Prof Gilbert LECTURE 7 CHEM 1211 Fall 10 Announcements Last time Homework Assignment 2 is due this Sunday Chemical Formulas from Percent Composition Calculations based on balanced chemical equations Key concept the stoichiometry of chemical equations describes mole ratios between reactants and products To use these ratios in calculations we may either 1 Convert mass ingredient A into into an equivalent s of moles then to moles of ingredient B and then into grams of B or 2 Convert mole ratios in the chemical equation into mass ratios by multiplying by the appropriate molar masses Then use these mass ratios to convert mass of A into mass of B This time Limiting reactants The ham and cheese metaphor How many ham and cheese sandwiches could you make from 8 slices of bread 3 slices of ham and 4 slices of cheese Inquiry you work for a manufacturer of the pesticide malathion To make a batch you first need to make PCl3 Into a gas tight reaction vessel you add 100 g P4 s and 300 g Cl2 g How much PCl3 could you make Yield Reactions don t always yield the quantities of products they re expected to Reasons 1 reaction comes to chemical equilibrium before all the reactants are consumed 2 reaction rates slow down as reactants are consumed and the chemist gets impatient 3 side reactions form different products than those desired To calculate the yield of a reaction take the ratio of the quantity of product that was made to the maximum quantity of product that could have been made Inquiry Consider the industrial synthesis of ammonia N2 g 3H2 g 2 NH3 g A sealed reaction vessel is charged with 210 pounds of N2 and 55 pounds of H2 If 167 pounds of NH3 is recovered what was the yield Tips for balancing chemical equations 1 For combustion reactions balance C then H and then O 2 For acids and bases be sure moles of H donors moles of H acceptors 3 For others make 1 the coefficient of the most complex ingredient and try to balance the others Fe3Si2O5 OH 4 s CO2 g H2O l FeCO3 s H4SiO4 aq Carbon cycle Starts with photosynthesis 6 CO2 g 6H2O l C6H12O6 aq 6O2 g This biomass dies or is consumed and reverts back to CO2 and H2O C6H12O6 aq 6O2 g 6 CO2 g 6H2O l Except about 0 01 is buried and not oxidized eventually turning into fossil C compounds including fossil fuels Adds up to about 1020 kg C Now combustion of fossil fuels is reintroducing almost 1013 kg C back into the atmosphere each year The impact Besides climate change there is the acidity problem CO2 g H2O l H2CO3 aq H aq HCO3 aq Workable solutions consume less fossil fuel or sequester more CO2
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