Stoichiometry is a section of chemistry that involves using relationships between reactant, Study notes of Physics

Download Stoichiometry is a section of chemistry that involves using relationships between reactant and more Study notes Physics in PDF only on Docsity! STOICHIOMETRY Law of conservation of mass  The balance became an important instrument in the eighteen century.  Balances measure mass, which is the quantity of matter in a material. You should note the distinction between the terms mass and weight in precise usage. Solution: Total mass of substances before reaction = Total mass of substance after reaction From the law of conservation of mass: Chemical Equations A chemical equation is a symbolic representation of a chemical reaction in terms of chemical formulas. eg. 2Na + Cl2  2NaCl A reactant is a starting substance in a chemical reaction A product is a substance that results from a reaction. Balancing Chemical Equations A chemical equation is said to be balanced, when the numbers of atoms of each element are equal on either side of the arrow. Consider the burning of natural gas: CH4 + O2 CO2 + H2O Balancing Simple Equations Balance the following equations: CS2 + O2 CO2 + SO2 C2H5OH + O2 CO2 + H2O NH3 + O2 NO + H2O Atomic Weights & Molecular Weights Central to Dalton’s atomic theory was the idea that each atom had a characteristic mass. We now know that each isotope has its own characteristic mass. Dalton actually calculated the average atomic mass, based on the relative abundance of each isotope. Dalton could not weigh individual atoms, instead he could find the average mass of one atom relative to the average mass of another. (using H as a base) Atomic mass unit - amu The atomic weight of an element, is the average atomic mass for the naturally occurring element, expressed in atomic mass units. Today based on carbon-12 Mass of 12C = 12 amu 1 amu = 12 mass of 12C 1 Mass and Moles of Substance  We have seen so far that atoms / molecules react in simple ratios.  Atoms and molecules are too small to count.  We need find another way of expressing such large numbers In pursuit of this end we are going to consider the following concepts:  Molecular weight  Formula weight  the Chemical Mole Formula and Molecular Weight The formula weight of a substance is the sum of the atomic weights of all atoms in a formula unit of the compound. The molecular weight of a substance is the sum of the atomic weights of all the atoms in a molecule of the substance. Note: the formula weight is applicable to both molecular and ionic compounds. Example Calculate the formula weight of the following compounds from their formulae: a) calcium hydroxide b) methylamine = CH3NH2 The molar mass of a substance is the mass of one mole of the substance For all substances, the molar mass in grams per mole is numerically equal to the formula weight in atomic mass units. Mole Calculations Because molar mass is the mass per mole we have the formula: Molar Mass = Mass Moles or Mr = m n Remember that mass must be expressed in grams here! Example Converting Grams of Substance to Moles Calcite is a mineral composed of calcium carbonate, CaCO3. A sample of calcite composed of pure CaCO3 weighs 23.6 g. How many moles of calcium carbonate is this? Determining formulae The percentage composition of a compound leads directly to its empirical formula. An empirical formula for a compound is the formula of a substance written with the smallest integer subscripts. Compounds with different molecular formulae can have the same empirical formula, and such substances will have the same percentage composition. Eg. acetylene = C2H2 benzene = C6H6 Empirical formula from Composition Consider the following flow-diagram: Percent composition Mass Composition Number of moles of each element Divide by smallest number of moles to find the molar ratios Multiply by appropriate number to get whole number subscripts Molecular Formula from Empirical Formula The molecular formula of a compound is a multiple of its empirical formula. Molecular weight = n x empirical formula weight where n = number of empirical formula units in the molecule. Determining the Molecular Formula from the Percentage Composition and Molecular Weight. We have already determined the mass composition and empirical formula of benzene. In a separate experiment, the molecular weight of benzene was determined to be 78.1. What is the molecular formula of benzene Example Stoichiometry Stoichiometry is the calculation of the quantities of reactants and products involved in a chemical reaction It is based on the chemical equation and the relationship between mass and moles. We can interpret a chemical equation in terms of number of molecules (or ions or formula units) or in terms of number of moles of molecules etc. depending on our needs. Remember that because moles can be converted to mass, we can also produce a mass interpretation of a chemical equation. The Steps in a Stoichiometric Calculation Mass of substance A Moles of substance A Moles of substance B Mass of substance B Use molar mass of A Use molar mass of B Use coefficients of A & B in balanced eqn Example Calculate the mass of sulfur dioxide (SO2) produced when 3.84 mol O2 is reacted with FeS2 according to the equation: 4FeS2 + 11O2 2Fe2O3 + 8SO2 Limiting Reactants The limiting reactant (or limiting reagent) is the reactant that is entirely consumed when a reaction goes to completion. The moles of product are always determined by the starting moles of the limiting reactant. When 100.0 g mercury is reacted with 100.0 g bromine to form mercuric bromide, which is the limiting reagent? Hg + Br2 HgBr2 Example Example More Challenging!! Iron can be obtained by reacting the ore haematite (Fe2O3) with coke (C). The latter is converted to CO2. As manager of a blast furnace you are told that you have 20.5 Mg of Fe2O3 and 2.84 Mg of coke on hand. (a) Which should you order first - another shipment of iron ore or coke? (b) How many megagrams of iron can you make with the materials you have? 2Fe2O3 + 3C 4Fe + 3CO2