YIELD OF CHEMICAL REACTIONS so far we have assumed that every atom or molecule undergoes the chemical reactions we wrote down would be an ideal case scenario but rarely happens this is called a reaction goes to completion in real life most reactions do not only form the desired products unreacted materials side products the amount of product formed is called the YIELD of a reaction PERCENT YIELD ACTUAL YIELD OF PRODUCT THEORETICAL YIELD OF PRODUCT x 100 CALCULATING PERCENT YIELD EX if 10 6g of Na2CO3 react with 7 2g of HCl and form 2 0g CO2 what is the percent yield of the reaction start by writing the equation and balancing all coefficients figure out moles of reactants use molar mass calculate theoretical yield using chemical stoichiometry LIMITING REACTANTS the last slide introduced us to an important concept unless we mix exact stoichiometric amounts of chemical there will be a LIMITING REACTANT this is the chemical that has the lowest number for the quantity number of moles coefficients the other reactant is the EXCESS REACTANT there is more than enough of it we often use one reactant in excess to help the reaction go to completion very important for industrial scale reactions especially if one reactant is cheap REACTIONS WITH LIMITING AMOUNTS OF REACTIONS Ex at a high temp ethylene oxide reacts with water to form ethylene glycol which is an automobile antifreeze and a starting material in the preparation of polyester polymers 3C2H4 aq 5H2O l 3C2H6O2 l 2H2O LIMITING REACTANT ethylene oxide because it is completely used EXCESS REACTANT water because there are extra molecules on the solution right side of the equation that did not react CALCULATIONS FOR LIMTING REACTANT PROBLEMS Ex how many grams of BaS can be formed through the reaction of 5 0g of Ba with 5 0g of Sulfur assume that the limiting reactant is completely consumed Ba s S s BaS s Formula Mass Ba 137 327 amu Molar Mass Ba 137 327 g mol Formula Mass S 32 065 amu Molar Mass S 32 065 g mol REACTIONS IN SOLUTIONS chemicals need to contact each other to react can be hard to achieve for not very soluble solids reactions are often carried out in solution to facilitate contact between chemicals In the liquid phase MUCH MORE MOBILE chemicals must be dissolved to carry out reactions in solution IMPORTANT CONCEPT MOLARITY to measure the amount of particular reactant of a solution we must know the solutions molarity how many moles of substance are dissolved in 1L of solution Unit symbol M always capital ex 3M HCl 3 molar HCl when talking about solution we need to know several other terms SOLUTION a homogenous liquid mixture SOLUTE the substance dissolved in a solution SOLVENT the major component in a solution HOW TO PREPARE SOLUTIONS we cannot just mix 1 mol of solute with 1L of solvent volume of solution is likely not 1L then THE ADVANTAGE OF USING MOLARITY molariy converts between mole of solute and liters of solution Molarity Moles of solute Volume of solution Volume of solution L Moles of solute Molarity Moles of solute Molarity x volume of solution L Ex what is the molarity of a solution that has 1 2 moles of ammonia per 250 mL NH4OH 1 2 moles 0 25 L ANS 4 8M IF THERE IS TOO CONCENTRATED OF A SOLUTION IN THE CHEM LAB ONE MAY HAVE TO DILUTE THE SOLUTION