Topic 4: Chemical Bonds IB Chemistry SL Ms. Kiely Coral Gables Senior High
5th PERIOD Bell Ringer - review of 4.1 Quiz (revised) 1. What is the formula of a compound formed by magnesium and phosphate? Mg₃(PO₄)₂ 2. Which substance has the lowest electrical conductivity? a) LiCl(s) b) LiCl(l) c) LiCl(aq) 3. What is the formula of a compound formed by lithium and nitrate? LiNO₃ 4. What is the name of the following compound Ni₃PO₄? nickel(i) phosphate HW: Read pgs 133-138 5. What is the name of the following compound (NH₄)₂SO₄? Ammonium sulfate
7th PERIOD Bell Ringer - review of 4.1 Quiz 1. What is the formula of a compound formed by calcium and phosphate? Ca₃(PO₄)₂ 2. Which substance has the lowest electrical conductivity? a) NaCl(s) b) NaCl(l) c) NaCl(aq) 3. What is the formula of a compound formed by lithium and nitrate? LiNO₃ 4. What is the name of the following compound Ti(HCO₃)? titanium(i) hydrogen carbonate HW: Read pgs 133-138 5. What is the name of the following compound (NH₄)₂SO₄? Ammonium sulfate
TODAY: POP Quiz based off lecture and HW at the end of the lecture Take out your HW and leave it on your desk until the quiz
Octet Rule is not always followed Incomplete octet: Some molecules are exceptions to the octet rule. Small atoms such as beryllium (Be) and the metalloid Boron, (B), form stable molecules in which the central atom has fewer than eight electrons in its valence shell. Molecules with incomplete octets are said to be electron deficient. Examples: Beryllium chloride BeCl₂, Beryllium fluoride BeF₂, Boron fluoride BF₃, and boron chloride BCl₃ -Boron (B) is a metalloid. -Beryllium (Be): is a metal, so BeCl₂ and BeF₂ seem like they should be ionic at first. This is an exception due to how their orbitals overlap when bonding! They end up bonding covalently.
Octet Rule is not always followed Electron deficient molecules tend to bond with atoms or other molecules that have a lone pair on them. The atom or molecule with the lone pair can share its lone pair via a coordinate covalent bond to the electron deficient molecule, forming a coordinate compound. Boron chloride, BCl₃, is actually a very important catalyst used in several synthetic reactions due to its ability to accept electrons. Below is an image of BCl₃ becoming stable by accepting the electron pair of the nitrogen atom of ammonia, NH₃. Coordinate covalent bond!
Electronegativity values determine type of bond IN GENERAL, the larger the difference in electronegativity between atoms in a compound, the more ionic that compound is. For example, it is generally recognized that a difference of 1.8 units or more will give a compound the is predominantly ionic.
Electronegativity values determine type of bond The electronegativity values are given to you for each pair of elements. They are the same ones you would find in your IB Data Booklet. Calculate the difference between each pair: In general, if the difference is 1.8 and above, then the compound is ionic.
2.4 1.5 0.2 1.8 At first, you might have thought that A was the answer, considering: A) 4.0 (F) - 1.6 (Be) = 2.4 whereas D) 2.6 (S) - 0.8 (K) = 1.8 HOWEVER, remember the exception with Beryllium fluoride (BeF₂)! BeF₂ and BeCl₂ are considered covalent always.
Bond length & Bond strength NOT ALL BONDS ARE CREATED EQUAL Every covalent bond is characterized by two values: -Bond Length: a measure of the distance between the two bonded nuclei. -Bond Strength: the strength of a bond, which is usually described in terms of bond enthalpy, which is a measure of the energy required to break a bond.
For example, let s consider the group 17 halogens, which are all diatomic. A fluorine (F) atom has the smallest radius of the halogens, having only two energy levels. An iodine (I) atom is much larger since it has 5 energy levels. This means that the valence electrons that are shared by F₂, although they repel each other, are much closer the nuclei of each F atom, causing them to get quite close to each other (short distance between them, meaning short bond length). Iodine s nucleus is not very effective at attracting the shared valence electrons inwards since there is so much shielding due the many more energy levels with electrons. This means the repulsion between the bonding electrons causing them to be quite distant from each other (longer bond length).
Based on the bond lengths, which of these four halogens do you think has the lowest bond enthalpy? Meaning, which of these DOES NOT require a lot of energy to break the bond?
Based on the bond lengths, which of these four halogens do you think has the lowest bond enthalpy? Meaning, which of these DOES NOT require a lot of energy to break the bond? Iodine (I₂) has the lowest bond enthalpy. It has the largest radius of these four halogens, and therefore has the longest bond length. It is easier to break long bonds than it is to break short bonds. Long bonds are weaker than short bonds since short bonds are short due to the strong attraction of the nucleus.
