Presentation on theme: "Introduction Reaction stoichiometry involves the mass relationships between reactants and products in a chemical reaction. It is based on chemical equations."— Presentation transcript:
Introduction Reaction stoichiometry involves the mass relationships between reactants and products in a chemical reaction. It is based on chemical equations similar to the ones studied in the last section. All reaction stoichiometry calculations start with a balanced equation. You will need to be familiar with gram/mole relationships as studied earlier this year.
The mole enables chemists to move from the microscopic world of atoms and molecules to the real world of grams. Stoichiometry problems are classified between the information given in the problem and the information you are expected to find, the unknown. The given and the unknown may be expressed in grams or moles. The masses in the reaction are usually expressed in grams. Definition of mole: mole of a substance = grams of substance/MW of substance
You will need to use: i.molar ratios in a balanced equation. ii.molar masses of reactants and products. iii.balancing equations. iv. conversions between grams and moles.
Mole Ratios A mole ratio converts moles of one compound in a balanced chemical equation into moles of another compound. All stoichiometry problems use mole ratios.
Example Reaction between magnesium and oxygen to form magnesium oxide. ( fireworks) 2 Mg(s) + O 2 (g) 2 MgO(s) (balanced) Mole Ratios: 2 : 1: 2
Stoichiometry (working with ratios) Ratios are found within a chemical equation. 2HCl + Ba(OH) 2 2H 2 O + BaCl moles of HCl react with 1 mole of Ba(OH) 2 to form 2 moles of H 2 O and 1 mole of BaCl 2 coefficients give MOLAR RATIOS
1) N H 2 ---> 2 NH 3 Write the mole ratios for N 2 to H 2 and NH 3 to H 2. Practice Problems
Review: Molar Mass A substance’s molar mass (molecular weight) is the mass in grams of one mole of the compound. CO 2 = grams per mole H 2 O = grams per mole Ca(OH) 2 = grams per mole
Review: Chemical Equations C 2 H 5 OH + 3O 2 2CO 2 + 3H 2 O reactantsproducts 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water
Types of Stoichiometry Problems
Problem Type 1: Given and unknown quantities are in moles Amount in moles of known substance Amount in moles of unknown substance How many moles of lithium hydroxide are required to react with 20 moles of CO 2
Given: amount of CO 2 = 20 moles Unknown: amount of LiOH in moles Amount of CO 2 in moles Amount of LiOH in moles mol CO 2 x mol LiOH / mol CO 2 = mol LiOH 20 mol CO 2 x 2 mol LiOH / 1 mol CO 2 = 40 mol LiOH mole ratio
When N 2 O 5 is heated, it decomposes: 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) a. How many moles of NO 2 can be produced from 4.3 moles of N 2 O 5 ? 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) 4.3 mol? mol Problem Type 1 Units match Mole – Mole Conversions
When N 2 O 5 is heated, it decomposes: 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) a. How many moles of NO 2 can be produced from 4.3 moles of N 2 O 5 ? = moles NO mol N2O5N2O N 2 O 5 (g) 4NO 2 (g) + O 2 (g) 4.3 mol? mol Mole – Mole Conversions Units match
b. How many moles of O 2 can be produced from 4.3 moles of N 2 O 5 ? = mole O mol N2O5N2O N 2 O 5 (g) 4NO 2 (g) + O 2 (g) 4.3 mol? mol
Problem Type 1: Given and unknown quantities are in moles Amount in moles of known substance Amount in moles of unknown substance How many moles of ammonia are produced when 6 moles of hydrogen gas react with an excess of nitrogen gas.
Given: amount of H 2 = 6 moles Unknown: amount of NH 3 in moles Amount of H 2 in moles Amount of NH 3 in moles (mol H 2 ) x (mol NH 3 / mol H 2 ) = mol NH 3 (6 mol H 2 ) x (2 mol NH 3 / 3 mol H 2 ) = 4 mol NH 3 Mole ratio
Problem Type 2: Given amount is in moles and unknown quantity is in grams Amount in moles of known substance Amount in grams of unknown substance Problem Type 3: Given amount is in grams and unknown quantities are in moles Amount in grams of known substance Amount in moles of unknown substance
Problem Type 2 mole ↔ gram In plants when carbon dioxide reacts with water it produces glucose and oxygen: 6CO 2 + 6H 2 O(l) C 6 H 12 O 6 (s) + 6O 2 (g) How many grams of C 6 H 12 O 6 is produced when 3.0 mol of water react with carbon dioxide? = g C 6 H 12 O mol H 2 O mol? grams Units match 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2
Problem Type 2 mole ↔ gram In plants when carbon dioxide reacts with water it produces glucose and oxygen: 6CO 2 + 6H 2 O(l) C 6 H 12 O 6 (s) + 6O 2 (g) How many grams of CO 2 is needed to react with 3.0 mol of water? = g CO mol H 2 O mol? grams Units match 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2
Problem Type 2 mole ↔ gram When magnesium burns in air, it combines with oxygen to form magnesium oxide according to the following equation: 2Mg + O 2 (g) 2MgO(s) How many grams of MgO is produced from 2.0 mol of magnesium? = g MgO 2.0 mol Mg mol? grams Units match 2Mg + O 2 2MgO
Problem Type 3 When N 2 O 5 is heated, it decomposes: 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) a. How many moles of N 2 O 5 were used if 210g of NO 2 were produced? = moles N 2 O g NO gram ↔ mole 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) 210g ? moles Units match
Problem Type 3 Nitric acid is produced from the catalytic oxidation of ammonia NH 3 (g) + O 2 (g) NO(g) + H 2 O(g) a. How many moles of NO were produced from 824g of NH 3 ? = moles NO 824 g NH gram ↔ mole 824g ? moles Units match 4NH 3 (g) + 5O 2 4NO(g) + 6H 2 O(g)
Steps Involved in Solving Mass-Mass Stoichiometry Problems Balance the chemical equation correctly Using the molar mass of the given substance, convert the mass given to moles. Determine the molar ratio. Using the molar mass of the unknown substance, convert the moles just calculated to mass. Amount in grams of known substance Amount in moles of known substance Amount in moles of unknown substance Amount in grams of unknown substance mw molar ratio
How many grams of N 2 O 5 are needed to produce 75.0 grams of O 2 ? = grams N 2 O g O2O N 2 O 5 (g) 4NO 2 (g) + O 2 (g) 75.0 g ? grams grams ↔ grams When N 2 O 5 is heated, it decomposes: Problem Type 4
Stoichiometry Problem- Type grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed. 1. Identify reactants and products and write the balanced equation. Al+O2O2 Al 2 O 3 a. What are the reactants? b. What are the products? c. What are the balanced coefficients?
