CHAPTER 3 STOICHIOMETRY - site.iugaza.edu.ps

1 / / CHAPTER3 STOICHIOMETRY :CHEMICALEQUATIONS STOICHIOMETRY :The area of study that examines thequantities of substances consumed and produced inchemical reactions. Remember:Atoms are neither created nor destroyedduring any chemical reaction or physical process. / / CHEMICALEQUATIONSC hemical equation represents the chemical recation .The arrow is called the yield A chemical equation must have an equalnumber of atoms of each element on each sideof the arrow. / / RULES/HINTS FORBALANCINGEQUATIONS1) Use correct ) The coefficients must be the lowest ) Do not change or add subscripts(number of atomsof each element in a molecule).4) Balance H and O ) If a polyatomic ion stays together on both sides ofthearrow, then keep it together when + Na2CO3 2 NaCl + 2H2O + CO2C2H5OH + 3O2 2CO2+ following diagram represents a chemical reaction in which thered spheres are oxygen atoms and the blue spheres are ) Write the chemical formulas for the reactants and products.

2 O2+ NO NO2(unbalanced)(b) Write a balanced equation for the reaction. O2+ 2NO 2NO2(balanced)(c) Is the diagram consistent with the law of conservation of mass? Yes; because there are equal numbers of both N and O atoms in the two boxes / / PRACTICEEXERCISEIn the following diagram, the white spheres represent hydrogenatoms, and the blue spheres represent nitrogen atoms. To beconsistent with the law of conservation of mass, how many NH3molecules should be shown in the right box?Answer: Six OFMATTER We use the symbols(g) for gas,(l) for liquid,(s) for solid,and(aq) for aqueousto identify the states of matter of thereactants and products. Catalysts, photons (h ), or heat ( ) may stand above thereaction arrow ( ).CaCO3(s)+2 HCl(aq) Ca2+(aq)+2Cl (aq)+ CO2(g)+ H2O(l) / / this equation:Na(s)+ H2O(l) NaOH(aq)+ H2(g)Answer:2Na(s) + 2 H2O(l) 2 NaOH(aq) + H2(g)_____PRACTICE EXERCISE:Balance the following equations:a.

3 Fe(s)+ O2(g) Fe2O3(s)b. C2H4(g)+ O2(g) CO2(g)+ H2O(g)c. Al(s)+ HCl(aq) AlCl3(aq)+ H2(g)SOMESIMPLEPATTERNS OFCHEMICALREACTIVITY 1) Combination Reactions: In synthesis/combination reactions two or moresubstances react to form one product. Generic Reaction:A + B AB Real Reaction:2Mg + O2 2 MgOExamples of Combination Reactions:N2 (g)+ 3 H2 (g) 2 NH3 (g)C3H6 (g)+ Br2 (l) C3H6Br2 (l) / / Examples of Combination Reactions:Decomposition Reactions:In a decomposition reaction one substance undergoes areaction to produce two or more Reaction:AB A + BReal Reaction:CaCO3 CaO + CO2 Examples:CaCO3 (s) CaO(s)+ CO2 (g)2 KClO3 (s) 2 KCl(s)+ O2 (g)2 NaN3 (s) 2 Na(s)+ 3 N2 (g) / / the balanced equations for the following reactions:a. The synthesis reaction that occurs when lithium metaland fluorine gas Li(s) +F2(g) 2 LiF(s)b.

