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Crash Course - Buffers

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A few months ago when we first talked about acid-base reactions We saw how acid rain could melt the face off a statue Which is better than melting your face off, which some acids can do. So consider yourself lucky Acid rains forms when sulphur dioxide emitted from burning fossil fuels reacts with water in the air to form sulphuric acid Infact when acid rain was at it's worst in the 80's and 90's it was the scourge of pristine waterways all over Europe and North America. Some rivers and streams were left completely devoid of algae and fish In Montana, where I live, saw quite a bit of acid rain too as that statue could attest. But oddly enough rivers and streams did not suffer the same horrible effects as other places This that I am standing in right now is the Clarke Ford river. It remains relatively unharmed by acid rain. Infact when the river that fed it had a pH of 4.5, the river remained basic. This is a dropful of dilute sulphuric acid. Think of it as our model for acid rain. And this is just distilled water with a pH indicator that is blue when neutral or basic, but will turn pink when a little acidic. Let us see how much acid it takes to turn our distilled water a little bit acidic. Let's do one drop... Oh wow, it's already pink, guys. It's already pink. One drop. Well let's try the same thing with the river water. Let's see how many drops it takes before it turns.. I'm just dumping it in here. Are we getting it? Is it starting to happening now? There, it's sort of half and half. Got a half and half transition after like 5 or 6 or 7 drops. That was a lot more. And we are not even there. Let's give it a little bit more. Little bit more. Come on. There we go. There, there now it's acidic. Why does this happen? It's because this river is protected by limestone. Which is calcium carbonate that is present throughout the entire riversystem. Acid rains dissolves the calcium carbonate in the limestone. That carbonate washes into the river and acts as a natural buffer A buffer solution resists changes to it's pH when a strong acid or base is added to it. So the Clark Ford geochemistry explains why acid rain isn't as devastating here compared to places where there is no limestone. Buffering is a big deal in chemistry. We buffer swimming pools to prevent the chemicals from damaging our skin And we buffer soda pop to prevent the acidic flavourings from damaging our teeth and tissues. We even have buffers in our blood to keep our internal pH constant and our cells healthy I think it is important to learn something that powerful, how about you? A buffer solution is a mixture of a weak acid plus its conjugate base or a weak base plus its conjugate acid These are called acid-base pairs Weak in reference to acids and bases means that they only partially dissociate It is actually their weakness that makes them great buffers. Since they don't fully dissociate in water the undissociated buffer can act as either a source or a sink for protons. Which helps to neutralize a strong acid or base that is added to the solution It's really all about equilibrium For example when a weak acid like ascetic acid is added to water a tiny fraction dissociaties into it's constituent ions. Acetate and hydrogen ions, or protons I was recently pouring a bunch of a solution like this all over my fish and chips Vinegar is just a 5% solution of ascetic acid. But the reaction is reversible as well, and that's where equilibrium comes in. Say we have a one molar solution of ascetic acid. To make the solution a buffer we need to increase the concentration of acetic acid's conjugate base, acetate, by adding sodium acetate to it. That would be a good way. Sodium acetate dissociates completely thereby providing a ton of acetate ions. To see how this works in practice let's add enough of that salt to make a 1 molar solution. We know how much acid and acetate we put in, but the important thing is the pH. And to determine that, we have to know how many protons there are. A rise table will help us keep track of eveything. The reaction, R, we are interested in here is the dissociation of ascetic acid. We can ignore the sodium from the sodium ascetate. Although it will stick around, it is just a spectator ion that won't take part in any reactions. Our solutions contains an initial concentration, I, of 1 molar acid and 1 molar ascetate. We haven't formed any hydrogen ions yet so that stays at zero for now We don't know how much the concentration will change, C, so we call it x. The acid loses x and both ions gain x. So our concentration at equilibrium, E, is then one minus x, one plus x and simply x in that order. Now let's put the numbers into the equilibrium formula and we get 1.76 x 10^-5 But when chemists work with dissociation equations for acids and bases they give the Keq a special name The acid (or base) dissociation constant. A symbol for this kind of equilibrium constant is Ka for an acid or Kb for a base So use the equilibrium expression for ascetic acid and put in the Ka and the equilibrium concentrations from the rise table With a little simple algrebra it simplifies... That doesn't look simplified. I guess you could call that simplifying but.. Bwadr That's like quadratics and I don't wanna do that, so here's a little trick to make this a lot easier See, ascetic acid is a really weak acid so only a tiny fraction of it disssociates which means that x is super small If it's so small that after rounding for significant digits it doesn't change or answer at all, then why not just drop it That first x for the hydrogen ion has more effect because it's multiplied