Chemistry Concepts: Bonds, Electronegativity, Lewis Structures, and Molecular Geometries, Exams of Chemistry

Download Chemistry Concepts: Bonds, Electronegativity, Lewis Structures, and Molecular Geometries and more Exams Chemistry in PDF only on Docsity! ,t/ Chemistrv 1035 | ,"r,, November 3-2005 THIS IS FORM A [FORM B #s in ( )] Name ALL LEWIS STRUCTURES ARE DRAWN AT THE END OF THE TEST. LOOK AT THE LEWIS STRUCTURES TO FOLLOW THE SOLUTTON TO EACH QUESTION. l. (3)Which of the following elements is most likely to form a cation by the loss of three electrons? l.K Answer: 3. Sc 2. Ca 3. Sc 4.N Some atoms lose electrons, becoming positively charged ions - cations. Atoms that lose electrons will lose the number of electrons required to obtain a noble gas configuration. The elements that lose electrons to obtain a noble gas configuration are metals. Nonmetals gain electrons to form negatively charged ions, anions, in order to obtain a noble gas configuration. K: [Ar]4st^ K will lose the 4sr electron to form K* which has a noble gas configuration. Ca: [Ar]4s2 Ca will lose the two 4s2 electrons to from Ca2* which has a noble gas configuration. Sc: [Ar]4s23dt Sc will lose three electrons - 4s23dt to form Sc3*which has a noble gas configuration. N: [He]2s22p3 N will eain three electrons to forrn the anion N3-. 2. (6)Which one of the following statements is correct when comparing the two bonds C:C and C= C? l. C= C has higher bond order, higher bond energy, and shorter bond length. 2. C= C has lower bond order, higher bond energy, and shorter bond length. 3. C= C has higher bond order, lower bond energy, and shorter bond length. 4. C= C has higher bond order, higher bond energy, and longer bond length. Answer: 1. C= C has higher bond order, higher bond energy, and shorter bond length. A double bond has 2 shared pairs and a bond order of 2. A triple bond has 3 shared pairs of electrons and a bond order of 3. The more bonds there are between the two atoms, the more pairs of electrons being shared between them. As the number of electron pairs between two atoms increases, the bond becomes stronger (the bond energy increases) and the atoms are pulled closer together (the bond length becomes shorter or decreases). Bond length decreases as the number of bonds increases (inversely proportionality) and bond energy increases as the number of bonds increases (direct proportionality). 3. (s)Which of the following bonds is most polar? 2. r{l 3. I-Br 4. Br{l A bond between two atoms of different elements is polar which means that the more electronegative atom in the bond pulls the shared electron cloud towards it, away from the less electronegative atom. Thus the more electronegative atom has apartial negative charge and the less electronegative atom has apartial positive charge. The electron cloud is shared unequally. The greater the difference between the electronegativity values on the two atoms in the bond, the more unequally the electrons are shared and the more polar the bond. The electronegativity trend in the periodic table is: --)-) -+-)-+ -+-+->lncreases Increases The electronegativity difference between I and Cl is greater than that for the other bonds since I and Cl are more widely separated in the periodic table. Therefore I-Cl is the most polar bond. 4. (2)Which of the following molecules has bonds that are best described as nonpolar covalent? l. HF 2. NaF 3. CCl4 4. Br2 5. H2O Answer: 4. Br2 A nonpolar covalent bond is one between two atoms with the same electronegativity value, like the Br-Br bond. The difference in electronegativity is zero and the electrons in the bond are equally shared between the two atoms. NaF has an ionic bond; An ionic bond is a bond between two elements with VERY different electronegativity values, a metal and a nonmetal. The nonmetal is so much more electronegative than the metal that the nonmetal "pulls"the electron completely away from the metal, forming an anion (nonmetal) and a cation (metal). HF, CCl4 and H2O all have polar covalent bonds. A polar covalent bond is a bond between two nonmetal atoms with some difference in electronegativity such that the electrons in the bond are unequally shared; the electron cloud is pulled towards tle more electronegative element so there is a partial positive end of the bond (the less electronegative element) and a partial negative end of the bond (the more electronegative element). The bond is covalent since electrons are shared but polar since the uneven electron cloud distribution results in two different ends of the bond. I. F-F Answer: 2. l-{'1 J t t 1 t t t F cl Br I 1 1. (8)What is the formal charge on sulfur in SO32-? l. -1 2. +3 3. +l 4.0 Answer: 3. +1 Look at the Lewis structure of SO32-at the end of the test. S has three bonds (6 shared electrons) and fwo unshared electrons. Formal charge : # of valence electrons - [# of unshared electrons + %(# of shared electrons)] F.C. : 6 - 12 +'/,(6)l : 6 - 5 : +l 12. (la)Which of the following species exhibits resonance? l. soo2- 2. NHa* 3. Xeo2 4. oz Answer: 4. 03 Look at the Lewis structures of these species at the end of the test.Soor , NH.* , and XeOz all have just single bonds and no possibility of resonance. In 03, a double bond is needed and can be put between either of the outside O atoms and the central O. There are two resonance structures. 