brightsky's Chem Thread

[Lasercookie]

Your textbook should have a page in the appendices explaining it. Otherwise have a look at this page Significant figures Guide.

There was a series of posts in a thread a month or two ago where Mao also explained significant figures in a lot of depth. I'll look for the link, I think you'll find it interesting. edit: this thread sig figs

It's actually pretty rare that a question will explicitly tell you how many significant figures. Basically, you just look at the numbers given in the question, use those values in your calculation (e.g. you'd just 10.0 in your calculations, rather than writing 10) and then you look for the number with the lowest amount of significant figures that you used in your calculation and round off to that at the end (so if there was another value given in the data, but you didn't use it in your calculations, you can ignore it). Also, values such as Molar Masses from the data book, if they happen to be the lowest amount of significant figures you used, then you use that value.

In this case the values you'd use would be 10.0 g (3 significant figures) and molar masses for Mg and Cl from the data book (so 3 significant figures). So you round off your final answer to 3 significant figures.

[brightsky]

ah okay thanks!!

and just to clarify: if one of the numbers in a question is given to say 10 s.f. (hypothetical!) and the molar masses given in the data book are to 3 s.f., we'd express our final answer to 3 s.f.?

(also does this mean that people with different data books (some periodic tables express molar masses to something like 5 s.f.) can in fact arrive at answers which have a varying number of s.f.?)

edit: mindf'ked by mao's "summary"....

[pi]

ah okay thanks!!

and just to clarify: if one of the numbers in a question is given to say 10 s.f. (hypothetical!) and the molar masses given in the data book are to 3 s.f., we'd express our final answer to 3 s.f.?

(also does this mean that people with different data books (some periodic tables express molar masses to something like 5 s.f.) can in fact arrive at answers which have a varying number of s.f.?)

Yep

(not at all, because everyone in VCAA exams will have the same periodic table in the data book )

[brightsky]

what is meant by the phrase 'absolute temperature'? how does this differ from normal temperature?

[Mao]

what is meant by the phrase 'absolute temperature'? how does this differ from normal temperature?

Absolute temperature refers to the temperature in Kelvin. The absolute temperature scale is defined with absolute zero as 0K.

But what is absolute zero?
We can predict where absolute zero is with thermodynamic laws (though using concepts not taught in VCE). Remember that temperature is just the average kinetic energy of a bunch of particles, so absolute zero is where all particles are motionless. Standard thermodynamics and quantum mechanics predict this temperature is -273.15 celsius, so we scale everything down to absolute zero and 0C becomes 273.15K.

[Mao]

and just to clarify: if one of the numbers in a question is given to say 10 s.f. (hypothetical!) and the molar masses given in the data book are to 3 s.f., we'd express our final answer to 3 s.f.?

Yes.

(also does this mean that people with different data books (some periodic tables express molar masses to something like 5 s.f.) can in fact arrive at answers which have a varying number of s.f.?)

Yes. This happens all the time!

edit: mindf'ked by mao's "summary"....

Which part? I could help to elaborate

[Mao]

I see...thanks for that. Yeah I sorta understand...although how would one determine the bond angles? I wrote 120 degrees, under the impression that both the electron pair geometry and the molecular shape of the molecule is trigonal planar, but the answers say that 'O-C-H bond angles are slightly more than 120 and H-C-H bond angles are slightly less than 120'. Why is this the case?

120 is approximately the right answer. The key thing in determining bond angle is to consider where the electrons are, as they repel each other.

In this case, you start with your trigonal planar molecule. On the oxygen side, you have a lot of electrons, that will repel the other electrons in the C-H bonds. On the hydrogen sides, you have fewer electrons, and so the H-C-H repulsion is less than the O-C-H repulsion. This will mean a slight change of angle with O-C-H slightly larger than 120, and H-C-H slightly less than 120.

Also, how would one work out the bond angles for SF4?

This one is tough. Sulfur has 6 valence electrons, if it makes 4 bonds, it will still have a lone pair left. (after making 4 bonds, S will have 10 electrons in its valence shell. It violates the octet rule, but that happens in some cases)

It turns out, the optimal shape for this arrangement is the slightly bent seesaw.

[Mao]

And is there a way of 'working out' the bond angles for more complicated molecules using maths or do we have to refer to the table every time?

We can always calculate the bond angles in idea geometrical shapes. However to work out the precise bond angles (such as the precise angles in formaldehyde), we'll have to do a very difficult calculation where we solve Schrodinger's wave equation for the whole molecule. (very difficult, requires a powerful computer)

Also, what is meant by the term radical? (The exact wording of the question is: 'Which of these species (NH3, H2CO, SF4, NO2) is a radical?')

A radical is a molecule with an odd number of electrons. Add them up and see!

so latent heat measures the energy required to change the state of a substance at its melting (fusion) or boiling (vaporisation) temperatures. for water, the values given are, for latent heat of fusion, 6 kJ mol and, for latent heat of vaporisation, 44 kJ mol. but according to another formula associated with heat capacity, Energy = SPC * mass * change in temperature.

my question is, according to the formula above, the latent heat values of a substance, say water, should be both 0 kJ/mol right, since there is no change in temperature from, for water, 0 C to 0C and 100 C to 100 C? what exactly is meant by saying changing the state at melting/boiling temperature. i would have thought that once the temperature of water passes a certain threshold, say 100 C, it would automatically turn to gas from a liquid. why do we need to put more energy in?

