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Stoichiometry

AP Chemistry. Stoichiometry. In the thermite reaction, a mixture of powdered aluminum and powdered iron(III) oxide react to yield iron and aluminum oxide. The reaction burns hot enough to be useful in under- water welding. 2 Al + Fe 2 O 3  2 Fe + Al 2 O 3 + energy.

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Stoichiometry

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  1. AP Chemistry Stoichiometry In the thermite reaction, a mixture of powdered aluminum and powdered iron(III) oxide react to yield iron and aluminum oxide. The reaction burns hot enough to be useful in under- water welding. 2 Al + Fe2O3 2 Fe + Al2O3 + energy

  2. Components of a Chemical Equation Subscripts Subscripts Subscripts PHYSICAL STATE (Yield) (Reactants) (Products) Sometimes you will see a “yields” sign that looks like this. What do you think it means?

  3. Writing Chemical Reactions Reactant ( )  Product ( ) Reactants • Always written on the LEFT side of the arrow • Represent the chemicals you START with BEFORE the reaction Products • Always written on the RIGHT side of the arrow • Represents the chemicals you END with AFTER the reaction  Shows the direction of the reaction; separates React. from Prod. ( ) Where you write the PHYSICAL STATE of each chemical Always written to the RIGHT of the chemical formula (s) = solid (l) = liquid (g) = gas (aq) = aqueous (dissolved in water!)

  4. Chemical Equations: • Shorter, easier way to show chemical reactions, using symbols instead of words • Chemical formula: a set of element symbols that shows the a amount of atoms in a compound • Subscript: small number, located on the right corner, next to the chemical symbol and indicates the number of atoms. Number never changes • Ex: H2 shows that there are 2 Hydrogen atoms • Coefficients- number of moles or molecules in a given substance and is a number placed in front of an element symbol. Alsoused to balance chemical equations • Ex: 3H2shows that there are 3 moles (or molecules) of hydrogen gas.

  5. Review just readHow do we know a chemical reaction has occurred? A CHANGE in the following will occur during the lab: • Change in color • Change in odor • Change in clarity (cloudy/clear) • Solid precipitate forms (cloudy) • Gas forms (bubbles, fizzing, foaming) • Change in temperature (all by itself!) • Light/fire is produced (combustion)

  6. Key Words to HELP Translate Word Problems  Yields, produces, creates, generates, evolves, Forms, converted, decomposes, synthesized  The reaction is heated The temperature the reaction is taking place at 25˚C is room temperature! Catalyst makes a reaction go faster Catalyst is NOT a reactant or a product

  7. Key Words to HELP Translate Word Problems  Precipitate, residue, powder, cloudy, metal, slightly soluble, insoluble (s) or  (g) or  Vapor, evaporates, foams, bubbles, fizzes (aq) Solution, soluble, aqueous, dissolved in water Reacts with, and , combines, added to, bubbled through +

  8. Translating Word Equations • Solid potassium reacts with water to produce hydrogen gas and aqueous potassium hydroxide. • Solid nickel reacts with aqueous lead (II) nitrate to produce a solution of nickel (II) nitrate and a residue of lead. • Nitrogen monoxide gas and water vapor are produced from a reaction between ammonia gas and oxygen gas. +  K(s) H2O(l) H2(g) + KOH(aq)  + Ni(s) Pb(NO3)2(aq) + Ni(NO3)2(aq) Pb(s)  NO(g) + H2O(g) + NH3(g) O2(g)

  9. Chemical Equations In a reaction: atoms are rearranged AND mass are conserved energy charge Balancing Chemical Equations law of conservation of mass same # of atoms of each type on each side of equation = Hint: Start with most complicated substances first and leave simplest substances for last.

