Chapter 5: Calculations and the Chemical Equation

 

·         5.1 The Mole Concept and Atoms

o   The Mole and Avogadro’s 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.

§  Avogadro’s 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

§  Let’s 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   Don’t 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