PHcatalyzed reaction, NH3 + HOCl --> NH2Cl + H20 , how do I plot log k_obs vs pH?
NH4 + HOCl --> NH2Cl + H20
rate expression : d(NH2Cl) / dt = k_obs (C_T,N)(C_T,OCL)
assume (C_T,N) and (C_T,OCL) concentrations are constant
pKa for NH4+ = 9.3
pKa for HOCl = 7.5
any and all help much appreciated
To plot log k_obs vs pH for the given reaction NH4+ + HOCl -> NH2Cl + H2O, you need to understand how pH affects the rate constant (k_obs) based on the rate expression and the pKa values provided.
To explain the process step by step:
1. Understand the significance of pH in the reaction:
In this reaction, NH4+ (ammonium ion) and HOCl (hypochlorous acid) are reactants. pH has an effect on the ionization of these species, specifically the NH4+ and HOCl. pH affects the concentration of NH4+ and HOCl species, which in turn affects the reaction rate and the rate constant (k_obs).
2. Calculate the concentration of NH4+ and HOCl:
Since the concentrations of (C_T, N) and (C_T, OCL) are assumed constant, you need to determine the concentrations of NH4+ and HOCl at different pH values.
a) NH4+ concentration: NH4+ is a weak acid, so you can use the Henderson-Hasselbalch equation to calculate its concentration at different pH values:
[NH4+] = [NH3] / (1 + 10^(pH - pKa_NH4))
where [NH3] is the concentration of ammonia (NH3) and pKa_NH4 is the pKa value for NH4+ (given as 9.3).
b) HOCl concentration: HOCl is a weak acid as well, so you can also use the Henderson-Hasselbalch equation to calculate its concentration at different pH values:
[HOCl] = [OCl-] / (1 + 10^(pKa_HOCl - pH))
where [OCl-] is the concentration of hypochlorite ion (OCl-) and pKa_HOCl is the pKa value for HOCl (given as 7.5).
3. Calculate the rate constant (k_obs):
The rate expression for the reaction is given as:
d(NH2Cl) / dt = k_obs (C_T,N)(C_T,OCL)
Since (C_T,N) and (C_T,OCL) are assumed constant, you can consider them as a constant factor, say K, thus simplifying the equation:
d(NH2Cl) / dt = k_obs K
The rate constant (k_obs) does not depend on [NH4+] and [HOCl] directly. Therefore, to plot log k_obs vs pH, calculate the rate constant (k_obs) at each pH value while keeping the concentrations of NH4+ and HOCl constant.
4. Choose a range of pH values:
Decide on a range of pH values you want to analyze and select a number of pH values within that range.
5. Calculate the rate constant (k_obs) at each pH:
For each chosen pH value, calculate the concentrations of NH4+ and HOCl using the Henderson-Hasselbalch equations from step 2. Then, substitute these concentrations into the rate expression to find the rate constant (k_obs) at each pH value.
6. Plot log k_obs vs pH:
Once you have calculated the rate constant (k_obs) for each pH value, plot log k_obs on the y-axis and pH on the x-axis to obtain the desired graph.
Note: It is important to keep in mind that this explanation assumes the reaction follows first-order kinetics with respect to NH4+ and HOCl. If the reaction order is different, the rate expression and calculations would vary accordingly.