  10. Solid lithium reacts w/oxygen to form solid lithium oxide. Li+ O2– 4 2 Li(s) + O2(g) Li2O(s) + Aqueous aluminum sulfate reacts w/aqueous barium chloride to form a white precipitate of barium sulfate. The other compound remains in solution. Ba2+ Al3+ SO42– Cl– 3 3 2 + + AlCl3 BaSO4 Al2(SO4)3 BaCl2 (aq) (s) (aq) (aq)

  11. CaC2(s) + H2O(l) C2H2(g) + CaO(s) CaSi2+ SbI3Si + Sb+ CaI2 Al+ CH3OH Al(CH3O)3+ H2 Methane gas (CH4) reacts with oxygen to form carbon dioxide gas and water vapor. Furnaces burn primarily methane. 2 2 CH4(g) O2(g) H2O(g) CO2(g) + + Practice Balancing equations Already balanced- leave as is 6 2 2 3 3 3 6 2 2 3

  12. Identifying Different Types of Reactions 1. Combustion C?H? + O2(g)  CO2(g) + H2O(g) What you will see: • Always a hydrocarbon and oxygen on the reactant side • Always carbon dioxide and water on the product side CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g) C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (g) methane propane

  13. alkali metal metal hydroxide hydrogen gas + water + 2 2 5 4 2 C2H2(g) + O2(g) CO2(g) + H2O(l) 5 3 4 C3H8 + O2CO2 + H2O 8 4 10 2 5 C4H10+ O2 CO2+ H2O 13 Complete combustion of a hydrocarbon, or of a compound containing C, H, and O (e.g., methanol, CH3OH) yields CO2 and H2O. Another pattern of reactivity: e.g., 2 Rb(s) + 2 H2O(l)  2 RbOH(aq) + H2(g)

  14. Identifying Different Types of Reactions 3. Combination (synthesis) A + B  AB AB + C  ABC A + B + C  ABC A + B  AB simpler substances combine to form more complex substances What you will see: • Several things on the reactant side • Only ONE thing on the product side + Na 2 2 NaCl Cl2 sodium + chlorine gas sodium chloride

  15. lithium chlorate lithium chloride + oxygen gas Identifying Different Types of Reactions complex substances are brokendown into simpler ones AB  A + B ABC  AB + C ABC  A + B + C 4. Decomposition AB  A + B What you will see: • Only ONE thing on the reactant side • Several things on the product side Li+ Li+ ClO3– Cl– 2 3 2 + O2 LiClO3 LiCl

  16. Identifying Different Types of Reactions 5. Single Replacement AB + C  AC + B What you will see: • A compound and a single element on the reactant side • A compound and a single element on the product side 2AlBr3 (s) + 3Cl2 (g)  2AlCl3 (s) + 3Br2 (l) Mg (s) + H2S (g)  H2 (g) + MgS (s)

  17. 6. Precipitation Reaction (aka Double Replacement) AB + CD  AD + CB What you will see: • Two different compounds on the reactant side • Two different compounds on the product side AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) 2HCl (aq) + Ca(OH)2 (aq)  2HOH (l) + CaCl2 (aq)

  18. formula weight: the mass of all of the atoms in a chemical formula (unit is amu) -- If the substance is a molecular substance (e.g., C3H8), then the term molecular weight is also used. molar mass: the mass of one mole of a substance (unit is usually grams) -- recall that 1 mole of any substance 6.02 x 1023particles of that substance =

  19. Just read: What is the “MOLE” in chemistry? The “MOLE” • A unit in chemistry created by a scientist named Avogadro • 1 mole = 6.02 x 1023 units • 602,000,000,000,000,000,000,000 is the number of atoms it took to reach the atomic mass (of any element) in grams! Atomic Number (# protons) Atomic Mass 12.01 amu = (avg mass of 1 carbon atom) 12.01 g = (mass of 1 mole of carbon)

  20. Find the molar mass and formula weight of ammonium phosphate. NH4+ PO43– N: = 42.0 g 3 (14.0 g) (NH4)3PO4 (1.0 g) H: 12 = 12.0 g m.m. = 149.0 g P: = 31.0 g 1 (31.0 g) O: = 64.0 g 4 (16.0 g) f.w. = 149.0 amu

  21. Practice Problems… Calculating Molar Mass = 32.00 g/mol 16.00 x 2 (# of atoms) O2= • Oxygen gas – Atomic mass x number of atoms • Potassium sulfide • Aluminum carbonate K = 39.10 x 2 = 78.20 K2S Add 110.26 g/mol S = 32.06 x 1 = 32.06 Al2(CO3)3 Al = 26.98 x 2 = 53.96 C = 12.01 x 3 = 36.03 Add O = 16.00 x 9 = 144.00 233.99 g/mol

