Chapter 5: Calculations and the Chemical Equation
· 5.1 The Mole Concept and Atoms
o The Mole and Avogadros Number
§ atomic mass unit (amu) - unit of measure for the mass of atoms.
· carbon-12 assigned the mass of exactly 12 amu
· 1 amu = 1.66 x 10-24 g
· periodic table gives atomic weights in amu.
§ Chemists usually work with much larger quantities.
· It is more convenient to work with grams than amu.
§ To make the connection we must define the mole.
§ The mole is abbreviated mol.
§ Avogadros number = 1 mol of atoms = 6.022 x 1023 atoms of an element
§ A mole is simply a unit that defines an amount of something
· Just as a dozen defines 12
· Just as a gross defines 144
§ Molar mass - The mass in grams of 1 mole of atoms.
§ What is the molar mass of carbon? 12.01 g/mol
§ This means if you counted out a mole of Carbon atoms (i.e, 6.022 x 1023 of them) they would have a mass of 12.01 g.
§ The average mass of one atom of an element in amu is numerically equivalent to the mass of one mole of an element expressed in grams.
· That is, 1 atom F is 19.00 amu and 1 mole of F is 19.00 g. Or,
· 19.00 amu/atom F and 19.00 g/mole F
· 5.2 Compounds
o The Chemical Formula
§ Chemical Formula - a combination of symbols of the various elements that make up the compound.
§ Formula unit - the smallest collection of atoms that provide two important pieces of information
· the identity of the atoms and
· the relative number of each type of atom
§ Lets look closely at the following formulas:
§ H2O, NaCl, Fe(CN)3, (NH4)3PO4, CuSO4.5H2O
· This is an example of a hydrate - compounds containing one or more water molecules as an integral part of their structure.
· 5.3 The Mole Concept Applied to Compounds
o Formula weight - the sum of the atomic weights of all atoms in the compound, as represented by its formula.
§ expressed in amu
o Molar mass applies to compounds also.
o What is the formula weight of H2O?
§ 16.00 amu + 2(1.008 amu) = 18.02 amu
o What is the molar mass of H2O?
§ 18.02 g/mol H2O
o When calculating the formula weight (or molar mass of an ionic compound, use the smallest unit of the crystal)
· 5.4 The Chemical Equation and the Information It Conveys
o A Recipe For Chemical Change
o Chemical Equation - shorthand notation of a chemical reaction.
o Reactants - (starting materials) - the substances that undergo change in the reaction.
o Products - substances produced by the reaction.
o Law of Conservation of Mass - matter cannot be either gained or lost in the process of a chemical reaction.
§ The total mass of products must equal the total mass of the reactants.
o We know that a chemical equation represents a chemical change. The following is evidence for a reaction:
§ Release of a gas.
· CO2 is released when acid is placed in a solution containing CO32- ions.
· H2 is released when Na is placed in water.
§ Formation of a solid (precipitate.)
· A solution containing Ag+ ions is mixed with a solution containing Cl- ions.
§ Heat is produced or absorbed (temperature changes)
· Acid and base are mixed together
§ The color changes
§ Light is absorbed or emitted
§ Changes in the way the substances behave in an electrical or magnetic field
§ Changes in electrical properties.
· 5.5 Balancing Chemical Equations
o Consider the following reaction:
§ hydrogen reacts with oxygen to produce water
o Write the above reaction as a chemical equation.
o You probably wrote the following:
§ H2 + O2 ® H2O
o Dont forget the diatomic elements.
o Is the law of conservation of mass obeyed as written? NO!
o Balancing chemical equations uses coefficients to ensure that the law of conservation of mass is obeyed.
o You may not change subscripts!
o WRONG: H2 + O2 ® H2O2
· 5.6 Calculations Using the Chemical Equation
o We will learn in this section to calculate quantities of reactants and products in a chemical reaction.
o Need a balanced chemical equation for the reaction of interest.
o Keep in mind that the coefficients represent the number of moles of each substance in the equation.
o Theoretical and Percent Yield
§ Theoretical yield - the maximum amount of product that can be produced
· Pencil and paper yield
§ Actual yield - the amount produced when the reaction is performed
· Laboratory yield
§ Percent yield: actual yield divided by theoretical yield X 100