Chemical Reactions and Stoichiometry. Ms. Grobsky

Transcription

1 Chemical Reactions and Stoichiometry Ms. Grobsky

2 Wrapping Up the Lab As we know, the function of the airbags is to protect the occupant from injuring themselves by hitting against the windshield, steering wheel or the dashboard Under inflation of the air bag would not protect the occupant Over inflation of the air bag may cause the bag to act as a brick wall during impact or rupture (making them useless) In the Air Bag lab, you investigated the relationship between the amount of gas produced and the effectiveness of an air bag What part of the reaction determined the amount of gas produced? You saw that the amount of gas produced was dependent on the amount of starting reactants

3 Making the Connection You also explored the fact that when you know the quantity of one substance in a reaction, you can calculate the quantity of any other substance consumed or created in the reaction Quantity meaning the amount of a substance in grams, liters, molecules, or moles This was done by looking at the balanced chemical equation for the reaction Remember, chemical equations tell us what amounts of reactants to mix and what amounts of products to expect for a particular reaction Kind of like a chemist s recipe

4 So, What is Stoichiometry? This process the calculation of quantities in chemical reactions by looking at a chemical equation - is called stoichiometry Greek for measuring elements When you were calculating the amount of sodium bicarbonate and acetic acid to mix in order to produce the right amount of carbon dioxide during the lab, you looked at the coefficients in the balanced chemical equation The coefficients indicated the number of moles of reactants and products in a chemical reaction The key to converting between amount of carbon dioxide (a product) and the amounts of acetic acid and sodium bicarbonate (the reactants) was the coefficient ratio or mole ratio

5 Another Example N 2 (g) + 3H 2 (g) 2NH 3 (g) This equation can be read as: 1 mole of N 2 reacts with 3 moles of H 2 to produce 2 moles of NH 3 N 2 and H 2 always react in this mole ratio So, if you start with 10 moles of nitrogen gas and 30 moles of hydrogen gas, you will form 20 moles of ammonia

6 In General Use the coefficients from the balanced chemical equation to make mole ratios Ratios of balanced coefficients = mole ratios Makes connections between reactants and products Using this information, you can calculate the amounts of the reactants involved and the amount of product you might expect Keep in mind that any calculation done with this process is a theoretical value The real world isn t always perfect

7 Converting From Mole to Mole Practice! The following reaction shows the synthesis of aluminum oxide: 3O 2 (g) + 4Al(s) 2Al 2 O 3 (s) If you wanted to produce 24 moles of product, how many moles of each reactant would you need? Given: 24 moles of Al 2 O 3 Unknown: moles of Al moles of O 2

8 Need 2 separate equations to solve for the unknowns: 3O 2 (g) + 4Al(s) 2Al 2 O 3 (s) 4 mol Al 24 mol Al 2 O 3 2 mol Al 2 O 3 = 48 mol Al 24 mol Al 2 O 3 2 mol Al 2 O 3 3 mol O 2 = 36 mol O 2

9 Practice for You 1. How many moles of hydrogen will be produced if 0.44 mol of CaH 2 reacts according to the following equation? CaH 2 + 2H 2 O Ca(OH) 2 + 2H 2 (.89 mol H 2 ) 2. Iron will react with oxygen to produce Iron III oxide. How many moles of Iron III oxide will be produced if 0.18 mol of Iron reacts? (.090 mol Fe 2 O 3 )

10 USING STOICHIOMETRY TO CONVERT FROM MASS TO MASS

11 MASS MASS CALCULATIONS No lab balance measures moles directly Generally, mass is the unit of choice From the mass of 1 reactant or product, the mass of any other reactant or product in a given chemical equation can be calculated You must have a balanced reaction equation! As in mole-mole calculations, the unknown can be either a reactant or a product

12 Basic Steps to Solve Mass-Mass Stoichiometry Problems 1. Balance the equation 2. Get to moles - Convert mass to moles 3. Set up mole ratios - Use mole ratios to calculate moles of desired compound 4. Convert moles to grams, if necessary

13 MASS MASS CALCULATIONS 1 Acetylene gas (C 2 H 2 ) is produced by adding water to calcium carbide (CaC 2 ): CaC 2 + 2H 2 O C 2 H 2 + Ca(OH) 2 How many grams of C 2 H 2 are produced by adding water to 5.00 g CaC 2?

