Wikipedia:Reference desk/Archives/Science/2012 April 25

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April 25

pH Determination

What caused compounds to have lower pH values than other compounds? — Preceding unsigned comment added by Bibibubsy06 (talk • contribs) 08:24, 25 April 2012 (UTC)

Well usually they are more acidic. For the common type of acids they will produce more H⁺ ions. This may be because the H+ has less force and energy attaching it to the rest of the acid. It may be because the rest of the acid molecule has lots of atoms that attract electrons, eg trifluoroacetic acid has a lower pH than acetic acid. Graeme Bartlett (talk) 11:13, 25 April 2012 (UTC)

pH is an property of a solution--generally speaking the greater concentration of "whatever dissolved chemical", the lower the pH if an acid or the higher if a base. The "acidity or basicity" of a compound is termed pKa. So Graeme's TFA is more acidic than acetic acid--the chemical has a lower pKa. But a dilute solution TFA might still be "only very slightly lowered" (for example, pH 6.8) because it is dilute even though the acid itself is a intrinsically stronger. DMacks (talk) 21:13, 25 April 2012 (UTC)

Chemical kinetics - why no 2nd order reaction data?

Considering a quantity of contained pure hydrogen gas, it will come to an equilibrium monotomic (H) and diatomic (H₂) mixture. Logically, it seems to me that the following 2nd order reactions will occur:-

H + H → H₂.....................(1)

H + H₂ → H + H + H.........(2) normally written as H₂ + M → H + H + M, [M = H]

H₂ + H₂ → H + H + H₂......(3) normally written as H₂ + M → H + H + M, [M = H₂]

and the following 3rd order reactions will occur:-

H + H + H → H₂ + H.........(4) normally written as H + H + M → H₂ + M, [M = H]

H + H + H₂ → H₂ + H₂......(5) normally written as H + H + M → H₂ + M, [M = H₂]

Since it seems that nobody has discovered H₃, thre is no need for any more reactions such as H + H₂ → H₃. Upon a search, I found reaction rate data (eg Arrhenius quation constants) for Reactions (2) through (5), for example in Journal of Physical Chemistry Reference Data Vol 12 1983 Page 531-590. I have found data in other publications as well. However, I cannot find reaction rate data for Reaction (1). Is there some reason why this reaction can be ignored? Given that a quantity of gas is represented by particles of size <<< the mean distance between them, the probability of two molecules coliding in a given length of time must be considerably greater than three molecules colliding. What have I not realised? Some databases (eg NIST Chemical Kinetics database) list Reaction (1) in indexing, but when you follow the trail, you'll find that they actually meant H + H + M → H₂ + M, and didn't bother to write the M, which suggests perhaps that either they were slack, or that users ought to know there should be an M, or both. Ratbone120.145.51.113 (talk) 08:44, 25 April 2012 (UTC)

What you are asking is basically why can't two hydrogen atoms collide and make H₂. This has been asked before, see http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2011_February_24#Question_on_collision_between_hydrogen_atoms_.28momentum_.2F_energy_question.29. Essentially, the answers there seemed to be that the sum of kinetic energy and bond energy must be conserved, and this means that if H₂ is formed the conditions exist to immediately split it again. To form H2, a third collision partner is required to carry off some kinetic energy so that the bond energy lost has somewhere to go. The transfered bond energy can then gradually transfer to the bulk of the gas by subsequent (non-raective) collisions. I have to say I'm not confortable with this explanation - why can't the newly formed H₂ molecule race off with increased velocity, to be dissipated in subsequent collisions? See also http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2012_January_17#Order_of_chemical_reaction. Keit120.145.175.169 (talk) 03:39, 26 April 2012 (UTC)

To answer your last question, it has to do with conservation of momentum. The momentum of the hydrogen molecule formed would have to be equal to the sum of the momenta of the hydrogen atoms that formed it; there's no way for it to suddenly pick up speed. TenOfAllTrades(talk) 04:30, 26 April 2012 (UTC)

I've wondered for a while: could the excess energy just be emitted electromagnetically? --Tardis (talk) 00:54, 27 April 2012 (UTC)

