VCE Chemistry Question Thread

[amanaazim]

hey whys i still don't get it. I'm confused

First, use the formula n = N/NA where n is moles, N is the number of particles and NA is Avogadro’s number. Once you have the number of moles, sub it into the formula n = m/M where n is moles, m is mass in grams and M is molar mass. This should help you solve all the above questions.

[Bri MT]

hey whys i still don't get it. I'm confused

Whys has described that for each question the first step is to divide the number of particles (this is the number you're given in the question) by Avogadro's number. This gives you the number of moles.

Once you have the number of moles, you can multiply this by how much mass 1 mole of that compound has (you find this for the specific compound using the periodic table) to give you the total mass.

Which part/s don't you understand?

[jammol7]

How much detail do we need to know for the carbon monoxide poisoning dotpoint on the SD?

[Evolio]

Hey!
Since the study design references this topic specifically, I would think that you would need to know this topic in detail.
That is: you would have to know what the competing equilibria are, the reactants and products involved, how the equilibrium system works (as in what happens when there is carbon monoxide) and how carbon monoxide poisoning can be treated by giving the affected individual oxygen.

Hope this helps.

[tigerclouds]

Hi guys,
This question has been driving me insane for the past I don't know how many hours so any help will be greatly appreciated.
With the diagram I attached below, showing the decomposition of N₂O₄, I've tried to find the K value but it's not aligning with the whole if K<1, then more reactants are forming and vice versa.
So I read the concentration of NO₂ at equilibrium off the graph (0.08 M) then squared it (0.0064 M) and divided that by the concentration of N₂O₄ at equilibrium (0.03 M), getting 0.213 M. Since this is less than 1, that should mean that more reactants are being formed but that doesn't make sense because the graph shows that the concentration of the product is much higher than the reactant at equilibrium, so I'm really confused. Am I misinterpreting something?

[whys]

Hi guys,
This question has been driving me insane for the past I don't know how many hours so any help will be greatly appreciated.
With the diagram I attached below, showing the decomposition of N₂O₄, I've tried to find the K value but it's not aligning with the whole if K<1, then more reactants are forming and vice versa.
So I read the concentration of NO₂ at equilibrium off the graph (0.08 M) then squared it (0.0064 M) and divided that by the concentration of N₂O₄ at equilibrium (0.03 M), getting 0.213 M. Since this is less than 1, that should mean that more reactants are being formed but that doesn't make sense because the graph shows that the concentration of the product is much higher than the reactant at equilibrium, so I'm really confused. Am I misinterpreting something?

Hi tigerclouds! So you have found the Kc value to be 0.213M, which is also what I got. Now, I am going to calculate Qc using the initial concentrations. Qc is 0 because the initial concentration of NO2 is 0 and 0 divided by anything is 0. As Kc > Qc, we can conclude that the forward reaction will be favoured (i.e. more products are being formed than reactants) to reach equilibrium. I have never heard of Kc being less than 1 meaning that more reactants will be formed. I am only aware of the rough constraints being a value greater than 10^4 meaning products are favoured, and a value less than 10^-4 meaning reactants are favoured, with anything in between likely to be slightly favouring a side or not favouring any side. To be honest, I feel that 1 is too small of a value to conclude which direction the reaction is shifting. Where did you get this K>1 or K<1 rule from?

[tigerclouds]

Hi tigerclouds! So you have found the Kc value to be 0.213M, which is also what I got. Now, I am going to calculate Qc using the initial concentrations. Qc is 0 because the initial concentration of NO2 is 0 and 0 divided by anything is 0. As Kc > Qc, we can conclude that the forward reaction will be favoured (i.e. more products are being formed than reactants) to reach equilibrium. I have never heard of Kc being less than 1 meaning that more reactants will be formed. I am only aware of the rough constraints being a value greater than 10^4 meaning products are favoured, and a value less than 10^-4 meaning reactants are favoured, with anything in between likely to be slightly favouring a side or not favouring any side. To be honest, I feel that 1 is too small of a value to conclude which direction the reaction is shifting. Where did you get this K>1 or K<1 rule from?

Hi whys, thank you for your reply. I was introduced to this rule from this video from Bozeman science: https://m.youtube.com/watch?v=xfGlEXWDRZE (it’s towards the very end). It makes sense to me, because you’re only going to get 1 if the ratio of the products is equal to the reactants. If you get a number greater than 1, more products are being formed because for you to get greater than 1, the numerator has to be greater than the denominator and the concentrations of the products are always in the numerator.

[jammol7]

Hey!
Since the study design references this topic specifically, I would think that you would need to know this topic in detail.
That is: you would have to know what the competing equilibria are, the reactants and products involved, how the equilibrium system works (as in what happens when there is carbon monoxide) and how carbon monoxide poisoning can be treated by giving the affected individual oxygen.

Hope this helps.

Thank you Evolio.

[whys]

Hi whys, thank you for your reply. I was introduced to this rule from this video from Bozeman science: https://m.youtube.com/watch?v=xfGlEXWDRZE (it’s towards the very end). It makes sense to me, because you’re only going to get 1 if the ratio of the products is equal to the reactants. If you get a number greater than 1, more products are being formed because for you to get greater than 1, the numerator has to be greater than the denominator and the concentrations of the products are always in the numerator.

