Ppt 11 Plan (PS5, 1-11 material) Meaning of Coefficients in a Balanced Equation Ratio of reaction (in FU or moles), not actual amounts “Standard” vs. nonstandard (difference between a product and leftover reactant) Nanoscopic Interpretation (FUs) [Not stressed in Tro?] Predict the equation from picture Predict final picture from initial (pic) and equation Macroscopic Interpretation (moles [of FUs]) (Tro, 4.2) Mole ratios as 4th “interconversion factor” mol A  mol B; mol A, B  g B, A; g A  g B Problems on old Stoichiometry Quiz How to Balance an Equation (Tro, 3.10) Ppt 11

Example—Nanoscopic Interpretation of a balanced equation Note: The same number of atoms are represented on the right side of the arrow as the left. N2 + 3 H2  2 NH3 means what? “For every N2 molecule that reacts three molecules of H2 react with it to form two NH3 molecules.” Ratio is 3 H2 lost : 1 N2 lost : 2 NH3 formed NO!! Does this tell you how much N2 you start with? Does this tell you how much H2 you start with? NO!! Does this tell you how much NH3 is made? NO!! Ppt 11

Coefficients Represent a ratio only (not actual amounts) Ratio is only of FU or moles (not grams!) Only when chemical change (rxn) takes place Not the ratio of amounts present at the beginning Not the ratio of amounts present at end If I could, I’d define something called an “equation unit” of reaction: Smallest amount of reaction that could possibly occur. Coeffs represent the exact number of each FU used and made when one “equation unit” of reaction occurs Ppt 11

“Standard” Balanced Equation Coefficients are in the lowest whole number ratio These equations are balanced! They’re just not in standard form. The same substance appears only once on each side of the equation If it’s on both sides, it didn’t actually change! Leftover reactant is not a product! Technically balanced, but gives wrong ratio and implies H2 is made! Ppt 11

Application of Ideas I—Nanoscopic Pics/Interpretation See PS4 & 5 Practice Worksheet, Problems #7 and #8 7. Reaction of A (open spheres) with B (black spheres) is shown schematically in the following diagram: Which equation best describes the stoichiometry of the reaction (the ratio in which substances react and form when the reaction takes place)? (a) A2 + 2 B → A2B2 (b) 10 A + 5 B2 → 5 A2B2 (c) 2 A + B2 → A2B2 (d) 5 A + 5 B2 → 5 A2B2 Ans. (c) It gives the ratio (lowest whole number). Equation is not meant to represent “actual amounts”. Ppt 11

Application of Ideas I—Nanoscopic Pics/Interpretation See PS4 & 5 Practice Worksheet, Problems #7 and #8 8. If the diagrams below represent a reaction that occurred in a closed container according to the equation: 2 NO(g) + O2(g) → 2 NO2(g), what would be left in the box after the reaction has gone as completely as possible? DONE ON OVERHEAD Ppt 11

Application of Ideas I—Nanoscopic Pics/Interpretation #8 Follow up. NOTE: 6 NO + 5 O2  6 NO2 + 2 O2 is not an equation in standard form! “Simplify” to: 6 NO + 3 O2  6 NO2 (O2’s “cancelled”) And then: 2 NO + O2  2 NO2 (ratio reduced) The ratio in which the reactants “reacted” is 2:1, not 6:5! O2 is not a product! Ppt 11

PS Sign-Posting The concepts and skills related to problems 1-3 on Written PS5 (and some Mastering Qs) were covered in the prior section of this PowerPoint. Give those problems a try now! Ppt 11

Application of Ideas II—Macroscopic Interpretation N2 + 3 H2  2 NH3 also means: “For every 1 mole of N2 (molecules) that react, 3 moles of H2 (molecules) react with them to form 2 moles of NH3 (molecules) Ratio is 3 moles of H2 lost : 1 mole of N2 lost : 2 moles of NH3 formed Ppt 11

Thus….Mole RATIOS Can Be Made (and Used!) From: N2 + 3 H2  2 NH3 you can create….mole ratio “conversion factors” And reciprocals: Ppt 11

See Stoichiometry Quiz (Chemistry 121 Quiz Used as PRACTICE WORKSHEET on "Early" Stoichiometry) 1. (8 pts) Given the following chemical equation: P4O10 + 5 CCl4  5 CO2 + 4 PCl3 + 4 Cl2 How many moles of CO2 will be formed if 3 moles of P4O10 react? How many moles of P4O10 would be used up if 1.9 moles of Cl2 were produced? How many moles of PCl3 form if 2.4 moles of CCl4 react? How many moles of CO2 will be formed if 0.247 moles of P4O10 react? How many grams of Cl2 would be formed if 3.2 moles of P4O10 were reacted? If 32.6 g of CCl4 reacts, how many grams of Cl2 form? Ppt 11