You are not the first to ask that. It was asked by BenRG in Refrence Desk on 24 Feb 2011, and nobody answered it. I don't know a direct answer either. However, any closed amount of hydrogen comes to an equlibribrium with the fraction of H and fraction of H₂ in a certain ratio depending on the temperature. The ratio can be calculated by using dissociation/recombination mathematics http://en.wikipedia.org/wiki/Dissociation_(chemistry) derived from first priciples. At equilibrium, the reactions H₂ + M → H + H + M, [M = H,H₂] sum to an equal rate to the sum of reactions H + H + M → H₂ + M, [M = H,H₂]. So any photons / electromagnetic energy emitted by H + H + M → H₂ + M must be immediately and totally absorbed by H₂ + M → H + H + M, since the system is stable and emits no photons. That does not seem likely. Nor do photons affect the equilibrium fractions, other than by simple heating. Keit124.182.22.228 (talk) 01:16, 27 April 2012 (UTC)

Inappropriate paint

Apparently an inappropriate paint was used on the cabinets above my stove, since the paint is peeling as a result of rising steam from cooking. When I repaint, what type of paint would be best to resist this effect ? StuRat (talk) 16:15, 25 April 2012 (UTC)

Hey, I can't believe you couldn't have done this. Richard Avery (talk) 16:59, 25 April 2012 (UTC)

I want paints which are steam resistant, not paints where the manufacturer claims they are. StuRat (talk) 18:06, 25 April 2012 (UTC)

I don't know about steam per se, but in terms of bathrooms and other areas where one expects moisture, a more important factor in preventing peeling is often the use of a good primer rather than the choice of paint itself. The primer creates a moisture barrier and improves adhesion to the surface, which helps resists peeling and cracking that could occur after frequent exposure to moisture. I assume that similar principles apply in the presence of steam. Did you use a good primer the first time? Dragons flight (talk) 18:24, 25 April 2012 (UTC)

It was already painted when I moved in. StuRat (talk) 18:38, 25 April 2012 (UTC)

Ita erat quando hic adveni. Heard that excuse many times before  ;-) --Aspro (talk) 19:10, 25 April 2012 (UTC)

Heus, modo itera omnia quae mihi nunc nuper narravisti, sed nunc Anglice? SpinningSpark 21:08, 25 April 2012 (UTC)

Well parried but not quite touché – “I only rented this cheap loco poco (little hovel) (or Poco Loco – a little crazy) because my daddy din'nt have job at the time and so threw me and my entourage out the house. Just look at the dump I've landed in.” Hardly the situation that Ovid would have found himself in -don'nt you think?--Aspro (talk) 17:56, 26 April 2012 (UTC)

If we are talking about a rental (or some low cost new construction), it isn't uncommon to see landlords doing cheap / quick painting that looks good for a bit but won't last. For example, a coat of the cheapest paint available with no priming and without cleaning up old layers. Often landlords aren't even required to maintain the look of a place. As long as there isn't a safety issue, the renter may be out of luck when the paint starts to peel. Dragons flight (talk) 19:53, 25 April 2012 (UTC)

Having been a tenant, and later a landlord, I can say that although you do occaisonally get bad tenants, and occaisonally there are skinflint landlords, the most common problem is ratbag managing agents in league with roque maintenance contractors. So if a tenant reports something needs some work, the managing agent calls his pet maintenance contractor, who does a shonky job (or even no job) for a cheap price, and the managing agent tells the owner/landlord that work needed to be done, and here's the (high price) cost. After a while, the tenants get pissed off due to the poor quality maintenance and having to make lots of complaints, and the owner thinks all the expense is due to a bad tennant. Good managing agents are worth their weight in gold. At the same time, as an owner you learn a lot if you have a beer with your tenants now and then. Wickwack58.167.243.91 (talk) 04:38, 26 April 2012 (UTC)

europa europa

Forgive me for my ignorance, but as far as I know, you cannot differentiate between an erect circumcised and uncircumcised penis. So, why did the protaganist(Jupp) in Europa Europa avoided sex with hot Leni.--122.161.40.115 (talk) 22:24, 25 April 2012 (UTC)

I expect you could if you looked closely enough. Also, don't forget the floppy-time, after. :-) StuRat (talk) 22:29, 25 April 2012 (UTC)

Humans can be highly emotional and irrational creatures. Bruce Hood performed a social experiment where he offered people money if they put on a sweater for 15 minutes that used to belong to a serial killer (not even really, but he said it was). More then half of the people asked wouldn't do it. Even if you can't tell the difference, there is a perceived "essence" to things which can emotionally repulse us, or attract us for that matter. In another experiment, children are asked if they mind if one of their toys is replaced with an identical copy. For the most part they don't mind, but if they have a favourite toy, they refuse to exchange it, even if the replacement is indistinguishable from the original. Vespine (talk) 22:58, 25 April 2012 (UTC)

