VCE Chemistry Question Thread

[RazzMeTazz]

Would it be correct to say that when you mix a solution of NaCl(aq) and AgNO3(aq) , a precipitate of AgCl(s) forms, because the energy released when the Ag+(aq) and Cl-(aq) ions form ionic bonds with each other, is greater than the energy released when these respective ions form ion-dipole bonds with the oppositely charged polar ends of H20 molecules.

 Hence, the two ions combine to form the insoluble precipitate of AgCl?

Thanks!

Also during gravimetric procedures, suppose I wanted to quantitatively analyse the NaCl content in chicken stock cube and so I measure this chicken stock cube and then dissolve it in de-ionised water, then react with AgNO3 to form a precipitate of AgCl. I then measure the mass of the AgCl and use various mole calculations to find the Na content in the original chicken stock cube. What would happen if I originally didn't dissolve the chicken stock cube in de-ionised water, but instead used normal water? Would the Na+, Cl-, Ag+ and NO3- ions in solution possibly form other precipitates that we would be unaware of?

And lastly, is there any benefits in using vacuum filtration over gravity filtration, other than speed?
Thanks!

[Kel9901]

Also during gravimetric procedures, suppose I wanted to quantitatively analyse the NaCl content in chicken stock cube and so I measure this chicken stock cube and then dissolve it in de-ionised water, then react with AgNO3 to form a precipitate of AgCl. I then measure the mass of the AgCl and use various mole calculations to find the Na content in the original chicken stock cube. What would happen if I originally didn't dissolve the chicken stock cube in de-ionised water, but instead used normal water? Would the Na+, Cl-, Ag+ and NO3- ions in solution possibly form other precipitates that we would be unaware of?

Basically, the problems when using normal water are this:
There could be Cl- or Ag+ ions already present in the water (most likely the former), and they could affect the results (if there are Cl- ions already present for example, the mass of precipitate is larger therefore calculated Na content is higher).

There could also be other precipitates that are formed when Ag+ is introduced into the solution; Ag+ can form precipitates with other ions already present in the water.

And lastly, is there any benefits in using vacuum filtration over gravity filtration, other than speed?
Thanks!

I don't think that's on the course

[Splash-Tackle-Flail]

When doing question that involve multiple calculations, for example if we have to calculate the number of moles of an element, then find the concentration of it (say we're given the values of the mass, molar mass and the volume), when we calculate the number of moles, and write it to correct significant figures, for the calculation to find the concentration (using the moles and volume), do we use the number of moles corrected with significant figures, or do we use the exact value?

Hope that made sense.

[RazzMeTazz]

Basically, the problems when using normal water are this:
There could be Cl- or Ag+ ions already present in the water (most likely the former), and they could affect the results (if there are Cl- ions already present for example, the mass of precipitate is larger therefore calculated Na content is higher).

There could also be other precipitates that are formed when Ag+ is introduced into the solution; Ag+ can form precipitates with other ions already present in the water.
I don't think that's on the course

Thanks for the expanation!

[RazzMeTazz]

When doing question that involve multiple calculations, for example if we have to calculate the number of moles of an element, then find the concentration of it (say we're given the values of the mass, molar mass and the volume), when we calculate the number of moles, and write it to correct significant figures, for the calculation to find the concentration (using the moles and volume), do we use the number of moles corrected with significant figures, or do we use the exact value?

Hope that made sense.

During Thushan's ATARnotes chemistry lectures, I am quite sure he said to use exact values in consequent calculations, except when you write your final answer you take into account the sig figs of all the different values you have used.

[Alter]

Reinforcing the above answer 100%. My teacher made this point really clear to us-you shouldn't round off significant figures on top of already rounded numbers as it is technically incorrect. Always use exact numbers in calculations, as it's pretty simple even if you can't 'drag' answers down on your scientific calculator.

[RazzMeTazz]

Are all precipitates forming in H2O hydrated to some extent (contain H2O molecules within them), because they all seem to be needed to heated to constant mass during gravimetric analysis?

Also when explaining this would it be correct to say that: "during the formation of the precipitate, some H2O molecules become trapped within the precipitate."

^ I thought this sounded kind of dodgy but I don't know if it is correct/how else I would explain it. 

[Splash-Tackle-Flail]

During Thushan's ATARnotes chemistry lectures, I am quite sure he said to use exact values in consequent calculations, except when you write your final answer you take into account the sig figs of all the different values you have used.

Reinforcing the above answer 100%. My teacher made this point really clear to us-you shouldn't round off significant figures on top of already rounded numbers as it is technically incorrect. Always use exact numbers in calculations, as it's pretty simple even if you can't 'drag' answers down on your scientific calculator.