  22. g element x 100 % of element = molar mass of compound percentage composition: the mass % of each element in a compound Equation: Find the percentage of oxygen, by mass, in calcium nitrate. Ca(NO3)2 % O = = 58.5% O

  23. shows the true number and type of atoms in a m’cule lowest-terms formula Empirical Formula and Molecular Formula CH2O C3H8 C2H5 C5H4 C12H22O11 C4H9

  24. Finding an Empirical Formula from Experimental Data 1. Find # of g of each element. 2. Convert each g to mol. 3. Divide each “# of mol” by the smallest “# of mol.” 4. Use ratio to find formula. “What’s your flavor of ice cream?”

  25. A sample of a compound contains 4.63 g lead, 1.25 g nitrogen, and 2.87 g oxygen. Name the compound. #2 Convert to mole #1 Molar Mass (Periodic Table) #3 Divide by smallest mole #1 Molar Mass (Periodic Table) #1 Molar Mass (Periodic Table) see the ratio PbN4O8 ? ? Pb(NO2)4 lead(IV) nitrite Pb? 4 NO2–

  26. A ruthenium/sulfur compound is 67.7% Ru. Find its empirical formula. #2 Convert to mole #1 Molar Mass (Periodic Table) Multiply #’s by 2 #3 Divide by smallest mole RuS1.5 Ru2S3 When the #’s are too close to the middle (0.4, 0.5, 0.6) Don’t Round… x2 to get a whole #

  27. To find molecular formula… (“What’s your flavor?”) A. Find empirical formula. B. Find molar mass of empirical formula. C. Find n = mm molecular mm empirical D. Multiply all parts of empirical formula by n. (“How many scoops?”) (How many empiricals “fit into” the molecular?)

  28. Calculating the Molecular Formula • A white powder was found to have an empirical formula of P2O5. The compound isolated has a molar mass of 283.88 g/mol. What is the molecular formula of the white powder? MM Product = 283.88 g/mol MM of EF (P2O5) = (30.97x2) + (16.0 x5) = 141.94 g/mol Molecular Formula = (P2O5)2 = P4O10

  29. A sample of a compound has 26.33 g nitrogen, 60.20 g oxygen, and molar mass 92 g. Find molecular formula. Solve problem NO2 NO2 92 g 46 g 46 g N2O4 = 2 =mmemp=

  30. Hydrates and Anhydrous Salts anhydrous salt: an ionic compound (i.e., a salt) that attracts water molecules and forms weak chemical bonds with them; symbolized by MN “anhydrous” = “without water” Same idea as with… desiccants in leather goods, electronics, vitamins hydrate: an anhydrous salt with the water attached -- symbolized by MN .? H2O CuSO4. 5 H2O BaCl2. 2 H2O Na2CO3. 10 H2O FeCl3. 6 H2O Examples:

  31. H2O H2O H2O H2O HEAT H2O MN H2O H2O MN + H2O H2O H2O H2O H2O hydrate anhydrous salt water ENERGY ENERGY + +

  32. Finding the Formula of a Hydrate 1. Find the # of g of MN and # of g of H2O. 2. Convert g to mol. 3. Divide each “# of mol” by the smallest “# of mol.” 4. Use the ratio to find the hydrate’s formula.

  33. is an anhydrous salt on which the following data were collected. Find formula of hydrate. Strontium chloride beaker = 65.2 g beaker + sample before heating = 187.9 g beaker + salt + water beaker + sample after heating = 138.2 g beaker + salt Cl– Sr2+ SrCl2 • Need to find: MN .? H2O 1st find the masses of water and NM • 187.9 g • -138.2 g = 49.7 g H2O • 187.9 g • - 65.2 g beaker = 122.7g salt & H2O • 122.7g salt H2O - 49.7 g H2O = 73.0 g NM (anhydrous salt)

  34. Strontium chloride is an anhydrous salt on which the following data were collected. Find formula of hydrate. beaker = 65.2 g beaker + sample before heating = 187.9 g beaker + salt + water beaker + sample after heating = 138.2 g beaker + salt Sr2+ Cl– SrCl2 Solve for the empirical formula of MN and water SrCl2. 6 H2O