13. (2)What is the molecular geometry of XeF3-? l. trigonal planar 2. T-shaped 3. trigonal pyramid 4. see-saw Answer: 2. T-shaped Look at the Lewis structure of this species at the end of the test. Xe has 5 regions of electron density (three shared pairs and two unshared pairs). 5 regions of electrons are arranged around the central atom in a trigonal bipyramidal geometry. Only three of the comers in the trigonal bipyramid are occupied by atoms in a T-shaped geometry. 14. (18)Which of the following species does re! have a tetrahedral molecular geometry? 1. PCl4' 2. SO42- 3. ICl4- 4. NI{4+ Answer: 3. ICl4- Look at the Lewis strucfures of these species at the end of this test. PCl4*, SO42- and NlIa* each have four regions of electron density (four shared pairs) around the central atom which are arranged in a tetrahedral geometry, every corner of which is occupied by an atom. All of these have a tetrahedral geometry. IClq- has six electron groups: 4 bonds (shared pairs) and 2 unshared pairs. These 6 groups are arranged around the central atom in an octahedral geometry, four corners of which are occupied by atoms in a square planar geometry. 15. (9)Which of the following does not obey the octet rule? l. ICl4- 2. PCl4* 3. Po33- 4. XeO2 Answer: 1. ICl4- Look at the Lewis structures of these species at the end of the test. All atoms in PCla*, PO:3- and XeO2 have eight electrons, but I in IClq- has 12 electrons (four shared pairs and two unshared electrons). 16.(21) Which one of the following statements is true about XeFrn? 1. The bonds are polar and the molecule is polar. 2. The bonds are polar, but the molecule is nonpolar. 3. The bonds are nonpolar and the molecule is nonpolar. 4. The bonds are nonpolar, but the molecule is polar. Answer: 1. The bonds are polar and the molecule is polar. See the Lewis structure and molecular geomeky for XeF3*at the end of this test. XeF3+ is T-shaped. All of the Xe-F bonds are polar since the two atoms have different electronegativity values. Two of the bonds in the T-shaped geometry "cancel" each other out, but the third Xe-F polar bond is not canceled. The electron cloud is pulled towards that F and the ion is polar. 17. (20)Which of the following species is nonpolar? l. POr3- 2. XeO2 3. H3O- 4. BF4- Answer: 4. BFr- Look at the Lewis structures of these species at the end of this test. XeO2 is bent so that the two polar bonds do NOT cancel and the molecule is polar. Both PO33- and H3O* are trigonal pyramid and again the polar bonds do NOT cancel and these are polar. But BF+-has a tetrahedral geometry and these are no unshared pairs of electrons on the central atom. The four polar bonds cancel, resulting in a nonpolar species. 18.(25) If a central atom in a particular molecule utilizes sp3 hybrid orbitals, that molecule could have which one of the following molecular geometries? l trigonal planar 2. trigonal pyramid 3. see-saw 4. square planar Answer: 2. trigonal pyramid sp3 hybrid orbitals are used by a central atom that needs to accommodate 4 groups of electrons since sp' hybrid orbitals always occur in sets of 4. The trigonal pyramid geometry arises when a central atom has 3 bonds and I unsharedpair for alotalof4 electron groups. Trigonal planar has 3 electron groups, square planar has 6 electron groups and see-saw has 5 electron groups. 19.(24) What fype of hybrid orbitals is used by the central atom in ICI+-? l. sp 2.tp' 3. rp' 4. sp3d 5. sp3d2 Answer: 5. sp3d2 See the Lewis structure at the end of this test. Iodine needs 6 hybrid orbitals to accommodate its 6 electron groups - four bonds (shared pairs) and two unshared pairs. sp3d2 occur in sets of6 orbitals. 20.(23)For acefylene, HCCH (atoms bonded in the order listed), how many sigma (o) and how many pi (n) bonds are there in the molecule? 1. three o; one 7[ 2. four o; one 7[ 3. t'wo o; one r 4. three o; two r[ Answer: 4. three o: two a See the Lewis structure of HCCH at tle end of this test. There are two C-H single bonds. These are both sigma bonds. There is I C= C triple bond; a triple bond is composed of I sigma bond and 2 pi bonds. So there is a total of 3 sigma bonds and two pi bonds. 21(22) Which of the following species has a geometry that is planar (flat)? 1. ICl4- 2. SO32- 3. SO+2- 4. BF4 Answer: ICla- See the Lewis structures are the end of this test. ICI+- has a square planar geometry which is flat. SO32- is trigonal pyramid while SO+2- and BF+- are both tetrahedral. Trigonal pyramid and tetrahedral are three-dimensional geometries. 22.(17) Which of the following species has a 120o bond angle? l. SO3 2. XeF3- 3. O: 4. H3O' Answer: l. SOr See the Lewis structures at the end of this test. Since S in SOr has three electron groups, it has a trigonal planar molecular geometry which has 120" bond angles. XeF3* is T-shaped with 90o and l80obond angles. 03 has three electron groups arranged in a trigonal planar arrangement but one of these groups of electrons is an unshared pair which results in a bent molecule with bond angles less than 120". H3O-has a tetrahedral arrangement of the four electron groups with bond angles of less than 109.5'due to the unshared pair.