Chem Summary Week 5 - Chemistry: Structure and Properties PDF

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Summary of chapter 5 with images to explain how to work practice problems. Focus of chapter is on compounds and formulas....

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Chem Summary Week 5 4.6 Ionic Compounds: Formulas and Names Because ionic compounds are charge neutral and because many elements form only one type of ion with a predictable charge, we can reduce the formulas for many ionic compounds. From their constituent elements. Four elements to be charge neutral the cations and anions must equal each other. Ionic compounds always contain positive and negative ions. In a chemical formula, the sum of the charges of the positive ions must equal the sum of the negative ions. The formula of an ionic compound reflects the smallest hole number ratio of ions. Procedure for writing formulas for ionic compounds 1. Write the symbol for the metal cat ion and its charge followed by the symbol for the nonmetal anion and its charge. Determine charges from the elements group number in the periodic table. 2. Adjust the subscript on each cation and anion to balance overall charge 3. Check to make sure the sum of the charges of the cations equals the sum of the charges of the anions Example 4.3 Writing formulas for ionic compounds Write the formula for the ionic compound that forms between aluminum and oxygen. Al+3 O2- = Al2O3. Cations: 2(3+) = 6+. Anions: 3(2-) = 6-. The charges cancel For Practice 4.3 Write the formula for the compound formed between potassium and sulfur. K+1 S-2 = K2S. Cations: 2(1+) = 2+. Anions: 1(2-) = 2Example 4.4 Writing formulas for ionic compounds. Write the formula for the ionic compound that forms between calcium and oxygen. Ca2+ O2- = CaO. Cations: 2+. Anions: 2For practice 4.4 Write the formula for the compound formed between aluminum and nitrogen Al+3 N-3 = AlN. Cations: 3+. Anions: -2 Naming ionic compounds. Some compounds have common names, which are nicknames of sorts learned by familiarity. Chemist have also developed systematic names for different types of compounds including ionic ones. The first step in naming an ionic compound is identifying it as one. Remember, ionic compounds are usually composed of metals and nonmetals. We categorize Ionic compounds into two types. Whenever the metal in this first type of compound forms an ion, the ion always has the same charge. We can infer the charges of most of these metals from their group number in the periodic table. Li, Na, K, Rb, Cs, Ag = +1

Mg, Ca, Sr, Ba, Zn = +2 Sc, Al = +3 The second type of ionic compound contains a metal ion with a charge that can differ in different compounds. These are transition metals. Therefore, we must specify its charge for a given compound. Binary compounds contain only two different elements. The name of binary ionic compounds take the form: name of action (metal), name of anion (nonmetal) + ide. KCl is potassium chloride Example 4.5 naming ionic compounds containing a metal that forms only one type of cation Name the compound CaBr2. Cation is calcium and the anion is bromine. The correct name is calcium bromide For practice 4.5 Name the compound Ag3N = Silver nitride More practice 4.5 Write the formula for rubidium sulfide = RbS The full names of compounds containing metals that form more than one kind of cation have the form: Name of cation (metal), (charge of cation (metal) in Roman numerals in parenthesis), base name of anion (nonmetal) + ide. Cr3+ is chromium (III) Table 4.3 Example 4.6 naming ionic compounds containing a metal that forms more than one type of cation Name the compound PbCl4 = The charge in Pn must be 4+ for the compound to be charge neutral with 4 Cl- anions. = Lead(IV) chloride For practice 4.6 Name the compound FeS = Iron sulfide For more practice 4.6 Write the formula for ruthenium(IV) oxide = RuO2 A polyatomic ion is an ion composed of two or more atoms. Hypochlorite is ClO-. The charge on the hypochlorite ion is a property of the whole ion not just oxygen. Conceptual Connection 4.4 Identify the polyatomic ion and its charge in each compound (a) KNO2 - NO2(b) CaSO4 - SO4^2(c) Mg(NO3)2 - NO3We name FeSo4, according to its cation, iron, it’s charge (II), and it’s polyatomic ion sulfate. It’s full name is iron(II) sulfate. Know table 4.4 Most polyatomic ions are oxyanions, anions containing oxygen and another element. For example, NO3- is nitrate and NO2- is nitrite Example 4.7 naming ionic compounds that contain a polyatomic ion.

Name the compound Li2Cr2O7. Cation is lithium and polyatomic ion is dichromate so the full name is lithium dichromate For practice 4.7 Name the compound Sn(ClO3)2 = Tin(II) Chlorate For more practice 4.7 Write the formula for cobalt(II) phosphate = Co(PO4)2 Some ionic compounds- called hydrates- contain a specific number of water molecules associated with each formula unit. For example, Epsom salts is MgSO4 • 7H2O and its systematic name is magnesium sulfate heptahydrate. The seven H2O molecules associated with the formula units are waters of hydration. Waters of hydration can usually be removed by heating the compound. CaSO4 • 1/2 H2O is calcium sulfate hemihydrate

4.8 Molecular Compounds: Formulas and Names In contrast to an ionic compound, the formula for a molecular compound cannot always be determined from it constitute elements because the same combination of elements may form several different molecular compounds, each with a different formula. Many molecular compounds have common names like H2O is water and NH3 is ammonia but because there are so many molecular compounds, there has to be a systematic approach to naming them. Molecular compounds are composed of two or more nonmetals. their names have the form: Prefix, name of 1st element, prefix, base name of 2nd element + ide. Prefixes: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6, hepta 7, octa 8, nona 9, deca 10. If there is only one atom of the first element in the formula, we normally omit the prefix mono. NO2 is nitrogen dioxide Example 4.8 Naming molecular compounds Name each compound (a) NI3 = nitrogen trioxide (b) PCl5 = phosphorus pentachloride (c) P4S10 = tetraphosphorus decasulfide For practice 4.8 Name the compound N2O5 = Dinitrogen pentaoxide For more practice 4.8 Write the formula for phosphorus tribromide = PBr3 Conceptual Connection 4.7 The compound NCl3 is nitrogen trichloride, but AlCl3 is simply aluminum chloride. Why? = the name is different because NCl3 is a molecular compound and AlCl3 is an ionic compound.

