2mol Mg x 40 g/mol = 80g MgO 2mol Mg x 2mol MgO/2mol Mg x 40 g/mol = 80g MgO

Sample Problem What mass of oxygen combines with 2.00 mol of magnesium in the reaction in sample problem 1 ? What mass of oxygen combines with 2.00 mol of magnesium in the reaction in sample problem 1 ? 2MgO  2Mg + O 2 2MgO  2Mg + O 2 2mol Mg x (1mol O 2 /2mol Mg) x 32g/mol = 32g O 2 2mol Mg x (1mol O 2 /2mol Mg) x 32g/mol = 32g O 2

Mass-Mole Calculations How many moles of mercury(II) oxide (HgO) are needed to decompose and produce 125g of oxygen? How many moles of mercury(II) oxide (HgO) are needed to decompose and produce 125g of oxygen? 2HgO  2Hg + O 2 2HgO  2Hg + O 2 (125g O 2 /32g/mol) x (2mol HgO/1mol O 2 ) = 7.8mol HgO (125g O 2 /32g/mol) x (2mol HgO/1mol O 2 ) = 7.8mol HgO

Sample Problem How many moles of mercury are produced in sample problem 1? How many moles of mercury are produced in sample problem 1? 2HgO  2Hg + O 2 2HgO  2Hg + O 2 (125g O 2 /32g/mol) x (2mol Hg/1mol O 2 ) = 7.8mol Hg (125g O 2 /32g/mol) x (2mol Hg/1mol O 2 ) = 7.8mol Hg Notice: the same coefficients mean the same number of moles! Notice: the same coefficients mean the same number of moles!

Mass-Mass Calculations How many grams of NH 4 NO 3 are required to produce 33.0 g of N 2 O by decomposition? How many grams of NH 4 NO 3 are required to produce 33.0 g of N 2 O by decomposition? NH 4 NO 3  N 2 O + 2H 2 O NH 4 NO 3  N 2 O + 2H 2 O (33g N 2 O/44g/mol) x (1/1) x 80g/mol = 60g NH 4 NO 3 (33g N 2 O/44g/mol) x (1/1) x 80g/mol = 60g NH 4 NO 3 How many grams of water are produced in the above problem? How many grams of water are produced in the above problem?

Additional Sample Problems 1. Determine the mass of carbon dioxide produced by the decomposition of 50.0 grams of calcium carbonate. 1. Determine the mass of carbon dioxide produced by the decomposition of 50.0 grams of calcium carbonate. 2. Determine the mass of calcium needed to burn in air to produce 14.0 grams of calcium oxide. 2. Determine the mass of calcium needed to burn in air to produce 14.0 grams of calcium oxide grams of magnesium ribbon burn in air grams of magnesium ribbon burn in air. a. How many moles of magnesium burns? a. How many moles of magnesium burns? b. How many moles of magnesium oxide are produced? b. How many moles of magnesium oxide are produced? c. What mass of magnesium oxide is produced? c. What mass of magnesium oxide is produced?

More Sample Problems grams of sodium hydroxide reacts with an excess of hydrochloric acid (HCl). Find the mass of sodium chloride precipitated from this reaction grams of sodium hydroxide reacts with an excess of hydrochloric acid (HCl). Find the mass of sodium chloride precipitated from this reaction grams of nitrogen gas reacts with hydrogen to produce ammonia (NH 3 ). How many grams of ammonia are produced? grams of nitrogen gas reacts with hydrogen to produce ammonia (NH 3 ). How many grams of ammonia are produced? Assignment: Page 311, 1- 5 Assignment: Page 311, 1- 5

Open Response Find the mass of calcium oxide produced from the decomposition of 750g of calcium carbonate. Find the number of moles of carbon dioxide produced also. Find the mass of calcium oxide produced from the decomposition of 750g of calcium carbonate. Find the number of moles of carbon dioxide produced also. CaCO 3 → CaO + CO 2 CaCO 3 → CaO + CO 2

