is one where the substance retains its identity. Examples of physical reactions: ◦ ◦ ◦ ◦ ◦ melting / freezing boiling / condensing subliming subdivision dissolving (solvation) Stoichiometry the atoms of one or more substances are rearranged to produce new substances with new properties. the atoms remain the same, but their arrangement changes: H2O H2 + O2 water is broken down into two substances, oxygen gas and hydrogen gas. The two new substances bear no resemblance to the water of which they are made. Stoichiometry a) b) c) d) a colour change gas production an energy change precipitate formation Stoichiometry refers to balancing chemical equations and the associated mathematics. is possible because of the law of conservation of mass. Stoichiometry Concise representations of chemical reactions Stoichiometry CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) Stoichiometry CH4 (g) + 2 O2 (g) Reactants appear on the left side of the equation. CO2 (g) + 2 H2O (g) Stoichiometry CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) Products appear on the right side of the equation. Stoichiometry CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) The states of the reactants and products are written in parentheses to the right of each compound. Stoichiometry CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) Coefficients are inserted to balance the equation. Stoichiometry Subscripts tell the number of atoms of each element in a molecule Stoichiometry Coefficients tell the number of molecules Stoichiometry Matter cannot be lost in any chemical reaction. ◦ all the atoms must balance in the chemical equation. same number and type on each side of the equation ◦ can only change the stoichiometric coefficients in front of chemical formulas. ◦ Subscripts in a formula are never changed when balancing an equation. Stoichiometry Balance the following equation: Na(s) + H2O(l) H2 (g) + NaOH(aq) count the atoms of each kind on both sides of the arrow: ◦ The Na and O atoms are balanced (one Na and one O on each side) ◦ there are two H atoms on the left and three H atoms on the right. Stoichiometry Balance the following equation: Na(s) + 2 H2O(l) H2 (g) + NaOH(aq) the 2 increases the number of H atoms among the reactants. O is now unbalanced. have 2 H2O on the left; can balance H by putting a coefficient 2 in front of NaOH on the right: Stoichiometry Balance the following equation: Na(s) + 2 H2O(l) H2 (g) + 2 NaOH(aq) H is balanced O is balanced Na is now unbalanced, with one on the left but two on the right. ◦ put a coefficient 2 in front of the reactant: Stoichiometry Balance the following equation: 2 Na(s) + 2 H2O(l) H2 (g) + 2 NaOH(aq) Check our work: ◦ 2 Na on each side ◦ 4 H on each side ◦ 2 O on each side Balanced !! Stoichiometry CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) Stoichiometry Pb(NO3)2 (aq) + 2 NaI (aq) PbI2 (aq) + 2 NaNO3 (aq) Stoichiometry Remember: ◦ when changing the coefficient of a compound, the amounts of all the atoms are changed. ◦ do a recount of all the atoms every time you change a coefficient. ◦ do a final check at the end. ◦ fractions are not allowed. Multiply to reach whole numbers. Stoichiometry (s) (l) (g) (aq) - (ppt) - solid liquid gas aqueous (meaning that the compound is dissolved in water.) precipitate (meaning that the reaction produces a solid which falls out of solution.) Reaction conditions occasionally appear above or below the reaction arrow ◦ e.g., "Δ" is often used to indicate the addition of heat). Stoichiometry Complete 3.11 to 3.14 Stoichiometry Two or more substances react to form one product Examples: N2 (g) + 3 H2 (g) 2 NH3 (g) C3H6 (g) + Br2 (l) C3H6Br2 (l) 2 Mg (s) + O2 (g) 2 MgO (s) Stoichiometry Stoichiometry One substance breaks down into two or more substances Examples: CaCO3 (s) CaO (s) + CO2 (g) 2 KClO3 (s) 2 KCl (s) + O2 (g) 2 NaN3 (s) 2 Na (s) + 3 N2 (g) Stoichiometry Rapid reactions that produce a flame Most often involve hydrocarbons reacting with oxygen in the air Examples: CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g) C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (g) Stoichiometry Complete Problems 3.15 to 3.20 Stoichiometry Sum of the atomic