What is stoichiometry? It comes from the Greek word stoicheion, which means element, and metron, meaning measure.

Transcription

1 Stoichiometry

2 What is stoichiometry? It comes from the Greek word stoicheion, which means element, and metron, meaning measure.

3 It involves the mass relationships between reactants and products in a chemical reaction. It depends on chemical equations and the law of conservation of mass. You must always start with a balanced chemical equation, which gives the relative numbers of moles of reactants and products.

4 Stoichiometry is basically the principle of knowing what mass of reactants you should use to produce a certain amount of product. Example: If you need to make 425 grams of Ca(OH) 2 how much of each reactant should you use to make it. It s easy to figure out if you have a balanced chemical equation.

5 Concept of Limiting and Excess Reactants Limiting Reactant The one you run out of first; It determines how much product you can make. Excess Reactant You will have some of this reactant left over; It will not all react.

6 Mole Ratio Every Problem will include a mole ratio. A mole ratio is a conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction. Mole ratios come from the coefficients.

7 Mole Ratio Lets make sure we understand this first (and review balancing equations, too). Balance, then list all of the mole ratios. Al 2 O 3 Al + O 2 2 Al 2 O 3 2Al 2 O 3 4Al 4Al 4 Al 3O 2 2Al 2 O 3 3O 2 3O 2 3O 2 2Al 2 O 3 4Al

8 One More.

9 Mole to Mole The mole ratio you use will be Moles UnKnown Moles known Moles of known X mole ratio

10 In the following reaction, how many moles of FeS are needed to produce 25 moles of H 2 S? FeS + HCl FeCl 2 + H 2 S How many moles of BaO will be produced from 3 moles of Ba 3 (PO 4 ) 2? Ba 3 (PO 4 ) 2 P2O5 + BaO

11 Moles to Mass Balance your equation. Circle your known and underline your unknown. Moles known X Mole Ratio X Mol. Weight unknown

12 Examples: Fe + O 2 Fe 2 O 3 How many grams of Fe 2 O 3 are produced from 3.2 moles of Fe? How many grams of O 2 are needed to completely react with 5.4 moles of Fe?

13 Mass to Moles Balance the equation. Circle the known (in grams) underline the unknown (moles). Start with grams; divide by molecular weight (to get moles); Mole ratio.

14 In the following reaction, how many moles of FeS are needed to produce 25 grams of H 2 S? FeS + HCl FeCl 2 + H 2 S How many moles of BaO will be produced from 3 grams of Ba 3 (PO 4 ) 2? Ba 3 (PO 4 ) 2 P2O5 + BaO

15 Mass to Mass Balance equation. Circle known (grams); Underline unknown (grams). Mass known X 1mole X mole ratio X mol. wt unknown mol wt (known) 1 mole

16 CaO + HCl CaCl 2 + H 2 O If you want 25 grams of CaCl 2, how many grams of CaO should you start with? If you want 25 grams of CaCl 2, how many grams of HCl should you start with?

17 Limiting Reactant The reactant that is used up first; this is the one that determines how much product is formed. Excess Reactant The reactant that is not completely used up. There is extra left over that does not react.

18 Limiting Reactant In these problems, you will be given the mass (in grams) of both reactants. You will do 2 separate problems, converting grams of reactant into grams of either product. Repeat for the second reactant, converting into the SAME product. The smallest answer shows the limiting reactant.

19 Cl 2 + KBr KCl + Br 2 If you have 12.0 grams of chlorine, how many grams of KCl will you produce? If you have 15.0 grams of KBr, how many grams of KCl will you produce? Which is the limiting reactant?

20 Cl 2 + KBr KCl + Br 2 If you have 14.5 grams of chlorine and 21.3 grams of KBr, how much bromine will you make? What is the limiting reactant?

21 Percent Yield Theoretical Yield The calculated mass of product. Using a mass-mass calculation, this is how much product you should get. Actual Yield When you do an experiment or lab, this is how much product is actually formed.

22 Percent Yield = actual yield X 100 theoretical yield Actual yield is always given. If theoretical yield is not given, it will have to be calculated by doing a gram to gram problem. You will use the mass of reactant (LR) to calculate the mass of the product (The same product as the actual yield). * See example 2

23 Example 1: What is the percent yield of a reaction with a theoretical yield of 46.98g and an actual yield of g? X 100 = 88.3% 46.98

24 Example 2: When 56.0 grams of silicon dioxide is heated with an excess of carbon, 35.5 g of silicon carbide is produced. What is the percent yield of this reaction? SiO 2 + C SiC + CO