102Example:How many milliliters of 6.00M hydrochloric acid is needed to completely react with25.0 g of sodium carbonate?1 - Convert 25.0 g of sodium carbonate to moles. Use FORMULA WEIGHT.2 - Convert moles sodium carbonate to moles HCl. Use CHEMICAL EQUATION.3 - Convert moles HCl to volume. Use MOLAR CONCENTRATION of HCl (and a L- mL conversion)The probkem asked for the volumein milliliter units, so we needed toconvert from L - mL after usingmolarity.
103CONCEPT OF LIMITING REACTANT- When does a chemical reaction STOP?MagnesiumstripFlame,oxygen fromairMagnesium oxidepowder- When does this reaction stop? When burned in open air, this reaction stops whenall the MAGNESIUM STRIP is gone. We say that the magnesium is LIMITING.- This reaction is controlled by the amount of available magnesium- At the end of a chemical reaction, the LIMITING REACTANT will be completely consumed,but there may be amount of OTHER reactants remaining. We do chemical calculations inpart to minimize these "leftovers".These are often called "excess" reactants, or reactants present"in excess"
104LIMITING REACTANT CALCULATIONS- To find the limiting reactant, calculate how much product would be produced from ALL givenreactants. Whichever produces the SMALLEST amouht of product is the limiting reactant, andthe smallest anount of product is the actual amount of product produced.Example: - Formula weightsIf you start with 100. g of each reactant, how much calcium carbide would be produced?114 grams of calcium carbide should be produced. When 114 grams of the carbide isformed, all 100 grams of CaO have been used up, and the reaction stops. We still haveleftover carbon, but there's nothing for it to react with! - no more product can be made!We say that CaO is "limiting" (it controls how much product we make), and C is present"in excess".
105PERCENT YIELD- Chemical reactions do not always go to completion! Things may happen that prevent theconversion of reactants to the desired/expected product!SIDE REACTIONS:This reaction occurs when there is a large amountof oxygen available. while this reaction is more favorable in low-oxygenenvironments!. so in a low-oxygen environment, you may produce less carbondioxide than expected!TRANSFER AND OTHER LOSSES- When isolating a product, losses may occur in the process. Example: filteringDuring each step of this process,some amount of product losswill occur!Pourthrough filter!. then scrape productoff paper!
106EQUILIBRIUM- Reactions may reach an equilbrium between prodcuts and reactants. We'lltalk more about this in CHM 111. The net results is that the reaction willappear to stop before all reactants have been consumed!- All of these factors cause a chemical reaction to produce LESS product than calculated.For many reactions, this difference isn't significant. But for others, we need to reportthe PERCENT YIELD.Determined EXPERIMENTALLY!Calculated based on the limiting reactant. (The chemicalcalculations you've done up to now have beentheoretical yields!). the percent yield of a reaction can never be greater than 100% due toconservation of mass! If you determine that a percent yield is greater than 100%, thenyou've made a mistake somewhere - either in a calculation or in the experimentitself!
107 - Formula weights22.4 gbenzene31.6 g ACTUALnitric acidnitrobenzene22.4 grams of benzene are reacted with excess nitric acid. If 31.6 grams of nitrobenzene arecollected from the reaction, what is the percent yield?To get percent yield, we need to calculate the THEORETICAL YIELD of nitrobenzene, startingfrom the 22.4 g of benzene we reacted.
10825.0 mL of acetic acid solution requires 37.3 mL of 0.150 M sodium hydroxidefor complete reaction. The equation for this reaction is:What is the molar concentration of the acetic acid?Since we already know the volume of acetic acid solution, we need to find the moles ofacetic acid. To do THAT, we'll start with the sodium hydroxide volume, since we can relatethat to moles (and then to the amount of acetic acid!)!To get CONCENTRATION, we divide the moles by the volume of the acetic acid (in L)