Rod's Chemistry 3/4 Questions Thread

[keltingmeith]

Your studied chemical will NOT appear in the exam - don't stress. Once the SAC is gone, throw out the notes without fear. But don't - the notes are useful to hold onto as possible examples for applications of equilibria.

You're close - I'm not sure that volume is actually related to collision theory. It's all about two things:

• orientation
• kinetic energy

Let's refer to the ideal gas law, PV = nRT. You see, when we increase pressure, we'll get a resulting increase in temperature. This means we'll see an increase in kinetic energy, and that's what's giving us more collisions, not our decrease in volume. That's what's increasing the rate of reaction, and vice-versa for a decrease in pressure.

[Rod]

Your studied chemical will NOT appear in the exam - don't stress. Once the SAC is gone, throw out the notes without fear. But don't - the notes are useful to hold onto as possible examples for applications of equilibria.

You're close - I'm not sure that volume is actually related to collision theory. It's all about two things:

• orientation
• kinetic energy

Let's refer to the ideal gas law, PV = nRT. You see, when we increase pressure, we'll get a resulting increase in temperature. This means we'll see an increase in kinetic energy, and that's what's giving us more collisions, not our decrease in volume. That's what's increasing the rate of reaction, and vice-versa for a decrease in pressure.

Understood! thanks pal

[Reus]

Let's refer to the ideal gas law, PV = nRT. You see, when we increase pressure, we'll get a resulting increase in temperature. This means we'll see an increase in kinetic energy, and that's what's giving us more collisions, not our decrease in volume. That's what's increasing the rate of reaction, and vice-versa for a decrease in pressure.

Amazing explanation.

[keltingmeith]

Thanks, hahah. I was absolutely horrible at 3/4 chem, and I've rapidly picked up my game in uni. It's only when you suck at something and become good at it that you can explain it to others.

(welp, if you're good to begin with you can still teach people, but I digress... )

[lzxnl]

Thanks, hahah. I was absolutely horrible at 3/4 chem, and I've rapidly picked up my game in uni. It's only when you suck at something and become good at it that you can explain it to others.

(welp, if you're good to begin with you can still teach people, but I digress... )

*ahem

[keltingmeith]

I added an adamendendum. I love our smart peoples, too. <3

[Rod]

Hey everyone

Can someone please explain why catalysts have no effect at all at equilibrium? thanks

[lzxnl]

Catalysts work by reducing the activation energy of a reaction. However, reducing the activation energy of the forward reaction also decreases the activation energy of the reverse reaction. As a result, both forwards and backwards reaction rates are sped up.

Or, if you want, a catalyst isn't mentioned in the equilibrium constant expression so its concentration shouldn't affect the equilibrium (a bit hand-wavy but hopefully it helps you remember)

Finally, catalysts aren't consumed in the reaction. Imagine you add a catalyst to an equilibrium mixture. The amount of catalyst hasn't changed. Therefore, there is no change to the system. Do you expect the equilibrium to shift?

[Rod]

Catalysts work by reducing the activation energy of a reaction. However, reducing the activation energy of the forward reaction also decreases the activation energy of the reverse reaction. As a result, both forwards and backwards reaction rates are sped up.

Or, if you want, a catalyst isn't mentioned in the equilibrium constant expression so its concentration shouldn't affect the equilibrium (a bit hand-wavy but hopefully it helps you remember)

Finally, catalysts aren't consumed in the reaction. Imagine you add a catalyst to an equilibrium mixture. The amount of catalyst hasn't changed. Therefore, there is no change to the system. Do you expect the equilibrium to shift?

thanks lzxnl

[Rod]

 When nitric acid acts as an oxidant, it oxidises another species and is itself reduced. As a result of being reduced there will be a decrease in the oxidation number of nitrogen.   In HNO 3 the oxidation number of N is +5   In NO 3 -, the oxidation number of N is +5   In N 2, the oxidation number of N is 0   In NO, the oxidation number of N is +2   In NO 2, the oxidation number of N is +4   Hence NO 3 - will not be produced when HNO3 acts as an oxidant.

Not following. If HN03 is to be reduced, then why does whatever HNO3 oxidises have to have nitrogen with less oxidation number than the nitrogen in HNO3?

Thanks

2;AM I know this is pissing me off

[thushan]

The only reason for that is that the maximal oxidation state of nitrogen is +5. Hence, if HNO3 were to oxidise another nitrogen-based species (eg NO, N2, NH3), then that species needs to be able to be oxidised. This other species cannot have an oxidation number of +5, because it cannot be oxidised to attain a higher oxidation number. Therefore, the oxidation number of this other species needs to be less than +5.

[Rod]

The only reason for that is that the maximal oxidation state of nitrogen is +5. Hence, if HNO3 were to oxidise another nitrogen-based species (eg NO, N2, NH3), then that species needs to be able to be oxidised. This other species cannot have an oxidation number of +5, because it cannot be oxidised to attain a higher oxidation number. Therefore, the oxidation number of this other species needs to be less than +5.

thanks man


Another question; here's a study design point:

Comparison of the renewability of energy sources including coal, petroleum, natural gas, nuclear fuels and biochemical fuels

I'm just planning to make a simple summary table for this. Our school is skipping it as well, apparently it doesn't really come up in the exam. Would a table be enough you reckon?

thanks

[thushan]

Yeah, just know which are renewable. Knowing what 'carbon-neutral' is doesn't hurt.

[Yacoubb]

Yeah, just know which are renewable. Knowing what 'carbon-neutral' is doesn't hurt.

Would a reasonable explanation for carbon-neutral be that there is no net increase in the CO2 levels in the atmosphere through the burning of these fuels?

[Rod]

Why does the accumulation of charge in one half cell in the galvanic cell stop electricity from being produced?