9.3.4 HSC Q Feedback

2001

Q21) Key idea – IONS in solution allow charge flow (conductivity) – at start barium and hydroxide ions carry current, but their concentration and thus conductivity decrease as acid is added.. At equivalence point = no free ions = no conductivity. After equivalence point increasing acid concentration increases sulphate and hydrogen ions = increasing conductivity.

2003

Q23a) You would think that I had never tried to trick you by giving you volumes in mL rather than litres. CONCENTRATION CALCULATIONS ARE IN LITRES!

Q24) Always define the key Chemistry term – ie neutralisation. Also give an example chemical and you could possibly also talk about skin spills.

2004

Q22b) As inconvenient as it sometimes is, you have to answer the question asked. If they ask for equations to show why dihydrogen phosphate is amphiprotic you have to do it. You can’t just substitute other chemicals!

2005

Q9) Buffers are a mixture of a weak acid and its conjugate base. Conjugate bases have ONE less proton than the acid (not two, hence dihydrogen phosphate and hydrogen phosphate are buffers but dihydrogen phosphate an phosphate are not).

Q21) Try and link the scientists together specifically noting what the previous model couldn't explain (ie in the scientific method new theories are proposed when old theories can't explain all observations), eg Lavoisier said they contained oxygen based on his experiments but Davy disproved this when.... Arrhenius developed Davy’s theory by noting that acids ionise to produce H+ in aqueous solutions.... But could not explain amphiprotic substances, acidic salts or acids in non-aqueous solutions. BL theory explained acids in all solvents by noting that they were proton donors. It is critical that you note that BL realised the importance of the solvent. Ie a substance would be an acid in a solvent IF it could donate a proton to that solvent. It is vital that you use chemical equations. Eg show how Arrhenius explained the acidity of HCl (by producing hydrogen ions) compared to BL (producing hydronium ions). And this means you have to be careful, Arrhenius said acids ionise to produce hydrogen ions NOT hydronium ions (that is  BL concept).

2006

Q9) Equivalence I when equal # of hydroxide and hydrogen ions have been added. Many strong acid/base neutralisation reactions are not 1:1 in terms of acid to base molecules (eg H₂SO₄:NaOH = 1:2) but are always 1:1 H⁺ to OH^-.

Q11) Arrhenius modelled acids as chemicals that ionised to produce hydrogen ions. The higher the % ionisation the stronger they were. pH was a concept that was developed from his model but was not part of is model.

Q21b) To show a given salt is acidic (or basic) you have to show one of its ions reacting with water to donate (or accept) a proton to produce hydronium (or hydroxide). The parent acid/base argument is useful for titration questions only. BUT NB these salts are weak acids and bases – the equations MUST have an equilibrium sign.

2007

Q25) Show two equations to show it is amphiprotic(ie reacting with OH^- and H₃O⁺) and could also that it is cheap (not absolutely critical but most people overlooked it).

2008

Q8) Arrhenius said that acids ionise to produce hydrogen ions, BL said they were proton donors

Q26) Define key terms ie define buffer

Q27b) See 2006 Q21b above

Q28c) NEVER, NEVER, NEVER use c₁v₁=c₂v₂ unless it is a dilution.

*Find the moles of one species

*Find the moles of the other via a mole ratio

*Do the calculation to find what it is you are asked to find.

2009

Q14) Excellent Q to do – make sure you can do this Q. I have a feeling a similar one is coming in this year’s HSC

Q21) – an excellent Q to do again for revision

a) The more base that has to be added to neutralise the more moles of acid there are. Because they are both in equal volumes you can determine their relative concentrations. The weaker acid will have an equivalence point at higher pH.

d) Answer the Q. The end point ideally happens at the exact equivalence point but as long as the colour change occurs within the ‘steep’ part of the curve, the endpoint (ie amount of base added hen the titration stops) will match the equivalence point.

2010

Q8) Davy said that acids contained replaceable hydrogen. Arrhenius said they IONISED in water to produce hydrogen ions.

Q28) You should show your calculation for the mass of standard. You must say how the conical flask, pipette and burette are cleaned. You have to say which indicator is used and should explain why. Don’t forget that titrations are repeated until 3values within 0.1ml and averaged.

2011

Q15) At equivalence point the moles of H⁺ = moles of OH^-. The number of moles of acid base needed only depends on the mole ratio, not on strength.

Q18) – this is very similar to 2001 Q23

Q26a) You actually need to calculate both concentrations (from solid and from titration) to show how the values are different, not just say that NaOH absorb water and CO₂ from the air.

Q29) If asked a Q about a scientist’s acid theory, you must outline the theory. Ie what ar the basics of BL and Arrhenius theory. In this case re why we still use Arrhenius NB that most everyday acid are aqueous and the Arrhenius model is simple and convenient to use (eg calculating pH, comparing strength etc).