CHE CHAPTER 4 AND 5 EXAM II
scaling factor used in these models? In other words, by approximately what number would you have to multiply the radius of an actual oxygen atom for it to be the radius of the sphere used to represent the oxygen atom in the water molecule shown here?
(10^8) Atomic radii range in the hundreds of picometers, while the spheres in these models have radii of less than a centimeter. The scaling factor is therefore about 108 (100 million).
Which ratio can be correctly derived from the molecular formula for water (H2O)? Explain your answer.
(2 mol H : 1 mol H2O)The chemical formula for a compound gives relationships between atoms or moles of atoms per mole of the molecule. The chemical formula for water states that water molecules contain 2 H atoms to every 1 O atom or 2 mol H to every 1 mol H2O.
Structural formula
-A structural formula uses lines to represent covalent bonds and shows how atoms in a molecule are connected or bonded to each other -Communicates the most information
Molecular formula
-Indicates the actual number of atoms of each element in a molecule of a compound
Empirical formula
-Indicates the relative number of atoms of each element in a compound -Communicates the least
Summarizing Ionic Compound Formulas:
-Ionic compounds always contain positive and negative ions. -In a chemical formula, the sum of the charges of the positive ions (cations) must equal the sum of the charges of the negative ions (anions). Ex: Al2O3 cations: 2(3+)=6+ anions:3(2-)=6- The charges cancel. -The formula of an ionic compound reflects the smallest whole-number ratio of ions
IONIC BOND CHARACTERISTICS
-Metal and nonmetal -Metal transfer one or more of its electrons to the nonmetal -Metal atom becomes a cation (a positively charged ion) -Nonmetal atom becomes an anion (a negatively charged ion) -The solid state is composed of a lattice—a regular three-dimensional array—of alternating cations and anions.
Lewis Model
-Represent valence electrons as dots -Focuses on valence electrons because chemical bonding involves the transfer or sharing of valence electrons between two or more atoms -Uses the simple octet rule
COVALENT BOND CHARACTERISTCS
-When a nonmetal bonds with another nonmetal, neither atom transfers electrons to the other. Instead, the two atoms share some electrons -Covalently bonded atoms form molecules, and the resulting compounds are called molecular compounds
Molecular models
1) Ball and stick molecular model 2) Space filling molecular model
Use the Lewis model to predict the formula for the compound that forms between calcium and chlorine.
1) Draw Lewis symbols for calcium and chlorine based on their number of valence electrons, obtained from their group number in the periodic table. 2) Calcium needs to lose its two valence electrons (to be left with an octet in its previous principal shell), while chlorine only needs to gain one electron to get an octet. Therefore, you must have two chlorine atoms for each calcium atom. The calcium atom loses its two electrons to form Ca2+ and each chlorine atom gains an electron to form Cl- In this way, both calcium and chlorine attain octets 3) Finally, write the formula with subscripts to indicate the number of atoms CaCl2
Types of chemical formulas
1) Empirical 2) Molecular 3) Structural
Classification of chemical bonds
1)Ionic bond: -Metal and nonmetal -Metal transfer one or more of its electrons to the nonmetal metal atom then becomes a cation (a positively charged ion), and the nonmetal atom becomes an anion (a negatively charged ion) 2)Covalent bond: - Between nonmetals -
Chemical formula
1)The quickest and easiest way to represent a compound 2)Indicates the elements present in the compound and the relative number of atoms or ions of each H2O
Hydrogen may replace gasoline as a fuel in the future. Most major automobile companies are developing vehicles that run on hydrogen. These cars are environmentally friendly because their only emission is water vapor. One way to obtain hydrogen for fuel is to use an emission-free energy source such as wind power to form elemental hydrogen from water. What is the mass of hydrogen (in grams) contained in 1.00 gallon of water? (The density of water is 1.00 g/mL.)
4.23x102 g H
calculate the mass of Cl in 1.00 kg CCl2F2, we use the following conceptual plan:
5.86x102 g Cl
CaSO4⋅12H2O BaCl2⋅6H2O CuSO4⋅5H2O
A. calcium sulfate hemihydrate B. barium chloride hexahydrate C.Copper (II) sulfate pentahydrate
The Formation of an Ionic Compound
An atom of sodium (a metal) loses an electron to an atom of chlorine (a nonmetal), creating a pair of oppositely charged ions. The sodium cation then attracts the chloride anion, and the two form a crystalline lattice.
