Stoichiometry
With your understanding of mole relationships and balanced equations you are now ready to apply your knowledge to chemical reactions. From our prior experiments you know that a new substance is produced in chemical reaction. Now we will be able to predict the amount of product formed in a reaction only knowing the initial amounts of reactants. Collectively these predictions, or calculations, which relate amounts of products to reactants in a balanced chemical equation are called stoichiometry.
Demos/Activities/Experiments
Experiment: Silver/Copper
Experiment: Percent yield
Reading #1 9.1-2 Chemical Formulas and Mole-Mole Relationships
Homework #1: p. 282-3: # 2-4, 6 ab, 7, 8, 10, 11 b, d, 14 a, d
Reading #2 9.3 Mass Calculations
Homework #2 : p. 284-5: #18, 23 a, 24 ab, 25 ad, 27, 28, 30, 31, 35, 37 p. 290 # 70,
Concepts: Chemical Equations: writing, balancing
Skills: ratios, factor label, chemical notation
Up to this point, it has been assumed that all the reactants/reagents have been consumed and transformed into products - that is, there are no reactants/reagents remaining or left over. However, the reality is that many times one (or more) reactant is available in an unlimited supply. For example, when wood burns in a campfire, the oxygen gas needed for combustion is readily available and in an unlimited amount. The oxygen gas can be referred to as the excess reagent. The campfire burns out when the wood is completely consumed. The wood then is referred to as the limiting reagent. A limiting reagent is defined as the reagent/reactant that limits the amount of product a chemical reaction can produce. It can also be thought of as the only reactant that is completely consumed and therefore causes the reaction to end, limiting the amount of product. For example, if you are making 4-wheeled carts and you start with 12 tires and 4 cart bodies you can only make 3 complete carts. Why? What is the limiting reagent in this example? What is in excess?
From your experimentation so far, you may have realized that perfect results are rare. There are many reasons why an experiment may not give a perfect result. Therefore the yield of a reaction is an important characteristic of a reaction. The yield is the amount actually produced in the experiment/reaction. Percent Yield is the ratio (as a percentage) of the amount actually produced as compared to the theoretical maximum amount that can be produced. For example, in your reaction creating MgCl2 from Mg metal and HCl, the balanced equation shows that 1.0 mole of Mg should make 1.0 mole of MgCl2. If, however, only 0.9 moles of MgCl2 is produced the percent yield of this reaction is 90%. Percent yield is an important concept in industry for determining the economic viability of a chemical process.
Reading #4 9.4 Limiting Reagent
Homework #4 : p. 286-90: 41, 45bd, 46ad, 54, 57, 86, 90
Reading #5 9.4 Percent Yield of a Chemical Reaction
Homework #5 : p. 288 : 60, 61, 63, 64, 71, 92
Ways of Writing Equations
Equations can be written in three ways molecular, ionic and net ionic. Molecular equations are ones written with all ions that are actually separated in solution as if they were together. Ionic are equations with all ions written as separate ions. Net ionic equations are written without the ions that do not change (spectator ions) in the transition from reactant to product. The following is an example of the same equation written each way.
Molecular AgNO3 (aq) + NaCl (aq) --> AgCl (s) + NaNO3 (aq)
Ionic Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) --> AgCl(s) + NO3-(aq) + Na+(aq)
Net Ionic Ag+(aq) + Cl-(aq) --> AgCl(s)
Concepts: observing and classifying chemical reactions
Skills: making observations, writing equations, classifying reactions
Demos/Activities/Experiments
Activity: Precipitate Formation
Reading #6 7.2 Reactions in which a Solid Forms
Homework #6 p 208-9 # 3-10, 11 a-c, 12 a-d,f, 13, 16, 17 b-d, 18 ef
Reading #7 7.3 Reactions in aqueous solution
Homework #7 p 210 # 21, 23, 24, 25, 29 p. 213 # 75
Reading #8 7.7 Classification of Reactions
Homework #8 p 212 # 55, 56, 58, 59 a, c, 61c, 63c , 67, 69 adef