Triple bonds are stronger and shorter than double bonds, which are shorter and stronger than single bonds. This is because multiple bonds, such as triple bonds, have a greater amount of shared electrons and so have a stronger force of electrostatic attraction between the bonded nuclei: due to more shared electrons between nuclei, there is a stronger pulling power from the electrons on the nuclei of the two bonding atoms, bringing them closer together.
Despite both of these being bonds between C and O, the double bond is stronger, causing it to be shorter, and also causing it to require more energy to break.
Polar and non-polar bonds NOT ALL SHARING IS EQUAL! For example, in the hydrochloric acid molecule, HCl, the shared electron pair that form the single bond between H and Cl is more strongly pulled by the Cl atom than the H atom, resulting in a polar molecule. Cl has a higher electronegativity value than H. This molecule could be described as having a dipole; the Cl is partially more negative than the H atom. The Greek letter, delta,, is used to represent a partial charge on a molecule. It has no fixed value.
In the last slide the polarity of the bond between H and Cl in hydrochloric acid was illustrated with the delta,, sign. This figure here is an example of another way to illustrate when bonds are polar. The vectors are pointing at oxygen to indicate that the electrons are more attracted to oxygen since oxygen has a higher electronegativity value due to its stronger nucleus.
How will you know if a bond is polar or nonpolar? 1) Make sure the bonding atoms are nonmetals (unless it is Be, which is considered to covalently bond). 2) Look at the difference in electronegativity values between the atoms. If there is a difference, then the bond is polar. The only bonds that are nonpolar are bonds between the same atoms, since clearly the difference in electronegativity is zero. Nonpolar bonds are often called pure covalent to express this. All other bonds have some degree of polarity, although it may be very slight. For example, the C-H bond, such as in methane CH₄, is often considered to be largely non-polar, although in fact the carbon atom is slightly more electronegative than hydrogen.
This is an example of a Paper 1 questions from last year s exam. Recall that in Paper 1 you will not have the IB data booklet, and will have to rely on what you know about electronegativity trends and in general about the polarity of bonds.
Answer: B At first you might think that the C=O bond is the least polar due to their close proximity in the periodic table. You have to remember what was written on the last slides: C-H bonds are often considered nonpolar since they have such a small difference in electronegativity, although Carbon does have a slightly higher electronegativity.
The presence of polar bonds in a molecule has a significant effect on the molecule s properties. For instance, since the bond is polar, its acting a lot like an ionic bond- so the more polar the bond, the more like an ionic compound the molecule behaves.
POP Quiz 5th Period - 10 minutes 1. Draw the Lewis dot structure of sulfur dioxide, SO₂, and state the types of bonds present in the molecule. (3 marks) 2. Based on electronegativity values, which bond is the most polar? A. B C B. C O C. N O D. O F (1 mark) 3. When C₂H₄, C₂H₂ and C₂H₆ are arranged in order of increasing C C bond length, what is the correct order? A. C₂H₆, C₂H₂, C₂H₄ B. C₂H₄, C₂H₂, C₂H₆ C. C₂H₂, C₂H₄, C₂H₆ D. C₂H₄, C₂H₆, C₂H₂ (1 mark)
POP Quiz 5th Period 1. 1 coordinate bond and 1 double bond. Important: Coordinate bonds are considered to be no different from other covalent bonds. So you will often see them illustrated just like a single covalent bond instead of an arrow. 2. B. C O because C and O are the only ones NOT right next to each other. 3. C. C₂H₂, C₂H₄, C₂H₆
POP Quiz 7th Period - Pop Quiz 1. Draw the Lewis dot structure of sulfur dioxide, SO₂, and state the types of bonds present in the molecule. (3 marks) 2. Based on electronegativity values, which bond is the most polar? A. B C B. O F C. N O D. C O (1 mark) 3. When C₂H₄, C₂H₂ and C₂H₆ are arranged in order of increasing C C bond length, what is the correct order? A. C₂H₆, C₂H₂, C₂H₄ B. C₂H₂, C₂H₄, C₂H₆ C. C₂H₄, C₂H₂, C₂H₆ D. C₂H₄, C₂H₆, C₂H₂ (1 mark)
POP Quiz 7th Period 1. 1 coordinate bond and 1 double bond. Important: Coordinate bonds are considered to be no different from other covalent bonds. So you will often see them illustrated just like a single covalent bond instead of an arrow. 2. D. C O because C and O are the only ones NOT right next to each other. 3. B. C₂H₂, C₂H₄, C₂H₆