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Topic: Some simple concepts I'm having difficulty with understanding  (Read 4382 times)

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Offline Bublik

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Hello,

This is my first post on this forum and I have noticed that there are most likely people who can help me understand some very basic concepts of chemistry. I am currently studying grade 12 AP chemistry, and I can solve most problems at this level with ease. However, I am having difficulty understanding why some things work the way they do.

First, I don't understand how to predict which compounds would give an acidic aqueous solution and which ones would give a basic aqueous solution. People seem to be able to tell right away, but I certainly can't. I understand how the measure of acidity works, how it is the log of the concentration of hydrogen ions, but let me give an example to explain my confusion:

I have heard that KCN, when dissolved in water, would give a basic aqueous solution. Why is this? I recognize that KCN will ionize into separate K+ and CN- ions. What I don't understand is how that will change the pH. In water, there are H+ and OH- ions which neutralize eachother and explain the neutral pH of 7. however, how does the addition of KCN affect this?

I mean, I understand when something like NaOH is added and why that augments the pH, because that is a direct OH- addition and in the above case there is no such thing. Can someone please explain that to me thoroughly?

Second, and this is a very specific question: Does the speed or weight of photons have anything to do with how energetic the corresponding radiation is?
X-ray radiation is more energetic than microwave radiation only because x-ray radiation has a higher frequency than does microwave radiation. Am I right about this? The characteristics of the photon pertaining to this question are irrelevant, right?

If that sounded confusing, then the specific problem I was looking at was:
X-ray radiation is more energetic than microwave radiation because:
A) photons of x-ray radiation travel faster than those of microwave radiation
B) photons of x-ray radiation are heavier than those of microwave radiation
C) x-ray radiation has a higher frequency than does microwave radiation
D) x-ray radiation has a longer wavelength than does microwave radiation
E) photons of x-ray radiation travel slower than those of microwave radiation

Thirdly, another question that I had is just why is radiation bad (to humans)? how are these electromagnetic waves, in some cases, so destructive?

Any help will be greatly appreciated. This isn't for any particular assignment or task, I just want to understand why and how some things work in chemistry.

Thanks in advance. 


Offline Jorriss

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Re: Some simple concepts I'm having difficulty with understanding
« Reply #1 on: July 17, 2012, 10:22:47 PM »
The energy of light/photons/electromagnetic radiation is given by E=hv where v is the frequency of the light. We see that the energy of light - be it microwaves, x-rays, whatever, is only based on the frequency of the light.

It almost had to be that way. Photons are massless and all frequencies travel at the same speed so those could not have effected the energy.

Light is dangerous at some frequencies because it causes certain unwanted chemical reactions.

Offline Bublik

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Re: Some simple concepts I'm having difficulty with understanding
« Reply #2 on: July 17, 2012, 10:33:29 PM »
The energy of light/photons/electromagnetic radiation is given by E=hv where v is the frequency of the light. We see that the energy of light - be it microwaves, x-rays, whatever, is only based on the frequency of the light.

It almost had to be that way. Photons are massless and all frequencies travel at the same speed so those could not have effected the energy.

Light is dangerous at some frequencies because it causes certain unwanted chemical reactions.

Thank you, that makes sense.

Offline camptzak

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Re: Some simple concepts I'm having difficulty with understanding
« Reply #3 on: July 20, 2012, 01:25:41 PM »
photons have mass
 e=mc2

but other than that yah true
"Chance favors the prepared mind"
-Louis Pasteur

Offline Bublik

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Re: Some simple concepts I'm having difficulty with understanding
« Reply #4 on: July 20, 2012, 02:02:15 PM »
camptzak, I believe you are incorrect with that statement. The photon is massless.

The following is an excerpt from wikipedia.

"The photon is currently understood to be strictly massless, but this is an experimental question. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c. Its speed would be lower and depend on its frequency. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the maximum speed that any object could theoretically attain in space-time.Thus, it would still be the speed of space-time ripples (gravitational waves and gravitons), but it would not be the speed of photons.
 
A massive photon would have other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra physical degree of freedom. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would cause the presence of an electric field inside a hollow conductor when it is subjected to an external electric field. This thus allows one to test Coulomb's law to very high precision. A null result of such an experiment has set a limit of m ≲ 10-14 eV/c2."

So Jorriss was right about photons being massless and all frequencies traveling at the same speed. That way, it makes sense.

Offline Jorriss

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Re: Some simple concepts I'm having difficulty with understanding
« Reply #5 on: July 21, 2012, 12:53:20 AM »
photons have mass
 e=mc2

but other than that yah true
E=mc^2 does not apply to a photon.

A more general formula for relativistic energy is E^2=(mc^2)^2+(pc)^2. A photon has zero rest mass so E=pc for a photon.

Offline XGen

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Re: Some simple concepts I'm having difficulty with understanding
« Reply #6 on: July 27, 2012, 05:09:55 PM »
In your response to the acidic/basic salts, the fact that you seem to be neglecting is that the cations and anions formed on salt dissociation can influence the H+ and OH- concentrations.

For example, in your given salt, KCN, the ions produced upon dissociation are K+ and CN-. Now, the cation will react with hydroxide, and the anion will react with hydronium. For the cation, the reaction is K+ + OH- -> KOH. However, KOH is a strong base and will also dissociate completely. Therefore, nothing will happen between potassium and hydroxide. However, when you look at H+ + CN- -> HCN, the product hydrogen cyanide is not a strong acid. Therefore, it will dissociate only partially. That means that some of the hydronium from the original pH 7 will no longer be in solution because the cyanide ion "took" them. This makes the hydronium ion concentration smaller than the original amount.

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