Chapter 9: Chemical quantities Chemistry 1020: Interpretive chemistry Andy Aspaas, Instructor

Interpreting chemical equations The coefficients in a chemical equation indicate relative numbers of molecules involved in the equation 2H 2 (g) + O 2 (g)  2H 2 O(g) This equation can be interpreted many ways –2 molecules H 2 gas react with 1 molecule O 2 gas to form 2 molecules H 2 O gas –2 dozen molecules H 2 gas react with 1 dozen molecules O 2 gas to form 2 dozen molecules H 2 O gas –2 mol H 2 gas react with 1 mol O 2 gas to form 2 mol H 2 O gas

Mole-mole relationships in reactions 2H 2 (g) + O 2 (g)  2H 2 O(g) If you want to produce 15.7 mol H 2 O, how many moles of O 2 gas are required in this reaction? Use the coefficients in the balanced chemical reaction to convert between moles of any components in a reaction Mole ratio: 1 mol O 2 (g) for every 2 mol H 2 O(g) from the equation Use that mole ratio as a dimensional analysis conversion factor

Mass calculations In chemistry, number of moles is the only unit that can be used to convert between quantities of constituents in a chemical reaction Stoichiometry: conversion of masses of different reactants and products in a chemical reaction If you’re given the mass of a reactant or product, you must first convert mass to moles If you’re looking to find the mass of a reactant or product, you must first find the moles, and then convert to mass

Practice mass calculation problems CO(g) + 2H 2 (g)  CH 3 OH(l) What masses or carbon monoxide gas and hydrogen gas are required to produce 6.0 kg of methanol? LiOH(s) + CO 2 (g)  Li 2 CO 3 (s) + H 2 O(l) What mass of CO 2 gas can 1.00 kg of lithium hydroxide react with?

Limiting reagent Whenever you are given quantities of more than one reactant, you must decide which will be consumed first (the limiting reactant) The cheese sandwich analogy –Decide quantity of products that each individual reactant is able to produce –The reactant that produces the smallest quantity of product is the limiting reactant

Limiting reactant Convert each reactant to moles Use mole ratios to convert each to moles of product Reactant which produces smallest value of product moles is limiting reactant Use limiting reactant to calculate product moles and finally product mass

Limiting reactant practice N 2 (g) + 3H 2 (g)  2NH 3 (g) Say 25.0 g nitrogen gas and 5 g hydrogen gas are reacted to form ammonia. What mass of ammonia can be produced? –Convert reactants to moles –Convert each to moles products –Identify limiting reagent –Product moles from limiting reagent –Convert to mass of product

Percent yield All calculations so far have given theoretical yields of products –The maximum amount of product that can be formed, assuming all of the limiting reagent reacts, and all product can be collected In reality, actual yield is almost always lower Percent yield: measure of experimental efficiency = [(actual yield) / (theoretical yield)] x 100%