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Download Unit 6- Math of Chemistry
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Formulas, Equations And The Math Involved • • • • • • • • • • • • Analysis Chemical change Coefficient Combustion Decomposition Diatomic molecule Double replacement Empirical Formula Formula Formula mass Gram formula mass Mole • • • • • • • • • • • • Molecular formula Molecule Percent composition Polyatomic ion Product Qualitative Quantitative Reactant Single replacement Subscript Symbol Synthesis Review: • What’s the SI unit for amount? – A mole • What do we know about moles? – # of atoms in 12 grams of C-12 – Avogadro’s number – 6.022 x 1023 • What is the gram formula mass? – Mass in grams of 1 mole of a substance – Units are g/mol….. The formula mass is in amu – Ex: Carbon 12g/mol • Is the atomic masses on the Periodic Table an average? – YES, it’s the average of the masses of each isotope Review: • How do you calculate average atomic mass? • Converting grams < -- > moles < -- > molecules Molecules Grams 6.022x1023 g/mol Moles What are chemical formulas used for? • To express composition – KBr 1:1 ratio (in ionic compounds the ratio is of cations and anions) – H2O 2:1 ratio (covalent) • To give ratios of polyatomic ions – KNO3 1:1 ratio because NO3 is a polyatomic ion • To calculate formula masses – ZnCl2 = 136.29g/mol • To calculate the percent composition of each element – Percent composition- % by mass of each element in a compound • What is the percent composition of O in KClO3? • 1st: Determine formula mass of element and of the compound • 2nd: % of element = element formula mass/ total formula mass x 100 Molecular vs. Empirical Formula • Empirical formula– Chemical formula that shows the composition of a compound in its simplest ratios – Ex: CH2O • Molecular formula– Chemical formula that shows the number and kinds of atoms in a molecule – Multiples of the empirical formula – Ex: C6H12O6 • You can find the molecular formula from the empirical formula – If a compound has a mass of 180 amu and an empirical formula of CH2O, what is the molecular formula? • Get formula mass of empirical formula (C + H +H +O = 30amu) • Divide molecular mass given by empirical formula mass (180amu / 30amu = 6) • Multiply subscripts by 6 (C 1x6 H 2x6 O 1x6 = C6H12O6) Chemical Equations and Reactions • What is a chemical reaction? – When 1 or more substances change into 1 or more substances whose chemical and physical properties differ from the original substances • Chemical formulas- use chemical symbols and #’s to show quantitative and qualitative info about a substance – Coefficients- written in front of a formula, tells you how many units of the formula are present • Ex: 2H2O 4 H’s and 2 O’s – Subscripts- written after an element, tells how many atoms of that element Describing a Chemical Reaction Indications of a Chemical Reaction – Evolution of heat, light, and/or sound – Production of a gas – Formation of a precipitate – Color change Signs of Chemical Reactions There are five main signs that indicate a chemical reaction has taken place: release input change in color change in odor production of new gases or vapor input or release of energy difficult to reverse Chemical Equations aluminum oxide Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O2(g) 2 Al2O3(s) The letters (s), (g), and (l) are the physical states of compounds. The numbers in the front are called stoichiometric coefficients. Chemical Equations and Reactions Con’t Reactants Products “yields/produces” • • • • 1st- word equation: carbon + oxygen carbon dioxide 2nd- formula eqn: C(s) + O(g) CO2(g) 3rd- balance: C(s) + O2(g) CO2(g) What law does this