Chemical Equations. Shorthand way of describing a chemical change or reaction. Reactants Products

Transcription

1 Chemical Equations Shorthand way of describing a chemical change or reaction. Reactants Products All chemical equations MUST be balanced: 1. Material balance - # atoms of each element must be the same on both sides of the equation. Ex. CH O 2 CO H 2 O 2. Charge balance net charge on reactant side must be equal to net charge on product side. Ex. H 3 PO H 2 O PO H 3 O + No set rules for balancing a chemical equation!! Guidelines for Balancing a Chemical Equation: A. Adjust the coefficients. Never change the subscripts. B. Start with most complicated species. OR Start with an element that appears in only one substance on both sides of the equation. C. Balance polyatomic ions as a group. D. By convention, write with smallest set of whole # coefficients. Balance the following chemical equations. A. O 2 + SO 2 SO 3 B. C 4 H 10 O + O 2 CO 2 O (complete combustion of butanol) C. Complete combustion of C 8 H 18 D. Al 2 O 3 SO 4 Al 2 (SO 4 O E. Na 3 PO 4 + Ba(NO 3 Ba 3 (PO 4 + NaNO 3 F. H 2 S + Fe(NO 3 Fe 2 + HNO 3 G. CaCl 2 + H 3 PO 4 + NaOH Ca 3 (PO 4 + NaCl O H. Complete combustion of C 8 H 6 N 2. I. Complete combustion of C 10 H 8 O. 1

2 Stoichiometry Use of the coefficients in a balanced equation to relate amounts of chemicals. Coefficients in balanced equation can be interpreted on two different scales.. Microscale atoms, molecules, formula units Macroscale moles Balance the chemical equation below and write microscale and macroscale conversion factors. Al 2 O 3 SO 4 Al 2 (SO 4 O (unbalanced) A. If mol sulfuric acid are reacted with excess Al 2 O 3, how many moles of aluminum sulfate can be produced? (Use eqn. shown above.) B. How many moles of sulfuric acid are needed to react with 1.07 mol Al 2 O 3? (Use eqn. shown above.) Examples (cont): C. What mass of phosphoric acid can be produced by the reaction of 8.5 mol diphosphorus pentoxide with excess water? How many moles of water are needed to completely react with mol diphosphorus pentoxide? What mass of phosphoric acid can be produced from 18 g of water reacting with an excess of diphosphorus pentoxide? 2

3 D. Magnesium nitride is formed by the reaction of magnesium metal with gaseous ammonia. What mass (in g) of magnesium nitride can be produced from g magnesium? (The other product is gaseous hydrogen.) What mass of gaseous ammonia is needed to react with the g magnesium? 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: High school dance.. Balanced Eqn: Actually Present: 1 girl + 1 boy 1 dancing couple 56 girls + 58 boys?? couples 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. 3

4 To identify a limiting reagent problem Amounts (mol, g, etc.) for two different reagents are given. A. Consider the balanced equation shown below. If 2.0 mol RuCl 3 is reacted with 5.0 mol Li 2 S, how many moles of Ru 2 can be formed? How many moles of which reagent are left? 3 Li 2 S + 2 RuCl 3 Ru LiCl B. If 0.10 mol RuCl 3 is reacted with 0.12 mol Li 2 S, how many moles of Ru 2 can be formed? How many moles of which reagent are left? C. 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 left? D. One step in the process for producing nitric acid is the following: NH 3 + O 2 NO O (unbalanced) What mass (in g) of NO can be formed by the reaction of g NH 3 with g O 2? After the reaction, what mass of which reagent is left? 4

5 Percent Yield Most reactions do not proceed 100% to completion. As a result, the percent yield is less than 100%. Percent Yield = Actual Yield x 100% Theoretical Yield Actual Yield: amount of product obtained experimentally Theoretical Yield: maximum amount of product that could be obtained based upon stoichiometric calculations (i.e. from amount of reagents). Actual Yield < Theoretical Yield due to.. 1. Mechanical losses (product stuck to glassware, filter paper, etc.) 2. Product evaporation 3. Competing reaction that uses up some of reagent to produce undesired product. 4. Reaction proceeds less than 100% to completion. A. When g chlorine are reacted with excess phosphorus (P), 26.7 g phosphorus trichloride are experimentally isolated. What is the percent yield of phosphorus trichloride? B. For the reaction shown below, the percent yield is typically 17% RuS 2. To experimentally isolate 15.0 g RuS 2, what mass of RuCl 3 should be reacted? 3 Li 2 S + 2 RuCl 3 S 2 RuS LiCl 5