Tbh I would just go with what the textbook says (which is the 10^4 rule, or some variance of this depending on the textbook you use). Having a Kc of 1 does not necessarily mean concentration of products = concentration of reactants. Why? Well, let's say I had a reaction as follows: A ---> 2B, and I've got 16M of A and 4M of B. This means my Kc is 4^2/16 = 1. My Kc value here is 1, but my concentrations of A and B are not the same (in fact, they are very different and the forward reaction is being favoured). However, using the logic you have mentioned, this would mean equal amounts of A and B are being formed if Kc = 1, but this is not true for the above reaction. Kc is not an exact ratio of the concentration of products to reactants, which is why we need extremely big or small numbers (like 10^4 and 10^-4) to make a judgement about whether more reactants/products are forming. This is because we put the concentrations to the power of the coefficients, so the only thing Kc is telling you is the extent of the reaction (i.e. how far the reaction will proceed at a given temperature). So, if the Kc value is in between 10^-4 and 10^4, there are significant concentrations of reactants and products but it's difficult to make a judgement about which side is favoured.

Hope this made sense.

[tigerclouds]

Tbh I would just go with what the textbook says (which is the 10^4 rule, or some variance of this depending on the textbook you use). Having a Kc of 1 does not necessarily mean concentration of products = concentration of reactants. Why? Well, let's say I had a reaction as follows: A ---> 2B, and I've got 16M of A and 4M of B. This means my Kc is 4^2/16 = 1. My Kc value here is 1, but my concentrations of A and B are not the same (in fact, they are very different and the forward reaction is being favoured). However, using the logic you have mentioned, this would mean equal amounts of A and B are being formed if Kc = 1, but this is not true for the above reaction. Kc is not an exact ratio of the concentration of products to reactants, which is why we need extremely big or small numbers (like 10^4 and 10^-4) to make a judgement about whether more reactants/products are forming. This is because we put the concentrations to the power of the coefficients, so the only thing Kc is telling you is the extent of the reaction (i.e. how far the reaction will proceed at a given temperature). So, if the Kc value is in between 10^-4 and 10^4, there are significant concentrations of reactants and products but it's difficult to make a judgement about which side is favoured.

Hope this made sense.

Oh ok, I didn’t consider the coefficients. Just to confirm, by 10^-4 and 10^4, do you mean a number multiplied by these values? Also, I’m having trouble visualizing Kc, what exactly is meant by it’s the extent of the reaction? What do Kc values mean, is it just an arbitrary number that helps us determine which direction the reaction is favoring? (This question is open to anyone btw)

EDIT: also can someone please explain why temperature changes the equilibrium constant? I don’t understand why the value of Kc decreases for exothermic reactions. I guess I can rationalize it for endothermic, because if the temp increases the kinetic energy of the molecules increase, meaning more molecules can overcome the activation energy and form products but I don’t understand why there are less products for exothermic reactions.

[ArtyDreams]

Oh ok, I didn’t consider the coefficients. Just to confirm, by 10^-4 and 10^4, do you mean a number multiplied by these values? Also, I’m having trouble visualizing Kc, what exactly is meant by it’s the extent of the reaction? What do Kc values mean, is it just an arbitrary number that helps us determine which direction the reaction is favoring? (This question is open to anyone btw)

Don't take my word for it - I'm new to this topic, but essentially:
If Kc is GREATER that 10^4, products are favoured.
If Kc is LESS than 10^-4, reactants are favoured.
If its in the middle of these, neither are favoured (I think!)

This does not refer to a number multipled by these values, its a number itself. i.e. if Kc = 2*10^5, this is greater than 10^4, so products are favoured. Kc is essentially just a number, but it is a number that references the concentrations of the reactants and products at equilibrium. This value helps as determine lots of things in relation to which way the reaction is favouring, as it is also compared to Qc. (same formula, but concentrations used to calculate are not necessarily at equilibrium.

Sorry if I got anything wrong but I hope this makes somewhat sense! Someone feel free to correct me, I'm still new to this!

[jammol7]

Dumb question but why does adding an inert gas have no effect?? Wouldn't having an inert gas = a decrease in volume?

[ArtyDreams]

Dumb question but why does adding an inert gas have no effect?? Wouldn't having an inert gas = a decrease in volume?

Adding an non reactive gas does not change effect the equilibirum position or constant. Even though the overall gas pressure increases as there's more gas added, the partial pressures of the individual gases involved in the reaction are unaffected.

Hope this helps!

[jammol7]

Adding an non reactive gas does not change effect the equilibirum position or constant. Even though the overall gas pressure increases as there's more gas added, the partial pressures of the individual gases involved in the reaction are unaffected.

Hope this helps!

Thank you ArtyDreams, this really helps.

[whys]

EDIT: also can someone please explain why temperature changes the equilibrium constant? I don’t understand why the value of Kc decreases for exothermic reactions. I guess I can rationalize it for endothermic, because if the temp increases the kinetic energy of the molecules increase, meaning more molecules can overcome the activation energy and form products but I don’t understand why there are less products for exothermic reactions.

If you increase the temperature in an exothermic reaction, then the endothermic reaction will be favoured. This is because an exothermic reaction results in a net release of energy, whereas an endothermic reaction results in a net absorption of energy. When you increase temperature, you add energy, so the system will attempt to partially oppose this change by trying to remove this energy (which it will do by favouring the reverse endothermic reaction). This is why Kc changes. An increase in temperature in an exothermic reaction will favour the formation of the reactants, so Kc will decrease.