Solution to (f) on prior worksheet If 32.6 g of CCl4 reacts according to the equation below, how many grams of Cl2 will form? P4O10 + 5 CCl4  5 CO2 + 4 PCl3 + 4 Cl2 Ppt 12

Calculating Mass of Reactants and Products Reacted or formed! Reacted or formed! Ppt 11

Bertrand Applet—Applying “Mole ratio” Idea to Chemical Reactions Try out the following applet to see if you really understand the meaning of a balanced chemical equation (and the difference between an equation and a chemical reaction)! http://web.mst.edu/~gbert/reactor/Areactor.html Ppt 11

Take home lessons from web exercise (prior slide) The amount of a reactant that is present to begin with is not necessarily equal to the amount that reacts Some might be left over (not reacted)! The amount of a product that is present at the end is not necessarily equal to the amount that formed Some might have been present to start with! Coefficient ratios apply only to the “change” in R’s or P’s Use “I C F” (Initial, Change, Final) table to help see this Ppt 11

PS Sign-Posting The concepts and skills related to problem 4 and 7* on Written PS5 (and more problems in Mastering) were covered in the prior section of this PowerPoint. Give those problems a try now! * Problem 7a and 7b have been covered, but the term “limiting reactant”, which is in part c of Q7, will be covered in Ppt12. Ppt 11

How To Balance a Chemical Equation-I Balancing Means “Adding Coefficients” Not subscripts!! Must ALREADY KNOW the substances that are reactants and products The FORMULAS must be determined FIRST! Write the formulas of the reactants on the left of the arrow and those of the products on the right Method of “Committed Coefficients” Idea (next slide) Ppt 11

How To Balance a Chemical Equation-II “Method of Committed Coefficients” Pick one of the most complex-looking formulas and make the coefficient a “1”. That coefficient is now “committed” 2. Find an atom type that occurs only in that formula (since it is “committed”), and in only ONE formula on the other side of the arrow (if possible). “Balance” that atom type by adding a coefficient (to make total atoms on each side equal). Now you have TWO committed coefficients! 3. Repeat the steps above, always looking first for atom types that appear in the fewest number of formulas  Always leave the formulas of ELEMENTS for last! Ppt 11

Method of Committed Coefficients (cont.) If you get to a point where you need to use a FRACTION to get the number of atoms you “need”, then use the fraction as a coefficient! At the end, multiply through all coefficients by the denominator of your fraction to end up with a whole number ratio. If you prefer to avoid fractions, you can “start over” with a “2” (or “3”) in place of the original “1” instead of doing “Step 4” here. Important tip: When counting up atoms before placing your next coefficient, only count those atoms that come from formulas with committed coefficients. i.e., Don’t just count up all the atoms of X on both sides of the equation to start off with! This will assume that all the coefficients are “1” when most of the time that will not be the case once the equation is balanced. Ppt 11

Example(s) Al + NH4ClO4 → Al2O3 + AlCl3 + NO + H2O 1 1 Look at right side: N, H, and Cl are “isolated”; O is in many places 1 Al + 1 NH4ClO4 → Al2O3 + AlCl3 + NO + H2O 1 2 O’s: on LEFT, 4 on RIGHT, 3 committed (don’t count Al2O3!)  Need ONE more  coefficient of 1/3 for Al2O3 Al’s: on RIGHT, 2/3 + 1/3 = 1  on LEFT, commit a “1” All coeffs done. Multiply whole equation (all coeffs) by 3: 3 Al + 3 NH4ClO4 → 1 Al2O3 + 1 AlCl3 + 3 NO + 6 H2O Ppt 11

Balancing Equations Examples/Practice (handout) ___ Cu2O + ___ Cu2S → ___ Cu + ___ SO2 ___ HCl + ___ Al(OH)3 → ___ AlCl3 + ___ H2O ___ CH3OH + ___ O2 → ___ HCHO + ___ H2O ___ P + ___ Fe2O3 → ___ P4O10 + ___ Fe ___ Pb + ___ PbO2 + ___ H2SO4 → ___ PbSO4 + ___ H2O ___ PbO + ___ PbS → ___ Pb + ___ SO2 Ppt 11

PS Sign-Posting The concepts and skills related to problems 5 and 6 on Written PS5 (and more problems in Mastering) were covered in the prior section of this PowerPoint. Give those problems a try now! Ppt 11