16.15
משוואת הנדרסון-האסלבאך
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Q1: How is the Henderson-Hasselbalch equation derived from the Ka expression?
The Henderson-Hasselbalch equation is derived by rearranging the equilibrium constant expression for Ka to solve for hydronium ion concentration. Taking the negative logarithm of both sides converts the expression into pH and pKa terms. This yields the equation: pH = pKa + log([base]/[acid]), which relates pH to the ionization constant and concentrations of the conjugate acid-base pair in a buffered solution.
Q2: When is it valid to use initial concentrations instead of equilibrium concentrations in the Henderson-Hasselbalch equation?
Initial concentrations can replace equilibrium concentrations when the change in hydronium ion concentration, x, is less than 5% of the initial concentrations of both the weak acid and conjugate base. This approximation is valid because the change is negligible, making the initial and equilibrium concentrations essentially equivalent for practical pH calculations.
Q3: What is the advantage of using the Henderson-Hasselbalch equation over an ICE table?
The Henderson-Hasselbalch equation is a quicker method to calculate buffer pH when a reaction involves a conjugate acid-base pair and the change in hydronium concentration is small. Unlike ICE tables, which require multiple algebraic steps, the Henderson-Hasselbalch equation directly incorporates the pKa and concentration ratio, streamlining calculations for buffer solutions.
Q4: How can the Henderson-Hasselbalch equation be used to prepare a buffer at a specific pH?
The Henderson-Hasselbalch equation shows the ratio of base to acid needed to achieve a desired pH. By rearranging the equation to solve for the ratio [base]/[acid], you can determine the exact proportions of conjugate base and weak acid required. This allows chemists to design buffers with precise pH values for specific applications.
Q5: How do you calculate pH for a buffer containing a weak base and its conjugate acid?
For a weak base and its conjugate acid, first convert the pKb to pKa using the relationship: pKa + pKb = 14. Then apply the Henderson-Hasselbalch equation using the calculated pKa and the concentrations of the conjugate acid and base. This allows pH calculation for basic buffers using the same logarithmic framework.
Q6: What does pKa represent in the Henderson-Hasselbalch equation?
The pKa is the negative logarithm of the acid dissociation constant (Ka). It represents the strength of a weak acid; lower pKa values indicate stronger acids. In the Henderson-Hasselbalch equation, pKa is the reference point around which buffer pH is calculated, determining the optimal pH range for a given acid-base pair.
Q7: How do you verify that the Henderson-Hasselbalch approximation is valid for a specific buffer?
After calculating pH using the Henderson-Hasselbalch equation, convert the result back to hydronium ion concentration using [H3O+] = 10^(-pH). Then verify that this concentration is less than 5% of the initial weak acid concentration. If this condition is met, the approximation is valid and the calculated pH is reliable for the buffer solution.