  35. Converting Between Various Units 1 mol = molar mass (in g) Mass (g) 1 mol = 22.4 L Volume (L or dm3) MOLE (mol) 1 mol = 22.4 dm3 (For gases only!) Particle (at. or m’c) at STP 1 mol = 6.02 x 1023 particles One-Substance Island Diagram

  36. Read: Unit Conversions… using the MOLE! • The amount of a substance can be measured using different units. • Micro-scale:particles (atoms or molecules) • Macro-scale:mole (mol), mass (g), volume (ml or cm3) • In order for scientists to compare their data the units must match  Molar Mass = 1 mole Avogadro’s # = 1 mole

  37. How do you use “moles” to convert units? Molar Mass (g) = 1 mole known unknown 1. How many grams are in 2.60 mol of copper(II) sulfate? 159.62 1 CuSO4 63.55 + 32.07 + (16x4) Conversion factor 3 sig figs

  38. How do you use “moles” to convert units? Molar Mass (g) = 1 mole 2. Calculate the number of moles in 9.21 g of SnO2. 1 SnO2 118.7 + (16x2) 150.7

  39. How do you use “moles” to convert to Particles units?Using Avogrado’s # 1 mole = 6.02 x 1023 atoms 3.How many moles are in 3.50 x 1024 molecules of FeCl3? 1 mole = 6.02 x 1023 molecules 1 6.02 x 1023

  40. How do you use “moles” to convert units? 1 mole = 6.02 x 1023 atoms 1 mole = 6.02 x 1023 molecules 4. How many atoms are in 0.250 mol of Na? 6.02 x 1023 1

  41. MOLE MAP FOR LONGER CONVERSIONSPut it all together Grams mol Particles (atoms or molecules) Grams/mole 6.02 x 1023particles/1mole (use periodic table) • Everything must go through “moles”!!! • (side by side numbers) = multiply • Top/bottom numbers =divide molar mass Avogrado’s #

  42.  Practice Problems  • Calculate the number of atoms in 43.15 g of carbon. Double T chart 6.02 x 1023 2.163 x 1024 Atoms C 1 12.01 1 Molar Mass Avogadro’s #

  43. PRACTICE PROBLEMS 2. What is the mass, in grams, of 2.00 x 1023atoms of NaCl? Molar Mass Avogadro’s # = 19.4

  44.  Practice Problems  Volume of Substance 3. How many mL of aluminum would need to be measured out in a graduated cylinder to have 4.0 moles of aluminum? (Density of aluminum is 2.70 g/ml.) Density 26.98 1 40. mL Al 1 2.70 Molar Mass density

  45.  Practice Problems  Volume of Substance 4. How many millimoles are present in a sample of calcium that has a mass of 0.0825 Kg? Density prefixes prefixes kg mmol 1 103 1 2.06 x 103 mmol Ca 1 40.0 10-3 prefixes molar mass prefixes

  46. ( ) ( ) ( ( ) ) aluminum sulfate 5. What mass is 6.29 x 1024m’cules ? Al3+ SO42– Al2(SO4)3 342.3 g 1 mol 6.29 x 1024 m’c 1 mol 6.02 x 1023 m’c = 3580 g 6.At STP, how many g is 87.3 dm3 of nitrogen gas? N2 1 mol 28.0 g 87.3 L = 109 g 22.4 L 1 mol

  47. Stoichiometry • Is the mathematical method used to figure out how much of a chemical you can make, or how much you need, during a chemical reaction Using the COEFFICIENTS in a BALANCED equation to set up a MOLE RATIO to calculate the amount of substance needed or produced

  48. What is a mole ratio? • It is the coefficients in a balanced equation. A mole ratio is used to convert the number of moles of one substance to number of moles of another substance in a chemical reaction.

  49. When going from moles of one substance to moles of another, use the coefficients from the balanced equation. Use coeff. from balanced equation in crossing this bridge mass mass vol. vol. MOL MOL part. part. SUBSTANCE “B” SUBSTANCE “A” (unknown) (known)

  50. The COEFFICIENTS tells us us the number of moles of each substance in the reaction. 2 moles of H2 gas reacts with 1 mole of O2 gas to produces 2 mol H20

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