1 chapter 3 chemical compounds. 2 chemical formulas; molecular and ionic substances the chemical...
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Chapter 3
Chemical Compounds
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Chemical Formulas; Molecular and Ionic Substances
The chemical formula of a substance is a notation using atomic symbols with subscripts to show the relative proportions of atoms of the different elements in a substance.
Consider the formula of aluminum oxide, Al2O3. This formula implies that the compound is composed of aluminum atoms and oxygen atoms in the ratio 2:3.
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A molecule is a definite group of atoms (two or more nonmetals) that are chemically bonded together – that is, tightly connected by attractive forces.
Chemical Formulas; Molecular Substances
Molecular substances
A molecular substance is a substance that is composed of molecules, all of which are alike.
A molecular formula gives the exact number of atoms of elements in a molecule.
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Molecular Compounds
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Naming Binary Molecular Compounds
• Binary molecular compounds consist of molecules that contain atoms of only two elements (nonmetals).
• In binary molecular compounds the more metallic element is listed first
Increasing metallic character
Increasin
g metallic ch
aracter
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Naming Binary Molecular Compounds
Nonmetal + Nonmetal
Prefixes
mono =1 hexa = 6
di = 2 hepta = 7
tri = 3 octa = 8
tetra = 4 nona = 9
penta =5 deca = 10
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Binary Molecular Compounds
(prefix)name of the first nonmetal +
(prefix)modified name of the second nonmetal
Oxygen - ox + ide = oxideChlorine - chlor + ide = chlorideFluorine – fluor + ide = fluorideHydrogen – hydr + ide = hydrideNitrogen – nitr + ide = nitrideSulfur – sulf + ide = sulfide
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Binary Molecular Compounds
CO - carbon monoxide
SO2 – sulfur dioxide
P2O5 – diphosphorus pentoxide
S2Cl2 – disulfur dichloride
N2O – dinitrogen monoxide
use prefixes in front of both elements; exception mono is never used in front of the first element
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Compounds that consist of ions
Chemical Formulas; Ionic Substances
Ionic substances
An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons.
When an atom loses an electron it becomes a positively charged - cationWhen an atom gains an electron it becomes a negatively charged - anion
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Formation of Ionic Compound, NaCl
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NaCl “table salt”
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The formula of an ionic compound is written by giving the smallest possible whole-number ratio of different ions in the substance.
Chemical Formulas; Ionic Substances
The formula unit of the substance is the group of atoms or ions explicitly symbolized by its formula.
Sodium chloride NaCl , not Na6Cl6
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Naming Binary Ionic CompoundsMetal + Nonmetal
Name of the metal + modified name of a nonmetal
Oxygen - ox + ide = oxideChlorine - chlor + ide = chlorideFluorine – fluor + ide = fluorideHydrogen – hydr + ide = hydrideNitrogen – nitr + ide = nitrideSulfur – sulf + ide = sulfide
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Naming Binary Ionic Compounds
Charges Group 1A – 1+Group 2A – 2+Group 3A – 3+Group 5A – 3-Group 6A – 2-Group 7A – 1-
Main group metallic elements have the charge equal to the group number.
Most nonmetallic elements have the charge equal to the group number – 8
( for example oxygen 6 - 8 = - 2)
We do not use prefixes for binary ionic compounds!!!
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Charges on Some Common Monatomic Cations and Anions
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Binary CompoundsMetal + Nonmetal
NaCl - sodium chloride
KBr – potassium bromide
LiI – lithium iodide
MgBr2 – magnesium bromide
Na2O– sodium oxide
Al2O3 – aluminum oxide
The total of charges = 0 Al2O3 2 x (+3) + 3 x (-2) = 0
3+ 2-Al O2 3
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Binary CompoundsMetal + Nonmetal
Transition Metals
FeO - Fe2O3 – CuO – Cu2O -
iron (II) oxideiron (III) oxidecopper (II) oxidecopper (I) oxide
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Mass and Moles of a Substance
The molar mass of a substance is the mass of one mole of a substance.
For all substances, molar mass, in grams per mole, is numerically equal to the formula weight in atomic mass units.
That is, one mole of any element weighs its atomic mass in grams.
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Molar Mass of Compounds
Calculate the molar mass of butane C4H10
C = 12.01 g/mol × 4 = 48.04 g/mol
H = 1.01 g/mol × 10 = 10.10 g/mol
C4H10 = 58.14 g/mol
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Determining Chemical Formulas
The percent composition of a compound is the mass percentage of each element in the compound.
We define the mass percentage of “A” as the parts of “A” per hundred parts of the total, by mass. That is,
%100whole the of mass
whole in A"" of mass A"" % mass
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Mass Percentages from Formulas
Let’s calculate the percent composition of butane, C4H10.
C % 82.63% 100g 58.14g 48.04
C %
4 moles of carbon × 12.01g/mol = 48.04 g
10 moles of hydrogen × 1.01 g/mol = 10.10 g
Total mass of 1 mol of C4H10 = 58.14 g
H % 17.37% 100g 58.14
g 10.10H %
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C
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Determining Chemical Formulas
We can calculate the percent composition by mass of a compound if we know its formula.
If we know the percent composition of a compound we can determine its empirical formula.
An empirical formula (or simplest formula) for a compound is the formula of the substance in in which the ratio of atoms or ions gives the smallest possible integer value
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Determining Chemical Formulas
Benzoic acid is a white, crystalline powder used as a food preservative. The compound contains 68.8% C, 5.0% H, and 26.2% O by mass. What is its empirical formula?
Cx HyOz
In other words, give the smallest whole-number
ratio of the subscripts (moles) in the formula
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Determining Chemical Formulas
For the purposes of this calculation, we will assume we have 100.0 grams of benzoic acid.
Since x, y, and z in our formula represent mole-mole ratios, we must first convert mass to moles.
Cx HyOz
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Determining Chemical Formulas
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Determining Chemical Formulas
3.501.63(8)C 728.5 mol
3.021.63(8)H mol95.4
1.001.63(8)O )8(63.1 mol
The empirical formula is C7H6O2 .
To calculate the ratio of moles:
Multiply each number by 2 to get integers.
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Determining Chemical Formulas
Determining the molecular formula from the empirical formula.
An empirical formula gives only the smallest whole-number ratio of atoms in a formula.
The molecular formula could be a multiple of the empirical formula
To determine the “true” molecular formula,
we must know the molar mass of the compound.
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Determining Chemical Formulas
C6H6
C2H6
C3H7O2
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Determining Chemical Formulas
A compound is 39.9% C, 6.70% H and 53.4% oxygen. The molecular weight of this compound is 60.0 g/mol. Determine the molecular formula of this compound.
CxHyOz