VCE Chemistry Question Thread

[p0kem0n21]

For this question
for b why when the volume is halved do all the concentrations stay the same and for a) why are the reactant decreasing shouldn't the reactant increase and the product decreases

We can solve this using Le Chatelier's principle. To begin, let's analyze our chemical equation. We are dealing with an endothermic reaction, which is significant for when we are changing temperature. There is also an equal number of molecules on both sides of the equation (2), which is important when considering what happens when volume is changed.

So for a), when temperature is increased, the reaction system will change in such a way that it partially opposes the change made (i.e. will try to lower the system's temperature). This is possible through an endothermic reaction, which will absorb this energy and therefore reduce the system's temperature. The forward reaction is endothermic, so we end up having some of our reactants converted to products, therefore reducing the concentration of reactants and increasing the concentration of products.

b) Halving the volume can basically be viewed as doubling the pressure of the system. With an increase in pressure, the reaction would shift to the side with fewer molecules in order to oppose this increase in pressure. However, both sides of the equation have the same number of molecules, so there is nothing which the system can do to oppose the change. The amount of reactants and products are the same. However, with the reduced volume, I would expect concentrations to increase just because you have the same amount of these substances in less available space. Either the answers you have given are wrong or I'm misinterpreting something. Perhaps someone else could elaborate on this?

[Chocolatepistachio]

i thought it was exothermic because heat is on the products side

[p0kem0n21]

i thought it was exothermic because heat is on the products side

Heat of combustion is always on the right side, but I suppose the fact that it did not list the symbol (or the units of kJ/mol) made it a bit ambiguous. I hope this is just a worksheet aha, tests would likely make a more clear distinction between the two. I just saw the forward reaction as endothermic from first nature; might be a bit dangerous now that I think about it 

[Coolgalbornin03Lo]

Discharge vs recharge?

Galvanic cells
Chemical———> electrical energy so recharge? Or is it discharge

[p0kem0n21]

Discharge vs recharge?

Galvanic cells
Chemical———> electrical energy so recharge? Or is it discharge

The conversion of chemical energy to electrical energy in galvanic cells is a process of discharge, as it 'discharges' electricity for use in a circuit or something else.

The conversion of electrical energy into chemical energy is the process of recharge which occurs in electrolytic cells (usually in secondary cells/rechargeable cells where you're recreating the original reactants so that they can once again be used for the discharge of electricity). Note that "recharge" may not always be the correct term to use with electrolytic cells (particularly for industrial purposes, where they're used for producing chemicals which are used somewhere else). A bit of a tangent there.

[ArtyDreams]

Discharge vs recharge?

Galvanic cells
Chemical———> electrical energy so recharge? Or is it discharge

Galvanic cells are primary cells so in general they cannot be recharged. They convert chemical energy to electrical energy as they discharge.
Electrolytic cells on the other hand can discharge as they're being used, and then recharged.
Recharge is electrical to chemical.
I like to think of it as when you plug in your phone to charge, you are supplying it with electrical energy than then converts itself to chemical energy as you use it.

EDIT: beaten by p0ken0n21 - will just leave this here but idk if its much help.

[Moonblossom]

Hi I'm having a bit of trouble with a practice question and even after looking at the answer and explanation, I'm not sure how to get to the answer. 

The question stem and question is: 3C₂H₅OH + 4HCrO₄^- + 16H⁺ -> 3CH₃COOH + 4Cr³⁺ + 13H₂O                     The rate of the above reaction can be determined by a technique called kinetic titration. In this technique, a standardised solution of a sodium thiosulfate of concentration 0.0130 M is loaded onto a burette. Potassium dichromate is weighed in an analytical balance and dissolved in a 3M HCl solution in a 500.0 ml volumetric flask. The dichromate ions are hydrolised to form hydrogenchromate ions. The flask was made up to the mark with 3M HCl solution. The final concentration of HCrO₄^- ions in the solution is 0.00822 M. Determine the mass in grams of K₂Cr₂O₇ that was weighed in the analytical balance.

The answer is 0.605g

[Coolgalbornin03Lo]

Hi I'm having a bit of trouble with a practice question and even after looking at the answer and explanation, I'm not sure how to get to the answer. 

Hi do you mind putting up your working out? I think it would be better for your understanding/learning to guide you or tell you where your going wrong rather than giving you the full answer 

MY QUESION

You guys know how there’s formulas for number of glucose monomer

180n - 18(n-1)

Or number of peptide links
(Amino acid monomers-1) correct next if wrong please

I was wondering where such a thing exists for tryclicerides at all? Thanks so much!!!

[keltingmeith]

MY QUESION

You guys know how there’s formulas for number of glucose monomer

180n - 18(n-1)

Or number of peptide links
(Amino acid monomers-1) correct next if wrong please

I was wondering where such a thing exists for tryclicerides at all? Thanks so much!!!

I mean, what would you want the equation to tell you? I can probably make one up, but it depends on what information you want to get from the equation. In some way, the answer is no - there is no equation, because the ones you're talking about are for polymers - triglycerides are not polymeric in the same sense. They don't have a repeating, regular, unit - they're three fatty acids (not necessarily the same acid, either!) attached to a glycerol backbone.

