Question
If a weak acid is say only 5% ionized at equilibrium, then the ionization rxn would be reactant favored, correct? I am a little on the fence about understanding this if anyone could better explain why this would be? I am assuming because it is not completely ionized, the amount of products would be less than the amount of reactants and so it would be reactant favored.
Answers
DrBob222
Here is the way I do it.
1. Write the reaction. Let's use acetic acid, CH3COOH (which I will call HAc), as an example.
HAc ==> H^+ + Ac^-
2. Write the Ka expression. For a weak base write the Kb expression. For an equilibrium reaction, write Keq. They all work the same.
Ka = 1.8 x 10^-5 = (H^+)(Ac^-)/(HAc).
3. Analyze.
It's as simple as this. Ka is small; therefore, the numerator of the fraction must be smaller than the denominator. Which means, of course, that the ions are less than the un-ionized material and that means the products are not favored (the reactants are favored). I look at it that the forward reaction is not favored; the reverse reaction is favored. (Another way I find useful is to say, "not many ions, a lot of un-ionized acid")
4. Students always want to know what I consider small; i.e., what Ka, Kb, Keq, is small and what is large. For a 1:1 set of coefficients, a Ka, Kb, Keq of 1 means numerator of fraction = denominator which means the reaction is 50%. So I use 1 as a benchmark although reactions that are not 1:1 vary slightly but as a quick assessment, K of 1 is a good number to choose.
1. Write the reaction. Let's use acetic acid, CH3COOH (which I will call HAc), as an example.
HAc ==> H^+ + Ac^-
2. Write the Ka expression. For a weak base write the Kb expression. For an equilibrium reaction, write Keq. They all work the same.
Ka = 1.8 x 10^-5 = (H^+)(Ac^-)/(HAc).
3. Analyze.
It's as simple as this. Ka is small; therefore, the numerator of the fraction must be smaller than the denominator. Which means, of course, that the ions are less than the un-ionized material and that means the products are not favored (the reactants are favored). I look at it that the forward reaction is not favored; the reverse reaction is favored. (Another way I find useful is to say, "not many ions, a lot of un-ionized acid")
4. Students always want to know what I consider small; i.e., what Ka, Kb, Keq, is small and what is large. For a 1:1 set of coefficients, a Ka, Kb, Keq of 1 means numerator of fraction = denominator which means the reaction is 50%. So I use 1 as a benchmark although reactions that are not 1:1 vary slightly but as a quick assessment, K of 1 is a good number to choose.