Thanks! was really hoping to go to the lecture but was overseas haha!

[keltingmeith]

What's the best way to go about answering questions such as these?

... Have a think? I honestly don't know, consider the experiment being done, and the best instrument for the job. Take into considerations such as speed of result delivery, cost of use/maintenance and appropriate use of machine opposed to just "will it give me an answer I want?"

For example, if I have a metal, and know it consists of Strontium or Nickel, would I use an AAS or flame test to figure out which one? Flame test, definitely. AAS takes longer to get results, costs more to use and is just plain overkill for the situation. Sure, it'll give you the answer you want, but the flame test would be a much better decision.

Yup that make sense. And self studied the unit 3 course content already.
Also it states that enthalpy is change in energy divide by mol. Except isn't enthalpy just the amount of energy released or absorbed by a reaction, and change in energy just the amount of energy needed to raise calorimeter by 1 degree Celsius. So why do we need to divide energy change by mol to get enthalpy.

Your red sentence basically says that enthalpy=energy (which, for a 3/4 course in chemistry, we can pretend they are the exact same). Your green sentence talks about a change in energy, which must be the same as a change in enthalpy, because energy and enthalpy are the same. Make sense?

Curiously, measuring the enthalpy at a single point is very hard to do - however, measuring the CHANGE in enthalpy is much, much easier.

And on another note: why is heat of combustion = energy change divide by gram. Isn't heat of combustion like enthalpy in that is the amount of energy released by a reaction= just that since it's an exothermic reaction it has to be positive.
Thank you once again. = I seriously believe textbook don't explain these concept well enough. Or just me..
thanks a lot Eulerfan101 for the help.

They are exactly the same. Hell, heat of combustion can easily be measured in mole. It honestly depends on if the question wants you to state it as /mol or /g. If it's something like a biscuit or product that is a mixture of MANY different elements, we go by /g because it's just easier than calculating the complex molecular formula and going by moles. If it's a particular compound, like NH4SO4, then we tend to go by /mol, because that makes calculations much easier.

[grannysmith]

When 1.92 g of a sample of gypsum (hydrated CaSO₄ ) was heated, it lost 0.51 g of H₂O . What was the degree of hydration in the sample?

I assumed m(CaSO₄) = 1.92 - 0.51 = 1.41 g so my answer ended up being 2.7 ~ 3.0. However, the solutions used 1.92 as the mass of CaSO₄ and got 2.

Can someone explain?

[knightrider]

i always see in textbooks and people saying that like forces repel and opposites attract.

Is there a reason to why this actually works?

[keltingmeith]

i always see in textbooks and people saying that like forces repel and opposites attract.

Is there a reason to why this actually works?

Hopefully these minutephysics videos are somewhat helpful. It's far beyond the course, though.

[knightrider]

For the image attached it tells us the maximum number of electrons in each shell.

It says shell 1=2 electrons,2=8e ,3=18e and 4=32e


but in my book for other examples they use the following pattern.

2,8,8,2

Why is this different to the image attached and why do they use this?

Also how can you tell which one you need to use?

[keltingmeith]

For the image attached it tells us the maximum number of electrons in each shell.

It says shell 1=2 electrons,2=8e ,3=18e and 4=32e


but in my book for other examples they use the following pattern.

2,8,8,2

Why is this different to the image attached and why do they use this?

Also how can you tell which one you need to use?

Because electrons don't fill according to "this shell has more space, let's jump in!"
Instead, you need to consider each sub-shell and their relative energy level. It just so happens that the 4s subshell has a lower energy level than the 3d subshell, hence why we only see 8 electrons in the third shell, even though we've started filling up the fourth.

To memory, you won't ever have to worry about subshells or the filling of atoms beyond the 4s subshell, so you can just not worry about this if you want (although I swear you're meant to learn it in unit 1?)

[Maths Forever]

For the image attached it tells us the maximum number of electrons in each shell.

It says shell 1=2 electrons,2=8e ,3=18e and 4=32e


but in my book for other examples they use the following pattern.

2,8,8,2

Why is this different to the image attached and why do they use this?

Also how can you tell which one you need to use?

2, 8, 8, 2 represents the electron configuration of Calcium.

The 's' and 'p' subshells are full within the third shell. Before filling the 'd' subshell of the third shell, electrons will fill the 's' subshell of the fourth shell.

But for scandium (atomic number 21), the electron configuration is 2, 8, 9, 2.

Once the 's' subshell within the fourth shell is full, electrons will then begin to fill the 'd' subshell of the third shell.

Refer to Figure 2.21 on page 30 of Heinemann Chemistry 1 for the order that electrons fill subshells.

I hope this helps!