Sulfuric Acid
Hydrochloric Acid
Acetic Acid
(systematically known as Ethanoic acid))
Citric acid
(systematically known as 2-hydroxypropane-1,2,3-tricarboxylic acid)
Worksheet
Q2) & Q3)
Unless the acid/base is strong (ie NaOH, KOH, HCl, HNO3, H2SO4) you MUST use the equilibrium arrow. AND you must include states!
Q2) & Q3) & Q4) & Q5)
Q6 & Q7)
It is good practise to show the H+ ion transfer in acid/base reactions - this is especially tru if you are having trouble identifying the conjugate pairs. The acid is what has donated the proton. The conjugate base is the resulting species that has one less H+. Similarly the base is the substance that accepts the proton. The conjugate acis is that new species with one extra H+.
Q8)
For stepwise dissociation, there can only be as many ionisations as there are 'donatable' protons. Citric acid is triprotic thus there will be three ionisations. Don't forget that the conjugate bases produced by these ionisations will have one less H and one less + charge each time. Finally - the questions specified that these were all weak acids so use the appropriate arrows.
Q9)
NEVER show an amphiprotic substance donating AND accepting protons to water. Water is either more acidic or more basic than the amphiprotic substance not both. Eg bicarbonate ions will always accept protons from water (making the solution slightly basic), but will not donate them to water. SO when showing how a species is amphiprotic write one reaction where it is accepting protons from H3O+ (strong acid) and one where it is donating protons to OH- (strong base).
Summary
The expression/language/communication in these summaries was generally very poor and many did not answer the questions that were asked. I can't stress enough how important it is to express yourself clearly in Chemistry. You need to know your definitions and terms thoroughly and need to use correct definitions and explanations. You notes are full of examples of explanations and definitions designed specifically for the HSC course. Please use them. But, as disappointed as I am, I now have an excuse to post my new Chemistry pic!
(A) You MUST know where the double bonds are in citric acid and you must know which H is removed from acetic and which H's from citric acid. You must remember to i) include and ii) draw correctly the hydroxyl group in citric acid. You were asked to draw the acetic acid and its conjugate so you need to do this and label them as such. In addition, the conjugate base should have a – sign next to where the H was removed. Finally - you needed to draw 'ball-and-stick' models, structural diagram are not sufficient (and it using coloured circles you need to include a key).
(B) Don’t even think about copying and pasting anything into a experiment summary ever again.
(C) This is the most important concept. It is vital you use the correct wording. Poor selection of words conveys that you don’t understand the difference between strong and weak, because IF you completely understood the difference THEN you would not use poor wording. Strong means that EACH acid molecule is ionized. It does NOT mean that ‘every hydrogen is removed’, or ‘all hydrogen’s ionized’. It is important that you understand this distinction. Weak acids have only SOME of their molecules ionized (or some of their molecules donate a proton). This is very different to the incorrect ‘ only some of their hydrogen’s are ionised’, or 'only some of the hydrogens came off the molecule; or ‘some of their hydrogen’s are removed’.
(D) Give the % ionisation equation! and make sure you define it correctly (see above). Also you need to give examples - ie you should specify the exact % ionisation for hydrochloric, acetic and citric acids.
(E) You need to say WHY a BASE is formed when an acid donates a proton (and vice versa) and you must define what a CONJUGATE PAIR is – i.e. answer the question
(F) You must explain WHY strong acids/bases produce very weak conjugates and why weak acids/bases produce weak conjugates – not just state that they do – ie again, answer the question
(G) The conclusion must answer the aim. SO -1) What did you use to model acids?, 2) How did you use those models to model strong and weak?
(H) Don't forget that you have to explain why structural formulae (diagrams) are more useful than molecular formulae for organic compounds. i) It allows you to see what the functional group is and where its is ii) it helps you to predict the properties of the compound. iii) there are many isomers of organic compounds and the structural diagram allows you to clearly determine the isomer. Eg see here for all the isomers of this simple molecular formula C4H8O2
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