Weak Base – Strong Acid Titration Curves

The titration of a weak base (NH3) with a strong acid (HCl) is shown below. Note that the curve has the same shape as the weak acid-strong base curve, but it is inverted. Thus, the regions of the curve have the same features, but the pH decreases throughout the process:

Curve for a weak base-strong acid titration. Titrating 40.00 mL of 0.1000 M NH3 with a solution of 0.1000 M HCl leads to a curve whose shape is the same as that of the weak acid-strong base curve,

but inverted. The midpoint of the buffer region occurs when [NH3] = [NH4+].

Methyl red is a suitable indicator here.

1.  The initial solution is that of a weak base, so the pH starts out above 7.00.

2.  The pH decreases gradually in the buffer region, where significant amounts of base (NH3) and conjugate acid (NH4+) are present. At the midpoint of the buffer region, the pH equals the pKa of the ammonium ion.

3.  After the buffer region, the curve drops vertically to the equivalence point, at which all the NH3 has reacted and the solution contains only NH4+ and Cl-. Note that the pH at the equivalence point is below 7.00 because Cl- does not react with water and NH4+ is acidic:

                                 NH4+(aq) + H2O(l) ⇌  NH3(aq) + H3O+(aq)

4. Beyond the equivalence point, the pH decreases slowly as excess H3O+ is added.

For this titration also, we must be more careful in choosing the indicator than for a strong acid-strong base titration. Phenolphthalein changes colour too soon and too slowly to indicate the equivalence point; but methyl red lies on the steep portion of the curve and straddles the equivalence point, so it is a perfect choice.

Why Warm Water Freezes Faster Than Cold

Nearly 50 years ago, Erasto B. Mpemba and Denis G. Osborne reported that if samples of water at 90 °C and 25 °C are cooled, the one starting at 90 °C begins freezing first. Many explanations for the “Mpemba effect” have been proposed, including ones based on evaporation, temperature gradients, impurities, and dissolved gases.

In warm water, weak hydrogen bonds break (top, red squiggles), leaving fragments

that easily reorganize into an ice lattice (bottom), a new study says.

A new computational study suggests that the effect arises from the liquid’s hydrogen bond network (J. Chem. Theory Comput. 2016, DOI: 10.1021/acs.jctc.6b00735). Southern Methodist University’s Dieter Cremer and colleagues investigated clusters of 50 and 1,000 water molecules, characterizing the types and strengths of the clusters’ 350 and more than 1 million hydrogen bonds, respectively. In (H2O)1,000 , raising the temperature from 10 °C to 90 °C led to fewer hydrogen bonds, as weaker, predominately electrostatic bonds broke.

That left behind cluster fragments with strong hydrogen bonds with more covalent character and proportionately more “dangling” or terminal hydrogen bonds. That hydrogen bond combination enables the fragments to easily reorganize and form the hexagonal lattice of ice.

Apart from learning what the name of this effect is and why it occurs, you can now answer two unit 1 past paper questions with the knowledge that:

1. Hydrogen bonds are largely electrostatic in nature.

2. Each water molecule forms four hydrogen bonds (from diagram).