Chapter 15 Exercises – Answers not in your text 18.) Rank MgO, NH3, PH3, and KCl in order of increasing boiling point Lowest Highest NH3 KCl MgO b. p. b. p. PH3 In ionic compounds (KCl, MgO), boiling involves breaking strong ionic bonds, so extremely high temperatures are required. The boiling point is highest for MgO because it has Mg2+ and O2‐ ions, which are more strongly attracted than the K+ and Cl‐ ions in KCl. The two molecular compounds, PH3 and NH3, have lower boiling points, because boiling only involves disrupting intermolecular forces, which are significantly weaker than ionic (or covalent) bonds. Between PH3 and NH3, intermolecular forces are weaker in PH3, which is less polar and cannot undergo hydrogen bonding; as a result, this substance boils at the lowest temperature. 30.) Although it is only slightly polar, ethanol (CH3CH2OH) is completely miscible in water – meaning that a homogeneous solution is formed – because ethanol and water can undergo hydrogen bonding, both with themselves and with each other. Hydrogen bonding of ethanol molecules with water molecules permits creation of a solution with uniform composition. 60.) The observation that H2S is a gas at ‐10 °C, while H2O is a solid results from the differing strengths of intermolecular forces operating in these substances. H2O is more polar than H2S – resulting in stronger dipole‐dipole forces – and it can engage in hydrogen bonding, whereas H2S cannot. As a result of its much stronger intermolecular forces, much higher temperatures are required to put H2O into the gas phase (i.e., 100 °C or above for H2O versus only ‐10 °C for H2S).