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WVU CHEM 115 - Lecture 10

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1 Limiting Reagent • Consider the reaction of N2 with H2 to form NH3: – N2(g) + 3H2(g) → 2NH3(g) • The stoichiometry suggests that for every mole of N2 we will need 3 moles of H2 to form 2 moles of NH3. • So what happens if these proportions are not met? The reaction proceeds, to use up one of the reactants (the limiting reagent) and but not all of the other reactant (it is in excess) • Why would an excess of a reagent be used? – Some reactions proceed better (more to completion) in the presence of an excess of one reagent. – One reagent may be expensive. Use an excess of the cheaper reagent to ensure that the more costly reagent is used up and not wasted. Limiting Reagent Problems One or more reagents is present in an excess amount and one reagent is limiting (i.e. limits the amount of product formed). Limiting reagent: Reagent that limits amount of product formed. All used up during the reaction. Excess reagent: Reagent that does not limit the amount of product formed. Some of this reagent is left over after the reaction. Example: Balanced Eqn: 2 wheels + 1 frame  1 bike Actually Present: 18 wheels + 7 frames ?? bikes2 To identify a limiting reagent problem  Amounts (mol, g, etc.) for two different reactants (reactants) are given. Example: Consider the balanced equation shown below. If 2.0 mol RuCl3 is reacted with 5.0 mol Li2S, how many moles of Ru2S3 can be formed? How many moles of which reagent are left? 3 Li2S + 2 RuCl3  Ru2S3 + 6 LiCl Step 1: Convert both reactants to moles of the same product. Step 2: Identify the limiting reagent by comparing moles of product determined in Step 1. The reactant that produces the least amount of product is the limiting reagent. The other reactant is the excess reagent. Limiting Reactant Steps Continued… Step 3: Use the limiting reagent to answer the problem.3 Learning Check If 7.0 mol hydrogen sulfide are reacted with 3.0 mol sulfur dioxide, how many moles of sulfur (S) can be formed (the other product is water)? How many moles of which reagent are


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