F334 (14/01/2013 1:30PM) - Salters Chemistry (Chemistry of Materials) Revision Thread

How did they expect us to work it out will be asking my chemistry teacher tomorrow.

This "kind of" explains it better

http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Reaction_Rates/The_Rate_of_a_Chemical_Reaction

Don't worry, we all make silly mistakes, and I find that with Chemistry, you can easily make a lot of silly errors.

I know I'm also going to have to retake to get the A I need, but I'm really going to try and emphasise my revision on exam technique; so I can cut down on silly errors.

You don't fail in life over this. Pick yourself up and focus on improving your grade now

I don't have the strength to even attempt to understand that right now
From what I gather, looks like rate of disappearance actually is the negative of rate of formation?

thank you for the pick-me-up! i think a resit is looking likely. not sure how that will fit in with my other exams, i think i'll have to consider perhaps dropping my 4th A2. it's doing me no favours continuing it under these circumstances.. i think i've easily lost about 20 - 25 marks on this paper, i got the units of the rate constant wrong as well i think.

How on earth does OCR expect us to know that?!

The first reactant was first because when you doubled that concentration the rate doubled. At that time the concentration of reactant 2 was kept the same and had no effect on the rate however when we halved the concentration of reactant 2, the rate fell by quadruple which is second order.

Ahh I was doing a fourth A2, but I soon found I couldn't handle the work load. I'm sure all universities don't need a fourth A2, so I would say drop the extra one.

I know, I think I lost around the same. You can't do anything to change it now though 'eh? Focus on the other exams. I got the units as mol^-1dm⁺³s^-1.

i thought it was just the rate of the creation of I2 multiplied by 2? because of the half equation and the stoichiometry of the overall equation.

yep, definitely mucked up the units, arggggh another mark lost! i'm not sure this work load is suiting me at all, especially when a resit looks so likely. i think i might have to give it a miss..

How many questions were on the paper, i finished I unually 12 minutes early. Did only go up to question 4?

I don't think its to do with the stoichiometry you know. Beep posted a link on the backward rate of reaction, it's somewhere on this thread.

I also doubled it, but I don't think it's the right answer.

1 I2 produced every second, 2 I- used every second ---> doubled

The first reactant was first because when you doubled that concentration the rate doubled. At that time the concentration of reactant 2 was kept the same and had no effect on the rate however when we halved the concentration of reactant 2, the rate fell by quadruple which is second order.

Second Order

Yah four questions only. Seems to be the new style of the paper, as it was also four questions in June 2012. I finished rather early as well; shockingly, as I found timing an issue before

Second Order

Lol

Just took a proper look and yeah it explains why its worth 2 marks in that case. 1 for the correct answer and the second for the negative sign maybe?

Frustrating how its not even in the specification :/ why OCR!

Did you work it out correctly in the exam?

I don't have the strength to even attempt to understand that right now
From what I gather, looks like rate of disappearance actually is the negative of rate of formation?