Polyatomic ion
An ion composed of two or more atoms
Oxyanions
Anions containing oxygen and another element Only two ions in the series more oxygen: More oxygen: -ate Fewer oxygen atoms: -ite NO3- Nitrate NO2- Nitrite More than two ions in the series: Less than: -Hypo More than: -Per ClO-hypochlorite ClO2-chlorite ClO3-chlorate ClO4-perchlorate
the average mass of an atom of an element ?
Atomic mass
Why Chemical bonds form?
Because the rest of the elements do not possess the stability of the noble gases, they form chemical bonds to become more stable (to lower the potential energy of the charged particles that compose atoms)
Without doing any calculations, list the elements in C6H6O in order of decreasing mass percent composition.
C > O > H Since carbon and oxygen differ in atomic mass by only 4 amu, and since there are six carbon atoms in the formula, we can conclude that carbon constitutes the greatest fraction of the mass. Oxygen is next because its mass is 16 times that of hydrogen and there are only six hydrogen atoms to every one oxygen atom.
The octet rule
Chemical rule of thumb that reflects observation that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas.
Based on what you learned in Chapter 1 about atoms, what part of the atom do you think the spheres in the space-filling molecular models shown in this chapter represent? If you were to superimpose a nucleus on one of these spheres, how big would you draw it?
Each sphere represents the electron cloud of the atom. The nucleus would be too small to see on the same scale.
Empirical formula
Empirical formula represents a ratio of atoms or a ratio of moles of atoms, NOT a ratio of MASSES. So the first thing mass (in grams)-- amount (in moles). How many moles of each element are present in the sample`
Some Common Polyatomic Ions
FeSO4 iron(II) sulfate NH4NO3 ammonium nitrate
Chemical Bond
Force that holds atoms together in a compound.
Molecular mass-weight/ average mass of a molecule (or a formula unit) of a compound
Formula mass
An aspirin tablet contains 325 mg of acetylsalicylic acid (C9H8O4)(C9H8O4). How many acetylsalicylic acid molecules does it contain?
Given:325 mg C9H8O4 Find: number of C9H8O4molecules ---------------------- C9H8O4 molar @mass=9(12.01)+8(1.008)+4(16.00)=180.15 g/mol @6.022×1023=1 mol @1 mg=10-3 g
Compound vs Mixture
In a compound, elements combine in fixed, definite proportions; in a mixture, elements can mix in any proportion whatsoever.
Molecular vs Ionic compounds
In contrast to an ionic compound, the formula for a molecular compound cannot always be determined from its constituent elements because the same combination of elements may form several different molecular compounds
Metals with only one type of ion
Metals with 1+ (alkali metals), 2+(alkali earth metals), 3+ (Sc, Al, Zn, Ag)
Metals forms more than one type of ion
Most of transition metals -Sn, Pb
Identify the polyatomic ion and its charge in each compound. (a) KNO2 (b) CaSO4 (c) Mg(NO3)2
NO2-, SO42-, and NO3-
Process of naming compounds is called
Nomenclature
Most common polyatomic ions are
Oxyanions
Ball and stick model
Represents atoms as balls and chemical bonds as sticks; how the two connect reflects a molecule's shape
Space filling molecular model
Represents atoms as balls and chemical bonds as sticks; how the two connect reflects a molecule's shape
Lattice Energy: The Rest of the Story
The lattice energy of an ionic compound is the energy associated with the formation of a crystalline lattice of the compound from the gaseous ions.
Way to express how much of an element is in a given compound?
The mass percent composition or mass percent
Molecular formula Structural formula Ball and stick model Space filling model
The molecular formula of methane indicates the number and type of each atom in the molecule: one carbon atom and four hydrogen atoms. The structural formula indicates how the atoms are connected: The carbon atom is bonded to the four hydrogen atoms. The ball-and-stick model clearly portrays the geometry of the molecule: The carbon atom sits in the center of a tetrahedron formed by the four hydrogen atoms. And finally, the space-filling model gives the best sense of the relative sizes of the atoms and how they merge together in bonding.