4 The decomposition reaction that occurs when solidbarium carbonate is (s) BaO(s) + CO2(g) / / PRACTICEEXERCISEW rite the balance equation for the following reactions:a. Solid mercury (II) sulfide decomposes into its elementswhen The surface of aluminum metal undergoes a combinationreaction with oxygen in the : (a) HgS(s) Hg(l) + S(s) (b)4 Al(s) + 3 O2(g) 2 Al2O3(s)COMBUSTIONREACTIONSC ombustion reactions are rapid reactions that produce require air (O2) as a hydrocarbons are combusted completely, theproducts areCO2and Reaction:CxHy+ O2 CO2+ H2 OReal Reaction:C3H8+ 5O2 3CO2+ 4H2 OExamples:CH4 (g)+ 2 O2 (g) CO2 (g)+ 2 H2O(g)2H2+ O2 2H2O / / the balanced equation for the reaction that occurswhen methanol, CH3OH(l), is burned in (l) + O2(g) CO2(g) + H2O(g)2CH3OH(l) +3O2(g) 2CO2(g) +4H2O(g)_____Practice Exercise:Write the balance equation for the reaction that occurswhen ethanol, C2H5OH(l), is burned in air.

5 Answer: C2H5OH(l) + 3 O2(g) 2 CO2(g) + 3 H2O(g)FORMULAWEIGHTSThe formula weight is a substance is the sum of the atomicweights of each atom in its chemical :H2SO4= 2(1 amu) + 32 amu + 4(16 amu) = 98 amuFor molecular compounds:The formula weight is alsocalled the molecular weight(MW).For ionic compounds:No separated molecules so it calledspecifically the formula weight(FW).For Elements:In case of isolated atoms is called atomicweight(AW).FW of NaCl =23 amu + amu = amu / / the formula weight of a. sucrose (C12H22O11)MW or (FW) of sucrose (C12H22O11) =12(12 amu) + 22(1 amu) + 11(16 amu) = 342 amub. Calcium nitrate .FW Ca(NO3)2= amu) + 2[14 amu+3(16 amu)] = amu_____Practice Exercise:Calculate the formula weight of (a) Al(OH)3 and (b) : (a) amu, (b) amuPERCENTCOMPOSITIONP ercent Composition is the percentage by masscontributed by each element in a :Example:For C2H6(MW = 30 amu)% element =(number of atoms)(atomic weight)(FW of the compound)x 100%C =(2)( amu)( amu) amu=x 100= =(6)( amu)( amu) amu=x 100= 100x 100 / / the percent of carbon, hydrogen, and oxygen (by mass)in :_____Practice Exercise: Calculate the percentage of nitrogen, by mass, in calcium : SNUMBER AND THEMOLEThe mole is just a number of daily life.

6 1 dozen = 12 things1 pair = 2 things1 Cross = 144 thingsIn Chemistry the unit for dealing with the number ofatoms, ions, or molecules, any particlesthemole, mol = x 1023representative particles / / Arrange the following samples in order of increasingnumbers of carbon atoms:12 g12C, 1 mol C2H2, 9 x 1023molecules of :12 g of12C = 1 mol of C atoms = 1023C mol of C2H2= 1023C2H2molecules=2 1023C x 1023molecules of CO2=9 x 1023C order:12 g 12C (1 mol C) <9 1023CO2 molecules ( mol C) <1 mol C2H2 (2 mol C).PRACTICEEXERCISEA rrange the following samples in order of increasing numberof O atoms: 1 mol H2O, 1 mol CO2, 3 x 1023molecules : 1 mol H2O (6 1023O atoms)< 3 1023molecules O3(9 1023O atoms) < 1 mol CO2(12 1023O atoms) / / Important Notes:1 molecule of H2SO4 2 atoms of H1 molecule of H2SO4 1 atom of S1 molecule of H2SO4 4 atoms of S1 mol of H2SO4 ( 1023) molecules of H2SO41 mol of H2SO4 2 moles of H 2( 1023) atoms of H1 mol of H2SO4 1 moles of S ( 1023) atoms of S1 mol of H2SO4 4 moles of S 4( 1023) atoms of the number of H atoms in mol of :moles C6H12O6 molecules C6H12O6 atoms HAnother solution: / / PRACTICEEXERCISEHow many oxygen atoms are in: a.

