The Mole Concept and Atoms

Transcription

1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 4 18 February 2014 Calculations and the Chemical Equation The Mole Concept and Atoms Atoms are exceedingly small Unit of measurement for mass of an atom is 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 g Periodic table gives atomic weights in amu 1

2 Mass of Atoms What is the atomic weight of one atom of fluorine? Answer: amu What would be the mass of this one atom in grams? amu F F atom 1amu F g F atom Chemists usually work with much larger quantities It is more convenient to work with grams than amu when using larger quantities g F The Mole and Avogadro s Number A practical unit for defining a collection of atoms is the mole 1 mole of atoms = x atoms This value is called Avogadro s number This has provided the basis for the concept of the mole Amedeo Avogadro

3 The Mole To make this connection we must define the mole as a counting unit The mole has symbol mol A mole is simply a unit that defines an amount of something dozen means 12 gross means 144 score means 20 ream means 500 mole means 6.022x10 23 Atomic Mass The atomic mass of one atom of an element corresponds to: The average mass of a single atom in amu The mass of a mole of atoms in grams 1 atom of F is amu amu/atom F 1 mole of F is g g/mol F amu F g F 1atom F 1amu F atom F 1mol F =19.00 g F/mol F or g/mol F 3

4 amu F g F 1atom F 1amu F atom F 1mol F =19.00 g F/mol F or g/mol F note: The name of the unit is mole The symbol is mol Calculating atoms, moles, and mass We use the following conversion factors: Density converts grams milliliters Atomic mass unit converts amu grams Avogadro s number converts moles number of atoms Molar mass converts grams moles 4

5 Strategy for Calculations Map out a pattern for the required conversion Given a number of grams and asked for number of atoms Two conversions are required Convert grams to moles 1 mol S/32.06 g S OR g S/1 mol S Convert moles to atoms mol S x (6.022 x atoms S) / 1 mol S Practice Calculations 1. Calculate the number of atoms in 1.7 moles of boron. (1.7 x 6.022x10 23 ) 2. Find the mass in grams of 2.5 mol Na (sodium). (2.5 x 23) 3. Calculate the number of atoms in 5.0 g aluminum. ( (5.0/27) x 6.022x10 23 ) 4. Calculate the mass of 5,000,000 atoms of Au (gold). ( (5,000,000/6.022x10 23 ) x 197) 5

6 Interconversion Between Moles, Particles, and Grams The Chemical Formula, Formula Weight, and Molar Mass 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 The relative number of each type of atom 6

7 Chemical Formula Consider the following formulas: H 2 2 atoms of hydrogen are chemically bonded forming diatomic hydrogen, subscript 2 H 2 O 2 atoms of hydrogen and 1 atom of oxygen, lack of subscript means one atom NaCl 1 atom each of sodium and chlorine Ca(OH) 2 1 atom of calcium and 2 atoms each of oxygen and hydrogen, subscript outside parentheses applies to all atoms inside Chemical Formula Consider the following formulas: (NH 4 ) 2 SO 4 2 ammonium ions and 1 sulfate ion Ammonium ion contains 1 nitrogen and 4 hydrogen Sulfate ion contains 1 sulfur and 4 oxygen Compound contains 2 N, 8 H, 1 S, and 4 O CuSO 4. 5H 2 O This is an example of a hydrate - compounds containing one or more water molecules as an integral part of their structure 5 units of water with 1 CuSO 4 (pentahydrate) Homework: What does sesqui mean? 7

8 Comparison of Hydrated and Anhydrous Copper Sulfate Hydrated copper sulfate Anhydrous copper sulfate Marked color difference illustrates the fact that these are different compounds Formula Weight and Molar Mass Formula weight - the sum of the atomic weights of all atoms in the compound as represented by its correct formula expressed in amu What is the formula weight of H 2 O? amu + 2(1.008 amu) = amu Molar mass mass of a mole of compound in grams / mole Numerically equal to the formula weight in amu What is the molar mass of H 2 O? g/mol H 2 O 8

9 Formula unit smallest collection of atoms from which the formula of a compound can be established When calculating the formula weight (or molar mass) of an ionic compound, the smallest unit of the crystal is used Formula Unit What is the molar mass of (NH 4 ) 3 PO 4? 3(N amu) + 12(H amu) + P amu + 4(O amu)= 3(14.01) + 12(1.008) (16.00)= g/mol (NH 4 ) 3 PO 4 Molar Mass Molar mass- The mass in grams of 1 mole of atoms What is the molar mass of carbon? g/mol C This means counting out a mole of carbon atoms (i.e., x ) they would have a mass of g One mole of any element contains the same number of atoms, x 10 23, Avogadro s number 9

10 The Chemical Equation and the Information It Conveys A Recipe For Chemical Change Chemical equation - shorthand notation of a chemical reaction Describes all of the substances that react and all the products that form, physical states, and experimental conditions Reactants (starting materials) the substances that undergo change in the reaction Products substances produced by the reaction Features of a Chemical Equation 1. Identity of products and reactants must be specified using chemical symbols 2. Reactants are written to the left of the reaction arrow and products are written to the right 3. Physical states of reactants and products may be shown in parentheses 4. Symbol over the reaction arrow means that energy is necessary for the reaction to occur 5. Equation must be balanced 10

11 Features of a Chemical Equation 2HgO( s ) 2Hg( l ) O2( g ) Products and reactants must be specified using chemical symbols Reactants written on the left of arrow energy is needed Products written on the right Physical states are shown in parentheses Everything is balanced number of atoms moles of stuff mass energy charge 11

12 The Experimental Basis of a Chemical Equation We know that a chemical equation represents a chemical change One or more substances are changed into new substances Different chemical and physical properties Evidence of a Reaction Occurring The following can be visual evidence of a reaction: Release of a gas CO 2 is released when strong acid is placed in a solution containing CO 3 2- ions Formation of a solid (precipitate) A solution containing Ag + ions mixed with a solution containing Cl - ions Heat is produced or absorbed Acid and base are mixed together Color changes 12

13 Subtle Indications of a Reaction Heat or light is absorbed or emitted Changes in the way the substances behave in an electrical or magnetic field before and after a reaction Changes in electrical properties Writing Chemical Reactions We will learn to identify the following patterns of chemical reactions: combination decomposition single-replacement double-replacement Recognizing the pattern will help you write and understand reactions 13