pH titration curves

[spudpeel]

I've asked my chem teacher this before, but still don't quite get it.

In titration curves, e.g:http://www.chemguide.co.uk/physical/acidbaseeqia/summary.gif

 

Why is there a sudden drop or rise in pH towards the equivalence point, is it due to the logarithmic nature of the pH scale, or am I missing something?

[Ilja]

It always a sudden drop or rise if you use a strong base or acid , you see the 4th curve appears not to have a sudden drop or rise because there is a weak acid/base act.

 

Addionally it has to do with the equivalence of acid and base , and when you add one more drop of strong base/acid it will result in a sudden rise/fall of your pH level.

 

Hope i could help

[woelen]

It indeed has to do with the logarithmic scale of the pH. I'll clarify with an example.

 

Suppose you have one liter of liquid, with 0.1 mol of acid (e.g HCl) in it. Now you add drops of concentrated base to this liquid. A concentrated solution of NaOH contains approximately 10 mol/l of NaOH. Each big drop of 0.1 ml contains 0.001 mol of NaOH.

 

Now suppose you add these drops to the liquid and after each drop you mix well. What you get is the following:

 

After 1 drop 0.099 mol of acid remains.

After 2 drops 0.098 mol of acid remains.

After 3 drops 0.097 mol of acid remains.

...

After 98 drops 0.002 mol of acid remains.

After 99 drops 0.001 mol of acid remains.

After 100 drops the liquid is neutral

After 101 drops 0.001 mol of base is in the liquid

After 102 drops 0.002 mol of base is in the liquid

...

After 200 drops 0.100 mol of base is in the liquid.

 

Now take the -log10() of the concentration of H(+) ions and you see what I mean. For simplicity neglect the increase in volume of the total liquid. It only increases from 1.00 to 1.02 liters, so that is not much of influence.

 

So, pH as function of number of drops (at 3-digit accuracy):

 

0 --> 1.000

1 --> 1.004

2 --> 1.009

3 --> 1.013

...

10 --> 1.046

...

20 --> 1.097

...

50 --> 1.301

...

80 --> 1.699

...

90 --> 2.000

91 --> 2.046

92 --> 2.097

93 --> 2.155

94 --> 2.222

95 --> 2.301

96 --> 2.398

97 --> 2.523

98 --> 2.699

99 --> 3.000

100 --> 7.000

101 --> 11.000

102 --> 11.301

103 --> 11.477

...

110 --> 12.000

...

200 --> 13.000

 

You see that the pH jumps from 3 to 11 with just two drops, while the first few drops and the last few drops hardly have any effect on the pH. This is just a property of the logarithmic scale. You add base linearly.

 

-------------------------------------------------------------------------------

 

If you did the same experiment with a weak acid, then the curve would be less steep around the neutral-point. This is because for weak acids you have an equilibrium shift when base is added and then not all base will be used for neutralization of the acid. The curve becomes 'softer/smoother'.

[jowrose]

Wow, that is really interesting. So you have to be very very close to equilibrium to get a pH of around 7? Even a few "drops" off and you'll get either a very acidic or basic indication.

[woelen]

Indeed, you have to be VERY close to equilibrium. But... this only is true for strong acids and strong bases. When weaker acids are used, then this need not be the case anymore. One can make so-called buffer solutions, which are designed to have a certain pH and to which quite some acid or base (beit strong or weak does not matter) can be added without a large change of the pH. Such buffer solutions also can be designed for pH close to 7.

[jowrose]

What constitutes a strong or weak acid? I know there are lists, but are there general rules? Or is it just through testing that scientists have determined which acids are strong and which acids are weak?

[spudpeel]

Excellent, thanks woelen, that's put my mind at rest.

 

Acids are strong if they dissociate readily to give H+ ions, and bases are strong if they readily accept the H+ ions.

[jdurg]

What constitutes a strong or weak acid? I know there are lists, but are there general rules? Or is it just through testing that scientists have determined which acids are strong and which acids are weak?

 

Because the number of strong acids is so incredibly small compared to weak acids, it's easy to just remember what the strong acids are:

 

HCl

HBr

HI

HNO3

H2SO4

HClO4

All other acids are basically 'weak' acids. Technically speaking, if in a one molar solution the acid dissociates 100%, then it is called a 'strong acid'. Anything less than 100% is called a weak acid.

[jowrose]

Ok, thanks for the help.

[woelen]

Because the number of strong acids is so incredibly small compared to weak acids' date=' it's easy to just remember what the strong acids are:

 

HCl

HBr

HI

HNO3

H2SO4

HClO4

All other acids are basically 'weak' acids. Technically speaking, if in a one molar solution the acid dissociates 100%, then it is called a 'strong acid'. Anything less than 100% is called a weak acid.[/quote']

You forget two other common strong acids, being HSO3F and HSO3Cl (fluorosulfonic acid and chlorosulfonic acid).

 

Besides these common strong acids there are more strong acids (e.g. borane based super acids), but from a practical point of view, all the acids mentioned above are strong.

 

Also quite strong (almost as strong as the 'strong acids') are HBrO4, H2SeO4, H5IO6, HClO3 and HBrO3. At 1 M concentration these are not ionized for 100% but close to 100%.

