Question

INTERMOLECULAR FORCES INTRODUCTION LEARNING OBJECTIVES To relate the effecet of intermolecular forces on relative melting points and vapour pressures of some organic substances THEORY Weak attractive forces that occur between individual molecules in substances are called intermolecular forces. These include London dispersion forces, dipole-dipole forces and hydrogen-bonding forces. Collectively, the three types of intermolecular forces are referred to as Van der Waals forces. These attractive forces are much weaker than the chemical bonds between atoms in a molecule. Note that the hydrogen-bonding force has an unfortunate and confusing name, since this type of intermolecular force is not really a chemical bond. For a description of what gives rise to each type of intermolecular forces read section 11.3 of your text book (Chemistry- A Molecular Approach, 1st Canadian Edition, by Tro, Fridgen and Shaw In a molecular solid, molecules are locked in a rigid crystal structure by intermolecular forces. The molecules cannot move around, but still have kinetic energy because they vibrate. When such a solid is warmed, a temperature is eventually reached where the molecules have sufficient kinetic energy to partially overcome the intermolecular forces holding the solid together. at which noint the molecules hegin to flow nast. RAW DATA AND OBSERVATIONS PART A -EVAPORATION AND INTERMOLECULAR ATTRACTION Table 1 (3 marks) T2 (°C) 2.6 11.1 19.5 3.7 18.9 0.8 AT (OC) 21.2 14.3 T1 (°C) 23.8 25.4 23.6 23.7 23.2 24.0 Substance methanol ethanol 1-butanol 20.0 4.3 23.2 pentane octane acetone PART B - MELTING POINTS OF SOME ORGANIC SUBSTANCES Substance (circle one in each row) Table 2 Temperature at which melting starts (°C) Temperature at which melting is complete (°C) 121.6 79.3 2-Naphthol OR Benzophenone 118.3 Naphthalene OR Benzil 76.9 QUESTIONS PART A EVAPORATION AND INTERMOLECULAR ATTRACTION 1.Complete the following table: [5.1 marks] Molecular Substance Formula Dipole Hydrogen Dipole (Yes or (Yes or No Structural Formula Mass (g mol1) Bonding 0 methanol CH3OH ethanol C2H5OH 1-butanol C4H3OH pentane CsH12 octane C8H18 acetoneCH3COCHa Two of the liquids, pentane and 1-butanol, had nearly the same molar mass, but significantly different AT values. Explain this based on their intermolecular forces. [2 marks] 2. Which of the alcohols (methanol, ethanol and 1-butanol) studied has the strongest intermolecular forces of attraction? The weakest intermolecular forces? Explain using the results of this experiment. [3 marks] 3. 4.Which of the alkanes (pentane and octane) studied has the stronger intermolecular forces of attraction? Explain using the results of this experiment. [2 marks] Acetone and ethanol have similar molar masses but significantly different ΔΤ values. Explain this based on their intermolecular forces. [2 marks] 5. 6. Based on your experimental results, rank the six substances studied in terms of the relative strengths of their intermolecular forces: [3 marks] weakest IMFs strongest IMFs PART B - MELTING POINTS OF SOME ORGANIC SUBSTANCES 7. Circle the pair of substances of which you measured the melting point range 2-naphthol (A) and naphthalene (B) benzophenone (A) and benzil (B) Complete the following table: [6 marks] Hint: Molecules containing only Cand H can be assumed to be nonpolar. Present in Present in Intermolecular Force Stronger in which substance? Why? (yes or no) (yes or no) London Dispersion Dipole Dipole Hydrogen Bonding Which of the two substances that you measured has the higher melting point? Why? [3 marks] To answer this question, you must summarize all of the information in the table above. Consider all of the intermolecular forces that are present, and also not present, in each substance. Then compare their relative strengths in both substances. Finally, assess both substances as to the total strength of their intermolecular forces. Please use the chemical's real names when answering, not A and B

Answer 1

mal) 32,04 ma e e 0 12 Octane His Acetone CH3 -CH3 n