OCR AS - Chemistry Unit F322 - Chains, energy and resource - REVISION!

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Reply 140

touran22

hey guys when ethane is oxidised to ethanal isit distilled or reflux?

Reply 141

Joseppea

Original post by touran22

hey guys when ethane is oxidised to ethanal isit distilled or reflux?

I think it's distilled.

I think you get ethanoic acid with reflux.

I could be wrong - probably am - so I've essentially confused you further. :cool:

I fail at this. xD

Reply 142

SteveCrain

Original post by touran22

hey guys when ethane is oxidised to ethanal isit distilled or reflux?

Distillation prevents full oxidation, so yes, it is ethanal. Ethanoic acid is formed under reflux, and when there is an excess of H+/Cr2O7 -2

Reply 143

student777

Original post by touran22

hey guys when ethane is oxidised to ethanal isit distilled or reflux?

It's definitely distilled.

Explanation (don't need to know, but just for interest): ethanal doesn't have hydrogen bonding, so it has a lower boiling point than ethanoic acid so it can be saparated by distillation.

Though I think you mean ethanol in your original question :wink:

Reply 144

kimmey

am i understanding this right....

basically if u want to work out enthalpy change of combustion and ur given values for enthalpy change of formation then do u do products - reactents

and also if u want to find enthalpy change of formation and ur given combustion values then do u do
reactents-products

?

Reply 145

SteveCrain

Original post by kimmey

Yep, that is correct

Reply 146

touran22

hey you guys, you see in the June 2010 paper Question 1 E iii....how do you work out the moloes for c2h2? im completly baffled?!

cheers!

Reply 147

SteveCrain

Original post by touran22

hey you guys, you see in the June 2010 paper Question 1 E iii....how do you work out the moloes for c2h2? im completly baffled?!

cheers!

link

Reply 148

touran22

Original post by SteveCrain

link

here dude : http://www.4shared.com/document/-csnNo5I/03_F322_JUN_10_QP.html

Reply 149

Killjoy-

Original post by touran22

hey you guys, you see in the June 2010 paper Question 1 E iii....how do you work out the moloes for c2h2? im completly baffled?!

cheers!

You have to use moles of gas= vol. of gas produced (given) / 24dmcubed.

24dm cubed is the molar volume of any gas at room temperature and pressure.

Reply 150

touran22

Original post by Killjoy-

You have to use moles of gas= vol. of gas produced (given) / 24dmcubed.

24dm cubed is the molar volume of any gas at room temperature and pressure.

ahhh yeah i see...i thought you had to divide by 24000 so i kept getting it confused :colondollar:

Also on that same paper Question 1 D ii - i got +128 when the answer is -128.
How come?

cheers.

Reply 151

Killjoy-

Original post by touran22

ahhh yeah i see...i thought you had to divide by 24000 so i kept getting it confused :colondollar:

Also on that same paper Question 1 D ii - i got +128 when the answer is -128.
How come?

cheers.

You have been given changes of formation and you need to find the enthalpy change of reaction, so sum the changes of formation for the products and then subtract the changes of formation for the reactants (you need to 'reverse' the arrows in a Hess' Law enthalpy change cycle.)

Remember to multiply the enthalpy change for the formation of H2O by two before subtracting.

As for your answer, I think you subtracted in the wrong order?
Mark scheme (if you don't have it) http://www.ocr.org.uk/download/ms_10/ocr_56096_ms_10_gce_f322.pdf

Paper from OCR http://www.ocr.org.uk/download/pp_10_jun/ocr_57549_pp_10_jun_gce_f322.pdf

Reply 152

touran22

Original post by Killjoy-

hi thanks for this...is there a certain rule we need to follow so if your givin Formation and they want Reaction its Products - Reactants

What about Formation and Combustion or Reaction and Combustion or any other combination?

Reply 153

Killjoy-

Original post by touran22

Correct, the general rule is to reverse the arrows, so if you are given enthalpy of formation (to calculate enathlpy change of reaction or combustion) you would have to reverse the arrows for the reactants.

So its sum of enthalpy change of formation of products take way sum of enthalpy change of formations of reactants.

And if you are given enthalpy change of combustion then you would have to do the opposite, look at the examiner tip boxes on pages 199 and 201 of the course book as it summarises it using notation that I cannot/do not know how to type out on a forum.

Oh, and if you are calculating enthalpy change of reaction, then it is associated with the molar quantities in the equation- you may have to multiply out your answer if it is calculated for one mole of a substance that does not appear as one mole in the equation.

Reply 154

warthogBE

Original post by touran22

Yup - I'm far to lazy to draw out Hess' cycle, so:

Enthalpy change of reaction = Sum of Enthalpy of formation (PRODUCTS) - Sum of Enthalpy of formation (REACTANTS)

Enthalpy change of reaction = Sum of Enthalpy of Combustion (REACTANTS) - Sum of Enthalpy of combustion (PRODUCTS)

Enthalpy change of reaction = Sum of Bond Enthalpy (REACTANTS) - Sum of Bond Enthalpy (PRODUCTS)

For all of the 'sum of' bits, just add up the figure as it's given - include any negatives etc, and then use the formulae. It'll give you the value for Enthalpy Change and the correct sign.

Hope this helps. :smile: