Prof Gilbert LECTURE 8 CHEM 1211 Fall 10 Announcements Homework Assignment 2 is due this Sunday Last time This time 1 Limiting Reactants 2 Yield Calculations 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 atmospheric CO2 and photosynthesis 6 CO2 g 6H2O l C6H12O6 aq 6O2 g The carbohydrate biomass glucose and compounds derived from it dies or is consumed and reverts back to CO2 and H2O C6H12O6 aq 6O2 g 6 CO2 g 6H2O l Except about 0 01 that is buried and not oxidized This eventually becomes 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 Chapter 4 Chemical Reactions in Solutions Nature of solutions 1 They are homogenous mixtures Ex sugar dissolved in water Red spheres represent molecules of sugar dissolving from a solid cube of it Note that they remain intact when they dissolve Now consider a sample of seawater collected from the surf on a sandy beach Suspended sediments make it far from a true solution though filtration could clear up the situation Case 1 The average NaCl concentration in seawater is 19 365 g kg Cl and 10 77 g kg Na Any guesses why the chloride concentration is higher Also chemical oceanographers prefer mol kg concentrations rather than mol L Any guesses why How can the solutes dissolved substances be separated from the solvent Seawater contains thousands of solutes The most abundant are ionic compounds When they dissolve they separate into their component ions Quantities moles of dissolved reactants can be defined by the volumes L the concentration molarities M of their solutions Other ways to express concentration mass volume or mass mass for example Case 2 Water in the Dead Sea between Israel and Jordan contains 32 5 g kg Na What is the molar concentration of Na Given the density of Dead Sea water is 1 24 kg L Case 3 How much NaCl g do you need to make up 2 50 L of normal saline same NaCl concentration as blood plasma which is about 0 157 M What mass of NaCl do you weigh out Dilution Concentration Case 4 Puddle 3 6 L of seawater is trapped in a depression in a rock as the tide goes out Puddle evaporates Vf 2 2 L What is final Na Key equation Vi Mi Vf Mf Case 5 Rain storm increases volume of water in depression to 2 8 L What is the value of Na then Lab scenario use pipets and volumetric flasks to deliver the desired volume Vi of concentrated Mi solution so that you have the desired volume Vf and concentration Vf of a more dilute solution Why do reactions in solution occur When solutes dissolve in solvent they are better able to react with each other Reactions happen spontaneously when they yield stable products such as 1 gases 2 insoluble solids precipitation reactions or 3 water as in neutralization reactions Strong electrolytes are ionic compounds that dissociate completely in water e g NaCl in the A panel below Nonelectrolytes e g ethanol in B are molecular compounds that do not ionize when they dissolve In between are weak electrolytes molecular compounds that either partially ionize e g acetic acid in the C panel or react a little with H2O to form ions e g ammonia completely If we insert electrodes into these solutions an apply a large voltage using say a 12 volt battery charger the positive ions cations will migrate toward the negatively charged electrode and the negative ions anions will migrate toward the positively charged electrode NOTE Many weak electrolytes are weak acids which means they partially ionize releasing H ions in aqueous solutions Most common of these is acetic acid The H atom that is part of the COOH group of atoms in its structure called a carboxyl group is the one that ionizes Hydration of ions When NaCl dissolves in water water molecules cluster around the sodium and chloride ions forming spheres of hydration Most monoatomic ions are surrounded by 6 water molecules though the smallest ones e g H have fewer