VCE Chemistry

The following notes detail the key underlying concepts and applications of the chromatographic techniques detailed in the VCAA Chemistry course:

Chromatography is, first and foremost, a separation technique. It separates mixtures of (usually) organic molecules on the basis of their polarity. The technique can be measured according to the distance the molecules to be separated adsorb onto a stationary phase in comparison to the movement of a mobile phase in the case of paper and thin layer chromatography (the Rf factor) or the time taken for the molecules to pass along a column (for GSC, GLC and HPLC) techniques – the retention time, Rt. In these cases, the separation of the molecules in question will depend on the extent to which they preferentially dissolve in either the polar stationary phase or the nonpolar mobile phase.

The key points to recognise with respect to paper chromatography are that the paper is the stationary phase and, being made from cellulose polymers (which in turn are made up of polar glucose monomers) is highly polar. The mobile phase is the solvent into which the paper is dipped: often water or a water/ethanol mixture. If the molecules to be separated are highly polar themselves they will adsorb more strongly onto the paper and so will move only a relatively short distance. In this scenario, the Rf factor is small. If the molecules are less polar, they will be less likely to adsorb (or ‘stick’) onto the paper and so will be carried a greater distance. In general, the presence of polar groups such as the hydroxyl (-OH) and amine (-NH2) AND/OR a larger molecular weight (and so more dispersion forces) increases the polarity of a molecule.

The same fundamental rules apply to column chromatography in the applications of GSC (Gas Solid Chromatography), GLC (Gas Liquid) and HPLC (High Performance or Pressure Liquid Chromatography). The stationary phase in these instruments is a highly polar solid with a high surface areas such as alumina (GSC) or a similar solid or inert material coated with a high molecular weight ester liquid (GLC and HPLC). The mobile phase will be an inert, nonpolar gas such as nitrogen or argon (GSC and GLC) or a nonpolar liquid (HPLC). The molecules to be separated will either dissolve more readily into the polar coating on the stationary phase (or the solid itself) if thy are polar OR will dissolve more readily into the mobile nonpolar phase (either a gas or a liquid).

In summary, polar molecules dissolve into the polar stationary phase and so have longer retention times. Nonpolar molecules dissolve more readily into the nonpolar mobile phase and so have short retention times, as they are ‘swept’ along the column more readily.

The following is a summary of the key concepts that should be known prior to attempting the Unit 4 VCE Chemistry examination. Chapter references are to Chemistry Dimensions 2.

Chapter 9   An introduction to Industrial Chemistry

• Producing chemicals on a large scale
• Particle motion (Boltzman distributions at varying temperatures)
• Energy changes in chemical reactions (heat change diagrams, exothermic and endothermic, activation energy)
• The rate of chemical reactions (factors that affect rate: temp, conc, surface area, catalysts, explain in terms of particle motion and energy)
• Catalysts (effect on activation energy, homo and heterogeneous catalysts)

Chapter 10     Chemical equilibrium

• Reactions are reversible
• The equilibrium law (K: how to calculate it given initial and/or equilibrium concentrations using the table rule I showed you, concentration fraction, manipulating equations to obtain another one and so calculate K for the new one
• The position of equilibrium: Le Chatelier’s Principle. This is critical! What is the effect of temperature, concentration, volume, inert gas on the position of equilibrium and the constant? Learn the rules!! Go over your Equilibrium SAC – this is a very useful one.

Chapter 11      Acid-base equilibria

• Self-ionisation of water and the pH scale (make sure you can interpret the graph of pH of water at various temperatures, practise pH questions for both acid and alkaline solutions, including diprotic acids and bases)
• Acid ionisation constants (calculating the pH of a solution of weak acid of known concentration given K_a, calculate K_a given the pH of a known concentration weak acid, percentage ionisation of a weak acid, pK_a)
• Acid-base indicators (make sure that you check the Data Table for the range of pH over which indicators change and their relevant colour changes
• Buffers (unlikely to come up, but I will discuss in class just in case)
• Competing equilibria (particularly in blood with the bicarbonate ion and carbon monoxide poisoning role on oxyhaemoglobin)

Chapter 12   Industrial chemistry case study

• Sulphuric acid (raw materials, rate and equilibrium considerations of the Contact Process equation, uses of sulphuric acid, waster materials, use of waste energy)

Chapter 13   Energy sources

• Energy changes in chemical reactions (endo and exothermic heat change diagrams, role of catalyst and activation energy/catalysts on these diagrams)
• Non-renewable energy sources (focus on fossil fuels, renewables, the idea of energy conversions and the loss of efficiency, generating electricity using turbines from coal, natural gas, nuclear or wind/hydro)
• Renewable energy sources

Chapter 14        Enthalpy of chemical reactions

• Energy changes in chemical reactions (Hess Law: manipulating given equations to obtain another equation and determining ΔH for the new one, calculating energy given ΔH, calculating ΔH given energy, calculations involving specific heat capacity)
• Thermochemical equations (recognise the difference between the heat of combustion of a fuel (heat from one mole) and a thermochemical equation for that fuel for which the actual equation matters)
• Latent heat and specific heat capacity (E = c x m x ΔT)
• The calorimeter (Important: solution and bomb calorimeters, calibration factor/constant, sources of error with calorimeters)

