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Author Topic: HSC Chemistry Question Thread  (Read 1040606 times)  Share 

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amandali

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Re: Chemistry Question Thread
« Reply #135 on: March 13, 2016, 08:03:00 pm »
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can anyone explain how to determine whether an acid/base can act as a buffer pair? thanks


Neutron

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Re: Chemistry Question Thread
« Reply #136 on: March 13, 2016, 08:07:33 pm »
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can anyone explain how to determine whether an acid/base can act as a buffer pair? thanks



Hey Amandali! I believe a buffer pair is commonly made up of a weak acid and its conjugate base (so for example, CH3COOH and CH3COO- are buffer pairs! Hope that makes sense :D

Neutron

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Re: Chemistry Question Thread
« Reply #137 on: March 13, 2016, 08:09:32 pm »
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Hey guys!

Sorry I have quite a long question, I was wondering whether someone could explain how a dry cell battery works or a lead-acid battery cell, I've read some textbooks about it but I'm still a bit confused! And also, what's a good way of knowing what citric acid and sodium hydroxide forms? Like what the products are.. Thank you! :D

Neutron

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #138 on: March 13, 2016, 08:33:01 pm »
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can anyone explain how to determine whether an acid/base can act as a buffer pair? thanks

Hey Amanda:

Good question! Actually a few weeks before this "buffer" concept was a little confusing for me too. But yes, like what neutron said, a buffer pair would be a weak acid and the conjugate base of that weak acid. You may ask, why not a strong acid? Because strong acids dont produce reversible reactions! Buffers are required to be weak acids or bases because we need the equilibrium reaction for excess acids/bases to be converted into neutral substances in order to minimise the added acid/base's impact upon the normal pH level of the system.

In our blood, H2CO3 and HCO3- are prevalent examples of buffers. Other buffer pairs can include CH3COOH/CH3COO- or H2PO42-/HPO4-.

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Happy Physics Land

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Re: Chemistry Question Thread
« Reply #139 on: March 13, 2016, 09:00:25 pm »
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Hey guys!

Sorry I have quite a long question, I was wondering whether someone could explain how a dry cell battery works or a lead-acid battery cell, I've read some textbooks about it but I'm still a bit confused! And also, what's a good way of knowing what citric acid and sodium hydroxide forms? Like what the products are.. Thank you! :D

Neutron

Hey Neutron!

First of all, thank you for answering amanda's question! It's really great to see everyone helping each other out! I certainly give you credit for that!

Ok, so for me, my chem teacher didnt really explain that dotpoint, so I really had to chew my jacaranda textbook and collect online informations. Im suspecting that you are perhaps thrown into the same situation too? (correct me if lm wrong haha) But dont worry, Im here to help!

If you understood galvanic cells, you should be able to reason with the chemical functionality of a dry cell battery. All it is, is just the electrode in the anode half-cell being oxidised and the electrode in the cathode half-cell being reduced, and hence producing a voltage. It works exactly like your galvanic cells, except with more complicated substances. In a dry-cell battery, the positive cathode at the centre consists of an inert graphite rod surrounded by MnO2(s) powder. The negative anode consists of the zinc casing of the cell. And there is a thick layer of moist electrolyte paste NH4Cl(aq) between the graphite rod and MnO2(s). What basically happens is essentially the reaction between manganese dioxide powder and Ammonium chloride and zinc casing and ammonium chloride . (2MnO2(s) + 2NH4+(aq) + 2e- --> Mn2O3(s) + H2O(l) + 2NH3(aq)) In this equation, the original oxidation state of Mn is +4 but in the product, the oxidation state of Mn becomes +3. This means that Mn has been reduced. In the zinc casing and ammonium chloride reaction (Zn(s) + 2NH4Cl-(aq) --> ZnCl2(s) + 2NH3(aq) + H2(g)), the original oxidation state of Zinc was 0, and after the reaction, its oxidation state becomes 2+. This means that Zinc has been oxidised. This redox reaction then produces a voltage, just like what happens in a galvanic cell.

In regards to citric acid + sodium hydroxide, you can very easily deduce what happens either by using the rule "acid + base --> salt + water" or just simply memorise what I got below! :)

C6H8O7(aq) + NaOH(s) --> NaC6H5O7(aq) + 3H2O(l)

Hope my answer has helped with your understanding of these areas of chemistry, and they are indeed hard concepts to master. Once again, thanks for being helpful in the community and if you have any further questions, please dont hesitate to ask! :)

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katherine123

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Re: Chemistry Question Thread
« Reply #140 on: March 13, 2016, 10:05:30 pm »
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Am i right in saying that:
-If all products are weak eg. H2PO4- + H3O+ --> H3PO4 + H20 then the reaction goes to completion
-some weak some strong eg. H2PO4- +H20 <--> OH- (strong) + H3PO4 (weak)
then reaction undergoes equilibrium
-weak acid eg. HF will always produce a strong conjugate base ( F-  )