But the other two.. Let's just forget they are there and see what happens. If we go back to where we plug in all the numbers and simply drop the two x'es that are being added and subtracted The rest of the problem cancels out leaving x equaling 1.76 x 10^-5 The rest is a breeze with a couple of taps to the calculator we find that the pH is 4.754 But is the question what if we tried to push the pH out of wack. The whole point of a buffer is to resist that, right? If I add a strong acid, like say hydrochloric acid, with the concentration of like 0.1 M to distilled water The pH will change very quickly to like very fast.. Dropping.. Yes, now we are like pH 3 Not a healthy environment for most organisms So let's think about what would happen to the pH with a buffer in place Even though HCl dissociaties completely in water releasing tons of hydrogen ions, when it's added to a solution that's buffered with acetate, the excess hydrogen ions join with the acetate ion to form ascetic acid. Thus that strong acid can't affect the pH much because the hydrogen ions are used up To neutralize 0.1 M of HCl or more specific 0.1 M of H+, The acetate concentration must decrease by 0.01 M per litre Simulatenously increasing the ascetic acid concentration by 0.01 M per litre That leaves us with 0.990 M of acetate and 1.01 M of ascetic acid. Well, pH is determined from the concentration of protons, which is in our equlibrium equation So let's just solve for that When you plug in the new ascetic acid and acetate concentrations and the Ka you get 1.8 x 10^-5 for the proton concentration Which translates to a pH of 4.746 It is obviously not a big change from 4.754. In fact I had to go out to the fourth digit to see the pH change, which isn't even justified if you watch CrashCourse Chemistry on significant figures But I have to confess vinegar and hydrochloric acid, they may make things easier to understand but they are not exactly super relevant in terms of practical chemistry Instead I want you to see how this works in the real, real world. So lets consider the buffering capacity of the Clark Ford river. There the reaction between dissolved limestone and acid rain really happens in three steps First the solid calcium carbonate reacts with the protons from acidic rain to form calcium and bicarbonate ions. The bicarbonate ions each have one hydrogen As as long protons are available the bicarbonate ions can grab a second proton to become uncharged carbonic acid. As you can see, this chain reaction uses up two protons per molecule of calcium carbonate That makes this process doubly powerful And that explains why the Clark Ford river is so resistant to acidification But of course buffers are not invincible. Add enough acid or base and even the best buffered solution will get overwhelmned. We can call that threshold the buffering capacity of the solution And we determine it in the lab using pH indicators through a process called titration Earlier I used a pH indicator that showed an acidic pH by turning pink, but there are lots of other indicators Each with different colors and a different pH where it changes. This is called the indicators end point Here we have a 100 ml of water from the river to which I have added some pH indicator It's actually a mix of two different indicators, which works well for this reaction. It will stay blue in color for as long as the bicarbonate ions are around to keep the pH buffered above the indicators end point of pH 4 This long glas thing here is a burette. It's for dispensing liquids in a very controlled way while also keeping track of how much you have dispensed At the moment it contains 25 mL of 0.10 M H2SO4 The thing to do is add acid very slowly, stirring as I go Which is being done for me by this automatic stirry thing, which is quite nice And you can do it a few ways If you have a guess at how much you can put in you can just sort of turn it on and let it go But I don't know, so I'm just gonna let it drip Very slowly You can turn it very slightly and you can actually get a sort of drippy-drip And then you are like waiting to see if there is some changing happening And there is a little bit of change happening but now it's going back So, we are getting closer And what I am doing right now is I am actually adding less than a drop at a time By going quickly past the opening in this little burrete thing You can actually see... You can SEE the reaction happening right before my eyes It's beautiful and then it goes back It's right now a paler blue than it was before Which is exciting, since we are getting very close This is the stuff I get excited about, I know I'm a nerd Ooh, it's almost white right now. It's like pinkish It's purple-y Aah, beautiful. That was perfect. The solution is now staying pink which means that I have exhausted it's buffering capacity And the pH has dropped below the indicators end point The volume in the pipette is now 22.4 ml, meaning I used 2.6 ml Or 0.0026 l of the acid solution The acid solution is 0.10 M so multiplying these numbers together tells me I used 0.00026 mol of H2SO4 Sulphuric acid reacts with a 1:1 ratio with calcium carbonate So assuming the calcium carbonate is the only buffer in the water The 100 ml of river water here must also contain 0.00026 mol of CaCO3 Making it a 0.0026 M or 2.6 mM solution I'm a big fan of protecting the environment But sometimes it is amazing to see what nature can do to protect itself We live in an amazing world, and I wouldn't trade it for anything Not even Mars.

Video Details

Duration: 11 minutes and 40 seconds
Country: United States
Language: English
Genre: None
Views: 211
Posted by: callesen58 on Nov 26, 2013


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