The Effect of pH on Acid Dissociation: Exploring Le Chatelier's Principle

In summary, the acid is most dissociated at a HIGHER pH than the lower pH, which to me seems strange intuitively! any good explanation of this?
  • #1
Kqwert
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Homework Statement
Calculate the pH for 1M HCN dissolved in water, and for 0.1M HCN dissolved in water. Is the acid most dissociated at high or at low pH?
Relevant Equations
.
So I calculated the pH for the two concentrations: pH=4.61 for the high concentration and pH=5.61 for the low concentration.

Regarding the dissociation: in the solution manual it is given that the acid is most dissociated at the higher pH than the lower pH, which to me seems strange intuitively! any good explanation of this?
 
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  • #2
Write out the equation for the dissociation of HCN then consider Le Chatelier's principle.
 
  • #3
Second question seems a bit unclear. Simple response, pH less than 7 if HCN is found dissolved in water, and no other salts added.

HCN + H2O⇔H3O+ + CN-

Ka is 4.9*10^-10 .

For the 1 M HCN part of the problem,
x, H+1 molarity(approximate) as well as CN-1 molarity
1-x, HCN molarity(x^2)/(1-x)=4.9*10-10Try to work with that.

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EDIT: small edit for subscript
 
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  • #4
I suppose what the statement refers to is the fact that ratio [itex]\frac{[A^-]}{[HA]}[/itex] goes up with pH.
 
  • #5
Yeah, that's correct @Borek. So we basically get more of the products at higher pH? I just find it weird how the acid is most dissociated at a HIGHER pH, when the concentration of [H+] will be higher at the lowest pH?
 
  • #6
High concentration of H+ doesn't have to come from the dissociation of the acid in question, imagine putting HCN in a solution of HCl - low pH, and HCN dissociation shifted way to the left (even more than usually, it is a very weak acid).
 
  • #7
Is it always like that? The higher the pH, the more of an acid will be dissolved? How can this be seen in light of Le Chatelier's principle? (as pointed out by @Yggdrasil)
 
  • #8
Anyone..?
 
  • #9
Kqwert said:
Is it always like that? The higher the pH, the more of an acid will be dissolved? How can this be seen in light of Le Chatelier's principle? (as pointed out by @Yggdrasil)
Try writing out the chemical equation for the dissociation of a weak acid and applying Le Chatelier's principle to see what happens as you increase or decrease the pH.
 
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  • #10
Ok, so if we have the following reaction:

HA <--> H+ + A-

Then if we decrease the pH - concentration of [H+] will increase, reaction will be shifted towards the left => less products.

If we increase the pH - concentration of [H+] will decrease, reaction will be shifted towards the right => less reactants.

Is this the correct way of explaining it?
 
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  • #11
Yes, or that seems to be the correct use of the concept.
 
  • #12
Kqwert said:
Is it always like that? The higher the pH, the more of an acid will be dissolved? How can this be seen in light of Le Chatelier's principle? (as pointed out by @Yggdrasil)
Kqwert said:
Anyone..?

You ask for answers but you also need to give some. E.g. the answers to #3 and what are your results you claim in #1 and how did you get them?

In your question you seem to be confusing "disassociation" with "dissolving". Two different things, each with a clear meaning (but you wouldn't be the first).

Confusingly maybe there is for, as it were secondary reasons a connection. Many acids are less soluble than the corresponding salts. Because ionised i.e. charged species interact with water better than the corresponding uncharged one. More than once I have seen a beginner or a technician think they are going to dissolve some organic or other acid and then bring it to pH to make a buffer. They tip the acid ( powder or crystals) into a beaker with water and leave it stirring. After an hour or a day they say " it won't dissolve!". They need to start adding alkali for that to happen and to get a buffer of the concentration required!
 
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1. What is dissociation of an acid?

The dissociation of an acid refers to the process in which an acid breaks apart into its constituent ions when dissolved in water. This results in the production of positively charged hydrogen ions (H+) and negatively charged ions of the acid's conjugate base.

2. How does the strength of an acid affect its dissociation?

The strength of an acid directly affects its dissociation. Strong acids, such as hydrochloric acid (HCl), completely dissociate in water, producing a high concentration of hydrogen ions. Weak acids, on the other hand, only partially dissociate, resulting in a lower concentration of hydrogen ions.

3. What factors influence the rate of dissociation of an acid?

The rate of dissociation of an acid is influenced by various factors, including temperature, concentration of the acid, and the presence of catalysts. Higher temperatures and concentrations generally result in a faster dissociation, while catalysts can increase the rate of dissociation by lowering the activation energy.

4. How is the dissociation of an acid measured?

The dissociation of an acid is measured by its dissociation constant, also known as its acid dissociation constant (Ka). This value represents the equilibrium constant for the dissociation reaction and is a measure of the strength of the acid. The higher the Ka value, the stronger the acid.

5. What is the significance of dissociation of acids in everyday life?

Dissociation of acids plays a crucial role in various everyday processes, such as digestion, cleaning, and industrial processes. In digestion, stomach acid (hydrochloric acid) dissociates to help break down food. In cleaning, acidic substances are used to dissolve dirt and grime. In industrial processes, acids are used in the production of various products, such as fertilizers and pharmaceuticals.

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