Titration Cures, Buffers and Indicator Theory：滴定曲线缓冲器和指标理论

1、titration curves, buffers and indicator theorychemistry 12read pages 595-597 (buffers), 608-612 (titration curves), 599-605 (indicators)titrationnis a technique to determine the concentration of an unknown acid or base. nslow addition of one solution where the to a until the reaction reaches a desir

2、ed level. colour change is close to equivalence point (hopefully)na color change from a ph indicator is reached or a direct reading using a ph meter. nthis information can be used to calculate the concentration of the unknown solution. strong acid strong base titrationnequivalence point is where nco

3、mplete neutralization of h+ (acid) with oh- (base) to make h2o.nideally, you want an indicator whose is very close to the titrations . (within 2 ph units)indicatorsna substance (often organic) that has a different colour in acidic and basic solutions and can be used to detect end points of titration

4、s.nit is a weak acid or basehow indicators work:example of litmus usednhin(aq) h+(aq) + in-(aq) redbluenin presence of acid = shift to left = red nin presence of a base = shift to rightnka = hinrearrange: = in- kathe colour of the indicator depends on h+ and also on ka value, when ph = pka indicator

5、 is in the middle of its colour change.useful indicatorsnphenolphthalein usednin this curve, you had acid in the flask (colourless b/c ph is less than 8.2) nyou are titrating (adding) base to raise the ph.nthe colour changes between 8.2 10.phvolume of base addedpractice choosing an indicator1.what c

6、olour will the solution be if its ph is 5.6 usingorangealmost redyellowtypes of titration curves:polyprotic acid with strong basewhy do the curves look different?nthey are all examples of acids with bases butacids, bases and neutral saltsnthe reason why a strong acid hcl and a strong base naoh can m

7、ake water and ph = 7 is because a neutral salt was also made naclnneutral salts are like spectator ions, they move around in solution but dont affect the ph. they are neutral.nany salt made from the following ions are neutralneutral salts ph = 7 for sa/sbbecause nacl is the neutral salt made, no hyd

8、rolysis occurs, leaving the ph to be only affected by the h+ from hcl and the oh- from naoh. doesnt matter which order it is titrated.hydrolysis and acidic/basic saltsnwhen weak acids and bases react, the relative strength of the conjugated acid-base pair in the salt determines the ph of its solutio

9、ns.n the salt solution formed can be acidic, or basic. na salt formed between a strong acid and a weak base is an , for example nh4cl.na salt formed between a weak acid and a strong base is a , for example nach3coo. hydrolysis of acidic saltsna salt formed between a strong acid and a weak base is an

10、 . nammonia is a weak base, and its salt with any strong acid gives a solution with a ph lower than 7. nfor example, let us consider the reaction: hcl + nh4oh nh4cl(aq) + h2o h+ + cl-+ nh4+ + oh- nh4+ +cl-+ h2o strong acid with weak base titrationsnbuffering region occurs where ph is changing gradua

11、llynafter the buffering region there is a drastic drop where the equivalence point is found.nnote that the ph is below 7 at equivalence point because of the hydrolysis of nh4cl.nnh3 reacts completely with hcl to form only nh4+ and cl- in solution. the nh4+ will hydrolyse making nh3 and h3o+nthe hydr

12、onium ion created adds to the acidity therefore lowering phnnh4+ + h2o nh3 + h3o+buffering regionsnareas on the titration curve where buffering occurs.buffersnthey work because they are made of a weak acid and a conjugate base mix or a weak base and a conjugate acid mix.nthink the weak acid can take

13、 up some base without changing ph and the conjugate base can take up some acid without changing ph to a point.buffering in bloodndissolved co2 in blood reacts with h2o to form hco3- ionsnhco3- dissociates in water to make co32-if excess is added hco3- reacts to make if excess is added hco3- reacts t

14、o make hydrolysis of basic saltsna basic salt is formed between a strong base and a weak acidnthe higher ph is due to the hydrolysis of the conjugate base of the weak acid used during the neutralization reaction.nfor example, lets consider the reactionnnach3coo will break up to form na+ and ch3coo-

15、ions, and the acetate ion will react with water to produce hydroxide.nweak acid and strong base titrations always have a ph greater than 7 strong base with weak acid titration: naoh + ch3cooh nach3coo + h2onph starts low, so acid was in flask.nadded base, so ph raisesnequivalence point is greater th

16、an 7 because of basic salt hydrolysis of nach3coocan you explain why ch3coo- undergoes hydroylsis?nremember that the naoh and the ch3cooh react and become neutral.nthen the salts made undergo their own reaction with water.polyprotic acid titrationsnfor acids with more than one hydrogen to be removed

17、, you can see multiple steps in the titration curve. neach step = a proton being removed by the base being added (in this example)nexample could be of h2so4 with naoh.diprotic h2so4 with naohnthe first ion will break off in water by the dissociation h2so4 h+ + hso4- writing equations for polyprotic/basic speciesnadd up the separate steps, then cancel out similar species to have the net ionic equation.nex: write balanced, net ionic equations