Oh I see- Thanks zsteve! That's very interesting
I wish I did UMEP Chem 
---
Any tips in drawing equilibrium graphs anyone?
Some papers draw beautifully curved lines, while others just draw straight jagged lines. (does that matter?)
I know that the magnitude of the change has to match that of the coefficient in front of the chemical specie, but that's about all I know. And that adding or removing a specie would equate to a sharp vertical lines as well as doing stuff with pressure. Temp changes would have more of a gradual change (so curved lines)
Let's say that the change to the equilibrium was "more Cl- was added" then on the equilibrium graph, can I make the sudden vertical spike as large as I like? Also when the question says, "equilibrium was re-established at 20 minute mark", does that mean I have to make sure that conc. changes and becomes stagnant exactly at 20 min (and draw a horizontal line after 20 minute to make the point that conc doesn't change after that and thus reached equilibrium)? Would it be wrong to have the conc. the same from like 18 min mark continuing to 20 min (cos that would be mean equilibrium "reached"at 18 min)
Sorry for being pedantic. It's all my teacher's fault 
Everything you've said sounds spot-on - except that, yes, in your example, you would need to make sure the concentrations reach a constant at 20 minutes, not any earlier. You don't have to draw a line to mark the point of the new equilibrium position, but it's a good idea to provide some clarity to your drawing. And yes, you want smooth curves, not "straight jagged lines".
Here's what VCAA said in the 2014 exam report regarding a question that required you to draw equilibrium graphs:
"These graphs proved challenging for many students. Some student struggled with the starting points of both graphs (based on initial concentrations), the final concentration for each graph (based on equilibrium concentrations), and the point at which the graphs levelled out (i.e. when equilibrium was reached). This latter point was often missed, with students overlooking the fact that all three species would reach their equilibrium concentrations at the same time. The difficulty was further compounded by vertical scale location, where it was required that the graphs levelled off within the correct vertical gradation."
Maybe this will help as well.
But yeah, I think the main points are:
- Smooth curves that plateau at the same time for all species at the time of the new equilibrium position
- Drops/rises in concentration that are in accordance with the stoichiometric ratios of the given equation at hand
- Sudden initial drops/rises in concentration for dilution, pressure changes, etc. but not for temperature changes which result in only gradual changes
My last piece of advice - something which sometimes gets overlooked by students - is ensure that your graphs are in accordance with Le Chatelier's principle (e.g., if the concentration of a species is increased, the concentration of that species might then decrease to oppose the change but the concentration at the new equilibrium position would still be greater than the initial concentration, as the change in concentration is only PARTIALLY opposed).
Hope this helps!
Home
Help
Search
Login