(original poster here) how is this related to the original question? Or are you trying to imply that he didn't screw lina for psychological reasons rather than getting himself caught pants (and foreskin) down.--122.161.40.115 (talk) 16:48, 26 April 2012 (UTC)

I always wondered what Bruce Hood did after he retired from the NHL. There's an old saying in regard to objects from questionable sources: "You don't know where it's been!" Or in some cases, you do, and that's sufficiently repellant. However, it also might depend on how much money was offered. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:32, 25 April 2012 (UTC)

Of course, I think he was only offering $10 or $20. Vespine (talk) 01:21, 26 April 2012 (UTC)

Wearing John Gacey's sweater should be harmless, provided it was certifiably, thoroughly laundered first. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:28, 26 April 2012 (UTC)

Err, you can tell the difference. Google it up if you are really curious. --Mr.98 (talk) 01:24, 26 April 2012 (UTC)

Google says that it is not possible to know if an erect penis is a circumcised one or an uncircumcised one under normal circumstances. Obviously a medical practitioner in a lab will find out as one of the difference is that the skin gets cornified in the case of circumcised ones. However, a normal girl in a sexual intercourse cannot find that out.--122.161.40.115 (talk) 16:48, 26 April 2012 (UTC)

Are you using a funny version of Google? Because I'm not seeing the same thing [1] (not exactly a neutral source but it's far more useful then stuff like Yahoo answers), [2] (while it doesn't comment on the appearance much, it does not the movement of the foreskin which again could be something she would notice particularly when touching the penis), [3], [4] and [5] all suggest it's sometimes/often? possible to tell even when erect (although the later notes it's not always easy even when not flaccid). In fact you seem to be contradicting yourself, by saying a medical practitioner in a lab will be able to tell but there's no risk she will notice something seems wrong. Remember as I noted below, we aren't discussing her simply looking at him in a pornographical movie or seeing it when he flashed her ([6]) or knowing with a high degree of certainty but simply the risk of her getting suspicious enough during a real world interaction with him naked, that it may lead to his being identified as circumscised (perhaps partially from the medical examination) at a time and place (and by a person) where it was a very bad thing, and therefore questions to be asked about his heritage which could lead to his death. (Note also I presume he was circumcised sometime in the 1920s so any scarring may be more visible then with more modern medical techniques.) Nil Einne (talk) 21:42, 26 April 2012 (UTC)

In addition to what the other two first level indenting posters have said, bear in mind as our Foreskin, the degree of retraction during an erection (without manual retraction) varies so in some individuals it may be as easy as with an unerect penis. And of course even if the foreskin does retract by itself, it doesn't mean it can't be moved to partially or completely cover the Glans during an erection, in fact this is a common masturbation technique and the similar movement during sexual intercourse is cited as one reason by circumcision opponents for the alleged harm. Of course some men with phimosis can't retract the foreskin fully at any time. Nil Einne (talk) 15:40, 26 April 2012 (UTC)

What I am saying is that Jupp could have passed off as a normal case. Am I wrong? --122.161.40.115 (talk) 16:48, 26 April 2012 (UTC)

I don't see any reason to think there wasn't a big risk. At the very least, if she had any experience she may be suspicious from both the look and at the lack of any obvious ability of the foreskin to retract or move at all even if all she saw was his fully erect penis. And there's no reason think that's all she would have seen. Even given his age and the alleged 'hotness' of the female involved, there's no guarantee he would have been fully erect from the moment she first saw his penis to when she last saw it. Pressure can do strange things, the pressure of being at risk of being killed likely even more so. Some men may dig the 'banging the hot chick who wants to kill me' angle, others may be disgusted by it.

And despite what you may see in the movies, particularly those of the pornographical nature, it's unlikely that he can be sure he would be fully erect by the time his pants came off. He could try more foreplay while fully clothed but what's he to do if she starts to remove his pants before he's ready? For that matter what if she or he fumbles and takes too long and he's starting to lose his erection? And of course he'd want to quickly put his clothes back on once he's done the deed. If he's luckly she may just be insulted, if he's unlucky she may be suspicious. And the risk is only likely to increase the more times he does it.

