VCE Chemistry Question Thread

[Syndicate]

Can someone please explain this in detail? Thank you.

The melting pointing of ionic compounds is quite dependent on the magnitude of charge of atoms (the greater charge in magnitude, the higher melting point), and the atomic radii (smaller radius results in a higher melting point). The increase in charge produces stronger ionic bonds, whereas having a smaller radius allows atoms to stay closer, which also results in a greater bond strength.

Ions such Mg and O has 2+ and 2- charge respectively. Whereas Na and Cl has a charge of 1+ and 1- respectively. Straightaway looking at the choices given, MgO has the highest melting point because both Mg and O have a charge of 2 in magnitude. Therefore the answer is A.

[vcestressed]

The melting pointing of ionic compounds is quite dependent on the magnitude of charge of atoms (the greater charge in magnitude, the higher melting point), and the atomic radii (smaller radius results in a higher melting point). The increase in charge produces stronger ionic bonds, whereas having a smaller radius allows atoms to stay closer, which also results in a greater bond strength.

Ions such Mg and O has 2+ and 2- charge respectively. Whereas Na and Cl has a charge of 1+ and 1- respectively. Straightaway looking at the choices given, MgO has the highest melting point because both Mg and O have a charge of 2 in magnitude. Therefore the answer is A.

But then how can you tell if Na2O has a lower melting point than MgO? Sorry for questioning, i am still a little confused.

[Syndicate]

But then how can you tell if Na2O has a lower melting point than MgO? Sorry for questioning, i am still a little confused.

A single atom of Na has a charge of 1+, whereas Mg has a charge of 2+(O has a charge of 2- in both cases, which is why we only need to examine Na and Mg ions). Therefore MgO has a higher melting point than Na2O.

If you had two molecules, where its ions had the same charges (ie. MgO, and CaO), then you compare their radius( In this case, we also only need to examine the Mg and Ca ions, as O will have the same radius). Mg has a smaller radius than Ca, therefore MgO has a higher melting point than CaO.

[vcestressed]

A single atom of Na has a charge of 1+, whereas Mg has a charge of 2+(O has a charge of 2- in both cases, which is why we only need to examine Na and Mg ions). Therefore MgO has a higher melting point than Na2O.

If you had two molecules, where its ions had the same charges (ie. MgO, and CaO), then you compare their radius( In this case, we also only need to examine the Mg and Ca ions, as O will have the same radius). Mg has a smaller radius than Ca, therefore MgO has a higher melting point than CaO.

Makes sense. Thanks.
Is this right?
- MgO has 2+ and 2- charges and the greater the charges, the higher the melting point
- MgCl2 has 2+ and - charges and is a less stronger charge which is why it is wrong (?)
- Na2O has charge 2+ and -. . MgO is more stronger
- NaCl because 1+ and 1- and therefore MgO is stronger --> it has high melting point? ? ?

[ringring]

Hey!
Would appreciate any help/explanation for the following question..

An electrolytic cell contained platinum electrodes and 100.0 mL of an aqueous solution containing magnesium, nickel(II), silver, and sodium ions. The concentration of each of the ions in the solution was 0.200 M. A current of 0.800 A was passed through the cell for 2.50 hours. After this time the mass of the cathode would have increased by...
 (answer is 3.33g btw)

[lzxnl]

Makes sense. Thanks.
Is this right?
- MgO has 2+ and 2- charges and the greater the charges, the higher the melting point
- MgCl2 has 2+ and - charges and is a less stronger charge which is why it is wrong (?)
- Na2O has charge 2+ and -. . MgO is more stronger
- NaCl because 1+ and 1- and therefore MgO is stronger --> it has high melting point? ? ?

Essentially, what you're looking at is force per ion. Higher charges will result in larger forces. The ions are all 'roughly' the same size, so the size of the charge is more important.
With Na₂O vs MgO, for instance, consider two ions in each lattice.
If you take one Na⁺ and one O^2- ion, the attraction is 2 * some unit.
If you take one Mg²⁺ and one O^2- ion, the attraction is 4 * some unit because the force is proportional to BOTH charges.

Hey!
Would appreciate any help/explanation for the following question..

An electrolytic cell contained platinum electrodes and 100.0 mL of an aqueous solution containing magnesium, nickel(II), silver, and sodium ions. The concentration of each of the ions in the solution was 0.200 M. A current of 0.800 A was passed through the cell for 2.50 hours. After this time the mass of the cathode would have increased by...
 (answer is 3.33g btw)

Well, you're going to be plating silver first because silver ions are the strong oxidants here. The amount of electrons flowing is
0.8 A * 2.5 * 3600 s = 7200 C = 7200/96485 mol approx 0.075 mol.

You have 0.1 L * 0.2 M = 0.02 mol silver ions, so you'll run out of silver. The reaction is Ag⁺ + e^- -> Ag
so the 0.02 mol silver ions take up 0.02 mol of electrons. Now there are 0.055 mol left.

There are 0.02 mol of nickel ions that will take up 0.04 mol of electrons because the equation for nickel is Ni²⁺ + 2e^- -> Ni
This means we've made 0.02 mol of nickel and 0.02 mol of silver. You can check that the mass is 0.02 * (58.69 + 107.87) = 3.33 g.

What about the remaining 0.015 mol of electrons? You can't electrolyse magnesium or sodium as water is a stronger reductant than either of them. Therefore, the last remaining electrons will go to forming hydrogen gas.