7 Mol of calcium nitrate? Answer: mol of sodium carbonate? Answer: 1024 MOLARMASS A mole is always the same number ( x 1023). But 1 mole sample of different substances will havedifferent masses. A mol of an element=AW (amu) in grams of thatelement. The mole in grams of substance calledthe molar massof the substance(g/mol).Each is 1 mole ofdifferent elementshavedifferentmasses but allcontainsamenumber of atoms= x 1023 / / is the mass in grams of mol of glucose,C6H12O6?Solution:MW glucose C6H12O6=6(12 amu) + 12(1 amu) + 6(16 amu) = 180 amuSo 1 mole of glucose = 180 gAnd the molar mass = 180g/mol_____Practice Exercise:Calculate the molar mass of calcium nitrate. Answer: the number of moles of glucose in mol Glucose g GlucoseConversion factor:1mol Gl 180 g Gl_____Practice Exercise:How many moles of sodium bicarbonate are in 508g of sodiumbicarbonate (also known as sodium hydrogen carbonate)?

8 Answer: mol NaHCO3 / / the mass, in grams. Of mol of g of Ca(NO3)2 mol Ca(NO3)2mol Ca(NO3)2 g Ca(NO3)2 PRACTICEEXERCISEWhat is the mass, in grams, ofa. mol sodium bicarbonate? Answer: 532 gb. x 10-5mol of sulfuric acid? Answer: 10 3g / / ) How many molecules are in of glucose??? molecules g glucoseb. How many oxygen atoms are in this sample??? O atoms g glucosePRACTICEEXERCISEa) How many molecules are in of nitric acid?Answer: 1022 molecules HNO3b) How many O atoms are in this sample?Answer: 1023atoms O / / EMPIRICALFORMULAS(EF)FROMANALYSESAn empirical formula is the lowest whole number ratio ofelements in chemical formula(written as subscripts).1. When given percentages, assume 100g so that thepercentages can be Convert grams into Divide all answers by the lowest number to If you get , or multiply all numbers you get or ,then multiply all numbers by3(in order to obtain smallest whole numbers).

9 Acid (vitamin C) C, H,and Oby mass. What is the empirical formula ofascorbic acid?Solution:Assume 100 g sample for simplicity:Divide on the smallest value:( ) / / Multiply by 3: The EF:PRACTICEEXERCISEA sample of methyl benzoate, a compound used inperfumes, C, H, and of the empirical formula of this substance? Answer: C4H4O / / MOLECULARFORMULA(MF) The molecular formula is the actual formula of a it can be the same as the empirical formula. MFcan be determined from theEFif we have the actualmolecular = n(EF) where nis whole numberExample:C2H6= 2(CH3)The whole number obtained by: MW= 30 for C2H6EW= 15 for CH3So= 30/15 = 2_____In the previous example :If the actual MW ofascorbic acid = 176 what is the MF of the acid?EFwas :C3H4O3EW [3(12) +4(1)+3(16) ]=88 Actual MW 176n = 176/88 = 2MF = 2 (C3H4 O3) = C6H8O6_____ / / , a hydrocarbon in crude oil, has an empiricalformula of C3H4.

10 The molecular weight of the substance is121 amu. What is the molecular formula of mesitylene?EF:C3H4EW [3(12) +4(1)]=40 Actual MW 121n = 121/40 = = 3 (C3H4) = C9H12 PRACTICEEXERCISEE thylene glycol, the substance used in antifreeze, is composed , H, and O by mass. Its molar mass is ) What is the empirical formula of ethylene glycol?Answers: CH3Ob. What is the molecular formula for the previous question?Answers: C2H6O2 / / COMBUSTIONANALYSIS Compounds containing C, H and O are routinelyanalyzed through combustion in a chamber likethis C is determined from the mass of CO2produced H is determined from the mass of H2O produced O is determined by difference after the C and H havebeen alcohol, a component of rubbing alcohol, is composedof C, H, and O. Combustion of of isopropyl alcoholproduces of CO2and H2O.