brightsky's Chem Thread

[brightsky]

Ah makes sense...though still a bit confused about the 'dimensional analysis' part. the textbook gives the formula ∆U = ∆H - P∆V (which at present makes no sense whatsoever to me), and says that - P∆V is the 'energy' required to push the piston up, though it no derivation of any of the formulas are given.

would you be able to show me how P∆V = Fd = W came about?

many times thankyou!

[Aurelian]

Ah makes sense...though still a bit confused about the 'dimensional analysis' part. the textbook gives the formula ∆U = ∆H - P∆V (which at present makes no sense whatsoever to me), and says that - P∆V is the 'energy' required to push the piston up, though it no derivation of any of the formulas are given.

would you be able to show me how P∆V = Fd = W came about?

many times thankyou!

A crude derivation would just look at units; P * V = N/m^2 * m^3 = Nm = F * d. A better derivation would include a diagram to make it clearer what's actually going on... But it's a bit hard to do that on here

[brightsky]

okays, i'll try searching it up on the net..

also, this might sound a bit retarded, but why is it that the POTENTIAL energy of products in an endothermic reaction is greater than that of reactants? i thought potential energy concerns only the position of the object. surely then, raising the beaker with the products in it will increase the potential energy, or maybe not?

[Mao]

@brightsky, you are asking very good questions, but these questions are very difficult to explain over the internet, especially since we'll have to start by teaching you the fundamentals of thermodynamics. If you wish to learn more about this topic, I suggest you to read an introductory physics textbook and an introductory thermodynamics textbook. You will find most of your questions arise from the inadequacy of definitions of concepts at the VCE level.

Re: some specific questions,

I might be mistaken, but I believe that often textbooks and whatnot will also use the term "heat of reaction" as a simple synonym for "enthalpy of reaction" (i.e., without it necessarily referring to the actual heat exchange associated with the reaction...). Especially common when referring to heats/enthalpies of formation...

The two terms are used interchangeably, but heat and enthalpy change are only equal under constant pressure conditions. Even though we usually perform measurements at 1 atm (thus everything is at constant pressure), the two terms do not mean the same thing in a general sense.

also, this might sound a bit retarded, but why is it that the POTENTIAL energy of products in an endothermic reaction is greater than that of reactants? i thought potential energy concerns only the position of the object. surely then, raising the beaker with the products in it will increase the potential energy, or maybe not?

You are thinking gravitational potential energy. There are many different potential energies, such as the spring potential energy, electric (Coulombic) potential energy, etc etc. In fact, for any force which has a field (i.e. non-contact force), you can derive a potential energy for that field.

In this particular case, the potential energy is calculated from the intramolecular forces in a molecule, and is usually called the chemical potential energy.

[brightsky]

A few more questions:

1) The following is a list of selected standard reduction potentials:
Cd^(2+) (aq) + 2e^(-) --> Cd(s)    E^(0) = -0.40 V
Zn^(2+) (aq) + 2e^(-) --> Zn(s)    E^(0) = -0.76 V
Ni^(2+) (aq) + 2e^(-) --> Ni(s)      E^(0) = -0.23 V
By referring to the standard reduction potentials above, which of the following species is the best oxidizing agent?
A. Cd(s)
B. Zb^(2+)(aq)
C. Ni^(2+)(aq)
D. Zn(s)
E. Ni(s)

[I don't understand standard reduction potentials at all! Anyone able to give a simple, reader-friendly explanation of what they are?]

2) It is known that carbon monoxide reacts exothermically with hydrogen gas to form methane at 400 degrees Celsius in the presence of catalyst.

CO(g) + 2H_2(g) <--> CH_3OH(g) + heat

A mixture of carbon monoxide, hydrogen gas and methanol placed under conditions described above achieves equilibrium in a closed container. If the reaction temperature is changed to 450 degrees Celsius, which of the following statements is correct?

A. The total number of molecules in the container decreases.
B. The reaction rates of both the forward and reverse reactions remain constant.
C. The average molecular mass of the gaseous mixture decreases.
D. Rate of formation of hydrogen decreases while the rate of decomposition of methane increases.
E. Total pressure within the container decreases.

3) Magnesium oxide has a solubility of 0.0086 g/100 mL. What pH will a saturated solution of magnesium oxide have?