follow? – Conservation of mass • Equations are like recipes – Show physical states and rx conditions Balancing Equations • Identify reactants and products • Count atoms – Treat polyatomic ions like a single unit • Insert coefficients – Try odd-even technique • Multiply odd by 2 gives an even – Can’t change subscripts** • Check results • Practice: Fe2O3 + H2 Fe + H2O NH3 + O2 NO + H2O HgCl2 + AgNO3 Hg(NO3)2 + AgCl CH4 + 2 O2 CO2 + 2 H2O Reactants 1 C atom 4 H atoms 4 O atoms Products 1 C atom 4 H atoms 4 O atoms Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride. 1) Write a word equation for the reaction. chlorine + sodium bromide bromine + sodium chloride 2) Write the correct formulas for all reactants and products. Cl2 + NaBr Br2 + NaCl 3) Determine the coefficients that make the equation balance. Cl2 + 2 NaBr Br2 + 2 NaCl Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate. 1) Write a word equation for the reaction. ? ? aluminum sulfate + calcium chloride calcium sulfate + aluminum chloride 2) Write the correct formulas for all reactants and products. Al2(SO4)3 + CaCl2 CaSO4 + AlCl3 3) Determine the coefficients that make the equation balance. Al2(SO4)3 + 3 CaCl2 3 CaSO4 + 2 AlCl3 Types of Chemical Reactions • Synthesis (combination) Rx: – – – – 2 Na Na Na 2 reactants form a single product A + B AB Ex: CO2(g) + H2O(l) H2CO3(aq) *dehydration synthesis + Cl2 2 NaCl Cl Na+ Cl - Cl Cl - Na+ • Decomposition (analysis) Rx: – – – – 1 reactant forms 2 or more products AB A + B Ex: 2NI3(s) N2(g) + 3I2(g) *airbags NaN3 +spark 2Na(s) + 3N2(g) 2 H2 O 2 H2 + H O H H O H + O2 • Combustion Rx: – Reaction of carbon based compound with oxygen to produce carbon dioxide and water vapor – Ex: burning of wood – Incomplete combustion= CO, unburned carbon(soot), carbon dioxide and water vapor Displacement reactions • Single and double replacement reactions • It’s necessary to use Reference Table J to determine if the reaction can take place • Element can replace those listed below it • Ex: F2 + 2NaCl Cl2 + 2NaF – F replaces Cl Ca Foiled again – Aluminum loses to Calcium ** H2- all metals above it will react with acids to release H gas and form a salt • Single replacement Rx: – 1 element replaces another in a compound – A + BX B + AX – Ex: Cu(s) + 2AgNO3(aq) 2Ag(s) + Cu(NO3)2(aq) • Reverse Rx won’t happen due to the reactivity of the metals Predict if these reactions will occur 3 Mg + 2 Al Cl3 2 Al + 3MgCl2 Can magnesium replace aluminum? YES, magnesium is more reactive than aluminum. Activity Series Al + MgCl2 No reaction Can aluminum replace magnesium? NO, aluminum is less reactive than magnesium. Activity Series MgCl2 + Al Therefore, no reaction will occur. No reaction Order of reactants DOES NOT determine how they react. The question we must ask is can the single element replace its counterpart? metal replaces metal or nonmetal replaces nonmetal. Single-Replacement Reactions Activity Series Fe + FeCl2 CuCl2 + Cu Can Fe replace Cu? Yes Zinc in nitric acid Zn + 2 3 HNO Can Zn replace H? Zn(NO3)2 + H2 Yes NO REACTION MgCl2 + MgBr2 Br2 Can Br replace Cl? + No General Form A + BC AC + B Cl2 Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H2 Cu Hg Ag Pt Au F2 Cl2 Br2 I2 • Double replacement Rx: – Usually involves 2 soluble ionic compounds that react in solution to produce a precipitate, a gas or a molecular compound such as water – AB + CD AD + CB – Ex: 2KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2KNO3(aq) Determining Unknowns • Follow Law of Conservation of Mass • Unknown reactant or product: – 2Na + 2H2O X + 2NaOH • You must have the same amount of each element on each side of the reaction – X = H2 • Missing mass: – How many grams of silver nitrate are needed to react with 156.2g of sodium sulfide to produce 595.8g of silver sulfide and 340.0g of sodium nitrate? 