I did part i but not sure how to part ii) what are the negative points?
i)
1.add 150ml of water into measuring cylinder A and 100ml of water into measuring cylinder B
2.transfer 10ml of water using 10ml pipette from measuring cylinder A to B (forward process)
3.Transfer 5ml of water using 5ml pipette from measuring cylinder B to A (backward process)
4.Repeat step 2 and 3 (one transfer cycle) until the volume of water remain constant in both measuring cylinder (equilibrium)
ii)Information collected is highly valid given that it is a qualitative model of an equilibrium reaction. In this procedure, it can be seen that the process occurs 2 directions (water transferred from measuring cylinder A to B or B to A) which is similar to an equilibrium. It is also observed that the volume of water in both measuring cylinder remain constant after transferring, like an equilibrium with no macroscopic changes. Moreover, no water is removed from the system which is similar to equilibrium which is a closed system
Providing the question could help here, thanks. I am not sure where "negative points" comes from just by looking at the answer.
Hey guys, just got a question on eutrophication and the methods used to determine the concentration of phosphates and nitrates/nitrogen.
The method for determining organic nitrogen within waterways is to convert it into ammonia. The nitrogen concentration is then found through acid-base titration. Is this in enough detail for exams or should i know about how they convert the nitrogen into ammonia? (heat in sulphuric acid and stuff.)
Back when I did the course, all I memorised was "Kjeldahl method" and nothing about the specifics at all. It is all that I felt was necessary.
You may find more useful information from another person.
im quite confused about the electrolysis of molten, concentrated and aqueous of NaCl
what info does the polarity of electrode give?
The polarity of the electrode determines which half-equation is occurring at the electrode. (Always remember that the polarity of the cathode/anode flip for an electrolytic cell; instead of a positive cathode and negative anode, we consider a negative cathode and positive anode.)
Recall the mnemonic RED-CAT AN-OX.
This means that the reduction occurs at the cathode, whereas the oxidation occurs at the anode.
We analyse three cases:
Case 1: Molten NaCl.
The only species present are Na
+ and Cl
-. This means that sodium ions will be reduced to sodium metal, and chloride ions will be oxidised to produce chloride gas.
+'ve anode: 2 Cl
- -> Cl
2(g) + 2e
--'ve cathode: Na
+ + e
- -> Na
(s)Case 2: Aqueous solution of NaCl.
The two species present are as above, and H
2O as well now. We use our data sheet to determine what is more likely to occur.
-'ve cathode: -2.71V is required to reduce sodium ions back to sodium. But -0.83V is required to reduce water into hydrogen gas and base (hydroxide ions). Clearly, it will take considerably less voltage for the latter equation to occur. Hence, provide water is present, that will ALWAYS be the reaction.
2 H
2O
(l) + 2 e
- -> 2 OH
- + H
2(g) +'ve cathode: -1.36V is required to oxidise chloride ions to chloride gas. -1.23V is required to oxidise water into acid (hydrogen ions) and oxygen gas. There is a voltage difference, however it's quite small, so we cannot say for sure which reaction will occur. We seperate up our cases:
Subcase 1: Concentrated aqueous solution (generally >2M, although some sources say >1M)
There is a good abundance of chloride ions present, hence they will be oxidised.
2 Cl
- -> Cl
2(g) + 2e
-Subcase 2: Dilute aqueous solution (generally <0.1M)
Not many chloride ions are present at all. The oxidation of water will be more dominant here.
2 H
2O
(l) -> 4 H
+ + O
2(g) + 4
e-(Note: The electrolysis of dilute NaCl solution is basically the electrolysis of water.)
Sometimes with examples in chemical equations, I get confused with the states. Like sometimes h2o it is in liquid state and sometimes it is in gaseous state. How do you know when it's in liquid state and when it's in a gaseous state (using h2o as an example)?
Thanks
Whilst a bit of this will require your knowledge of chemistry (e.g. aluminium reacts specifically with STEAM), more often than not it should be implied as to what state water is in. You may choose to provide examples to voice your confusion, which I can walk through.