Personally, I'm not a fan of these equations, because they can often detach people from the physical chemistry that's actually happening. For example, WHY is the number of peptide links = number of monomers - 1? Well, think of it like links in a chain. If you have two loops in a chain, how many chain links do you have? Well, there's only two loops, so there must be a link between them - so you have one link. What if there's three loops? Well, you can think of this as having two loops - the big loop from before, and now one smaller loop. Two loops are easy - there's one link. BUT, there's also one link in the big loop from before, so in total, we have two links. What if there are four loops? Well, we have one big loop of three links, and another link - again, two loops are easy, there's one link. BUT, there's also two links in the big loop, so in total we have three links.

See the pattern? There's always going to be n-1 links in a chain of n loops - because the amount of links is just what happens when you put something between each loop. The same goes for peptides - the number of peptide links is just the amount of things you can put between each amino acid.

Okay, so what about the glucose equation? Well, 180n-18(n-1) tells you the molecular mass of a glucose polymer of size n glucose units. What are those numbers? Well, the molecular mass of molecular glucose (that is, the monomer) is 180 g/mol. The molecular mass of 18 is of water. So why would the total molecular mass be 180n-18(n-1)? Well, like the chain example, let's take this one step at a time. If I want to make a dimer, I have the equation:



This makes sense - what happens is two glucose units meet, and they make a link. To make that link, one molecule of water is removed - it's just like the chain example. That means that molar mass of this dimer should be what you get from both of those glucose monomers, minus the one water, yeah? Well, what about three glucose units? Similar equation:



So in this case, we have three glucose units, and we lose TWO waters - so the trimer's molar mass is going to be the molar mass of three glucose, minus the molar mass of two water. We can generalise this, but to do that we have to know - how many waters will we lose for n glucose molecules? Well, we just learned you lose water each time you have a link. How many links in n glucose monomers? Well, we learned from before - there's n-1. So, the total molar mass is 180*n, minus 18(n-1).

IMO, you should be able to derive these equations yourself - there's no mathematical trickery about them, it's just an application of the chemistry information.

Having said that, if there's an equation you want, I can still show you how you might create that equation - but hopefully this explains why you haven't seen one for triglycerides, and why you have for these other macromolecules.

[Moonblossom]

Hi do you mind putting up your working out? I think it would be better for your understanding/learning to guide you or tell you where your going wrong rather than giving you the full answer 

I did: n(K₂Cr₂O₇) = c x V = 3 x 0.500
                                         = 1.5 mol
m(K₂Cr₂O₇) = n x M = 1.5 x (78.2 + 104 + (16 x 7)) = 441.3g

Thank you!

[p0kem0n21]

I did: n(K₂Cr₂O₇) = c x V = 3 x 0.500
                                         = 1.5 mol
m(K₂Cr₂O₇) = n x M = 1.5 x (78.2 + 104 + (16 x 7)) = 441.3g

Thank you!

Hey! The first thing to realize here is that 3 M is the concentration of HCl, not potassium dichromate. So it's not as simple as just chucking your values into n=cV. I've attached my working on paper, but I'll walk through the steps (hopefully you don't mind a huge spoiler).

Spoiler

To begin, let's work backwards. The only useful value we really have for this part of the question is the concentration of hydrogen chromate ions, which we can convert into moles since there's a volume of 500mL. However, how would we relate that to dichromate ions? Through a chemical equation, of course! Hmmmmm...but there isn't one. Thankfully, the question somewhat hints that we can create our OWN chemical equation, through "hydrolisation". Not too sure what this type of chemical reaction is, but surely it has to do with water?? Thus, I just tried to balance it kind of like a redox reaction (treating potassium ions as spectator ions), and that's how I got that second chemical equation. We already know that we can calculate the numbers of moles of hydrogen chromate ions, so we can just do some simple stoichiometry using that new equation to find the number of moles of dichromate ions. We can then infer that the number of moles of potassium dichromate is the same as that of dichromate ions. So, now that we have n(potassium dichromate), we can just use a little m=nM magic to get the mass of potassium dichromate, 0.605 g. You already know how to calculate the molar mass, so you're good there 

[Unknown-111]

Calculate the mass of water that is produced when 2.8 g of methane is burned in air.
Hey guys can someone work this out for me? I don't understand it and whats worse is I have a terrible teacher

[Owlbird83]

Calculate the mass of water that is produced when 2.8 g of methane is burned in air.
Hey guys can someone work this out for me? I don't understand it and whats worse is I have a terrible teacher

Hey, I hope this helps explain all the steps clearly, but let me know if you need clarification on any steps or have any questions!

Spoiler

also let me know if I stuffed up and did something wrong haha

[Unknown-111]

Hey, I hope this helps explain all the steps clearly, but let me know if you need clarification on any steps or have any questions!

Spoiler

also let me know if I stuffed up and did something wrong haha

Thanks

[Unknown-111]

Determine the mass of carbon dioxide produced when 0.85 grams of butane (C4H10)
reacts with oxygen according to the following balanced chemical equation:
2 C4H10 (l) + 13 O2 (g) --> 8 CO2 (g) + 10 H2O (g)

Determine the mass of sodium nitrate produced when 0.73 grams of nickel (II)
nitrate reacts with sodium hydroxide according to the following unbalanced
chemical equation:
Ni(NO3)2 (aq) + NaOH (aq) --> Ni(OH)2 (aq) + NaNO3 (aq)

Can someone show me how to do those questions? Also for some reason I am still finding stoichiometry really difficult  
During class whenever my teacher is talking I have no idea what is even happening around me, when I get home I really struggle to work things out on my own.