If his only risk was being slapped or perhaps dumped, perhaps it would be an acceptable tradeoff. But all in all, speaking as a male, I'd gladly screw the screwing and not put myself at risk of being killed, and it hardly seems surprising if he would do likewise. (Of course there would likely be a risk him refusing to do it would lead to her spreading rumours that may lead to his identification. But on the whole, it would seem a lower risk. Notably, the risk of her doing so would likely increase if he refuses to do it again and she already had some minor suspicions from the first time.)

Nil Einne (talk) 21:19, 26 April 2012 (UTC)

Realised I forgot to consider something. Even if it were true he has little risk of being identified if he had sex (& to be clear, I still suggest if we consider it from a real life more realistic & messy sex POV, it's way to risky to have sex with someone if your circumcised and being identified as such by the person you're having sex with puts you at strong risk of being killed); it's quite doubtful that a teenage? boy in the 1940s would have any real idea that it were so. And definitely not something he'd want to ask people about. Nil Einne (talk) 03:17, 9 May 2012 (UTC)

Bicycle wheel spoke breakage load question

Hi. I have a pretty standard bicycle, with (AFAICS) standard steel spokes on the wheels. One broke the other day. How much force does this take? I would estimate, using the figure for carbon steel on Ultimate tensile strength of 500 MPa, and an area of $10^{-6}{\rm {m}}^{2}$ a breaking load of 500 Newtons, which doesn't seem very much. I would have estimated 5000 Newtons. Have I erred in my calculation, or is my expectation wrong? Robinh (talk) 23:26, 25 April 2012 (UTC)

110 pounds breaking force? Sounds about right. --Carnildo (talk) 01:01, 26 April 2012 (UTC)

Is tensile strength the right measure? You weren't stretching the spoke. Depending on how it broke, it was either due to compressing the spoke (so you want the compressive strength, which doesn't have any examples other than concrete, unfortunately) or it broke because it was pushed sideways, in which case I'm not sure what measure of strength is relevant. --Tango (talk) 01:46, 26 April 2012 (UTC)

Tensile strength is the right meassure. In any case, in metals, compressive strength is equal to tensile strength, in theory. But you almost never get to meassure compresive strength in metals because in compression they will bend. A bycicle spoke is such a long thin specimen that compressive strength is totally irrelevant. In my experience, broken spokes are a common occurance and occur due to travelling over severe bumps or misjudging a curb jump. This puts sudden compression in adjacent spokes, which merely bend, and sudden high tension in opposite spokes, which break. Wickwack58.170.143.160 (talk) 02:44, 26 April 2012 (UTC)

(OP) thanks guys. I'm relieved to discover that broken spokes are common. The spoke "popped" when I was cycling along, not going over a bump or anything. It just went sproing and then I noticed that the wheel had buckled (badly enough to exert the brakes every revolution). Is this normal too? Robinh (talk) 07:34, 26 April 2012 (UTC)

If a spoke (or more) breaks, then yes, it is normal for the wheel to be defromed.

As a temporary fix, you can adjust the length of the other spokes to get the shape of the wheel closer to normal. This takes a tool and a lot of practice. You must aim for the wheel to be planar so that the breaks can work, but the wheel will likely still have a bump, giving an uneven ride. (You must also remove the broken spoke from the wheel.) This is a temporary fix not only because of the uneven ride, but also because the rest of the spokes and the rim will now have to take more stress, and thus other spokes can break or the rim can get damaged.

As a permanent fix, you must replace the broken spoke with a new one, then adjust the spokes again. You can have this done in a bicycle service if you don't have enough practice. Obviously if the rim is very damaged, then you might have to buy a completely new wheel instead.

– b_jonas 09:32, 26 April 2012 (UTC)

Bicycle spokes nearly universally break not because of overloading, but because of metal fatigue due to the repeated loading and unloading of the spoke, with corresponding stress in particular at the knee of the spoke. Spokes in a well-build wheel rarely break, but if spoke tension is too low, the loading/unloading effect during rotation is more pronounced, and the spoke will break earlier. If the spoke tension is high enough, all spokes will remain under tension permanently, so the metal will not deform significantly. Except for high-end models, modern bicycle wheels are machine-build, and should be re-tensioned after a few hundred kilometers. Unfortunately, doing this is a job for an expert. But it is well worth the money. --Stephan Schulz (talk) 18:03, 26 April 2012 (UTC)

Thanks guys! I was using the tensile strength of the un-fatigued metal, whereas the spoke was presumably very fatigued when it broke.

Resolved

Robinh (talk) 21:49, 26 April 2012 (UTC)