1. Chemistry 521 Chapter 4

2.  Stoichiometry is the study of the relative quantities of reactants and products in chemical reactions.  Ex:  Two hydrogen molecules and one oxygen molecule produce two water molecules.

3.  Stoichiometry is also the ratio of atoms and/or molecules as shown by the chemical equation.  Recall the Law of Definite Proportions – atoms combine in definite fixed proportions.  So then the chemical equation is also a ratio:

4.  Consider the analogy on Page 111 ◦ 3 slices of toast + 2 slices of turkey + 4 strips of bacon = 1 sandwich ◦ 3:2:4:1 ratio The ratio can be multiplied. ◦ 6:4:8:2 ◦ 9:6:12:3 The equation provides very useful information.

5.  Consider the equation to produce ammonia.  1 molecule N 2 : 3 molecules H 2 : 2 molecules NH 3  You can multiply this ratio: ◦ By 2: ◦ 2 molecule N 2 : 6 molecules H 2 : 4 molecules NH 3

6.  By 10:  10 molecule N 2 : 30 molecules H 2 : 20 molecules NH 3  By 2.63 × 10 14 :  2.63 × 10 14 molecule N 2 : 7.89 × 10 14 molecules H 2 : 5.26 × 10 14 molecules NH 3  Every chemical equation ratio always holds true no matter what you multiply by.

7.  Suppose you wanted 20 molecules of ammonia, how many molecules of N 2 are required? H 2 ?  The chemical equation is:  Therefore the ratio is: 1N 2 : 3H 2 : 2NH 3

8.  To get 20 NH 2, multiply everything by 10 10 N 2 : 30 H 2 : 20 NH 3 OR create conversion factors: 20 molecules NH 3 × 1 molecule N 2 = 10 molecules N 2 2 molecules NH 3 20 molecules NH 3 × 3 molecules H 2 = 30 molecules H 2 2 molecules NH 3

9.  Do Practice Problem #1 together on Page 114  Do Practice Problems on Page 114 #s 2 & 3

10.  Consider:  As we have said the coefficients represent a ratio of molecules 1 molecule N 2 : 3 molecules H 2 : 2 molecules NH 3  These coefficients from the chemical equation would also represent the number of moles of each atom or molecule.

11.  6.02 × 10 23 (1 molecule N 2 : 3 molecules H 2 : 2 molecules NH 3 )  Would still give the same ratio molecule to molecule.  Mole Ratios are the relationships between moles in a balanced equation.  Note- each equation about mole ratios may only involve 2 components of the chemical equation. It is assumed that the other atoms/molecules will follow in the same trend and that there is enough of each reactant.

12.  Do Practice Problem #4 as an example on Page 11  Do Practice Problems on Page 115 #s 5-7  Mole to Mole Worksheet

13.  Recall the Law of Multiple Proportions – that when 2 elements combine two or more different compounds may result – depending on the conditions and the amount of reactants available.

14.  Ex: carbon and oxygen can combine to form carbon dioxide or carbon monoxide.

15.  Do Sample Problem on Page 116  Do Practice Problems on Page 117 #s 8-10

16.  In Chapter 2 you learned that once you have a number in moles you could convert to grams, volume, number of particles or vice versa.  The coefficients in a chemical equation are considered the number of moles.  Therefore, if you have two moles of N 2, you can calculate the mass of N 2 in that reaction.

17.  Consider: ◦ 1 mol of N 2 × 28.02 g = 28.02 g N 2 1 mol ◦ 3 mol of H 2 × 2.02 g = 6.06 g H 2 1 mol ◦ 2 mol of NH 3 × 17.04 g = 34.08 g NH 3 1 mol

18.  According to the Law of Conservation of Matter – matter is neither created nor destroyed, it only changes form  Therefore the mass of the reactants is equal to the mass of the products in a chemical equation which is the Law of Conservation of Mass

19.  Whether you add up the mass of the reactants or the products, you will always get the same result.  Note – for stoichiometric problems the equations MUST be balanced. N 2 + 3H 2  2NH 3 28.02g + 6.06 g2(14.01 +3(1.01)) g = 34.08 g

20.  If you know the quantity of one substance in a chemical reaction (in particles, moles, grams, or liters), you can calculate the quantity of any other substance in the reaction.  Stoichiometry is so useful because you can use it to make predictions.  One purpose of stoichiometric calculations is to determine how much of a reactant is needed to carry out a reaction.

21.  Stoichiometric analysis involving mass is called gravimetric stiochiometry.  Stoichiometric calculations involving volume of gases is called gas stoichiometry.

22.  Do Sample Problem on Page 120  Do Practice Problems on Page 120 #s 11-14  Do Sample Problem on Page 121  Do Practice Problems on Pages 122-123 #s 15-18

23. 1. Write the balanced equation. 2. Convert the number given in the problem to moles (use Mole Chart from Ch. 2). 3. Convert between atoms/molecules using mol ratio based on the chemical equation. 4. Convert this mole value to what was asked for in the question.