How much does a reaction really produce? Limiting Reactants and Percent Yield limiting reactant - the reactant that controls the amount of product formed in a chemical reaction limiting reactant - the reactant that controls the amount of product formed in a chemical reaction excess reactant - the substance that is not used up completely in a reaction excess reactant - the substance that is not used up completely in a reaction

Sample Problem Some rocket engines use a mixture of hydrazine (N 2 H 4 ) and hydrogen peroxide (H 2 O 2 ) as the propellant system. The reaction equation is Some rocket engines use a mixture of hydrazine (N 2 H 4 ) and hydrogen peroxide (H 2 O 2 ) as the propellant system. The reaction equation is N 2 H 4 (l) + 2 H 2 O 2 (l)  N 2 (g) + 4H 2 O (g) N 2 H 4 (l) + 2 H 2 O 2 (l)  N 2 (g) + 4H 2 O (g) a. Which is the limiting reactant in the equation when mol of N 2 H 4 reacts with mol of H 2 O 2 ? a. Which is the limiting reactant in the equation when mol of N 2 H 4 reacts with mol of H 2 O 2 ? b. How much of the excess reactant, in moles, remains unchanged? b. How much of the excess reactant, in moles, remains unchanged? c. How much of each product, in moles, is formed? c. How much of each product, in moles, is formed?

Percent Yield theoretical yield - the maximum amount of product that can be produced from a given amount of reactant theoretical yield - the maximum amount of product that can be produced from a given amount of reactant actual yield - the measured amount of product obtained from a reaction actual yield - the measured amount of product obtained from a reaction percent yield - the ratio of the actual yield to the theoretical yield, multiplied by 100% percent yield - the ratio of the actual yield to the theoretical yield, multiplied by 100%

Sample Problem Methanol can be produced through the reaction of CO and H 2 in the presence of a catalyst catalyst Methanol can be produced through the reaction of CO and H 2 in the presence of a catalyst catalyst CO(g) + 2H 2 (g) > CH 3 OH(l) CO(g) + 2H 2 (g) > CH 3 OH(l) If 75.0g of CO reacts to produce 68.4g of CH 3 OH, what is the percent yield of CH 3 OH? If 75.0g of CO reacts to produce 68.4g of CH 3 OH, what is the percent yield of CH 3 OH? Assignment: Page 318, 1-4 Assignment: Page 318, 1-4

Stoichiometry Formulas Mole-Mole Mole-Mole coeff unk coeff unk mol unk = mol kn x mol unk = mol kn x coeff kn coeff kn Mole-Mass Mole-Mass coeff unk coeff unk mass unk = mol unk x x mm unk mass unk = mol unk x x mm unk coeff kn coeff kn

Stoichiometry Formulas Mass-Mole Mass-Mole mass kn coeff unk mass kn coeff unk mol unk = x mol unk = x mm kn coeff kn mm kn coeff kn Mass – Mass Mass – Mass mass kn coeff unk mass kn coeff unk mass unk = x x mm unk mass unk = x x mm unk mm kn coeff kn mm kn coeff kn

Open Response Quicklime, CaO, is used by farmers to increase the pH of soil. It reacts with water according to the reaction equation Quicklime, CaO, is used by farmers to increase the pH of soil. It reacts with water according to the reaction equation CaO + H 2 O → Ca(OH) 2 CaO + H 2 O → Ca(OH) 2 Suppose 100Kg of CaO is spread onto a field. Suppose 100Kg of CaO is spread onto a field. A. How many moles of CaO are in 100Kg? A. How many moles of CaO are in 100Kg? B. How many moles of Ca(OH) 2 are produced? B. How many moles of Ca(OH) 2 are produced? C. What mass of Ca(OH) 2 is produced? C. What mass of Ca(OH) 2 is produced? Support your answers with detailed calculations. Support your answers with detailed calculations.