Topic 4: Chemical Bonds IB Chemistry SL Ms. Kiely Coral Gables Senior High
Bell-Ringer Draw an example of each type of intermolecular force using the following molecules: TOPIC 4 TEST NEXT CLASS MONDAY 19th! 1) chlorine, Cl₂ molecules 2) hydrogen fluoride, HF molecules 3) water, H₂O molecules HW: Study and READ pgs. 148-159, especially because we will not cover Metallic Bonds in class! Paper 2 review questions available on website
Intermolecular forces There are three major types of intermolecular forces. The type of force present in covalent matter will depend on the polarity and the size of the molecules involved. Nonpolar molecules are held together by London dispersion forces. Polar molecules are held together by dipole-dipole forces. Any molecules that contain hydrogen covalently bonded to fluorine, nitrogen, or oxygen will be held together by hydrogen bonds. London dispersion forces and dipole-dipole forces are collectively called Van der Waals forces.
London dispersion forces are forces of attraction between nonpolar molecules caused by a dance that electrons do inside of molecules! For instance, despite each chlorine atom in Cl₂ sharing their electrons equally, electrons are still very mobile particles; one moment they might be shifted to one side of a molecule and the next moment they have shifted to the other side. When this occurs, a weak dipole known as a temporary or spontaneous dipole is formed. This will not last for more than a moment, but it can still affect a neighboring atom or molecule.
London dispersion forces are the weakest of the intermolecular forces. Their strength increases with increasing molecular size; this is because larger and larger molecules have more and more electrons, which increases the probability of temporary dipoles developing. London dispersion forces are the only forces that exist between nonpolar molecules; however since all atoms have electrons, when asked to name the forces between molecules one should always mention London dispersion forces despite the other forces that are also present.
Dipole-dipole attractions are forces of attraction between polar molecules. Polar molecules have permanent dipoles due to the large electronegativity differences between the atoms in the molecule. The negative side of one molecule will be attracted to the positive side of another molecule and so on. Note the dipole-dipole interactions can occur between any combination of polar molecules. Dipole-dipole attractions are stronger than dispersion forces since the dipoles of polar molecules are permanent, while the dipoles of nonpolar molecules are temporary.
Hydrogen bonding only occurs between molecules in which a hydrogen atom is bonded directly to fluorine, nitrogen, or oxygen. Fluorine, nitrogen, and oxygen are highly electronegative, and they also have lone pairs of electrons on them that can be shared via a coordinate bond to a hydrogen atom on another molecule. They are the strongest of the intermolecular forces
Properties of molecules due to Intermolecular forces The physical properties of covalent compounds are largely a result of their intermolecular forces. -For two substances of similar molecular mass, the more polar substance will have the higher boiling point. Can you guess why? -While examining the boiling points of different substances to see the effect of different intermolecular forces, it is important to compare substances with similar molecular mass. Otherwise the difference in boiling point could be attributed solely to stronger London dispersion forces in the larger molecules that have more electrons. Can you guess why?
Properties of molecules due to Intermolecular forces Generally, covalent compounds have lower melting/boiling points than ionic compounds. The forces to be overcome to separate the molecules are relatively weak intermolecular forces, which are significantly easier to break than that of an ionic lattice. This is why many covalent substances are liquids or gases at room temperature. Make sure that you realize that when a covalent substance melts or boils (changes state), it is the bond between the molecules that break, not the covalent bonds within the molecules.
Covalent molecules are not good conductors of electricity; however, some polar covalent molecules, in conditions where they can ionize, will conduct electricity. For example, HCl, hydrogen chloride, is a covalent molecule. However, when you put hydrogen chloride in water it becomes aqueous hydrochloric acid HCl(aq). Hydrochloric acid, HCl(aq) is actually ionic and can therefore conduct electricity when properly dissolved.
Review
ANSWER
1. This question is about Period 3 elements and their compounds. (a) Explain, in terms of their structure and bonding, why the element sulfur is a non-conductor of electricity and aluminium is a good conductor of electricity. [4 marks] (b) Explain, in terms of its structure and bonding, why silicon dioxide, SiO2, has a high melting point. [2 marks]
ANSWER 1. (a) sulfur is (simple) molecular; (contains) covalent bonds/no delocalized electrons/all (outer) electrons used in bonding; While aluminium contains positive ions and delocalized electrons; (delocalized) electrons move (when voltage applied or current flows); [4 marks] (b) silicon dioxide is macromolecular/giant covalent, therefore it has many/strong covalent bonds that must be broken; [2 marks] Award max 1 mark if no mention of covalent. Do not accept weakened instead of broken.
2. The boiling points of the hydrides of group 16 elements increase in the order H₂S, H₂Se, H₂Te, and H₂O. Explain the trend in the boiling points in terms of bonding. [3 marks]
ANSWER 2. for H₂S, H₂Se, and H₂Te, as their size/mass/molar mass (Mr) increases, so do the amount of van der Waals forces holding them together, causing their boiling points to increase, respectively. Meaning, the largest one has the highest boiling point due to having the most van der Waals forces that must be broken. H₂O contains Hydrogen bonding, meaning it requires even more energy to break since H-bonds are the strongest of the intermolecular forces. H bonding is stronger than van der Waals. [3 marks]
3. Identify the strongest type of intermolecular force in each of the following compounds. CH₃Cl CH₄ CH₃OH [3 marks]
ANSWER 3. CH₃Cl: polar molecule = dipole-dipole attractions CH₄: nonpolar molecule = dispersion/ London dispersion forces/van der Waals CH₃OH: O-H bond present = hydrogen bonding [3 marks]
4. An important compound of nitrogen is ammonia, NH3. The chemistry of ammonia is influenced by its polarity and its ability to form hydrogen bonds. Polarity can be explained in terms of electronegativity. (i) Explain the term electronegativity. [2 marks] (ii) Draw a diagram to show hydrogen bonding between two molecules of NH3. The diagram should include any dipoles and/or lone pairs of electrons [3 marks] (iii) State the H N H bond angle in an ammonia molecule. (iv) Explain why the ammonia molecule is polar. [1 mark] [1 mark]
ANSWER (i) the measure of an atom s attraction for bonded electrons/ for electrons in a bond; [2 marks] (ii) [3 marks] (iii) accept anywhere between 107-109 degrees (iv) molecule is asymmetrical [1 mark] [1 mark]