2AgNO3 + Na2S Ag2S + 2NaNO3 X + 156.2g = 595.8g + 340.0g X = 595.8g + 340.0g - 156.2g = 779.6g Stoichiometry • Proportional relationship between 2 or more substances during a chemical reaction – mass relationships between substances in a chemical reaction – based on the mole ratio • Mole ratio – indicated by coefficients in a balanced equation 2 Mg + O2 2 MgO Proportional Relationships 2 1/4 c. flour 1 tsp. baking soda 1 tsp. salt Conversion Factor 1 c. butter 3/4 c. sugar 3/4 c. brown sugar 1 tsp vanilla extract 2 eggs 2 c. chocolate chips Makes 5 dozen cookies. I have 5 eggs. How many cookies can I make? Ratio of eggs to cookies 5 eggs 5 dozen 2 eggs 150 dozen cookiescookies = 12.5 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Visualizing a Chemical Reaction 2 Na 10 mole Na ___ + Cl2 5 mole Cl2 ___ 2 NaCl 10 ? mole NaCl ___ Formation of Ammonia Stoichiometry Steps 1. Write a balanced equation. 2. Identify known & unknown. 3. Line up conversion factors. – – – – – Mole ratio Molarratio mass-Mole Molarity Molar volume - moles moles molesmoles grams moles moles liters soln moles liters gas Core step in all stoichiometry problems!! 4. Check answer. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Molar Volume at STP 1 mol of a gas=22.4 L at STP Standard Temperature & 0°C and 1 atm Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Pressure Molar Volume at STP LITERS OF GAS AT STP Molar Volume (22.4 L/mol) MASS IN GRAMS Molar Mass (g/mol) 6.02 1023 MOLES particles/mol NUMBER OF PARTICLES Molarity (mol/L) LITERS OF SOLUTION Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Stoichiometry Problems • How many moles of KClO3 must decompose in order to produce 9 moles of oxygen gas? 2KClO3 2KCl + 3O2 ? mol 9 mol O2 2 mol KClO3 3 mol O2 9 mol = 6 mol KClO3 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem 2 1. 5 mol 2 Sb + 3 Cl2 2 SbCl3 excess 5 mol 7.5 excess x mol mol = 3 x mol 2 x = 15 x = 7. 5 mol x mol How many moles of chlorine gas are required to react with 5 moles of antimony? x mol Cl2 = 5 mol Sb 3 mol Cl2 2 mol Sb = 7.5 mol Cl2 How many moles of SbCl3 are produced from 5 moles of antimony and excess Cl2? x mol SbCl3 = 5 mol Sb 2 mol SbCl3 2 mol Sb = 5 mol SbCl3 How many moles of SbCl3 are produced from 7.5 moles of Cl2 and excess Sb? x mol SbCl3 = 7.5 mol Cl2 2 mol SbCl3 3 mol Cl2 = 5 mol SbCl3 2. 2 Mg + O2 2 MgO 10 mol xL x mol How many moles of magnesium oxide are produced from the burning of 10 mol of Mg? x mol MgO = 10 mol Mg 2 mol MgO 2 mol Mg = 10 mol MgO How many liters of oxygen are needed to burn 10 mol of Mg? Assume 1 mol O2 = 22.4 L x L O2 = 10 mol Mg x L O2 = 10 mol Mg 1 mol O2 2 mol Mg = 5 mol O2 1 mol O2 22.4 L O2 2 mol Mg 1 mol O2 22.4 L O2 1 mol O2 = 112 L O2 = 112 L O2 3. CaCl2 Ca + Cl2 8 mol x mol How many moles of calcium metal and chlorine gas are produced from the decomposition of 8 mol of calcium chloride? x mol Ca = 8 mol CaCl2 1 mol Ca 1 mol CaCl2 = 8 mol Ca How many moles of calcium metal and chlorine gas are produced from the decomposition of 8 mol of calcium chloride? x mol Cl2 = 8 mol CaCl2 1 mol Cl2 1 mol CaCl2 = 8 mol Cl2 2C2H6 + 7O2 4CO2 + 6H2O 1. How many liters of CO2 gas will be produced from the complete combustion of 30.0L of C2H6 according to the equation? vol C2H6(g)= 30L vol of CO2(g) = x ratio C2H6(g) to CO2(g) = 2:4 30L/ 2 = X/4 X= 60L of CO2 2. How many moles of H2O will be produced from the complete combustion of 3 mol of C2H6 according to the equation? mol C2H6 = 3 mol mol water = x mol ratio ethane to water = 2:6 3mol C2H6/ 2 mol C2H6 = x/6mol water X=9mol water