24.  Do Sample Problem on Page 124  Do Practice Problems on Page 125 #s 19-22  Do Section Review on Pages 126-127  Worksheet

25.  Recall the clubhouse sandwiches from the previous section. By knowing the base formula you could predict how much of each ingredient you need to make multiple sandwiches. 3 slices of toast + 2 slices of turkey + 4 strips of bacon = 1 sandwich  What if you had 6 slices of toast, 12 slices of turkey, and 20 strips of bacon? How many sandwiches could you make?

26.  Taking all the ingredients you have into account and the formula, since you only have 6 slices of toast, you could only make 2 complete sandwiches.  There would be 8 slices of turkey and 12 strips of bacon left over or extra.  The toast would be the limiting reactant in the sandwich equation.

27.  The limiting reactant is the one to run out first.  Once the toast was used up you were unable to make any more sandwiches.  The slices of turkey and strips of bacon would be left over or in excess.  The excess reactant is the one that is left unreacted.

28.  Do Thought Lab on Page 129  The concept of limiting reactant and excess reactant come up in real life situations.  Zinc is extracted from zinc oxide by adding carbon.

29.  In industrial settings, they do not try to ensure the reactants are in a 1:1 ratio. They just make sure they have more carbon (charcoal) than zinc oxide.  This reaction will produce zinc until one reactant runs out and to ensure the most profit they do not want to waste any zinc oxide.

30.  Often you have to identify which of the reactants is the limiting reactant.  One method is to predict how much product each reactant could, at most, produce.  The reactant that produces the least amount of product is the limiting reactant.

31.  Do Sample Problem on Page 130  Do Practice Problems on Page 131 #s 23 - 26

32.  Stoichiometry is useful because it can make predictions.  Chemists need to know how much product they can expect from a certain reaction.  Often experimental results are compared to the calculated predicted results.  By analyzing an impure substance with a known substance the predicted expected mass is compared to the actual mass of the substance.

33.  Since most chemical reactions are not balanced inside a beaker, you must first determine the limiting reactant to solve stoichiometric problems.

34.  Do Sample Problem on Page 133  Do Practice Problems on Pages 134-135 #s 27-30  Do Section Review on Pages 135-136  Limiting Reactant Worksheet

35.  When you get a mark on a test, how was that mark calculated? Also recall mass percent.  Your mark × 100% Total Marks

36.  The stoichiometric calculations show us the theoretical yield. This would be how much product is produced under ideal conditions.  In the real lab, the experiment may not produce as much product as you would have expected. This is the actual yield.

37.  Why does the actual yield differ from the theoretical yield? 1.Competing reactions 2.Experimental design and technique 3.Impure reactants 4.Faulty measuring devices  These 4 factors lead to inaccurate results.  The accuracy of an experiment is measured by how close your result is to the expected value.

38.  The precision of the results refers to the reproducibility of the results. The results were precise if they can be repeated again and again.  Precision depends on the measuring devices, as in how many decimal places they go to and your ability to use the devices properly.

40.  Competing reactions can occur during experiments.  Ex: phosphorus and chlorine gas forming phosphorus trichloride. Sometimes the PCl 3 then reacts with some of the Cl 2 and another product, PCl 5 is formed.  Therefore the yield of the product, PCl 3, is less than calculated.

41.  The percentage yield is telling you how close you are to the expected yield.  First find the limiting reactant and use it to find the amount of product. This is the theoretical yield. The actual yield will be given in the problem.

42.  Do Sample Problem on Page 138  Do Sample Problem on Page 138 #31  Do Practice Problems on Page 139 #s 32 & 33

43.  Using the same equation, if the percentage yield is known and the theoretical yield can be calculated, then chemists can predict the actual yield from a sample.

44.  Do Sample Problem on Page 140  Do Practice Problems on Page 141 #s 34-37

45.  The percentage yield of a reaction is necessary in the pharmaceutical industry.  Pharmaceutical companies developing new drugs may only get a small percentage yield of product.  In the R&D phase that is reasonable, as scientists are working on developing a process or perfecting a reaction.  But the higher the percentage yield then more money is made.

46.  The lower the percentage yield the more costs are associated.  Once a new drug is ready, it would then be manufactured in large quantities (more cost effective).  At this point a small difference in percentage yield can cost companies thousands of dollars.

47.  When doing stoichiometric calculations, we assume ideal conditions.  However, in reality the reactants are not pure and this can lower the actual yield.  Ex. A 1.00 g sample of sodium chloride (NaCl) may have absorbed some water. So the sample is not 1.00 g of just NaCl. You really do not know how much NaCl you actually have.

48.  The percentage purity of a sample tells you how much of the sample is the element of compound that you are looking for. ◦ This concept is used in mining situations. ◦ Ore is mined but is not pure. ◦ There is often rock, dirt, or other waste within a sample.  To find percentage purity, calculate the amount of “important” element or compound in the sample.

49.  This is the theoretical mass and use it to find the percentage purity.

50.  Do Sample Problem on Page 146  Do Practice Problems on Page 147 – 148 #s 38 – 40  Do Section Review on Page 148 #s 1-3  Chapter 4 Review on Pages 149 – 150 #s 1 – 10, 14 - 21