[sjmax]

Hi, i'm new to this forum so i hope i have put this post in the right place!

I had my unifying concepts exam today (exam board OCR)  and i just wondered if anyone could explain this question to me. It asked me to explain about buffers using NH3 and NH4Cl. I wasn't sure how this would work. It was a seven mark question and you had to use equations.

Thanks 

[Will]

I was so happy about that question.

You just had to simply define what a buffer solution is, write equations for the dissociation of NH₃ and NH₄Cl in water (saying how much/little they dissociate), and show how adding small amounts of H⁺ or OH^- would only effect the pH to a very small extent. Look at how buffers work here. You should've been taught this as it was part of the syllabus.

Hope you did well otherwise . I found it much easier than other past papers, but the organic paper was much harder than past papers (I was so lucky I remembered the structure of Terylene, I can't believe they didn't give it to you!), although I still think I got a high A in it.

[sjmax]

that was the annoying thing! i no about buffers but i just couldn't understand how it would work with NH3. because thats a base and we had only really done it with acids! i knew how to do it with say ethanoic acid, but i wasn't sure how NH3 would dissociate!! 

Hmmmm i dunno i didnt really like the whole paper!!! I thought it was harder than the past papers!!    hehe oh well


Did you resit chains and rings then?

[Will]

I didn't resit AS chains and rings, I just did the A2 chains, rings and spectroscopy paper, which was also this afternoon; have you already done it?

NH₃ dissociates in water like this:
NH_3(aq) + H₂O_(l) <--> NH₄⁺_(aq) + OH^-_(aq)
But, NH₃ is a weak base, so dissociates to a very small extent (eqm. lies far to the left), but the salt completely dissociates to NH₄⁺ and Cl^-, (OH^- soaks up H⁺, NH₄⁺ soaks up OH^-, but better wording!). This is the same principle as weak acids and their salts.

Its unfortunate you didn't get taught about how weak bases and their salts act as buffers.

I have a feeling most people did badly (or not-so-great!) in both papers, so it should be easier to get the higher grades.

[sjmax]

yea i did the A2 chain and rings in january!

Oh right thankyou for clearing that up for me. 
i sought of put that but i messed up the first bit. i put that NH3 goes to NH2 and H+ (based on acid knowledge). but yey i said about NH4 dissociating to NH4+ and cl-. so all is not lost!
i said that when acid is added this is 'mopped' up by the NH2, which is obviously wrong. but i said that when alkali is added the oH- ions will combine with H+ to make water.?