4) Consider the following three scenarios:
i) 2.3 g of pure Na_2CO_3 reacts completely with a certain volume V1 of 0.1 M hydrochloric acid to give sodium chloride, carbon dioxide and water.
ii) 2.3 g of a sample containing Na_2CO_3 but also contaminated with some K_2CO_3, similarly reacts completely with a certain volume V2 of 0.1 M hydrochloric acid.
iii) 2.3 g of a sample containing Na_2CO_3 but contaminated with some NaOH, similarly reacts completely with a certain volume V3 of 0.1 M hydrochloric acid.

What is the relationship between the volumes V1, V2 and V3? (Use greater than or less than signs).

5) Which of the following groups of ions can coexist in significant quantities in aqueous solution without reacting with each other?
A. HSO_4^(-), K^(+), Na^(+), HCO_3^(-)
B. K^(+), CO_3^(2-), Cl^(-), Al^(3+)
C. K^(+), Ba^(2+), NO_3^(-), OH^(-)
D. Na^(+), Ba^(2+), H^(+), SO_4^(2-)
E. Ca^(2+), K^(+), CO_3^(2-), Cl^(-)

6) A gaseous mixture containing ethane and methane underwent complete combustion to produce 5.28 g of carbon dioxide and 3.78 g of water. What is the volume ratio of ethane to methane in the original mixture?

Thanks!

[brightsky]

For the question:

Find pH of 10^(-7) M HCl.

What exactly is it that makes it wrong for us to proceed as per usual? Why is this not the case for questions like:

Find pH of 0.1 M HNO_3.

Thanks!

[pi]

Well, think about it this way, HCl is ALWAYS going to be acidic, it's an acid, no matter what the concentration is (at 25deg) is will always be acidic.

pH = -log(10^-7) = -(-7) = 7

This implies that HCl solution is neutral (again, assuming 25deg). Which cannot be true as it's an acid.

Take a more extreme example: pH of 10^-10 HCl, plugging it in -> pH = -log(10^-10) = -(-10) = 10. Obviously it is not going to be basic! Rather, it'll be very close to 7, approaching it from the acidic side.

[brightsky]

Well, think about it this way, HCl is ALWAYS going to be acidic, it's an acid, no matter what the concentration is (at 25deg) is will always be acidic.

pH = -log(10^-7) = -(-7) = 7

This implies that HCl solution is neutral (again, assuming 25deg). Which cannot be true as it's an acid.

Take a more extreme example: pH of 10^-10 HCl, plugging it in -> pH = -log(10^-10) = -(-10) = 10. Obviously it is not going to be basic! Rather, it'll be very close to 7, approaching it from the acidic side.

Thanks!

So at what point do we need to rule out the 'normal method' and resort to the method used to determine pH for the first question I posed? 10^(-6)? 10^(-7.2)?

[illuminati]

Well, think about it this way, HCl is ALWAYS going to be acidic, it's an acid, no matter what the concentration is (at 25deg) is will always be acidic.

pH = -log(10^-7) = -(-7) = 7

This implies that HCl solution is neutral (again, assuming 25deg). Which cannot be true as it's an acid.

Take a more extreme example: pH of 10^-10 HCl, plugging it in -> pH = -log(10^-10) = -(-10) = 10. Obviously it is not going to be basic! Rather, it'll be very close to 7, approaching it from the acidic side.

Thanks!

So at what point do we need to rule out the 'normal method' and resort to the method used to determine pH for the first question I posed? 10^(-6)? 10^(-7.2)?

If the "normal" method doesn't provide the answer, I suggest adding the concentration of said HCl with the hydronium ions from your self-ionisation of water (10^-7M).
In this question you'd get a concentration of about 2 x 10^-7M and use that in your log to find pH
The reason this differs from the normal method is because the concentration of the solution is usually high enough that the additional hydronium from the self-ionisation is irrelevant. e.g. 0.1M + 0.0000001M makes almost no change to the concentration and your pH.

[Mao]

If the "normal" method doesn't provide the answer, I suggest adding the concentration of said HCl with the hydronium ions from your self-ionisation of water (10^-7M).
In this question you'd get a concentration of about 2 x 10^-7M and use that in your log to find pH
The reason this differs from the normal method is because the concentration of the solution is usually high enough that the additional hydronium from the self-ionisation is irrelevant. e.g. 0.1M + 0.0000001M makes almost no change to the concentration and your pH.

