Ch 3 Chemical Reactions and Reaction Stoichiometry Stoichiometry Area of study that examines quantities of substances consumed and produced in chemical reactions Law of conservation of mass o Foundation of stoichiometry o Antoine Lavoisier in 1700s o Atoms are neither created nor destroyed during a chemical reaction 3 1 Chemical Equations Chemical equations Represent chemical reactions means reacts with and arrow means produces Reactants Left of arrow starting substances Products Right of arrow substances produced in reaction Coefficients Relative numbers of molecules of each kind Balanced Equal number of atoms of each element on each side of equation Number of atoms is obtained by multiplying each subscript in chemical formula by coefficient for the formula Balancing Equations Smallest possible number whole number coefficients Never change subscripts when balancing an equation Placing a coefficient in front of formula changes only amount not identity Balance elements that occur in the fewest chemical formulas of equation first Indicating the States of Reactants and Products Symbols g l s and aq Sometimes symbols representing conditions under which reaction proceeds appear above or below reaction arrow 3 2 Simple Patterns of Chemical Reactivity Combination and Decomposition Reactions Combination reactions Two or more substances react to form one product o Combination reaction between metal and nonmetal produces ionic solid o A B C Decomposition reactions A single reactant breaks apart to form two or more substances o C A B o Ex Decomposition of CaCO3 by heating to produce CaO and CO2 o Ex Sodium azide NaN3 inflates safety air bags by release of nitrogen gas by decomposition Combustion Reactions Combustion reactions Rapid reactions that produce a flame Hydrocarbons combusted in air react with O2 to form CO2 and H2O o Whether water is gas or liquid depends on reaction conditions H 2O g forms at high temp in an open container Oxygen containing derivatives of hydrocarbons also produce CO 2 and H2O Reactions with intermediate steps described as oxidation reactions 3 3 Formula Weights Chemical formulas and equations have a quantitative significance in that subscripts and coefficients represent precise quantities Formula and Molecular Weights Formula weight FW Sum of the atomic weights AW of atoms in the chemical formula of substance Molecular weight MW Another name for formula weight when dealing with a molecule Ionic substances can t be referred to as molecules because ions exist as 3 D arrays of ions Percentage Composition from Chemical Formulas Percentage composition elemental composition Percentage mass contributed by each element in the substance o 3 4 Avogadro s Number and the Mole Mole Amount of matter that contains as many objects atoms molecules etc as number of atoms in 12 g of isotopically pure 12C Avogadro s number NA Amount of objects atoms molecules etc in one mole of substance o 6 022 x 1023 Ex Molar Mass 1 mol samples of different substances have different masses The atomic weight of an element in atomic mass units is numerically equal to the mass in grams of 1 mol molar mass of that element Molar mass Mass in grams of one mol of a substance Interconverting Masses and Moles Molar mass provides factor for converting grams to moles or moles to grams Interconverting Masses and Numbers of Particles Molar mass and Avogadro s number used as conversion factors to convert grams to moles and then moles to atoms Calculate number of atoms typically very large 3 5 Empirical Formulas from Analyses Ratio of numbers of moles of all elements in a compound gives the subscripts in the smpd s empirical formula Ex Subscripts of empirical formula are smallest integers that express ratio of atoms present in cmpd Molecular Formulas from Empirical Formulas Obtain molecular formula of cmpd from empirical formula if molecular weight or molar mass of cmpd is known Subscripts in molecular formula of substance are always whole number multiples of subscripts in empirical formula Whole number multiple molecular weight empirical formula weight Combustion Analysis Dimensional analysis to change mass of CO2 to mass of C and mass of H2O to mass of H Mass of sample subtracted by mass of C and H produces mass of O All masses converted to moles divided by smallest amount of moles to find relative numbers of moles 3 6 Quantitative Information from Balanced Equations The coefficients in a balanced chemical equation indicate both relative numbers of molecules or formula units in reaction and relative numbers of moles stoichiometrically equivalent to 3 7 Limiting Reactants Limiting reactant Reactant completely consumed in reaction which determines amount of product formed Excess reactant Reactant that is present in larger amounts that limiting reactant Theoretical and Percent Yields Theoretical yield Quantity of product calculated to form when all of a limiting reactant is consumed Actual yield Amount of product actually obtained in a reaction o Almost always less than theoretical yield Percent yield Relates actual and theoretical yields o