Working a Stoichiometry Problem 6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed? 4 Al + 3 O 2 2Al 2 O 3 = 6.50 g Al ? g Al 2 O 3 1 mol Al g Al 4 mol Al 2 mol Al 2 O 3 1 mol Al 2 O g Al 2 O x 2 x ÷ ÷ 4 =12.3 g Al 2 O 3
How many grams of acetylene are produced by adding water to 5.0 grams of CaC 2 ? = grams C 2 H g CaC CaC 2 + 2H 2 O C 2 H 2 + Ca(OH) 2 Balanced? 5.0 g? grams Acetylene gas (C 2 H 2 ) is produced by adding water to calcium carbide:
How many moles of CaC 2 are needed to react completely with 49.0 g H 2 O: CaC 2 + 2H 2 0 C 2 H 2 (g) + Ca(OH) 2 How many moles of CaC 2 are needed to react 49.0 g of H 2 0? = moles CaC 2 49 g H 2 O 1.36 CaC 2 + 2H 2 0 C 2 H 2 (g) + Ca(OH) g? moles Units match
Suppose you want to figure out how many grams of ammonia you can produce if you react 60.0 g of hydrogen gas with excess nitrogen. Remember the balanced chemical equation gives the mole relationship between reactants and products. N H 2 2 NH 3 1) Take the grams of hydrogen gas (recall that it is diatomic) and convert it to moles: 2) Convert the moles of hydrogen gas to moles of ammonia, using the coefficients in the balanced chemical equation: 3) Convert from moles of ammonia to grams of ammonia using the molecular mass and solve the equation: Stoichiometry Review Problem
Calculating the Percent Yield The predicted amount of product – which we have been doing with our stoichiometry problems has been for 100% yield, or the theoretical yield of a reaction. The theoretical yield is the maximum amount of product that can be produces in a given reaction. When chemical reactions take place, they are almost never 100% complete. A reaction may not go to 100% due to not all the reactants becoming involved, impurities in the reactants, competing side reactions, loss of product due to filtering, or just not getting it all out of the vessel. The actual yield is how much product can be collected – and measured. The percent yield is a ratio given by: Percent Yield = Actual Yield x 100% Theoretical Yield
Calcium carbonate is decomposed by heating, as shown in the following equation: 1) What is the theoretical yield of this reaction if 24.8 g CaCO 3 is heated to yield 13.1 g CaO? Handle like a typical stoichiometry problem: grams to grams 2) What is the percent yield? Ignore the 13.1 g CaO to solve. This is the actual yield.
Chlorobenzene is used in the production of many different chemicals, such as aspirin, dyes and disinfectants. One method of preparing chlorobenzene is to react benzene C 6 H 6, with chlorine according to the following equation: 1) When 36.8 g of C 6 H 6 react with excess of Cl 2, the actual yield of C 6 H 5 Cl is 38.8g? What is the theoretical yield? Handle like a typical stoichiometry problem: grams to grams 2) What is the percent yield? C 6 H 6 + Cl 2 C 6 H 5 Cl + HCl
Standard Molar Volume Equal volumes of all gases at the same temperature and pressure contain the same number of molecules. - Amedeo Avogadro At STP (Standard Temperature and Pressure): 1 mole of a gas occupies 22.4 liters of volume
Gas Stoichiometry #1 If reactants and products are at the same conditions of temperature and pressure, then mole ratios of gases are also volume ratios. 3 H 2 (g) + N 2 (g) 2NH 3 (g) 3 moles H mole N 2 2 moles NH 3 3 liters H liter N 2 2 liters NH 3
Gas Stoichiometry #2 How many liters of ammonia can be produced when 12 liters of hydrogen react with an excess of nitrogen? 3 H 2 (g) + N 2 (g) 2NH 3 (g) 12 L H 2 L H 2 = L NH 3 L NH
Gas Stoichiometry #3 How many liters of oxygen gas, at STP, can be collected from the complete decomposition of 50.0 grams of potassium chlorate? 2 KClO 3 (s) 2 KCl(s) + 3 O 2 (g) = L O g KClO 3 1 mol KClO g KClO 3 3 mol O 2 2 mol KClO L O 2 1 mol O