weights for the atoms in a chemical formula So, the formula weight of calcium chloride, CaCl2, would be Ca: 1(40.08 amu) + Cl: 2(35.45 amu) 110.98 amu These are generally reported for ionic compounds Stoichiometry Sum of the atomic weights of the atoms in a molecule For the molecule ethane, C2H6, the molecular weight would be C: 2(12.01 g/mol) + H: 6(1.01 g/mol) 30.08 g/mol Stoichiometry formula and molecular weights are often called molar mass. The formula weight (in amu) is numerically equal to the molar mass (in g/mol). Always report molar mass in g/mol. Always round the atomic weights to 2 decimal places. Stoichiometry Complete questions 3.21 and 3.22 Stoichiometry Percentage composition is obtained by dividing the mass contributed by each element (number of atoms times AW) by the formula weight of the compound and multiplying by 100. (number of atoms)(atomic weight) % element = (molar mass of the compound) x 100 Stoichiometry So the percentage of carbon in ethane (C2H6) is… %C = (2)(12.01 g/mol) (2(12.01) + 6(1.01))g/mol = 24.02 g/mol x 100 30.08 g/mol = 79.85% Stoichiometry • the formula weight is 98.09 amu (do calculation) • % H = 2(1.01 amu) x 100 = 2.06 % H 98.09 amu • % S = 32.07 amu x 100 = 32.69 % S 98.09 amu • % O = 4(16.00 amu) x 100 = 65.25 % O 98.09 amu the sum of the percent compositions will always equal 100 % Stoichiometry Complete questions 3.24 and 3.26 Calculate the % composition of each element in each compound. Stoichiometry 6.022 x 1023 1 mole of 12C has a mass of 12 g Stoichiometry By definition, these are the mass of 1 mol of a substance (i.e., g/mol) ◦ is the atomic weight on the periodic table ◦ The formula weight (in amu’s) will be the same number as the molar mass (in g/mol) Stoichiometry Moles provide a bridge from the molecular scale to the real-world scale Stoichiometry Using the mass and molar mass to calculate the number of MOLES of a compound or element. ◦ m ◦ n ◦ M Mass amount of matter Molar mass number of moles n = = grams moles grams per mole g mol g/mol mass molar mass m M Stoichiometry If you have 20.2 grams of Potash, how many moles do you have? Molar Mass KCl (calculated): 74.55 g/mol Mass KCl: 20.2 g 20.2 g = 0.271 moles KCl 74.55 g/mol if the units work out, you are correct. Stoichiometry How many moles are present in 0.750 g of ammonium phosphate? Ammonium phosphate: (NH4)3PO4 Molar mass: (3(14.01) + 12(1.01) + 30.98 + 4(16.00)) g/mol = 149.13 g/mol Number of moles = 0.750 g = 5.03 x 10-3 mol 149.13 g/mol (NH4)3PO4 Stoichiometry What is the mass of 12.31 mol of NaOH ? ◦ Step 1. Calculate molar mass 1 x 22.00 g/mol 1 x 16.00 g/mol 1 x 1.01 g/mol 40.00 g/mol ◦ Step 2. Calculate mass (12.31 mol)(40.00 g/mol) = 492.4 g NaOH Stoichiometry What is the mass of 2.1 x 10-4 mol of diarsenic trioxide? Diarsenic trioxide: As2O3 Molar mass: (2(74.92) + 3(16.00)) g/mol = 197.84 g/mol Mass = (2.1 x 10-4 mol)(197.84 g/mol) = 0.042 g As2O3 Stoichiometry How many moles of caffeine (C8H10N4O2 ) is required to reach the 12.0 g lethal dose in a 140 pound female? Step 1. Molar Mass Step 8 10 4 2 x 12.01 g/mol x 1.01 g/mol x 14.01 g/mol x 16.00 g/mol 194.22 g/mol 2. Calculate Moles 12.00 g 194.22 g/mol = 0.06179 mol caffeine Stoichiometry We can also find the number of atoms of a substance using Avogadro’s Number: 6.022 x 1023 atoms/mol Example: Calculate the number of atoms in 0.500 moles of silver? 0.500mol Ag x 6.022x1023 atoms = 3.01 x 1023 atoms mol of Ag Stoichiometry For molecules! Avogadro’s number represents the number of particles, whether atoms or molecules. Eg. Calculate the number of molecules in 4.99mol of methane. 4.99 mol x 6.022 x 1023 molecules = 3.00 x 1024 molecules mol of CH4 Stoichiometry How many molecules are present in 0.045 mol of H2O? Number of atoms = (0.045 mol)(6.022 x 1023 particles/mol) = 2.7 x 1022 molecules H2O Stoichiometry How many moles are present in 8.75 x 1026 molecules of Vitamin C? Moles = 8.75 x 1026 molecules = 1450 mol 6.022 x 1023 particles/mol Vitamin C Stoichiometry What is the mass of 2.1 x 1022 molecules of N2O? Step 1. Convert molecules to moles: ◦ Moles = 2.1 x 1022 molecules 6.022 x 1023 particles/mol = 0.0349 mol N2O Step 2. Convert moles to mass: ◦ Mass = (0.0349 mol)((2(14.01) + 16.00) g/mol) = 1.54 g N2O Stoichiometry How many molecules are present in 4.65 t of CO2? Step 1: Convert mass to moles ◦ Moles = (4.65 t)(1000 kg/t)(1000 g/kg) = 1.06 x 105 (12.01 + 2(16.00)) g/mol mol CO2 Step 2: Convert moles to molecules ◦ Molecules = (1.06 x 105 mol)(6.022 x 1023 particles/mol) = 6.36 x 1028 molecules CO2 Stoichiometry Complete questions 3.32 to 3.42, even (leave 3.32 for last) Stoichiometry One can calculate the empirical formula from the percent composition Stoichiometry 1. Assume we start with 100 g of sample. 