[exit]

Hi!! For this, you would need to know the periodic table trend of electronegativity.
(Image removed from quote.)
Excluding noble gases, the halogens (Group 17) are the most electronegative elements that basically want electrons and attract to hydrogen.


There are 3 natures of molecules: Polar, non-polar, and ionic.
Polar Molecules
Polar molecules almost always have a BENT shape.
These molecules can be identified by looking for electronegative elements bonded to hydrogen.
For example, water:
(Image removed from quote.)
It has a bent shape.
But why is it bent?
This is because oxygen has high electronegativity because it is on the top-right of the periodic table.
It has a 2 lone pairs of electrons that repel the 2 hydrogen atoms into a "bent shape".
Because it is bent, one side of the molecule is negatively charged by oxygen's lone pairs; and the other side is positively charged by hydrogen being a proton. This is denoted by delta+ and delta-.
Water can dissolve salts and other polar molecules because it can form dipoles and hydrogen bonds due to its polar nature, however it cannot dissolve non-polar molecules like oil.

Non-polar Molecules
Non-polar molecules are usually linear in shape because it contains low electronegativity elements.
Examples of this are hydrocarbons like butane.
(Image removed from quote.)
Butane is a linear non-polar molecule with no electronegative elements. Carbon has little to no effects on hydrogen, hence its straight shape.
Butane cannot dissolve in water because it is not polar or ionic, thus it will float separaed in water if it were liquid.

Also worth mentioning, some molecules are BOTH polar and non-polar, such as ethanol and other alkanols.
(Image removed from quote.)
The C2H5- ethyl group can form dispersion forces and is non-polar, whereas the -OH hydroxyl group is polar with a bent shape.

Hope this helps with identifying polar/non-polar molecules
Let me know if more explanation is needed

HI for some reason the pictures don't show up anymore. Can you fix this?

[pmmenotes]

why are carbon chains in alkanes not straight as commonly drawn in structural formulas?

[keltingmeith]

why are carbon chains in alkanes not straight as commonly drawn in structural formulas?

You may remember learning about a little thing known as VSEPR theory in unit 1, which stands for "Valence Shell Electron Pair Repulsion".

In chemistry, we often talk about molecules having certain amounts of energy. Methane, for example, has a much higher amount of energy than carbon dioxide. If a carbon atom was told to pick between the two, it would much prefer to exist as carbon dioxide, because then it will have less energy. Molecules like having less energy, because the more energy they have, the more things they have to do. Picture yourself at home - you'd much rather lie around watching TV than doing homework, because then you have less energy you have to be using.

So, let's think back to our methane atom. VSEPR theory states that the closer electron pairs (this includes both lone pairs and bonds) are to each other, the higher the energy of that molecule will be. So, our four hydrogen atoms will want to be as far apart from each other as possible, because then the electron pairs will also be as far away as possible from everything. It just so happens (because maths), then when the carbon atom puts on a tetrahedral geometry (google if unsure), which is when each bond is at an approximately 109 degree angle.

It just so happens this theory applies to more than just molecule with all the same things bonded to it. Take propane as our next example - the middle carbon wants to bond tetrahedral so that it can be in its lowest energy state. This means that the carbon chain must have kinks at every carbon atom with an angle of 109 degrees, which is why even though in VCE you'll often draw straight chains (nobody after high school draws straight chains. It's universally agreed as bad notation. No clue why VCE enforces it so much), the actual chains won't look like that on a molecular level.

[miguel_1]

Can someone tell me what the worded form of this equation is:
3CO + Fe2O3 → 2Fe + 3CO2

[keltingmeith]

Can someone tell me what the worded form of this equation is:
3CO + Fe2O3 → 2Fe + 3CO2

What part of putting it into worded form are you struggling with?

[Sine]

Can someone tell me what the worded form of this equation is:
3CO + Fe2O3 → 2Fe + 3CO2

Carbon monoxide + Iron (III) Oxide ----> Iron + Carbon Dioxide

[miguel_1]

What part of putting it into worded form are you struggling with?

The Fe2O3. I knew it was iron oxide but I was looking for the another term for Iron (III) Oxide which I just googled. (A: Ferric oxide)

[21stressedout]

Hi I need some help!

We did an electrolysis of copper (II) sulfate, with a stainless steel cathode and a copper metal anode.

One of our questions is: If an inert carbon electrode was used instead of a strip of copper metal, would the experiment have worked?
And: Calculate the current efficiency of the cell, how do I do this? It's supposedly is the actual mass of copper deposited / theoretical amount of copper deposited. How do I calculate the theoretical amount?

Thanks!

[keltingmeith]

The Fe2O3. I knew it was iron oxide but I was looking for the another term for Iron (III) Oxide which I just googled. (A: Ferric oxide)

Tbh, wouldn't use ferric oxide. Only geologists will still use weird terminology like that, it's going out of phase for iron (III) which is a lot more straightforward.

Hi I need some help!

We did an electrolysis of copper (II) sulfate, with a stainless steel cathode and a copper metal anode.

One of our questions is: If an inert carbon electrode was used instead of a strip of copper metal, would the experiment have worked?
And: Calculate the current efficiency of the cell, how do I do this? It's supposedly is the actual mass of copper deposited / theoretical amount of copper deposited. How do I calculate the theoretical amount?

Thanks!

You may be getting stumped by the context - the trick with chemistry is, if they ask you something and it looks like you haven't learned it, you need to strip away all the scary stuff and just work with what you have.

What is an electrolysis? Just a type of chemical reaction. So how would you calculate the theoretical mass? Writing up the chemical equation, see how much you started with, know that at completion the amount of mole should be the same.