PAUL ROBERTS: Hi, I'm Paul Roberts. I'm a chemistry teacher here at Andover College.
And today, I'm going to talk you through how to carry out core practical neutralizing an
acid. Now although this is a very straightforward practical, I'm going to give you some hints
and tips of how you can get some really good results.
So first of all, we're going to take 50 centimeters cubed of 1 molar hydrochloric acid and add
it to a beaker. Good idea to use a 250 centimeter cubed beaker for this as you're going to be
stirring it and it gives the solution room to move, just with a glass stirring rod. It
doesn't have to be glass, but I'm just going to put a dot of the acid on a piece of universal
indicator, wait a few moments, and see the color change.
OK, so that's our initial pH. And looking at my swatch card here, it has a pH of round
about 1. So we're going to start the neutralization. And as I mentioned before, the neutralization
is going to be using calcium hydroxide. Now for each time you perform the neutralization,
you're going to add 0.3 of a gram of calcium hydroxide. I measured some out already on
the balance here. I took time to tare the balance before weighing it and just add it
into the pot.
Using the glass rod now to stir, and you'll notice a couple of things happening. It'll
start off being quite cloudy. And this is because the calcium hydroxide isn't very soluble.
It takes time for it to dissolve. But over time, it will dissolve. And once all the calcium
hydroxide is dissolved into solution, and then reacted with the acid, the solution should
become colorless again.
You might notice a very small amount of fizzing taking place in the beaker. And that's because
calcium hydroxide typically has a little bit of calcium carbonate impurities in it. So
don't be alarmed if you see a few bubbles appearing.
OK, so now, you can see the solution is colorless and I can again perform another pH check.
Noticing that the pH hasn't really changed. It's still around pH 1. You could alternatively
at this point be using a pH meter. You're going to get a more precise result, but still
we'd expect still a low pH.
And now, we're going to continue adding portions of 0.3 grams of calcium hydroxide and stirring.
Each time you stir, it will go cloudy. And then as the calcium hydroxide reacts with
the acid, the precipitation should disappear and it become colorless. You can always gently
tap on the precipitate in the bottom of the beaker, and it should clear it up. Continue
stirring until there's no more noticeable solid. And once we get to that point, you
can then tap and measure the pH.
I'm going to continue again. This is my third addition of the calcium hydroxide. Each time,
I'm adding 0.3 of a gram. And once there is no more precipitate, I'll check the pH of
the solution, and it still remains fairly acidic.
This is my fourth addition. And after the fourth addition, the calcium hydroxide again
will react with the hydrochloric acid. This means hydrochloric acid is essentially in
excess. It's taking longer each time though. Once again, I'm going to check the pH after
my fourth addition, and it's still around about pH 1.
My fifth addition of calcium hydroxide. And I go through the stirring process. And this
time, I may notice a slightly more neutral pH. It's all gone into solution. That's a
good thing. I'm going to tap it there. Note that we still have very much an acidic-- a
very strongly acidic-- pH. OK, that's five additions.
This is my sixth addition. When this calcium hydroxide is unreacted or the calcium hydroxide
is in excess, there will be a much more alkaline pH. But stick with it. Keep an eye on the
precipitate in the base of the beaker. And give it a stir. See how much is gone after
each stir. It may not be the most exciting chemistry, but if you're patient, you can
get some very clean results. It's just about all gone into solution now.
And let's have a look at the pH. OK, so the pH is looking slightly more orange there.
If you're comparing the initial pH to the pH after six additions, you're definitely
seeing a closer neutralization. So the next addition, I'm expecting to see the final neutralization.
OK, now, I'm expecting there to be excess calcium hydroxide this time. As a result,
if there's excess calcium hydroxide, no matter how much I stir it, the calcium hydroxide
won't react and it will remain as a precipitate. So I'm stirring it. The solution isn't clearing.
And this means that all the acid has reacted and now, we have excess calcium hydroxide.
This isn't dissolving. I'm going to check the pH now, and I'm expecting quite a alkali
pH. And there we have a pH. This is looking a pH around about 10-- 10 to 12. It's not
particularly easy to see. In the center, it looks around about 12.
So now, we can see that neutralization has taken place and we have excess unreacted calcium
hydroxide. I can add another proportion of 0.3 of a gram of calcium hydroxide just to
confirm my results. And this time when I add it and stir, I know that the calcium hydroxide
won't dissolve and I'm just going to be adding to my excess. I can now check my pH, and I'm
expecting a similar pH. In fact, it's become ever so slightly more alkaline.
So what do we have now? A range of colored pH paper. And we can compare this to our swatch
and determine the pH at each addition of calcium hydroxide. A graph can now be plotted of pH
versus mass of calcium hydroxide and determine the mass of calcium hydroxide required for
neutralization.
To summarize the practical, we've added calcium hydroxide base to hydrochloric acid, and we've
continued to add it until neutralization has taken place. This has given us data to enable
us to draw a graph of pH versus mass of calcium hydroxide added.