Consider this. In a solid, all the particles are locked in a lattice. In a liquid, particles can move around.

How hard is it to go from solid to liquid? To do this, we'll need to take one of the particles in the lattice out and put it somewhere else. In doing so, we *must* supply extra energy, as we are breaking bonds inside the lattice and putting the particle in a liquid where there are fewer bonds. The temperature has not changed, but we still need to supply energy.

The same argument runs for liquid --> gas, as liquids are more dense, so there are more bonds.

The specific heat equation only works for temperature changes without phase transition. When there is a phase transition, we need to supply extra energy to overcome the intermolecular bonding energy.

(this is a very simplified account of why there must be latent heats. there are other important components of latent heats that I haven't touched on, such as the change in entropy)

[Mao]

and also, can anyone explain to me how a photon can have a frequency? i've read up on how light can behave like a particle and a wave, but i'm finding this idea hard to conceptualise. for example, i think of a photon as being like a small ball that moves around in space. how can a small ball have a frequency...does it sort of move around in a wave-like motion, or?

Possibly one of the hardest concepts in physics. There isn't much that can be said for this. There is no fundamental physical form, if you keep zooming in you don't eventually find a small ball. You have to accept that any particles can have a momentum (like a particle) and a wavelength (like a wave).

[brightsky]

Thanks so much Mao!

Another: What is the difference between enthalpy of reaction and heat of reaction? It says in the book that only under constant pressure are the two equal. Why is this the case?

Thanks!

[paulsterio]

and also, can anyone explain to me how a photon can have a frequency? i've read up on how light can behave like a particle and a wave, but i'm finding this idea hard to conceptualise. for example, i think of a photon as being like a small ball that moves around in space. how can a small ball have a frequency...does it sort of move around in a wave-like motion, or?

Possibly one of the hardest concepts in physics. There isn't much that can be said for this. There is no fundamental physical form, if you keep zooming in you don't eventually find a small ball. You have to accept that any particles can have a momentum (like a particle) and a wavelength (like a wave).

If you run through a door, you will, technically, diffract

To be honest, even I can't imagine this concept, it just occurs naturally to me now that light has both wave like and particle like properties, just like particles can also have wave like properties.

[Mao]

Another: What is the difference between enthalpy of reaction and heat of reaction? It says in the book that only under constant pressure are the two equal. Why is this the case?

The enthalpy of a reaction can be thought of as the net amount of energy a reaction releases.

The heat of reaction is the amount of heat energy that is transferred.

Not all energy released has to be heat. There could be pressure changes which consumes some energy, or other processes which will affect the temperature changes.

[Aurelian]

The enthalpy of a reaction can be thought of as the net amount of energy a reaction releases.

The heat of reaction is the amount of heat energy that is transferred.

Not all energy released has to be heat. There could be pressure changes which consumes some energy, or other processes which will affect the temperature changes.

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...

[brightsky]

Another: What is the difference between enthalpy of reaction and heat of reaction? It says in the book that only under constant pressure are the two equal. Why is this the case?

The enthalpy of a reaction can be thought of as the net amount of energy a reaction releases.

The heat of reaction is the amount of heat energy that is transferred.

Not all energy released has to be heat. There could be pressure changes which consumes some energy, or other processes which will affect the temperature changes.

Hmm...what do you mean by transferred? I sort of understand what you're getting at...so does that that mean that the enthalpy of a reaction is always greater or equal to the heat of reaction?

Also, I don't quite get the last bit. How can pressure changes consume energy?

And while we're at it, what exactly is pressure in the context of a reaction/gases? I always thought pressure = force/area, but I don't really see how I can apply that to this particular context.

Thanks again!

[Aurelian]

Also, I don't quite get the last bit. How can pressure changes consume energy?

And while we're at it, what exactly is pressure in the context of a reaction/gases? I always thought pressure = force/area, but I don't really see how I can apply that to this particular context.

Thanks again!

For the first question, I'm not sure how up to date your physics knowledge is, but if you recall from year 10, work = force * distance - work is a form of energy. Consider a closed container with a piston on top which can go up an down (sort of like a syringe). Say a reaction occurs inside between some gaseous reactants, resulting in a nett production of gas molcules. Since the piston can be pushed out by the gas, the pressure will remain constant but the volume will increase to accommodate the new gas particles. The reaction is thus doing work by pushing the piston up - an energy transfer distinct from heat flow. Some simple dimensional analysis will also show you that P∆V = Fd = W. Hm, not sure how clear that was - trying to give a non-formal explanation haha

As for the second question, pressure is still force/area in this context, as all contexts. To illustrate this, consider a balloon the air inside of which is at some pressure - this represents the force the gas molecules are exerting per unit area against the surface of the balloon.

Does that help...?