4.12 Organic Compounds

Early chemist divided compounds into two types: organic and inorganic. Organic was things that originated from living things and inorganic originated from the earth. Today, chemists can synthesize organic and inorganic compounds. Perfumes, spices, fuel, drugs, and foods come from organic compounds. They are major components of living organisms. Organic compounds are composed of carbon and hydrogen and a few other elements including nitrogen oxygen and sulfur. The key element is carbon which always forms four bonds. The simplest organic compound is methane or CH4. Carbon frequently bonds to itself to form chain, branched, and ring structures. Carbon can also form double bonds and triple bonds with itself and other elements. The simplest organic compounds are hydrocarbons and they are composed of carbon and hydrogen. (fuels) Common hydrocarbons: methane, propane, n-butane, n-pentane, ethene, and ethyne. N stands for normal which means straight chain. Table 4.5 5.2 Electronegativity and Bond Polarity when the Lewis structure for hydrogen fluoride is shown, it looks like the atoms are equally shared between hydrogen and fluorine, but we know they are not. Hydrogen has a slight positive charge and fluorine has a slight negative charge. The electron is unequally shared. The bond is polar- having a positive pole and a negative pole. A polar covalent bond is intermediate in nature between a pure covalent bond and an ionic bond. The ability of an atom to attract electrons to itself in a chemical bond (which results in polar and ionic bonds) is electronegativity. Electronegativity generally increases across a period in the periodic table. Electronegativity generally decreases down a column in the periodic table. Fluorine is the most electronegative atom. Francium is the least electronegative element (sometimes called electropositive) Conceptual Connection 5.1 periodic trends in electronegativity. Arrange these elements in order of decreasing electronegativity: P, Na, N, Al. = N, P, Al, Na The degree of polarity and a chemical bond depends on the electronegativity difference between the two bonding atoms. The greater the electronegativity difference, the more polar the bond. If two atoms with identical electronegativities form a covalent bond, they share the electrons equally and the bond is purely covalent or nonpolar. Table 5.1 Electronegativity difference Small (0-0.4) covalent Cl2 Intermediate (0.4-2.0) polar covalent HCl Large (2.0+) ionic NaCl A dipole moment (u) occurs anytime there is a separation of positive and negative charge. The magnitude of a dipole moment created by separating two particles of equal but opposite charges

of magnitude q by distance r is given by the equation : u=qr. (q= 1.6 x 10^-19) r = 130 pm (approx. length of short covalent bond) is (1.6 x 10^-19C) (130 x 10^-12m) = 2.1 x 10^-29 c•m = 6.2 D By comparing the actual dipole moment of a bond to what the dipole moment would be if the electron were completely transferred from one atom to another, we can get a sense of the degree to which the electron is transferred. The percent ionic character is the ratio of a bond actual dipole moment it would have if the electron were completely transferred from one atom to the other, multiplied by 100. Perfect ionic character = measured dipole moment of bond/ dipole moment if electron were completely transferred x 100%. Example 5.1 classifying bonds as pure covalent, polar covalent, or ionic. Is the bond between each pair of atoms covalent, polar covalent, or ionic? a. Sr and F = ionic (Sr is 1.0 and F is 4.0. 4-1 = 3 = ionic) b. N and Cl = covalent (N is 3 and Cl is 3. 3-3=0 = covalent) c. N and O = polar covalent (N is 3 and O is 3.5. 3.5-3 = 0.5 = polar covalent) For Practice 5.1 Is the bond between each pair of atoms pure covalent, polar covalent, or ionic? a. I and I b. Cs and Br c. P and O Conceptual connection 5.2 perfect ionic character. The HCl (g) molecule has a bond length of 127 pm and a dipole moment of 1.08 D. Without doing detailed calculations, determined which is the best estimate for its perfect ionic character. D= 130-pm, 6.2 D 1/6 x 100% = 15% 5.3 Writing Lewis Structures for Molecular Compounds and Polyatomic Ions. 1. Write the correct skeletal structure for the molecule 2. Calculate the total number of electrons for the Lewis structure buy summing the valence electrons of each item in the molecule 3. Distribute the electrons among the atoms, giving octets (or duets in the case of hydrogen) to as many atoms as possible. 4. If any items like an octet, form double or triple bonds as necessary to give them off if any atoms like an octet, form double or triple bonds as necessary to give them octets 5. Check Example 5.2 writing Lewis structures

Write the Lewis structure for CO2 O-C-O (four dots on each side of both oxygens both double bonded to carbon) For Practice 5.2 Write the Lewis structure for CO Example 5.3 writing Lewis structure Write the Lewis structure for NH3 H-N-H | H (with the dots over nitrogen) For practice 5.3 Write the Lewis structure for H2CO. For polyatomic ion ‘s, add one electron for each negative charge and subtract one electron for each positive charge. Write the Lewis structure for a polyatomic ion with them brackets with a charge of the ion in the upper right-hand corner outside the bracket. Example 5.4 writing Lewis structures for polyatomic ions Write the Lewis structure for NH4 + ion. [H | H-N-H | H]+ For Practice 5.4 write the Lewis structure for the hypochlorite ion, ClO-...