Gen Chem Ch. 9 Review Questions

27 Do resonance structures always contribute equally to the overall structure of a molecule? Explain.

No, the resonance structure with the overall lowest formal charge on the atoms and the most negative formal charge on the most electronegative atom in the structure will contribute most to the overall structure

23. What is the magnitude of the dipole moment formed by separating a proton and an electron by 100 pm? 200 pm?

[(1.6*10^-19 C) (100*10^-12 m)](1D/ 3.34*10^-30 C*M)=4.8 D

19What is electronegativity? What are the periodic trends in electronegativity?

electronegativity is the ability of an atom to attract electrons to itself in a chemical bond this results in a polar bond electronegativity increases across a period from left to right and decreases down a column(group) most electronegative element-fluorine

32 Explain the difference between endothermic reactions and exothermic reactions with respect to the bond energies of the bonds broken and formed.

endothermic: bonds broken exothermic: bonds forming

31 What is bond energy? How can you use average bond energies to calculate enthalpies of reaction?

energy required to break one mole of the bond in the gas phase breaking bond is endothermic forming bond is exothermic sum of enthalpy changes associated with the bond breaking the required bonds in the reactants and forming the required bonds in the products add bond energies of product with bond energies of reactant

29 Why does the octet rule have exceptions? Give the three major categories of exceptions and an example of each.

1) odd octets-electron species, molecules, ions with an odd number of electron ( NO) 2) incomplete octets-molecules/ions with fewer than 8 electrons around an atom ( BF3) 3) expanded octets - molecules or ions with more than eight electrons around an atom ( AsF5)

21. Explain what is meant by the percent ionic character of a bond. Do any bonds have 100% ionic character?

A percent ionic character is the actual dipole moment of the atom divided by the dipole moment if the electron was completely transferred. A bond in which an electron is completely transferred form one atom to another would have 100% ionic character. No bond is 100% ionic. Percent ionic character increases with electronegativity differences increase. bonds with greater than 50% ionic character are referred as ionic bonds

5. Describe the octet rule in the Lewis model.

Bonds are formed when atoms attain a stable electron configuration. B/c the stable configuration has 8 e- in the outermost shell, this is known as the octet rule.

11. What is the Born-Haber cycle? List each of the steps in the cycle and show how the cycle is used to calculate lattice energy.

Born Haber cycle is a hypothetical series of steps that represent the formation of an ionic compound from its constituent elements. The steps are chosen so that change in enthalpy of each step is known excel for the last one, which is lattice energy. For formation of NaCl step 1: formation of gaseous Na from solid Na (heat of sublimation of Na) step 2: formation of Cl atom from Cl molecule (bond energy of Cl) step 3: Na becomes Na+ (first ionization energy) step 4: Cl becomes Cl- ( electron affinity) step 5: formation of crystalline solid from gaseous ions (lattice energy) The overall reaction is the formation of NaCl(s), so we can use Hess's Law to determine the lattice energy. heat of sublimation+ 1/2 bond energy+ first IE+ EA+Lattice Energy All the terms are known except for the lattice energy is starting with the reactants in its elemental form and undergoing multiple steps that change the enthalpy and end up with the solid ionic compound

34 How does the electron sea model explain the conductivity of metals? The malleability and ductility of metals?

However, the electrons are mobile, and no individual electron is confined to any particular metal ion. When a metal wire is connected to the terminals of a battery, electrons flow through the metal toward the positive terminal and into the metal from the battery at the negative terminal. The high heat conductivity of metals is also accounted for by the mobility of the electrons, which permits ready transfer of kinetic energy throughout the solid. The ability of metals to deform (their malleability and ductility) can be explained by the fact that metal atoms form bonds to many neighbors. Changes in the positions of the atoms brought about in reshaping the metal are partly accommodated by a redistribution of electrons.

7. How do you draw an ionic Lewis structure?

In Lewis theory, we represent ionic bonding by moving electron dots from the metal to the nonmetal and allowing resultant ions to form a crystalline lattice composed of alternating cations and anions. The cations lose its valence electrons and is left with an octet in the previous principal energy level ; the anion gains electrons to form an octet. The Lewis structure of the anion is usually written with brackets with charge in the upper right hand corner, outside the brackets. positive and negative charges attract one another, forming compounds.

28 What is formal charge? How is formal charge calculated? How is it helpful?

The formal charge of an atom in a Lewis structure is the charge it would have if all the electrons were shared equally bet. bonded atoms.

13 How does the ionic bonding model explain the relatively high melting points of ionic compounds?

We modeled ionic solids as a lattice of individual ions held together by coulombic forces, which are equal in all directions To melt the solid, these forces must be overcome. This requires a significant amount of heat. The model accounts for high melting point of solids.

33 What is the electron sea model for bonding in metals?

When metal atoms bond together to form a solid, each metal atom donates one or more electrons to an electron sea.

8. How can Lewis structures be used to determine the formula of ionic compounds? Give an example.

With the Lewis structure, you can see how many of the cations or anions are needed for every ion to fulfill their octet. For example Na has one electron and Cl has seven electrons. Na+ and Cl- attract one another and fulfills their octet.

15 Within a covalent Lewis structure, what is the difference between lone pair and bonding pair electrons?

bonding pair electrons are the electrons shared between two atoms lone pair of electrons is associated with only one atoms and not involved in bonding

6. According to the Lewis model, what is a chemical bond?

chemical bonds allow atoms to get to a more stable state, lowers the potential energy bet. charged atoms

17How does the Lewis model for covalent bonding account for why certain combinations of atoms are stable while others are not?

combinations of atoms that can satisfy the octet rule on each atom are stable while combinations that do not satisfy the octet rule are not stable

18 How does the Lewis model for covalent bonding account for the relatively low melting and boiling points of molecular compounds (compared to ionic compounds)?

covalent bond is not as strong as ionic compounds

4. How do you determine how many dots to put around the Lewis symbol of an element?

dots=# of valence electron an element has

16In what ways are double and triple covalent bonds different from single covalent bonds?

double and triple are greater in strength than the single bonds double and triple are shorter than the single bonds

14. How does the ionic bonding model explain the nonconductivity of ionic solids, and at the same time the conductivity of ionic solutions?

in solutions the ionic solids breaks into cations and anions and electricity flows between them when the electrons are transferred ionic solids do not conduct electricity b/c the ions are locked into a rigid lattice or array the ions cannot move out of the lattice so solid cannot conduct electricity.

9. What is lattice energy?

lattice energy is associated with forming a crystalline lattice of alternating cations and anions from gaseous ions. B/c cations are positive and anions are negative, there is a lowering of potential and the ions come together to form a lattice. The energy is emitted as heat when the lattice forms.

12. How does lattice energy relate to ionic radii? To ion charge?

lattice energy is directly related to charge of ions and inversely related to the ionic radii of the ions

3. What are the three basic types of chemical bonds? What happens to electrons in the bonding atoms in each type?

metallic bond: metal+ metal, sea of electrons, characterized by electrons being pooled ionic bond: metals+ nonmetals, electron transferred covalent bond: nonmetals + nonmetals, electron shared

30 What elements can have expanded octets? What elements should never have expanded octets?

ones with energy level of 3 or greater can have expanded octets b/c these levels have the d subshell ones with energy level 1 or 2 should never have expanded octets

26 What are resonance structures? What is a resonance hybrid?

resonance hybrid is the average of all the resonance structure resonance structure occur when bonds are equivalent are intermediate in strength and length of a double and single bond resonance hybrid: actual structure of the molecule, intermediate of the resonance structures

22. What is a dipole moment?

separation of positive and negative charge