2. The mass percent then translates as the number of grams of each element in 100 g of sample. 3. From these masses, calculate the number of moles (use atomic weight from Periodic Table) 4. The lowest number of moles becomes the divisor for the others. (gives a mole ratio greater than 1) 5. Adjust mole ratios so all numbers are whole (1, 2, etc) 6. The lowest whole-number ratio of moles is the empirical formula. Stoichiometry The compound para-aminobenzoic acid (you may have seen it listed as PABA on your bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA. Assumptions: 1. These are % by mass; in 100g of the compound, the percentages represent masses in grams. 2. Mass divided by molar mass gives moles of each atom. Stoichiometry Assuming 100.00 g of para-aminobenzoic acid, each percentage converts into a mass: C: 61.31 g x H: 5.14 g x N: 10.21 g x O: 23.33 g x 1 mol 12.01 g 1 mol 1.01 g 1 mol 14.01 g 1 mol 16.00 g = 5.105 mol C = 5.09 mol H = 0.7288 mol N = 1.456 mol O Stoichiometry Calculate the mole ratio by dividing by the smallest number of moles: C: 5.105 mol 0.7288 mol = 7.005 7 H: 5.09 mol 0.7288 mol = 6.984 7 N: 0.7288 mol 0.7288 mol = 1.000 O: 1.458 mol 0.7288 mol = 2.001 2 Stoichiometry These are the subscripts for the empirical formula: C7H7NO2 Stoichiometry Complete questions 3.43 to 3.46 Stoichiometry Once we know the composition of a compound the next step is to determine the molar mass. From these two pieces of information the actual molecular formula can be determined: ◦ take the ratio of the actual molar mass divided by the molar mass determined by the empirical formula ◦ multiply the atoms in the empirical formula by the results of the ratio. Stoichiometry Complete questions 3.47 to 3.50 Stoichiometry Compounds containing C, H and O are routinely analyzed through combustion in a chamber like this ◦ C is determined from the mass of CO2 produced ◦ H is determined from the mass of H2O produced ◦ O is determined by difference after the C and H have been determined Stoichiometry Compounds containing other elements are analyzed using methods analogous to those used for C, H and O Stoichiometry Complete questions 3.51 & 3.52 Stoichiometry The coefficients in the balanced equation give the ratio of moles of reactants and products Stoichiometry From the mass of Substance A you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed (if it’s a product) or used (if it’s a reactant) Stoichiometry C6H12O6 + 6 O2 6 CO2 + 6 H2O Stoichiometry Work on questions 3.57 to 3.66 Copy examples done in class. Stoichiometry You can make cookies until you run out of one of the ingredients Once this family runs out of sugar, they will stop making cookies (at least any cookies you would want to eat) Stoichiometry In this example the sugar would be the limiting reactant, because it will limit the amount of cookies you can make Stoichiometry The limiting reactant is the reactant present in the smallest stoichiometric amount Stoichiometry The limiting reactant is the reactant you’ll run out of first (in this case, the H2) Stoichiometry In the example below, the O2 would be the excess reagent Stoichiometry Complete questions 3.69 to 3.76 Stoichiometry The theoretical yield is the amount of product that can be made ◦ In other words it’s the amount of product possible as calculated through the stoichiometry problem This is different from the actual yield, the amount one actually produces and measures Stoichiometry A comparison of the amount actually obtained to the amount it was possible to make Actual Yield Percent Yield = Theoretical Yield x 100 Stoichiometry Complete questions 3.77 to 3.80 Stoichiometry