Chemistry 3/4 2013 Thread

[clıppy]

If we have a reaction that's at equilibrium, and we increase the temperature, the system will try to counter this with a reaction that reduces the temperature. Well, an endothermic reaction does that, so the endothermic reaction will be favoured by temperature rises.

Thanks nliu that cleared it up!

[Yuki N.]

If we have a reaction that's at equilibrium, and we increase the temperature, the system will try to counter this with a reaction that reduces the temperature. Well, an endothermic reaction does that, so the endothermic reaction will be favoured by temperature rises.

yup, just think of temp as a reactant to be countered by LCP

[Scooby]

Why do we need to graph the results obtained from calibrating a calorimeter? Shouldn't the calibration factor we calculate be the roughly the same regardless of whether we use the temperature change at 90 seconds, or at 120 seconds, or whenever? If not then at which time do we say "yeah, let's use this change in temperature for our calculations"? Or does the temperature change we use have something to do with why we draw a graph...

If we calibrated a solution calorimeter using 100 mL of water, what would happen if the reaction mixture we then added afterwards was only 90 mL? Or 110 mL?

What's the difference between the molar enthalpy of combustion and the heat of combustion? Why do they have the same symbol?

In mass spectrometry, why do we assign the base peak a relative abundance of 100? And is there any way to determine at what m/z the base peak will appear for a particular organic compound?

[Snorlax]

I have a 1/2 question about when Hydrogen burns in oxygen. What state is the water molecule that's produced?

[Stevensmay]

Gas.

[Snorlax]

Gas.

Haha, I should of added I know the answer is gas, but why?
Is it because it states that hydrogen burns in oxygen?

[Stevensmay]

This is just a guess here. As the reaction is exothermic the released energy has to go somewhere. I'm thinking this extra energy is what causes it to be expressed as water vapour and not liquid.

[lzxnl]

The enthalpy of formation of water is -285.8 kJ/mol. If we catalysed a reaction between hydrogen and oxygen at 298 K, for instance, if we formed 1000 mol of water and hence released 285800 kJ of energy, 1000 mol of water is 18 kilograms.
285800 kJ = 4.18 J K^-1 g^-1 * 18000g * temperature change
Temperature change = around 3.8*10^3 K
Which is huge. This explains why we don't normally form liquid water as a by-product of the reaction.

[lolipopper]

so for chemistry SAC, i had the following questions.

Q1. burning coal is being replaced by many non-fossil fuel resources.
    select one and give one advantage.

So i selected nuclear power and said that the advantage is that we have large reserves. The teacher didn't give me the mark.

Q2. which is more sustainable, bio-diesel or uranium?

So i said uranium because it produces less pollution and is more capable of supplying todays energy needs. The teacher game me no mark and said bio-diesel was correct.

Do i have a point worth arguing against his corrections in these situations?

[Conic]

so for chemistry SAC, i had the following questions.

Q1. burning coal is being replaced by many non-fossil fuel resources.
    select one and give one advantage.

So i selected nuclear power and said that the advantage is that we have large reserves. The teacher didn't give me the mark.

Q2. which is more sustainable, bio-diesel or uranium?

So i said uranium because it produces less pollution and is more capable of supplying todays energy needs. The teacher game me no mark and said bio-diesel was correct.

Do i have a point worth arguing against his corrections in these situations?

For Q1, you might have an argument, but the main advantage is the lack of pollution produced by power plants. For Q2, we can't make any more uranium after we use it up. Bio-diesel is produced from renewable chemicals so it is more sustainable.

[SocialRhubarb]

1.) According to Wikipedia, the amount of recoverable coal in the world (948 billion short tons of recoverable coal reserves) is greater than the amount of recoverable uranium in the world (5.5 million tonnes of uranium exists in ore reserves that are economically viable). I understand that you don't need as much uranium to power a nuclear plant, but you'd probably need to mention that if you wanted to argue abundance of fuel for this question.

2.) Bio-diesel is renewable. Uranium is not. Even if uranium is more efficient or practical, it's difficult to argue that it's more sustainable.

[Homer]

which of the following best describes the source of energy in a nuclear power station
a)nuclei are split into smaller particles and energy is released in the process
b) nuclei are burnt in oxygen, releasing energy in the process
c) nuclei fuse with each other to make bigger particles, releasing energy in the process
d) nuclei release neutrons one at a time and release energy in the process.

how do we answer questions like these?

[clıppy]

which of the following best describes the source of energy in a nuclear power station
a)nuclei are split into smaller particles and energy is released in the process
b) nuclei are burnt in oxygen, releasing energy in the process
c) nuclei fuse with each other to make bigger particles, releasing energy in the process
d) nuclei release neutrons one at a time and release energy in the process.

how do we answer questions like these?

I can only answer from prior knowledge learnt in physics about nuclear power plants.
The answer (i think) is A.
I'd suggest doing some research or asking someone on the physics board who probably knows more than me but the gist of it is (using Uranium as an example) you bombard the Uranium sample with neutrons making the atom unstable and so it breaks into two smaller (daughter) nuclei and releases energy in the process. This way of creating power used in nuclear power plants is nuclear fission whereas fusing atoms together and releasing energy is nuclear fusion.
Hopefully if I'm wrong someone will clarify, but that's all I would think you'd need to know

[Homer]

Water self ionises according to the equation H2O(l)+H2O(l) (forward and back arrow) OH-(aq) + H3O+(aq) enthalpy +57 kJ mol -1. At 25 degrees the pH of pure water is 7.00. at 35 degree the pH of pure water is:

answer is less than 7 because the reation has shifted to the right; however, the solution is still neutral.

What I don't get: well if the pH is decreasing with the increase in temperature how can the solution still be neutral? If the solution is less than 7 wouldn't the H30 concentration be higher than the OH? How can it be neutral? :-/

[psyxwar]

Water self ionises according to the equation H2O(l)+H2O(l) (forward and back arrow) OH-(aq) + H3O+(aq) enthalpy +57 kJ mol -1. At 25 degrees the pH of pure water is 7.00. at 35 degree the pH of pure water is:

answer is less than 7 because the reation has shifted to the right; however, the solution is still neutral.

What I don't get: well if the pH is decreasing with the increase in temperature how can the solution still be neutral? If the solution is less than 7 wouldn't the H30 concentration be higher than the OH? How can it be neutral? :-/

pH = -log [H⁺]

So you could have more H+ (and thus a lower pH) than at 298K, but also more OH- to counter it. So the solution is still neutral, just with more of both as water self-ionises more at higher temperatures.