Sciencemadness Discussion Board

Reaction Speed (help for school)

Blind Angel - 30-1-2003 at 06:51

Sorry about bothering you about my school project, but today we had a laboratory (test) on speed reaction so the teacher took all of us in the lab and told us to do whatever we want as long as we proove how Speed of reaction can be mesured and modified. We have access to almost every hardware, for chemical product we have:
Zn (powder and scrape)
Mg (Ribbon, powder and scrape)
HNO3 (1mol/l)
HCl (1mol/l)
NaOH (solution: 1mol/l)
CH3COOH (1mol/l)

Personaly i would have tried to do it with HBr and a base (the Bromine coloration would have disapeared gradualy so we would be able to time it), but we don't have Bromine.

What i have tried:
I put a tube on a erlenmeyer and finished it with a U glass tube. I put the other end in a long glass tube (about half an inch of diameter) filled with water with the end submerged in a becher also full of water. Since the long glass tube was graduated we decide to time of long it will take to reach a certain predefined level. But here is the problem: or the glass tube was all filled in one shot or is was filling by big shot wich gave us too much imprecision. In the erlenmeyer I tried to put:
50ml HCl/1g Mg(scrape)
25ml HCl/1g Mg(scrape)
25ml HCl/1g Mg(scrape)/1g Zn(powder)
10ml HCl/0.5g Mg(scrape)
10ml HCl/0.05Mg(scrape)

None of these tried work, but we have a second hour to work on it Monday. Any idea that could help me? Thanks

P.S Is HNO3 1mol/l can be usefull or it's too weak (for Nitration)?

I hope it's not late

KABOOOM(pyrojustforfun) - 5-2-2003 at 19:18

u can do it without measuring the volume of hydrogen evolved just measure the mass of Mg (g) and the time it takes until it completely disappears (s).
Dn = mass/24.305
then use this formula:

RMg = - ----

RH2 = - RMg

RHCl = 2RMg

RMgCl2(aq) = - RMg

ahlok2002 - 22-2-2004 at 07:21

speed of reaction in chemistry term rate of reaction....
normally to do so we need to identified the reaction equation of the particular reaction.
that may involved gas, liquid, or solid form whether the reactants or products.
the to measure the rate of the reaction we can measure in the form like pH , colour, volume, mass, concentration change....
that is many ways and instruments to do so like pH meter, optical density measurment....etc.

thunderfvck - 22-2-2004 at 15:04

25 ml's HCl/0.5g Mg (or something like that)
25 ml's HNO3/0.5 Mg
25 ml's acetic/0.5 Mg

Time all the reactions, stop timer when metal has completely reacted. From this you can say that stronger acids will react at a faster rate.

Then you can use powder instead of ribbon (or ribbon instead of powder) and you can use this to say how surface area affects the rate.

Then you can use the CaCO3, Mg, Zn, use them with some constant volume of HCl (or whatever), and see which one goes fastest. This has to do with the effect of acid on different reagents.

I'm sure you can write up a nice report with that. And I don't think 1 molar HNO3 would be incredibly useful but if you were to obtain a large volume you could always distill it to purify.

The highest expression of brilliance is the logical application laziness.

Hermes_Trismegistus - 22-2-2004 at 16:04

The fastest and easiest way to measure rate of reaction is to arrest that reaction in progress.

For instance you could begin with a known weight of metal strip and calculate its surface area exposed to reactants.

"length x width x height"

Allow to digest in acids for a series of set periods of time and then withdraw from solution, wash with distilled water and then mass remaining metal.

Graph the results.

Dilute the acids to known dilutions and repeat.

graph results.

Tabulate rate of reaction v.s. dilution of acids.

graph results.

Remember Hermes in your Nobel Prize acceptance speech.

unionised - 23-2-2004 at 12:12

Yep, remember Hermes, he's the one who gave you the expression for the volume rather than the area :D
Apart from that it's a neat solution.

BTW, if the problem with the original method was the reaction being too fast you can try diluting the acid.
Now, all I need is a time machine so I can post this in time for his lesson.
Nevermind, teachers tend to set the same pieces of work each year.

more complications

Polverone - 23-2-2004 at 13:52

If you've dropped a block of metal into an acid before, you'll notice that the block gets smaller and less block-like over time. That is, the surface area and the geometry are both changing as the reaction progresses.

You need to keep the geometry/area from changing. If you have metal scraps, you might be able to work like this:

Coat each metal scrap in hot wax.

In a well-defined area, remove the wax by solvent or mechanical means.

Allow acid to act on the scraps for a limited amount of time.

This should give you a reasonably uniform geometry/surface area for all of your samples. The flat surface exposed to the acid should be far greater than the minor amount of perhaps-irregular material around the edges of the wax-protected area.

Of course this advice may be impractical, since 'wax' is not listed among your available materials, and it's probably too late anyhow.