Question

1. a) Describe the difference in observed pH changes upon adding a small amount of strong acid or base to 25.0 mL of water vs to 25.0 mL of your buffer. b) Explain why there was a difference.

buffer is HC2H3O2/NaC2H3O2

difference in PH: big change when added to buffer. small change when added to water. why?

why such a bug change when you add an acud or base to water but not to butfer?

Answer 1

In case of water:

If we add a strong acid or strong base to water, the pH will change dramatically. For instance, adding a strong acid such as HCl to water results in the reaction HCl + H2O → H3O+ + Cl-. In other words, the proton (H+) from the acid binds to neutral water molecules to form H3O+ raising the concentration of H+. The resulting large concentration of (H+) makes the solution more acidic and leads to a dramatic drop in the pH.

Solid NaOH consists of Na+ and OH- ions packed into a crystalline lattice. When this solid is added to water, the ions float apart leading to extra OH- ions in the water: NaOH → OH- + Na+. The resulting large concentration of OH- makes the solution more basic and leads to a dramatic increase in the pH. (Remember that since the product of concentrations, [OH-][H+], remains fixed at Kw=10-14, as the concentration of OH- ions goes up, the concentration of H+ ions goes down.)

In case of buffer:

A buffer is a solution containing either a weak acid and its salt or a weak base and its salt, which is resistant to changes in pH.

If we mix a weak acid (HA) with its conjugate base (A-), both the acid and base components remain present in the solution. This is because they do not undergo any reactions that significantly alter their concentrations. The acid and conjugate base may react with one another, HA + A- → A- + HA, but when they do so, they simply trade places and the concentrations [HA] and [A-] do not change. In addition, HA and A- only rarely react with water. By definition, a weak acid is one that only rarely dissociates in water (that is, only rarely will the acid lose its proton H+ to water). Likewise, since the conjugate base A- is a weak base, it rarely steals a proton H+ from water.

So, the weak acid and weak base remain in the solution with high concentrations since they only rarely react with the water. However, they are very likely to react with any added strong base or strong acid.

If a strong base is added to a buffer, the weak acid will give up its H+ in order to transform the base (OH-) into water (H2O) and the conjugate base: HA + OH- → A- + H2O. Since the added OH- is consumed by this reaction, the pH will change only slightly.

If a strong acid is added to a buffer, the weak base will react with the H+ from the strong acid to form the weak acid HA: H+ + A- → HA. The H+ gets absorbed by the A- instead of reacting with water to form H3O+ (H+), so the pH changes only slightly.

That's why there is a bug change when you add an acid or base to water but not to butfer.