Name the compound Li2Cr2O7
The name for Li2Cr2O7 is the name of the cation, lithium, followed by the name of the polyatomic ion, dichromate. Its full name is lithium dichromate. Li2Cr2O7 lithium dichromate
The compound NCl3 is nitrogen trichloride, but AlCl3 is simply aluminum chloride. Why?
The name forms differ because NCl3 is a molecular compound and AlCl3 is an ionic compound. Prefixes such as mono, di, tri, etc are used for molecular compounds but not for ionic compounds. In nomenclature you must first determine whether the compound is an ionic compound or a molecular compound, and then name it accordingly. NCl3 is a molecular compound (two or more nonmetals), and therefore you must use prefixes to indicate the number of each type of atom—so NCl3 is nitrogen trichloride. The compound AlCl3, in contrast, is an ionic compound (metal and nonmetal), and therefore does not require prefixes—so AlCl3 is aluminum chloride.
Combustion Analysis
The sample to be analyzed is placed in a furnace and burned in oxygen. The water and carbon dioxide produced are absorbed into separate containers and weighed.
Ionic Bonding: Models and Reality
We model an ionic solid as a lattice of individual ions held together by coulombic forces that are not directional (which means that, as we move away from the center of an ion, the forces are equally strong in all directions). To melt the solid, these forces must be overcome, which requires a significant amount of heat. Therefore, our model accounts for the high melting points of ionic solids. In the model, electrons transfer from the metal to the nonmetal, but the transferred electrons remain localized on one atom. In other words, our model does not include any free electrons that might conduct electricity (the movement or flow of electrons in response to an electric potential, or voltage, is electrical current). In addition, the ions themselves are fixed in place; therefore, our model accounts for the nonconductivity of ionic solids. When our idealized ionic solid dissolves in water, however, the cations and anions dissociate, forming free ions in solution. These ions can move in response to electrical forces, creating an electrical current. Thus, our model predicts that solutions of ionic compounds conduct electricity (which in fact they do).
Use the ionic bonding model developed in this section to determine which has the higher melting point, NaCl or MgO. Explain your answer.
You would expect MgO to have the higher melting point because, in our bonding model, the magnesium and oxygen ions are held together in a crystalline lattice by charges of 2+ for magnesium and 2- for oxygen. In contrast, the NaCl lattice is held together by charges of 1+ for sodium and 1- for chlorine. According to Coulomb's law, as long as the spacing between the cation and the anion in the two compounds does not differ that much, the higher charges in MgO result in lower potential energy (more stability), and therefore a higher melting point. The experimentally measured melting points of these compounds are 801 °C for NaCl and 2852 °C for MgO, in accordance with the model.
Hydrated Ionic Compounds
contain a specific number of water molecules associated with each formula unit
prefixhydrate
indicates the number of water molecules associated with each formula unit. hemi=1/2 mono=1 di=2 tri=3 tetra=4 penta=5 hexa=6 hepta=7 octa=8
Conversion Factors from Chemical Formulas
mass (in grams) of Cl contained in 25.0 grams CCl2F2CCl2F2. The relationship inherent in the chemical formula (2 mol Cl : 1 mol CCl2F2CCl2F2) applies to the amount in moles, not to mass. Therefore, we first convert the mass of CCl2F2 to moles CCl2F2. Then we use the conversion factor from the chemical formula to convert to moles Cl. Finally, we use the molar mass of Cl to convert to grams Cl. (14.7)
The general form for solving problems where we are asked to find the mass of an element present in a given mass of a compound is:
mass compound→ moles compound→ moles element→ mass element
The mass percent composition or mass percent of an element
mass percent of element X= ((molar mass g/mol of element X subscript of element indication X subscripts in 1 mole of compound) % molar mass of the compound g/m) ×100%
Naming a molecular compound
molecular compounds are composed of two or more nonmetals Ex: NO2----nitrogen dioxide N2O----dinitrogen monoxide NI3----nitrogen triiodide PCl5--phosphorus pentachloride P4S10----tetraphosphorus decasulfide
Calculating Molecular Formulas for Compounds
molecular formula=empirical formula×n,where n=1,2,3,...
Formula Mass
sum of the atomic masses of all the atoms in its chemical formula.
empirical formula molar mass
the sum of the masses of all the atoms in the empirical formula 1) molar mass=empirical formula molar mass×n 2) n=molar mass% empirical formula molar mass