QUANTITIES IN CHEMICAL REACTIONS - The Mole o The mole is the SI unit that is used to measure the amount of a substance o 1 mole (mol) of a substance is the amount of substance that contains as many particles [atoms, molecules, ions, or formula units] as the number of atoms in exactly 12 g of the isotope C-12; or 6.02 x 1023 particles of the substance (which is called Avogadro's constant) NA = 6.02 x 1023 o The representative particles for pure, monatomic elements are atoms Ex. 1 mol of Fe (s) has 6.02 x 1023 atoms o The representative particles for diatomic molecules and molecular compounds are molecules Ex. 1 mol of O2 has 6.02 x 1023 molecules Ex. 1 mol of H2O has 6.02 x 1023 molecules o The representative particles for pure ionic compounds are formula units Ex. 1 mol of NaCl has 6.02 x 1023 formula units - n = N/NA Where n is amount in moles N is number of individual particles NA is Avogadro constant Ex. Hydrazine, N2H4 (l), is used in airbags and we obtained a chemical sample of 3.65 mol. a. How many molecules are in the sample? b. How many atoms are in the sample? Ex. If a teaspoon of [Mg(OH)2] stomach medicine contains 4.1 x 1021 formula units, what amount in moles of magnesium hydroxide is in the teaspoon? Mass and The Mole o Recall: That 1 atom of C-12 has a mass of exactly 12 u, and 1 mol (6.02 x 1023 atoms) of C-12 has a mass of exactly 12 g The atomic mass unit is defined using the C-12 isotope and the atomic masses of all the other elements are defined using C-12 as the standard Therefore 1 mol of any element has a mass that is numerically equal to the elements atomic mass expressed in grams o o Ex. An atom of iron has an atomic mass of 55.85 u, and 1 mole of iron atoms has a mass of 55.85 g. The term for the mass of 1 mol of substance is molar mass, given the symbol M, and unit is g/mol i.e. 1 mol of mass of atoms is atomic molar mass, 1 mol of mass of molecules is molecular mass, and 1 mol of mass of formula units is formula unit molar mass o Ex. What is the molar mass of water? of Al2(SO4)3? The molar mass represents the mass of 6.02 x 1023 particles Therefore, m= n x M --- If n = N/NA then, m = (N x M)/NA -3 Ex. Calculate the mass of 1.28 x 10 mol of glucose, C6H12O6 Ex. What is the mass of 4.27 x 1023 formula units of CrI3 (s)? - Chemical Proportions and Percent Composition o The law of definite proportions states that a chemical compound always contains the same proportions of elements by mass i.e. water is the same formula, H2O, if found in rain, snow, oceans, etc. o This law deals with the proportion of each element in a compound by mass Expressed as a mass percent Mass of an element in a compound, expressed as a percent of the total mass of the compound o Ex. Calculate the mass percent of both elements in dinitrogen tetroxide - Empirical and Molecular Formulas o A molecular formula for a compound shows the number of atoms of each element that makes up a molecule of that compound Ex. Hydrogen peroxide, H2O2 --- 2 atoms of H and 2 atoms of O o An empirical formula shows the smallest whole number ratio of the elements in a compound o o o Need to reduce ratios if possible Ex. H2O2 --- 2:2, becomes 1:1, so the empirical formula is HO Note: for some compounds the empirical formula may be the same as the molecular formula Ex. H2O - molecular formula Ratio is 2:1, therefore empirical is H2O Recall ionic compound can only have 1 possible atomic configuration, therefore, only represented by empirical formula's Ex. of 6 compounds with the empirical formula, CH2O CH2O - formaldehyde, C2H4O2 - acetic acid, C3H6O3 - lactic acid, C4H8O4 erythrose, C5H10O5 - ribose, C6H12O6 – glucose o Molecular formulas We use percent decomposition to determine the empirical formula Rules Convert percent into mass by assuming total mass of sample to be 100 g Determine number of moles of each element Convert moles of each element into whole numbers that become subscripts by dividing each amount in moles by the smallest amount If subscripts are not whole numbers, determine the least common multiple that will make the decimal values into whole numbers o