CH 101, Science and Society Spring 2019

Chapter 8: water properties review

1. Explain why in water hydrogen bonds are intermolecular bonds, whereas covalent bonds are intramolecular bonds.

Hydrogen bonds are formed between water molecules, whereas covalent bonds are formed within individual water molecules.

2. Explain how a hydrogen bond is arranged: what atoms are required, what bonds are required, and why is the bond made.

Hydrogen bonds require a polar covalent bond between a highly electrogenative atom (such as oxygen, fluorine, or nitrogen) and hydrogen. The hydrogen in such a bond is partially positively charged. The hydrogen is the bond donor. In addition, another O, N, or F is required to accept the bond, and because the O, N, or F is strongly electronegative, it would carry a partial negative charge. The positive and negative partial charges attract each other, and this attractive force is a hydrogen bond.

3. Indicate the polarity, if any, of the following bonds. Include the negative and positive partial charges. Consult the electronegativity table as needed. If a bond has an electronegativity difference of less than 0.5, it is considered non-polar.

The arrow points to the negative end of the bond. ← ← non-polar → non-polar → O—H F—N Si—H P—Cl S—H C—N

4. Would the following aqueous solutions conduct electricity, i.e., include electrolytes?

Sucrose C12H24O12 no, this is a molecular compound (made of non-metals)

Table salt NaCl yes, this is a soluble ionic compound (made of a metal and a non- metal)

Ethanol C2H5O no, molecular compound

Baking soda NaHCO3 yes, ionic compound

Aluminum chloride AlCl3 yes, ionic compound

Ammonium nitrate NH4NO3 yes, ionic compound (made of ammonium and nitrate, two polyatomic ions)

Benzene C6H6 no, molecular compound

5. What ions are present in hard water? What problems do they cause? What benefits do they provide?

Ca2+ and Mg2+ are hard water ions. Problems include precipitation of compounds containing the ions and plugging of water lines. Benefits include nutritional value and in some applications favorable taste.

6. How many acid or base equivalents do the following compounds produce when dissolved completely in water?

Nitric acid (HNO3) one Carbonic acid (H2CO3) two

Hydrobromic acid (HBr) one Hydrofluoric acid (HF) one

Sodium hydroxide (NaOH) one Ammonia (NH3) one (base, + - because NH3 + H2O → NH4 + OH )

7. Determine the pH of the following solutions of strong acids or bases.

0.01 M HCl 10 mM HBr -log(0.01)=2 10 mM x (1M/1000mM) = 0.01M -log(0.01) = 2

1.3 M HNO3 0.5 M NaOH -log(1.3)=-0.11 pOH=-log(0.5)=0.3 pH=14-pOH=13.7

0.57 mM KOH 1.3 M NH4OH pOH=-log (0.57)=0.24 pOH=-log(1.3)=-0.11 pH=14-pOH=13.76 pH=14-pOH=14.11