I would almost say that's correct, though you must take into account the self-ionisation of water (which is an equilibrium), so the actual [H+] of a 10^-7 M HCl solution is somewhere in between 10^-7 M and 2*10^-7 M (if you do the math, [H+]=1.6*10^-7 M)

The point is, we generally don't need to worry about protons already present in neutral water, nor the equilibrium of self ionisation of water. Experimental concentrations (as small as millimolar solutions) are order of magnitudes greater than 10^-7, and we can safely neglect the contributions.

Students have not been taught how to deal with the equilibrium of self ionisation of water (the relevant theories are taught, but no one expects a student to solve a quadratic formula in a chemistry exam). As far as the students are concerned, they only need to be aware of how the usual method breaks down at sub-micromolar concentrations.

Since the students do not actually do the calculations, there is no clear-cut way of knowing where the usual method breaks down. However, the examiners are aware of this. They won't ask a question on 10^-6 M HCl, because students cannot reasonably assess how large the errors are (it turns out to be a 1% error), so they will either use concentrations that are much larger than 10^-7, or use an obviously small  concentration.

But, for the more curious, I've tabulated some data for your enjoyment:

[illuminati]

If the "normal" method doesn't provide the answer, I suggest adding the concentration of said HCl with the hydronium ions from your self-ionisation of water (10^-7M).
In this question you'd get a concentration of about 2 x 10^-7M and use that in your log to find pH
The reason this differs from the normal method is because the concentration of the solution is usually high enough that the additional hydronium from the self-ionisation is irrelevant. e.g. 0.1M + 0.0000001M makes almost no change to the concentration and your pH.

I would almost say that's correct, though you must take into account the self-ionisation of water (which is an equilibrium), so the actual [H+] of a 10^-7 M HCl solution is somewhere in between 10^-7 M and 2*10^-7 M (if you do the math, [H+]=1.6*10^-7 M)

The point is, we generally don't need to worry about protons already present in neutral water, nor the equilibrium of self ionisation of water. Experimental concentrations (as small as millimolar solutions) are order of magnitudes greater than 10^-7, and we can safely neglect the contributions.

Students have not been taught how to deal with the equilibrium of self ionisation of water (the relevant theories are taught, but no one expects a student to solve a quadratic formula in a chemistry exam). As far as the students are concerned, they only need to be aware of how the usual method breaks down at sub-micromolar concentrations.

Since the students do not actually do the calculations, there is no clear-cut way of knowing where the usual method breaks down. However, the examiners are aware of this. They won't ask a question on 10^-6 M HCl, because students cannot reasonably assess how large the errors are (it turns out to be a 1% error), so they will either use concentrations that are much larger than 10^-7, or use an obviously small  concentration.

But, for the more curious, I've tabulated some data for your enjoyment:

haha yeah, i couldn't be bothered solving a quadratic so i took the easy way out and just added them together
that and i had no idea whether the question was based on u2 or u4, cos in u2 equilibrium hasn't been taught yet...

[brightsky]

A few questions:

1) What exactly is the difference between an electric field and a magnetic field?

2) According to quantum mechanics, light exhibits both wave and particle properties. Exactly what wave properties and what particle properties does light exhibit? And while we're at it, what exactly IS a wave? My understanding is that it is simply a collection of particles moving in a 'wave-like' manner, but apparently it is totally distinct from a particle.

3) What causes light to exhibit those properties? I've been told the electromagnetic field has some effect on the behaviour of light, but how?

Thanks! (And forgive my ignorance if some of these questions sound a bit derpy!)

[Stick]

These sound like Physics questions to me (I'd help you but conveying my Physics knowledge is not a strength of mine). I guess you may find your answers if you're willing to have a quick flick through a Physics textbook.

[Mao]

A few questions:

1) What exactly is the difference between an electric field and a magnetic field?

2) According to quantum mechanics, light exhibits both wave and particle properties. Exactly what wave properties and what particle properties does light exhibit? And while we're at it, what exactly IS a wave? My understanding is that it is simply a collection of particles moving in a 'wave-like' manner, but apparently it is totally distinct from a particle.

3) What causes light to exhibit those properties? I've been told the electromagnetic field has some effect on the behaviour of light, but how?

Thanks! (And forgive my ignorance if some of these questions sound a bit derpy!)

These are very difficult questions, but very good questions. My 'explanations' here aren't by any standards correct, they are simply interpretations of the equations in classical and quantum theories of electrodynamics.

