Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread

include water if all the reactants are liquids
don't include water if water is the solvent (i.e. if the reactants are (aq))
hope I helped

Hey! COuld someone tell me when to and when not to include H2O in the equilibrium constant expression? Because in the CHem U6 exam, we were supposed to include water in the expression, I thought the Ionization of water is a constant. Isn't that so?

You need to include water in the K_c expression when it is a product of the reaction, for example in an esterification reaction. The reason water is mostly not included is that it's normally the solvent, meaning that it's in such an excess that the concentration doesn't change at all during the reaction.

any hep with this one

OMG!!! Thanks so much!!!

Could you also explain 2)c)i) of this paper?

https://ks5chemistry.files.wordpress.com/2011/01/carboxylic-acid-lots.pdf

OMG I did this question like 5 minutes ago! Let me just take a photo of what I wrote

1. Calculate decrease in moles in HCOOH
2. Subtract this figure from the original mole value for C2H5OH
3. Add the figure from step 1 to the 0 for the ester
4. Do the same for H2O

You do this because there is 1 mol of each reactant and of each product
If there were to be 1 mole of a reactant and 2 moles of a product, you would calculate the increase in product moles by adding double the value of the decrease in the moles of reactants

Sorry if this doesn't make much sense

Help, I have no idea how you do this.

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Thanks. You condense your answer down so well. When I answer these questions I end up writing a full paragraph on one peak . This is super helpful especially for structuring the answer correctly. Good luck tomorrow.

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Hey guys, can someone please explain what the heck this question is on about? How are we supposed to know to compare experimental and theoretical lattice energies?!

Hi, do you know which past paper this is from?

June 2012 I think

Any help on this one please? Lool I've seen so many water questions I am now starting to confuse myself!

Question 2: When talking about thermodynamic feasibility, this is ALWAYS talking about entropy. You chose to talk about activation energy, which is to do with kinetics.

So something that is kinetically stable will have a high activation energy.

Something thermodynamically stable will have a more negative ΔS(total).

Question 5: count the moles of gas on each side of the equilibrium equation, and you should be able to figure that one out. Why it isn't A is because it isn't B.

Lol jk but if a gas is more pressured then surely it helps it go through the reactor anyway? Much like pushing harder to get onto a packed train will make people want to move out of the way so they aren't so squashed.

The pH is temperature dependent. If we increase the temperature, the value for Kw will increase(Kw is a type of equilibrium constant) as this is an endothermic reaction so increasing the temperature would favour the forward direction hence increasing the concentration of hydrogen ions and thus lowering the pH. However, it is not acidic because concentration of hydrogen and hydroxide ions are equal.

When you say 'Solvent' do you mean we don't include water when it is a reactant?
How about when it is a gas and is a reactant?