Question
For each of the following, use appropriate chemical principles to explain the observation.
(a) Sodium chloride may be spread on an icy sidewalk in order to melt the ice; equimolar amounts of calcium chloride are even more effective.
(b) At room temperature, NH3 is a gas and H2O is a liquid, even though NH3 has a molar mass of 17 grams and H2O has a molar mass of 18 grams.
(c) C (graphite) is used as a lubricant, whereas C (diamond) is used as an abrasive.
(d) Pouring vinegar onto the white residue inside a kettle used for boiling water results in a fizzing/bubbling phenomenon.
▶️Answer/Explanation
Ans:
(a) The addition of a solute lowers the freezing point of water.
A mole of NaCl contains (dissociates into) 2 moles of ions/particles, whereas a mole of CaCl2 contains (dissociates into) 3 moles of ions. Therefore CaCl2 is more effective.
(b) Hydrogen bonding is the most important intermolecular attractive force between molecules of H2O and between molecules of NH3.
Water is a liquid because the hydrogen-bonding forces are stronger between adjacent H2O molecules than between adjacent NH3 molecules.
Further explanations for the stronger hydrogen bonding in H2O include the larger dipole moment (or more polar character) of H2O compared to NH3 and the fact that 0 is more electronegative than N is.
(c) Graphite’s structure consists of 2-dimensional sheets of covalently bonded carbon atoms. The attractive forces between sheets (layers) are weak London (dispersion) forces, which allow the sheets to slide easily over one another.
Note: must indicate layers and sliding to earn point.
Diamond consists of an extended 3-dimensional covalent network of carbon atoms. This makes diamond a very hard substance.
(d) Vinegar, a dilute solution of acteic acid, reacts with the white solid, which contains metal carbonates, in a neutralization reaction to form gaseous CO2.