Chapter 15        Galvanic cells

• Galvanic cells (oxidation/reduction, oxidant/reductant, role of electrodes and salt bridge, standard conditions, these cells as exothermic, spontaneous energy producing reactions, RC+ rule)
• The electrochemical series (make sure you can use the Electrochemical series to predict whether or not a reaction will occur)
• Commercial cells (primary/secondary/fuel cells and what these terms mean, general structural features of these types of galvanic cell, discharge and recharge equations, pH changes in the sulphuric acid electrolyte of the lead acid accumulator, acidic or alkaline electrolyte equations in fuel cells, electrodes as catalysts in fuel cells)

Chapter 16     Electrolysis

• The electrolytic cell (recognise these cells are effectively the ‘opposite’ of galvanic cells, RC-, endothermic, nonspontaneous energy-requiring reactions, molten or aqueous electrolytes, note that water can undergo oxidation/reduction)
• Industrial applications of electrolysis (focus on the Diaphragm cell for the electrolysis of concentrated salty water, look at Down’s cell (electrolysis of molten NaCl) and Hall-Heroult cell (electrolysis of molten alumina in cryolite)
• Electrolysis calculations (Faraday’s Laws, colulombs, Faradays of charge, current, voltage, E =VQ, using Faraday’s Laws to calculate mass, time, charge on cation species and (advanced) Avogadro’s Constant

It is at this time, when you are working through practice exams that you can get a little discouraged if your marks aren’t what you were hoping for.  Occasionally, my students experience these frustrations and wonder where they should go from there.  The following provides some suggestions that I might make to a student in this predicament. Remember, you are not alone! Over 8000 students are studying for this Chem exam and many are feeling the same as you might possibly be right now.

“I can understand and appreciate that this must be both frustrating and upsetting. Your knowledge of the course work is solid, as you have shown consistently this semester in your SAC results. So why are you achieving at a sub-optimal level on your exam preparations? Examinations require a level of precision in your responses; you need to know exactly what you are talking about to achieve full marks. Note that many of the practice exams are written by experienced teachers for that very purpose; to allow students to dissect their knowledge and apply a forensic examination to it. So this means that you do not know what you are doing? Not at all, it just means that you need to hone your knowledge and fill in any little gaps that still may be present in either understanding or technique.”

“And how, pray tell, am I to achieve that?!”, asks the student.

“You already know the answer, of course.  At this point, practice examinations are your greatest friend. Look carefully at the Examiner’s reports (from VCAA website at vcaa.vic.edu.au) and see where other students struggled with questions. Did they make the same mistakes as you? What kinds of things were they getting wrong? What does the Examiner suggest? Read over the solutions that I prepared for this year’s Neap exam. When you get something wrong, classify the error as (a) I had and still have no idea! {Okay, forget it or email/ask your teacher}. (b) I was on the right track but I chose the wrong option or made a few small errors. {You are human and are in the same situation as the majority. Practice makes perfect} (c) I disagree with the answer and I think I am right. {Possible, but unlikely. Email/ask your teacher so that they can have a look at it}.”

Best wishes for your upcoming exams,

Mr Ellett

Dear Kilbreda girls

I hope that you are all feeling okay with the June Examination not far away now.  In class on Friday I said that I would try to get the solutions to the exam for you.  Unfortunately I do not have the official solutions but you can access my draft copy at the link here.

Good to hear from Michelle earlier today. I will keep an eye on the email and website if you want to communicate with me over the long weekend, girls.

Best wishes, Mr Ellett

This message is to the Year 12 students of Kilbreda College whom I shall be teaching this year. Welcome to the new year and I look forward to getting to know all of you as soon as possible and to a positive and successful year. Our first topic for the year will be Modern Instrumental Analysis, with specific focus on the spectroscopic techniques of Atomic Absorption Spectroscopy (AAS) and UV/visible spectrophotometry. This is done to prepare us for the visit on Friday 6 February of a guest presenter in these areas. This task will also constitute part of your first major assessment.

Best wishes, Mr Ellett

To Salesian boys

When term 4 begins we will get started on our penultimate topic for the year: The Atmosphere. This topic is fairly descriptive and looks at the gases that make up our atmosphere and how these key gases can be produced on both an industrial scale and in the laboratory.

Look forward to seeing you all next week. Regards, Mr Ellett

This site is the work of Brian Ellett, currently Head of Science at Salesian College, Chadstone and co-author of Chemistry Dimensions 1 (Year 11) and 2 (Year 12), published by Pearson Education. In 2009 I will be taking up an appointment as Assistant to the Principal (Teaching and Learning) at Kilbreda College Mentone. This site will be an integral part of my program at the college.

Prior to this appointment I worked for many years as the Year 12 Co-ordinator and Senior Chemistry teacher at Loreto Mandeville Hall in Toorak, Melbourne.