Predicting Acid and Base Equilibrium

The holy grail of acid-base chemistry is to be able to draw the mechanism and predict the equilibrium for an acid-base reaction. Believe it or not, you’re ready to do this. 

Concept: The 3 steps for determining the direction of acid and base equilibrium.  

6m
Video Transcript

So now we've got into its pretty much the Holy Grail of acid base chemistry and that is predicting acid-base equilibrium. So let's go ahead and get started on this. For these types of questions what you are going to be asked is, “What's the direction of the reaction? Is it going to go to the right, is if going to go the left?” I even know it sounds like, “Hey, I’ve got 50-50 shot. It can't be that bad.” They are surprisingly tricky. So we’re going to learn a set of rules and we're going to use pKa information in order to figure out these questions.
So basically here is, I’m just going to give you an example. Here's a common question that you can see in an exam. Would the following reaction go to the left or would it go to the right? Label all the species. Draw the correct arrows. There's a lot in there and there's a lot you have to understand. So let's go ahead and break it down one step at a time. The very first thing you always want to start off with is identifying the Lewis acid and the Lewis base. Remember that I said the Lewis is the most general definition of acids and bases. So that just means that you’re just going to find on the left hand side which one is the acid which one is the base. Now how do you do this? A lot of times that could be the trickiest part. Well, many times there’s going to be known charges or a known acid will present. So if there’s charges or a known acid that makes it easy.
So for example if you see a positive charge, that’s always going to be acid. And if you see a negative charge that's always going to be the base. So that part is easy. Also let's say that you have a carboxylic acid. If you have a carboxylic acid then that would make it easy as well or HCl or whatever.
But sometimes you're not going to have a negative charge and you’re not going to have a positive charge So if you don't, then you go to the second step. The second step is to make sure that all spectator ions
are dissociated. Do you guys remember spectator ions from Gen Chem? Those were just your first column cations. So that would be Lithium (positive), Potassium (positive) and Sodium (positive). Now some professors even go as far as to add Cesium. I have seen that before. Cesium is also in that first column. Alright, so these actually, I mean I know that I said that it was three but actually it could be four depending on how tricky your professor wants to get. These are cations that always associate in solutions to these basically make ionic bonds.
So for example if I had NaOH and I was trying to determine if that was my acid my base I would be confused possibly because I would say, “Well, I don't see a negative charge. I don't see a positive charge. What is it?” Well you have to disassociate it first. So that means you have to take this and make it Na+ and that means my OH becomes OH-. And now member that any positive is a spectator ion that I don't care about. So that means that now I have a base. Does that make sense? So if you don't see an obvious positive or negative you have to go ahead and start dissociating.
But now let’s say that you do both those steps and your compounds are both still neutral. So let’s say there's nothing to dissociate, there's no charges, have no clue what to do. So for example if you had something like, let me think a good example, so let's say that you had… so let's say that you had an H2O and you were comparing it to carboxylic acid and you didn't remember that carboxylic acid was an acid. Let’s just say you had a brain fart and forgot that carboxylic acid was an acid. Well then you can just go ahead and look at pKa’s then you could say—that’s basically the next rule. If they’re still neutral, assign the lowest pKa as the acid.
So then I would say the pKa of my water is 16. The pKa of my carboxylic acid is five. So that means that my carboxylic acid is going to be the acid and that means that my water is going to be the base. Does that make sense? So basically, if you are still neutral at the end just go with the lowest pKa. So that's going to really take care of you.
The next step is now that you know your acid and your base are, go ahead and label the conjugates. That means draw them out if you have to draw them. Or just label them based on which one is the acid, that one is the conjugate base, the base becomes the conjugate acid.
So then the last step is compare the acidity or pKa—basically acidity and pKa are same thing—of the Lewis acid to the conjugate acid. And remember that in order to go in that direction you're going to have to go from stronger to weaker. Now one way I like to say that is that the strongest acid is going to be the one at the lowest pKa. So I always say that the pKa should go from lower on the left to higher on the right pKa. Because remember pKa is the opposite of strength. So you want a low pKa to start with and you want a high pKa at the end.

STEP 1. Identify the acid and base

  • Many times charges or a known acid will be present (+) = Acid ( -) = Base
  • If you see no charges, dissociate ALL spectator ions, these are the cations Li+, Na+, K+  and Cs+
  • If both compounds are still neutral, assign the one with the lowest pKa as the acid.

STEP 2. Label the conjugate acid and base. Remember:

  • Acids LOSE their most acidic proton to become conjugate bases
  • Bases GAIN a proton to become conjugate acids

STEP 3. Compare the acidity/pKa of the acid to the conjugate acid. The equilibrium will favor the side with the weaker acid (arrow towards the highest pKa)

That’s all there is to it! Watch me solve the first example so you can get the hang of it. 

Example: Would the following reaction go to the right or left? Label ALL species. Draw arrows in the correct direction.

4m

This next one is all you.

Example: Would the following reaction go to the right or left? Label ALL species. Draw arrows in the correct direction.

3m

Predicting Acid and Base Equilibrium Additional Practice Problems

Consider the reactions below and answer the following questions:

a. Provide the major organic products.

b. Show the mechanism. You will need to redraw the molecules to show relevant bonds and lone pairs.

c. Determine which way the equilibrium lies and why.

Watch Solution

In the rounded boxes, write the products for each of the following acid-base reactions.

a) These reactions will be in equilibrium, and the equilibrium can favor one side or the other or be balanced (i.e. nearly equal). Indicate which side of the reaction is favored in the equilibrium either by use of arrows ⇌ or writing on one side of the reaction, “favored”.

b) Draw the curved arrows to show the flow of e- for each of these reactions.

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Predict the favorable direction of the two following acid base equilibria:

A) I→  and II→

B) I→ and II←

C) I← and II←

D) I← and II→

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Examine the reaction below. Identify the Bronsted acid, Bronsted base, conjugate acid, and conjugate base. Would the reaction work in the direction of the arrow? Explain why or why not.

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Indicate the shift of the acid-base equilibriums below by enlarging one of the equilibrium arrows.

 

 

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Predict the direction of the following acid-base equilibriums.  Be sure to label all species and include all necessary pKa’s. 

Watch Solution

Predict the direction of the following acid-base equilibriums.  Be sure to label all species and include all necessary pKa’s. 

Watch Solution

Write the products for the following acid-base reaction.

a) These reactions will be in equilibrium, and the equilibrium can favor one side or the other or be balanced (i.e. nearly equal). Indicate which side of the reaction is favored in the equilibrium either by use of arrows ⇌ or writing on one side of the reaction, “favored”.

b) Draw the curved arrows to show the flow of e for each of the reaction.

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Determine the side of the reaction that is favored for each reaction:

Following letters express the direction of equilibrium:

R = Right

L = Left

 

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Which ionization state predominates at thermodynamic equilibrium? Based on the conclusion, predict whether the substance is a solid, liquid or gas at room temperature (Note: (CH3)2CHNHand C2H5COOH are both liquids at room temperature.)

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Which reactions proceed forward?

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Which of the following can deprotonate acetylene? You are given the pKa values of the conjugated acid in parentheses. 

A. Only I

B. Only I and II

C. Only I and III

D. Only II and III

E. Only II and IV

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Use the reaction below to answer A-C.

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Does the equilibrium lie to the left or right? (Choose one)

 

 

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Does the equilibrium lie to the left or right? (Choose one)

 

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