28. Explain, in terms of equilibrium shifts, how this buffer mixture resists changes in pH when small quantities of both a strong acid and a strong base are added.
The buffer mixture was made up of:
monosodium phosphate(NaH2PO4) and disodium phosphate (Na2HPO4)
H2PO4^- is the acid
HPO4^2- is the base.
In strong acid is added you have
HPO4^2- + H^+ ==> H2PO4^- ; i.e,, when a strong acid is added to the buffer mixture the weak base gobbles up the acid so the pH doesn't change much.
In strong base the rxn is
H2PO4^- + OH^- ==> HPO4^2- + H2O; i.e., the weak acid gobbles up the strong bsse in a netralization reaction and form water and the pH doesn't change much.
To understand how this buffer mixture resists changes in pH, let's first look at how buffers work in general. A buffer is a solution that can maintain a relatively stable pH even when small amounts of acid or base are added to it. Buffers are composed of a weak acid and its conjugate base (or a weak base and its conjugate acid).
In the case of the buffer mixture you mentioned, the weak acid is monosodium phosphate (NaH2PO4), and the conjugate base is disodium phosphate (Na2HPO4). When these two components are combined, they form a buffer system.
Now, let's consider what happens when a small quantity of a strong acid is added to the buffer. The strong acid will dissociate completely in water, producing hydrogen ions (H+). These additional H+ ions will react with the conjugate base (Na2HPO4) in the buffer system, forming the weak acid (H2PO4-) according to the equation:
H+ + Na2HPO4 -> H2PO4- + 2Na+
This reaction consumes the additional H+ ions, preventing the pH from decreasing significantly. The weak acid produced is able to maintain the pH of the buffer solution because it only partially dissociates.
Similarly, if a small quantity of a strong base is added to the buffer, it will react with the weak acid (NaH2PO4) in the buffer system, forming the conjugate base (HPO42-) according to the equation:
OH- + NaH2PO4 -> HPO42- + Na+ + H2O
This reaction consumes the additional OH- ions, preventing the pH from increasing significantly. The conjugate base formed is able to maintain the pH because it has the ability to react with any additional acid that may be introduced.
In both cases, the buffer mixture effectively resists changes in pH by utilizing the equilibrium shifts between the weak acid and its conjugate base. These equilibrium reactions occur in opposite directions to counteract the addition of acid or base, keeping the pH relatively stable.