Why is 10 M HCl more reactive than 95% H2SO4?

In my chemistry class today, I was showing the reactivity of metals. 10M hydrochloric was very reactive yet 95% sulfuric was significantly less reactive.
I would have thought that with concentrated sulfuric producing 2 hydrogen ions instead of one that it would have been more reactive.
Any ideas?

I would venture that the reactivity of the metals is with the free hydrogen ions (really H3O+) present in the acid solution. With 95% H2SO4 there just isn’t enough water to allow for disassociation of hydrogen from the hydrogensulfate. With 10M HCl at 70% water this allows for ample disassociation with lots of hydrogen ions (H3O+)around

Thanks, that makes a lot of sense. Any idea how I could dilute H2SO4 to show its true strength as an acid?

What was it reacting with?

Not every since molecule of acid is going to give up the hydrogen ions, though. That depends on the pKa. This determines the reactivity of the acid more than anything else. HCl has a pKa of -8, sulfuric acid is 1.92.

I would like to see a reference for your numbers here. I have a pKa of -7 for HCl and -9 for H[sub]2[/sub]SO[sub]4[/sub]. Cited from Advanced Organic Chemistry, third edition by Jerry March. The source for this data is Bell, “The Proton in Chemistry.” 2nd Ed., Cornell University Press, Ithica N.Y., 1973 and Finston and Rychtman, "A New View of Current Acid-Base Theories, " Wiley, New York. 1982. I must admit, these references are a little more convoluted than is standard for March.

Of course when you get down to pKa’s that are that low the difference between your -8 and my -7 is pretty understandable due to the methods of determination, but your 1.92 and my -9 for sulfuric acid are not reconcilable.

er - you add water :smack: about 2 volumes water, 1 volume acid

but make sure you add the acid slowly to the water - never the other way round as the water /acid mixture will superheat and can explode in your face

I pulled it off Wikipedia while refreshing myself on pKa’s. They give a pKa1 of -3.00 and pka2 of -1.99 (somewhere else on the internet gave 1.92). If you’re certain that you have a better value, Wikipedia is editable, go nuts. http://en.wikipedia.org/wiki/Acid_dissociation_constant

So you’re saying that HCl is a much stronger acid that H2SO4. What is the pKa for acetic acid since that is another acid we tested in my demonstration?
Did the idea that concentrated 95% H2SO4 is too low in water to liberate its ions have any validity as stated by** Sigene**? Sorry Sigene.

Yes I’m pretty sure Sigene made a a valid point, however I don’t feel like going thru the math to demonstrate it. For acetic acid, Wikipedia shows a pKa of 4.76 which is obviously higher (thus weaker) than HCl or H2SO4.

Seriously? Why would it it superheat if you add water to acid but not if you add acid to water?

The reaction generates the same amount of heat no matter which way you do it. However, water is much better at dispersing the heat. Thus it’s better to add acid to an excess of water… the heat is instantly dispersed in the larger volume of water. If you add a small volume of water to acid, the heat has nowhere to go. The water can instantly boil, splattering acid everywhere.

If it makes it easier to imagine, imagine pouring water onto a hot grill. Hot water splattering everywhere. Then imagine dropping a similarly superheated piece of metal into a sink full of water. Nothing happens except maybe a bit of a sizzle. Similar concept.

It’s something I note regularly. Concentrated (>90% w/w) sulfuric acid can be stored safely in carbon steel tanks. Even down to 80% if the velocity and temperature is low. Sulfuric acid forms ferrous sulfate on the surface of carbon steel. Ferrous sulfate is passivating and prevents further acid attack. FeSO is soluble in water though, and can be removed by turbulence. The solubility of FeSO increases with temperature and water concentration. If it goes into solution as quickly as it is formed, you’re going to have problems.

In this case, since you are dealing with aqueos solutions, it makes no real difference since they are both strong acids (They completely dissociate in water.)

I must admit, every other number I have agrees with you, though both editions of March give a value of -9. I wish I had the original source.

This is why I asked what it was reacting with. Sulfates tend to form passivating surfaces that inhibit reaction whereas chlorides do not.