14 MASS MASS CALCULATIONS 1 What do we know? Given mass = 5.0 g CaC 2 Mole ratio: 1 mol CaC 2 = 1 mol C 2 H 2 MM of CaC 2 = 64.0 g CaC 2 MM of C 2 H 2 = 26.0g C 2 H 2 What are we asked for? grams of C 2 H 2 produced

15 Step 1 - Get to Moles! In this case that can be done by using the Molar Mass of your given compound: 5.0 g CaC g CaC 2 1 mol CaC 2 = mol Step 2 - Convert from moles of our given to moles of unknown using the mole ratio: CaC mol 1 mol C 2 H 2 = mol CaC 2 1 mol CaC 2 C 2 H 2

16 Step 3 - Since we are asked for mass of our unknown in this problem, we need to use our molar mass of our unknown and convert our newly calculated moles into grams: mol 26.0 g C 2 H 2 = 2.03 g C 2 H 2 C 2 H 2 1 mol C 2 H 2 Summary of 3 Steps: 1. Get to Moles 2. Mole Ratio 3. Get to desired final unit

17 MASS MASS CALCULATIONS 2 The double replacement reaction between lead (II) nitrate and potassium iodide produces a bright yellow precipitate that can be used as a color additive in paint. How many grams of potassium iodide would we need to completely react 25.3 g of lead (II) nitrate? Pb(NO 3 ) KI PbI KNO 3 mass A mols A mols B mass B

18 MASS MASS CALCULATIONS 2 1mol Pb(NO 3 ) g Pb(NO 3 ) g Pb(NO 3 ) 2 2mol KI 1mol Pb(NO 3 ) g KI 1mol KI = 25.4 g KI

19 Practice for You 1. What mass of barium chloride is needed to react completely with 46.8 g of sodium phosphate according to the following reaction equation? BaCl 2 + Na 3 PO 4 Ba 3 (PO 4 ) 2 + NaCl 2. Use the following equation to determine what mass of FeS must react to form 326g of FeCl 2. FeS + HCl H 2 S + FeCl 2 (89.2 g BaCl 2 ) (226 g FeS)

20 Recall, a balanced reaction equation indicates the relative numbers of moles of reactants and products We can expand our stoichiometric calculations to include any unit of measure that is related to the mole The given quantity can be expressed in numbers of particles, units of mass, or volumes of gases at STP! The problems can include mass-volume, volume-volume, and particle-mass calculations

21 In any of these problems, the given quantity is first converted to moles Then, the mole ratio from the balanced equation is used to convert from the moles of given to the number of moles of the unknown The moles of the unknown are the converted to the units that the problem requests The next slide summarizes these steps for all typical stoichiometric problems

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23 MORE MOLE EXAMPLES How many molecules of O 2 are produced when a sample of 29.2 g of H 2 O is decomposed by electrolysis according to this balanced equation: 2H 2 O 2H 2 + O 2

24 MORE MOLE EXAMPLES What do we know? Mass of H 2 O = 29.2 g H 2 O 2 mol H 2 O = 1 mol O 2 From balanced equation MM of H 2 O = 18.0 g H 2 O 1 mol O 2 = 6.02x10 23 molecules of O 2 What are we asked for? Molecules of O 2

25 Mass A Mols A Mols B Molecules B 29.2 g H 2 O 1 mol H 2 O 18.0 g H 2 O 1 mol O 2 2 mol H 2 O = 4.88 x x10 23 molecules O 2 1 mol O 2 molecules O 2

26 MORE MOLE EXAMPLES The last step in the production of nitric acid is the reaction of NO 2 with H 2 O: 3NO 2 +H 2 O 2HNO 3 + NO How many liters of NO 2 must react with water to produce 5.00x10 22 molecules of NO?

27 MORE MOLE EXAMPLES What do we know? Molecules NO = 5.0x10 22 molecules NO 1 mol NO = 3 mol NO 2 From balanced equation 1 mol NO = 6.02x10 23 molecules NO 1 mol NO 2 = 22.4 L NO 2 What are we asked for? Liters of NO 2

28 Molecules A Mols A Mols B Volume B 5.0x10 22 molecules NO 1 mol NO 6.02x10 23 molecules NO 3 mol NO 2 1 mol NO 22.4 L NO 2 1 mol NO 2 = 5.58 L NO 2