Balancing Chemical Reactions Chemistry Review

Balancing Chemical Reactions: To balance equations, start by balancing atoms other than hydrogen and oxygen. Balance hydrogen atoms next to last, and oxygen atoms last. When balancing oxygen, make sure left is same as right, usually subtract and divide by 2 for left side. To find molecular formula from empirical: Take sum of empirical mass, and divide by molecular mass (mass of each element). With that quotient, multiply all subscripts of empirical formula by quotient. Calculating Empirical Formula: 1. 2. 3. 4. Convert from percentages to grams of element Convert from grams to moles Take smallest mole, and divide all moles by smallest. Your quotient needs to be rounded to the nearest whole number. Usually __.9 so round up. ***If you can’t round, multiply the quotient by a multiple that will get you closest to a whole number. REMEMBER to multiply all of the other quotients of the other moles by that same multiple. When writing empirical formula Carbon always comes first, oxygen last. Least  most electronegative. Combustion Reaction: always yields water, carbon dioxide, and energy source. CxHyNoOz + O2  H2O + CO2 + NO2 Combustion analysis problems: Ex: A .2500 g sample compound known to contain carbon, hydrogen, oxygen, undergoes complete combustion to produce .3664 g of CO2 and .1500g H2O. What is empirical formula of this substance? 1. Find grams of each element in compound by multiplying mass by (mass of element/mass of compound) **For hydrogen its (2gH/18gH2O) because 2 hydrogens in H2O ** do hydrogen and carbon first, then add those grams and subtract from TOTAL sample to get mass of oxygen. 2. Convert those grams to moles, then divide each mole by smallest to get empirical formula. To find molecular formula, take mass of empirical and multiply it by a factor so it fits the ballpark given Limiting Reactant: the reactant you’ll run out first To Find limiting Reactant: 1. Compare the number of moles of each reactant to its ratio in the balanced equation ***convert from grams to moles (multiply grams by 1mol/molar mass of compound), then divide the mole by the coefficient of corresponding compound in balanced equation. Do this for all compounds ***the quotient that is the smallest is the limiting reactant 2. Use the limiting reactant’s moles to solve all stoichiometric problems. Ex: Theoretical Yield: what you should get in lab experiment. Different from actual yield- amount one actually produced and measured. (Actual Yield / Percent yield) X 100 = Percent Yield Molarity: M= moles of solute/liters of solution. *when compounds not the same, make equation with H2O and salt as product, then balance, don’t forget criss cross salt in product. Use mole ratio to solve problem!!! *CONVERT FROM mL to Liters *pay attention if asking for grams or moles!! If grams need to do extra step ( ) to convert from moles to grams Concentration= Molarity How many grams of Na2SO4 are required to make 0.350 liters of .500 M Na2SO4 How many grams of Ca(OH)2 are needed to neutralize 25mL of .100M HNO3 If 45.7 mL of .500 M H2SO4 is required to neutralize 20.0 mL of NaOH solution what is the concentration of NaOH solution? Oxidation numbers: Elements in elemental form have oxidation number of 0 (ex F2) Oxidation number of monoatomic charge is same as charge Nonmetals tend to have negative oxidation numbers, but can be positive certain compounds or ions Oxygen has oxidation # of -2 except in peroxide ion which has -1 (when ends in -O2 usually -1 oxidation) Hydrogen -1when bonded to metal (gains electron), +1 when bonded to nonmetal (loses electron) The sum of oxidation #’s in a neutral compound is 0, in a polyatomic ion is the charge on the ion Fluorine always has an oxidation of -1 **First look at elements see if they lost or gained electrons- means redox!!! (lost electrons +1, gain electrons -1) Double Replacement Reactions: Non-Redox (when both reactants switch to make products) Single Replacement: Redox. Zn+CuCl2 ZnCl2 + Cu Combustion reactions: Redox Synthesis/Decomposition- can be either redox or non-redox Neutralization Reactions: Product always H2O and salt. balance it out, check if weak acid or weak base (DO NOT break apart if either of those!!) before crossing shit out after balancing put it in ionic form (subscripts become coeff, if coeff and subscript: multiply and make product coeff), then cross shit out to get net ionic form Usually if strong acid and base are mixed always yields some form of H + OH = H2O (can be multiple) Ex: Perciptate problems: Reactants should be aqueous use chart to determine whether product is soluble or not is not solid. When writing balanced equation do double replacement for products. (methasis- ‘to switch’= Doub.Rep *** SOLID, LIQUID, GAS are not broken up at ALL Write ionic equation, then net ionic equation Ex: Displacement reactions: forward reaction occurs but reverse doesn’t. Dilution: Mconcentrated x Vconcentrated = Mdiluted x VDiluted Titration: Write out equation with water and salt as product. Then convert from grams to moles, mole ratio, then compare mole ratios, convert to liters. If asking for liters do (liters/ mols) for grams do (mol/L)