IB DP Chemistry: HL复习笔记18.1.2 pH Curves-翰林国际教育

IB DP Chemistry: HL复习笔记18.1.2 pH Curves

During a titration, a pH meter can be used and a pH curve plotted
A pH curve is a graph showing how the pH of a solution changes as the acid (or base) is added

The features of a pH curve

All pH curves show an s-shape curve
pH curves yield useful information about how the acid and alkali react together with stoichiometric information
The midpoint of the inflection is called the equivalence or stoichiometric point
From the curves you can:
- Determine the pH of the acid by looking where the curve starts on the y-axis
- Find the pH at the equivalence point
- Find volume of base at the equivalence point
- Obtain the range of pH at the vertical section of the curve

There are four combinations of acids and alkalis that you should know about:
- strong acid + strong base
- weak acid + strong base
- weak base + strong acid
- weak acid + weak base

In this example, sodium hydroxide, NaOH (aq), is being added to hydrochloric acid, HCl (aq)

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

The pH intercept on the y axis starts at a low pH, roughly 1, due to the relative strength of the hydrochloric acid
As the NaOH (aq) is added, there is a gradual rise in pH until the titration approaches the equivalence point
In this case, the pH at equivalence is 7
- The equivalence point is in the middle of the vertical section of the pH curve
Once all of the acid has been neutralised, the curve flattens out and continues to rise gradually
At the end of the titration, the pH will be high due to the relative strength of the sodium hydroxide

18.1-Strong-acid-strong-base-1

Strong acid - strong base pH curve

In this example, strong sodium hydroxide, NaOH (aq), is being added to weak ethanoic acid, CH₃COOH (aq)

NaOH (aq) + CH₃COOH (aq) → CH₃COONa (aq) + H₂O (l)

The pH on the intercept on the y axis starts at roughly 3 due to the relative strength of the ethanoic acid
The initial rise in pH is steep as the neutralisation of the weak acid by the strong base is rapid
Ethanoate ions (conjugate base to ethanoic acid) are formed which then creates a buffer
- A buffer consists of a weak acid and its conjugate base or a weak base and its conjugate acid
At this point, the buffer formed will resist changes in pH so the pH rises gradually as shown in the buffer region
The half equivalence point is the stage of the titration at which exactly half the amount of weak acid has been neutralised
- [CH₃COOH (aq)] = [CH₃COO^- (aq)]
- At this point, it is important to note that the pK_a of the acid is equal to the pH
  - pK_a = pH at half equivalence
The equivalence point in a weak acid - strong base titration is above 7

18.1-Weak-acid-strong-base-1

Weak acid - strong base pH curve

In this example, strong hydrochloric acid, HCl (aq), is being added to weak ammonia, NH₃ (aq)

NH₃ (aq) + HCl (aq) → NH₄Cl (aq)

The pH on the intercept on the y axis starts at roughly 11 due to the relative strength of the ammonia
The pH will fall as the ammonia begins to be neutralised and the conjugate acid, NH₄⁺ (aq), is produced
This again creates a buffer region so the pH will only fall gradually
The half equivalence point is the stage of the titration at which exactly half the amount of weak base has been neutralised
- [NH₃ (aq)] = [NH₄⁺ (aq)]
- At this point it is important to note that the pK_b of the base is equal to the pOH
  - pK_b = pOH at half equivalence
The pH at equivalence for a weak base-strong acid is below 7

18.1-Weak-base-strong-acid

Weak base - strong acid pH curve

In this example, weak ethanoic acid, CH₃COOH (aq), is being added to weak ammonia, NH₃ (aq)

NH₃ (aq) + CH₃COOH (aq)→ CH₃COONH₄ (aq)

The starting pH of roughly 11 for the weak base will fall as it begins to neutralise
The change in pH for this titration is very gradual
- Note the that the vertical section of this pH curve is not steep as with other three so the equivalence point is difficult to determine
- Therefore this titration is not performed
The pH at equivalence for a weak acid -weak base is roughly 7 but is difficult to determine

18.1-Weak-acid-weak-base