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #141 on: March 13, 2016, 10:12:55 pm »
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Am i right in saying that:
-If all products are weak eg. H2PO4- + H3O+ --> H3PO4 + H20 then the reaction goes to completion
-some weak some strong eg. H2PO4- +H20 <--> OH- (strong) + H3PO4 (weak)
then reaction undergoes equilibrium
-weak acid eg. HF will always produce a strong conjugate base ( F-  )

Hey Katherine:

In regards to your first two points, I find it a bit difficult to memorise it that way. All you need to know is whenever there is a strong acid or a strong base involved in the reactants, then the reaction is always going to go to completion because strong bases/acids completely ionise in water. Whenever both reactants are weak acids/bases, the reaction is always going to become reversible (i.e. equilibrium reaction) because they dont fully ionise in water.

I do agree with your third point though! Very well stated! Weak acids will for strong conjugate bases because they only partially ionise in water.

Hope this give you a clearer insight into acid-base reactions! :)

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qwerty222

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Re: Chemistry Question Thread
« Reply #142 on: March 13, 2016, 11:04:34 pm »
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hello,
if i wanted to do some self-study at home wat textbook would u recommend?

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #143 on: March 14, 2016, 12:01:34 am »
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hello,
if i wanted to do some self-study at home wat textbook would u recommend?

Hey qwerty222!

Jacaranda, definitely jacaranda.
It has the most detailed information on each dotpoint, a whole load of content, if you make notes out of the textbook and be familiar with them, you will be very familiarised with all your modules. The two disadvantages are its cost (60-70 dollars l remember) and the overwhelming amount of content which may bore you. If you want to obtain a fast grasp onto the chemistry knowledge I will recommend Excel chemistry textbook because it has all the information you would need for each dotpoint and present it in very succint manner. But of course it is not as extensive as jacaranda. I havent really used any other textbooks as just yet, but l do recommend these two textbooks, especially jacaranda.

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katherine123

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Re: Chemistry Question Thread
« Reply #144 on: March 14, 2016, 10:45:29 pm »
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why is HNO3 and NO2- not  a conjugate acid/base pair   and conjugate acid-base buffer pair

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #145 on: March 14, 2016, 11:25:43 pm »
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why is HNO3 and NO2- not  a conjugate acid/base pair   and conjugate acid-base buffer pair

Hey Katherine!

A conjugate acid/base pair will have exactly the same chemical composition except it will either lose or gain a proton (i.e. a H+ atom). So in this case, when HNO3(aq) loses a H+ atom, it becomes NO3-, not NO2-. So HNO3 and NO3- in this case will be conjugate pairs.

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katherine123

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Re: Chemistry Question Thread
« Reply #146 on: March 15, 2016, 06:50:46 am »
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Is this info wrong in saying that CH3COO-   since weak acids always produce strong conjugate bases

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #147 on: March 15, 2016, 08:52:00 am »
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Is this info wrong in saying that CH3COO-   since weak acids always produce strong conjugate bases

I would say that the information is wrong. The chemistry that we have been taught in year 12 tells us that a weak acid would yield a strong conjugate base. Hence CH3COO- is likely to be a strong conjugate base.

Sorry, update. The information is actually correct. We got taught that a weak acid would yield a strong conjugate base. But just like what rui said, only an extremely weak acid would yield a strong conjugate base. However, the conjugate base of the weak acetic acid is still stronger in comparison to water (i.e. it will react with water) despite compared to other bases it is still a weak base. Sorry for my misunderstanding of the knowledge!
« Last Edit: March 15, 2016, 10:38:51 am by Happy Physics Land »
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WLalex

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Re: Chemistry Question Thread
« Reply #148 on: March 15, 2016, 09:06:35 am »
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Hi, i was wondering if you have any resources/information of the lead-acid battery. The chemistry of it was quite confusing to me and the way the teacher taught it to us was in a hurry and she didn't really explain much. We also did the silver button cell

Thanks
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RuiAce

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Re: Chemistry Question Thread
« Reply #149 on: March 15, 2016, 10:10:57 am »
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Is this info wrong in saying that CH3COO-   since weak acids always produce strong conjugate bases

The information is very much correct.

A weak acid (CH3COOH) yields a weak base conjugate CH3COO-

A strong acid (HCl), specifically, however has an extremely weak conjugate base (Cl-).

I would say that the information is wrong. The chemistry that we have been taught in year 12 tells us that a weak acid would yield a strong conjugate base. Hence CH3COO- is likely to be a strong conjugate base.

It actually isn't, basically.

No pun intended.