1. Electrostatic interactions and magnetic interactions are aspects of electromagnetism. Electric fields describe the forces felt by a charge at some point in space due to the location of other charges. Magnetic fields describe the forces felt by a charge at some point in space due to the motion of other charges. I can expand a bit more on the origin of these forces, but the take home message here is that electric fields and magnetic fields are part of the same force, but they are separated mathematically as it is convenient to think about them separately. They really represent how charges affect other charges.

2. Light diffracts (wave-like property), but also has a momentum (particle-like). There is also the photoelectric effect and various other effects showing light behave like particles with a defined quanta of energy (like individual particles).

What is a wave? A wave is something that oscillates in time and space, and transfers energy. Examples are sound waves (density oscillations in air), mechanical waves (e.g. the motion of a slinky spring), and light (EM-field oscillations). These oscillations can transfer energy by exerting a force on another body. For sound, the density oscillations exert forces on hairs in your inner ear, transferring mechanical energy (if you have really loud speakers, you can make your walls shake by the same principle). For the slinky spring, energy is transferred when the end of the spring impacts another body, transferring mechanical energy. For light, the EM-field exerts electromagnetic forces on other charged particles (e.g. electrons in your retina), transferring EM potential energy.

3) Nothing 'causes' light to exhibit those properties, much like nothing 'causes' gravity to attract two masses. Light *has* these properties.

I must stress that these aren't easy questions, and the small length of forum posts aren't suitable for answering these questions. Many many books have been written in attempt to explain these questions. These concepts take years of pondering and discussion to 'understand' (and the level of understanding changes dramatically as you learn more about it). Indeed many brilliant physicists agree there is no 'correct' interpretation of the equations of quantum mechanics.

I will be happy to discuss these with you further, but a lot of my understanding is based off higher level mathematics that you probably haven't been exposed to, so I may not be much use here.

[brightsky]

These are very difficult questions, but very good questions. My 'explanations' here aren't by any standards correct, they are simply interpretations of the equations in classical and quantum theories of electrodynamics.

1. Electrostatic interactions and magnetic interactions are aspects of electromagnetism. Electric fields describe the forces felt by a charge at some point in space due to the location of other charges. Magnetic fields describe the forces felt by a charge at some point in space due to the motion of other charges. I can expand a bit more on the origin of these forces, but the take home message here is that electric fields and magnetic fields are part of the same force, but they are separated mathematically as it is convenient to think about them separately. They really represent how charges affect other charges.

2. Light diffracts (wave-like property), but also has a momentum (particle-like). There is also the photoelectric effect and various other effects showing light behave like particles with a defined quanta of energy (like individual particles).

What is a wave? A wave is something that oscillates in time and space, and transfers energy. Examples are sound waves (density oscillations in air), mechanical waves (e.g. the motion of a slinky spring), and light (EM-field oscillations). These oscillations can transfer energy by exerting a force on another body. For sound, the density oscillations exert forces on hairs in your inner ear, transferring mechanical energy (if you have really loud speakers, you can make your walls shake by the same principle). For the slinky spring, energy is transferred when the end of the spring impacts another body, transferring mechanical energy. For light, the EM-field exerts electromagnetic forces on other charged particles (e.g. electrons in your retina), transferring EM potential energy.

3) Nothing 'causes' light to exhibit those properties, much like nothing 'causes' gravity to attract two masses. Light *has* these properties.

I must stress that these aren't easy questions, and the small length of forum posts aren't suitable for answering these questions. Many many books have been written in attempt to explain these questions. These concepts take years of pondering and discussion to 'understand' (and the level of understanding changes dramatically as you learn more about it). Indeed many brilliant physicists agree there is no 'correct' interpretation of the equations of quantum mechanics.

I will be happy to discuss these with you further, but a lot of my understanding is based off higher level mathematics that you probably haven't been exposed to, so I may not be much use here.

Thanks so much Mao!! Really, really appreciate it! I'll try and look into quantum mechanics a little bit more before discussing it any further with you; otherwise, I'd probably be wasting your time. :p

I hate it how VCE requires you to 'know' stuff but not to KNOW it if you know what I mean.

Another quick question:

Would anyone be able to explain to me in clear terms the exact steps involved in UV-vis spectroscopy and how this particular technique helps to identify the identity of chemicals in a particular sample of stuff? I *sort of* understand what the textbook is talking about, but it doesn't explain the procedure in a step-by-step manner; it's a bit all over the shop.

Thanks!