what is the approximate ph at the equivalence point of (a) curve? This is a topic that many people are looking for. secret-life.org is a channel providing useful information about learning, life, digital marketing and online courses …. it will help you have an overview and solid multi-faceted knowledge . Today, secret-life.org would like to introduce to you Acid Base Titration Curves. Following along are instructions in the video below:
The simplest terms. A titration curve is basically just a plot of the volume of of titrant that youre adding to the analyte versus ph and what the general of a titration curve looks like is kind of like this and if you havent seen. My titration stoichiometry problem video.
Yet i highly recommend that you look at that because in that video. I explained how titrations work so back to this this is a curve of a strong acid and it is being titrated with a strong base and you can tell that its starting out with a strong acid. Because the ph is very low and remember that the lower the ph is the stronger.
The acid is so here were going to add more and more titrant or more. And more the strong base and then here you see a big jump up to a very high ph. So over here.
Its extremely basic and here. Its very acidic and then here is what we call the equivalence point. Its pretty much halfway up this huge jump.
So this is the equivalence point all right. And whats happening is we have our acid. So lets say hcl because thats a strong acid.
And when you have it in solution. Its present as h. Ions and cl.
Minus ions. But when you add a strong base to it. Lets say naoh that goes into solution.
As na and oh h. Minus. Ions.
Now here the na and cl. Our spectator ions because they dont really do anything or change chemically.
Its just these h. And which minus ions are going to combine to neutralize each other and make h2o. So whats happening here is we have a lot of hcl and not that much ending of each.
Yet so because we have a lot of hcl. Which is a strong acid thats causing the solution to be very acidic now here. The equivalence point is where the moles of hcl equal.
The moles of naoh so as you can see the equivalence point is at a ph of around 7. Which is perfect because we have a strong acid and a strong base when they have equal moles. Then the ph is 7 or neutral.
And then up here we have a lot of nuh. So now naoh is in excess. And its causing the solution to be basic.
But lets see what happens when we have a strong acid and a weak base as opposed to a strong acid and a strong base ok. Im back and now were going over a titration curve for a strong acid and a weak base. So here you see that the ph is still really low meaning that we have our strong acid to start with so lets go with hcl again and for our weak base lets go with nh nh.
3. Which is ammonia and we see that our equivalence point is around here now notice that this isnt at 7. Like the other titration curve that we looked at its a little below 7.
So that means that at the equivalence point. The solution is slightly acidic and the reason for that is because we have a weak base instead of a strong one thats balancing out the effects of the strong acid and also if you write this out the equation for nh3 when its placed in water is it partially dissociates or ionizes into nh 4 plus and os now since this doesnt go to completion some of it stays at nh3 and some of it change nh four plus and since nh three is a weak base its conjugate. Acid then h.
Four plus is pretty its a little stronger than the strength of nh three so since this is a stronger acid. Its also causing the solution to be a little acidic and youll see something similar to that when we look at the weak acid and strong base titration curves. Okay.
So weve got the same general shape for a titration curve with a weak acid and a strong base. But notice that the ph isnt as low as what it was before before it was like around here.
So because were starting out with a weak acid. Its the ph isnt going to be as low plus. When we look at the equivalence point.
When you go lets say its around here and move over to where it says the phs. The ph is a little higher than seven. Now so that means that its going to be a little bit basic following.
The logic that we used for the strong acid weak base titration curve. When we have a weak acid such as acetic acid. So each c two h.
Three o. Two. That partially dissociates into acetate and each plus ions.
But remember its only a partial dissociation. So because acetic acid is a weak acid its conjugate base is stronger. So since acetate is more of a strong base.
Its going to asset to protons and change back into this so. Its going to cause the g ph at the equivalence point to be a little higher than seven. Also id like to point out that in here you have what is called a buffer buffer region.
So whats happening here is when we titrate the to the weak acid with a strong base. We have a conjugate acid base pair. So whats happening is if we have a strong base such as naoh.
Its going to steal the h ions from the hc to h3o. So whats that so what thats going to create is a bunch of acetic acid and a bunch of acetate and thats going to create a buffer because their conjugate acids and bases so in the middle of the buffer region. Which is half of the equivalence point.
Were actually going to see that the ph is equal to pka and remember from the undersand hasselbeck equation which is ph equals pka plus log of concentration of the base over the concentration of the conjugate acid that whenever these two are the same that just simplifies to one and the log of one is just 0. So what we get from here is a ph equals pka. Meaning that the concentration of the conjugate conjugate base.
And the conjugate acids are the same .
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