 

What about HMnO4? Isn't that a strong acid?

[Borek]

HNO₃ is reatively weak with pKa = -1. In 1M solution 8.4% is not dissociated.

 

Best,

Borek

--

Chemical calculators at www.chembuddy.com

equation balancer and stoichiometry calculator

www.pH-meter.info

[jdurg]

While those are examples, in a high school or undergraduate lab you will pretty much never use those.

 

Also, I like to go by the KISS (Keep It Simple Stupid) method of teaching chemistry to people. Having them memorize the entire list of strong acids when in reality they will never need to learn them all is just a waste. If someone sees a list of 8 items and is told to memorize it then they're much more likely to do that than if you give them a list of 16 items, 8 of which they will probably never come across. Yes there are exceptions to many rules, but in many cases there is no need to memorize those exceptions. A good analogy is with the structure of the atom. When someone is first learning about atomic structure, you tell them that there is a nucleus which is orbited by electrons. That's pretty much all they need to know. If you started putting in the full reality about quantum mechanics in there, NOBODY would ever bother to continue on. I see this type of thing happening a lot on message boards in regards to various subjects. Someone will come in and ask a question about a topic while someone else goes and gives them 3-years worth of the topic in one post mentioning every exception to the rule and every single possible item that falls into the category. I can pretty much guarantee that the person asking the simple question either didn't understand the response or didn't care anymore about it after seeing the response. It's crucial to look at the question asked by the poster and provide an answer based on that question. None of us learned everything about chemistry in one day. We learned piece by piece and over the course of a few years we gained the knowledge we have today. The human brain learns by piecing things together like a jigsaw puzzle. Keeping things simple until the complexity is needed is really the best way to go.

 

I didn't "forget" about HBrO4, or H2SeO4, or HMnO4, or peroxymonosulfuric acid, or antimonicpentafluoride acid (I think that's the name), or the other acids mentioned above. I just don't see why someone who was asking "what are strong acids?" would need to know that at this time. Had he asked "I know that HCl, HBr, HI, H2SO4, etc. are strong acids, but are there any others?" then yeah, telling him about all these odd acids that many chemists will never deal with would be worth it. Based on his question, however, telling him that HCl, HBr, HI, HNO3, H2SO4, and HClO4 are the strong acids is good enough. (BTW, HNO3 is one of those numerous exceptions to the rule and one of the cases where you really don't need to bother knowing that it has a pKa of -1 as in nearly all reactions involving HNO3 the equillibrium will eventually cause all of the HNO3 to dissociate).

[jowrose]

Yeah the common strong acids are fine for me. I won't come into contact with the more exotic samples for a long time... But thanks for the info anyways, it's still nice to know. I doubt my chemistry teacher knows about all the extra ones.

 

I know this is off-subject, but where can one get nitric acid? Do you have to order it from a chemical-supply store, or is there an easier way?

[jdurg]

Nitric acid can be purchased through certain web-based sites but it's a bit pricey in small quantities due to all the Hazmat fees. It's also a real pain to store which is a reason why I haven't gone through and purchased more. I think the e-bay seller al-chymist (sp?) has his own website where you can get a liter of concentrated nitric acid for about $50 or $60 I think. I just don't have a dedicated storage area for it and because HNO3 is such a potent oxidizer I don't want it anywhere near any organic material.

[woelen]

All other acids are basically 'weak' acids. Technically speaking, if in a one molar solution the acid dissociates 100%, then it is called a 'strong acid'. Anything less than 100% is called a weak acid.

Jdurg, I agree with your lengty post on more detailed knowledge than necessary, but then you should have formulated things a little bit different. Scientists need to be precise . If you say that "All other acids are basically 'weak' acids...", then you give the impression that your list is the full list and that is why I posted my message. If you had written something like "these are the most common strong acids and others are rarely encountered in practice" then I would not have jumped onto this. You give the impression that these are all of them, which is not correct.

[jdurg]

Jdurg, I agree with your lengty post on more detailed knowledge than necessary, but then you should have formulated things a little bit different. Scientists need to be precise . If you say that "All other acids are basically 'weak' acids...", then you give the impression that your list is the full list and that is why I posted my message. If you had written something like "these are the most common strong acids and others are rarely encountered in practice" then I would not have jumped onto this. You give the impression that these are all[/i'] of them, which is not correct.

 

Point taken. (I actually realized the poor wording after it was too late to edit my post. )

[greysteel_M6]

well, amongst the garbage that's collecting in here, I'll explain

With respect to strong acids/bases, we have a species that has a great affinity to either take or give hydrogens away with respect to itself, a strong acid maintains a high affinity to donate its protons thus analogously it has a low affinity to give its protons away.

 

the graphs with a sharp curve and sudden change of pH correspond to

the analyte being consumed, therefore nothing is hindering the change of pH other than water

[nitroglycol]

HNO₃ is reatively weak with pKa = -1. In 1M solution 8.4% is not dissociated.

I always found that one funny, but yeah, it's technically true. One definition of a strong acid is any acid with a pKa greater than that of the hydronium ion (-1.74).