Multiply all subscripts by this common multiple Use these numbers as subscripts to complete empirical formula Ex. Assume that a compound is 50.91% Zn, 16.04% P, and 33.15% O; determine its empirical formula. Ex. Determine the empirical formula for a compound that is found by analysis to contain 27.37% Na, 1.2% H, 14.3% C, and 57.14% O Ex. Chemical analysis indicates that a compound is 28.64% sulfur and 71.36% bromine. The molar mass of the compound is 223.94 g/mol. Determine the molecular formula. o - A hydrate is a compound that has a specific number of water molecules bound to each formula unit Often when a crystal forms from a water solution, water molecules are trapped in the crystal in a specific arrangement Ex. CaSO4· 2H2O (s) Ionic compounds in solid state can be hydrates or anhydrous compounds (without water) The water molecules usually do not interfere with the chemical activity of the compound Ex. A 50.0 g sample of Ba(OH)2 · xH2O contains 27.2 g of Ba(OH)2. Calculate the percent by mass of water in Ba(OH)2 · xH2O (s) and find the value of x. What is Stoichiometry? o Stoichiometry is the study of the quantitative relationship among the amounts of reactant used and the amounts of products formed in the chemical reaction A balanced chemical equation is required for any calculations; we use them in ratios o The coefficients in front of the chemical formulas in a balanced chemical equation represent the relative numbers of particles involves in the reaction Ex. 2H2 (g) + O2 (g) → 2H2O (g) Ex. The combustion of octane, C8H18. If 450 molecules of water are produced, how many molecules of carbon dioxide are produced Ex. What amount in moles of CuO (s) forms when 0.0045 mol of malachite, Cu2(CO3)(OH)2 (s) decomposes completely according to the following equation: Cu2(CO3)(OH)2 (s) → CO2 (g) + H2O (g) + 2 CuO (s) Ex. An astronaut produces an average of 1.0 x 103 g of CO2 each day. What mass of oxygen would need to be produced by photosynthesis is each day? Mass of water? Mass of glucose? - Limiting and Excess Reactants o A limiting reactant is completely consumed during a chemical reaction, limiting the amount of product that is produced o An excess reactant remains after a reaction is over o The limiting reactant is not necessarily the reactant that is present in the smaller amount, but the one that forms the smaller amount of product Ex. The production of ammonia is an important industrial process in the manufacture of fertilizers. If 4.2 g of N2 (g) reacts with 0.75 (g) of H2 (g), which is the limiting reactant - Ex. 2 Al (s) + Fe2O3 (s) → Al2O3 (s) + 2 Fe (l) If 113 g of aluminum powder is mixed with 279.50 g of iron (III) oxide, what mass of molten iron forms? Reaction Yields o Stoichiometric calculations allow you to calculate the amount of product that forms, in theory, in a chemical reaction Called theoretical yield o However the theoretical yield is not always the same as the amount of product that actually forms o The actual amount of product that forms in a chemical reaction during a lab is the actual yield o Factors that affect yield: Competing reaction A reaction that occurs along with the main reaction Involves the same reactants or products and therefore lowers the yield of the main (principal) reaction o Ex. Principal reaction C3H8 + 5 O2 → 3 CO2 + 4H2O Competing reaction o 2 C3H8 + 7 O2 → 6 CO2 + 8H2O [incomplete combustion] o Ex. 2 P + 3 Cl2 → 2 PCl3 PCl3 + Cl2 → PCl5 o o Reaction rates Speed of reaction can be affected by surface area, temperature, pressure, and reaction vessel conditions Purity of reactants Ex. Lab grade acetic acid compared to vinegar o Vinegar is 5% acetic acid, 95% water Laboratory techniques used to collect final products If product is soluble or clings to glassware Calculate % yield % yield : Ex. If 20.0 g of glucose reacts but only 1.40 g of ethanol is produced, what is the % yield of the reaction. C6H12O6 (aq) → 2 C2H5OH (aq) + 2 CO2 (g) Ex. If 49.0 g of potassium phosphate is recovered after 49.0 g of phosphoric acid reacts with 49.0 g of potassium hydroxide, what is the % yield of the reaction?