Chemistry 3/4 2013 Thread

[lolipopper]

don't we need the moles too, to calculate the concentration? I shall just wait for someone else to answer :s

i thought i was going dumber lol but yeah dont we?

[chemdeath]

don't we need the moles too, to calculate the concentration? I shall just wait for someone else to answer :s

Yeah true sorry!!
 
 sodium tripolyphosphate=0.25M
2M= HCl
and just water
Thanks !

[Conic]

Find the amount of sodium thiosulfate:



The total volume of the solution is 50mL, or 0.05L, so using this and the amount we can find the concentration:

[bucklr]

Hello,
for equilibrium reactions  such as
CO(g) + 2H₂(g)  <-->  CH₃OH(g)

If you add more carbon monoxide to the system at constant volume why do the answers say the yield changes???

I thought it didn't...
my reasoning was:
when you add more reactants you increase the theoretical maximum, however more actual products also form. Therefore because both actual and theoretical increase at the same rate there is therefore no net increase in yield.

Edit: upon reflection, wouldn't it also be reliant on whether H₂ is the limiting reagent? The question does not go into anymore detail... if there was a reaction such as one with a single reactant  that created a single product, would my ideology hold true?

many thanks.

[Homer]

im not sure if im right, but the more CO there is, the more CH3OH is made. this would increase the K value and hence would make it a net forward reaction providing more yeild :s

[bucklr]

im not sure if im right, but the more CO there is, the more CH3OH is made. this would increase the K value and hence would make it a net forward reaction providing more yeild :s

My understanding was that only temperature can change the equilibrium constant. :S

[lzxnl]

I'll analyse this situation for a special case.
If the hydrogen gas was already the limiting reactant, then increasing the amount of CO does not increase the theoretical yield, but it does push the equilibrium further to the right, so the percentage yield would increase.

What does the question mean by yield though? If it just means the amount of product formed, then a simple application of Le Chatelier's principle would suffice. If it means percentage yield, then you'll have to take into account situations when the reaction is an excess reaction or when either reagent is in excess. That does not look like fun.

And yes, the K value is constant if you add CO, but the denominator of the fraction increases, so the numerator must increase too.

[bucklr]

I'll analyse this situation for a special case.
If the hydrogen gas was already the limiting reactant, then increasing the amount of CO does not increase the theoretical yield, but it does push the equilibrium further to the right, so the percentage yield would increase.

What does the question mean by yield though? If it just means the amount of product formed, then a simple application of Le Chatelier's principle would suffice. If it means percentage yield, then you'll have to take into account situations when the reaction is an excess reaction or when either reagent is in excess. That does not look like fun.

And yes, the K value is constant if you add CO, but the denominator of the fraction increases, so the numerator must increase too.

Perhaps i'm reading too much into the question... your probably right, that there is a difference between "percentage yield" and "yield".

Thankyou.

[academicbulimia]

Just a quick question-If I was to draw up a temperature vs rate of reaction graph or a concentration vs rate of reaction graph would the rate of reaction be on the y or x-axis? Or does it not matter?

Thanks in advance!

[saba.ay]

Just a quick question-If I was to draw up a temperature vs rate of reaction graph or a concentration vs rate of reaction graph would the rate of reaction be on the y or x-axis? Or does it not matter?

Thanks in advance!

The independent variable is generally on the x-axis and the dependent on the y-axis. Therefore, in scenario 1 (temp. vs. reaction rate), for example, the reaction rate changes based on temperature. Therefore, temperature would be independent and on x-axis and reaction rate on the y-axis.
Generally/all questions I've seen, have this order

[academicbulimia]

The independent variable is generally on the x-axis and the dependent on the y-axis. Therefore, in scenario 1 (temp. vs. reaction rate), for example, the reaction rate changes based on temperature. Therefore, temperature would be independent and on x-axis and reaction rate on the y-axis.
Generally/all questions I've seen, have this order

Okay thanks 
Also what units is rate of reaction generally measured in?

[lzxnl]

Generally rate of reaction is rate of change of concentration with respect to time. Go figure.

[Homer]

[academicbulimia]

Generally rate of reaction is rate of change of concentration with respect to time. Go figure.

Yeah i knowwww

Nah I don't think you guys get me don't worry I got the answer the units are in sec^-1

[lzxnl]

What ARE you looking for? If you mean the rate constant in rate equations, that will depend on the order of the reaction. If you just want then M/s will be fine.