The most common issue among students new to chemistry is writing equations; but more precisely, predicting the products of a reaction and then balancing equations.  There are a few key pieces of information which help this process which I am presenting here today.

The basics
Predicting chemical reactions starts first with knowing the basics of atoms, molecules, ions and compounds.  Get familiar with elements in the periodic table - their names, chemical symbols, their states at room temperature, and their potential charges as ions.

To get you started:

  • Most metals can exist in their elemental form, with an oxidation state of zero, in solid state, e.g. Al(s), Fe(s), Ag(s), etc.  The notable exception is Mercury, which exists at room temperature as a liquid, Hg(l) (some thermometers contain mercury to tell the temperature).
  • Noble gases (Group VIII) exist as gases, but are generally not involved in chemical reactions, unless you are considering ionization energies or similar concepts.
  • Non-metals which are not noble gases generally bond covalently with other non-metals to form molecules.  Small molecules like N2(g), O2(g), CO2(g) and NO(g) all exist as gases at room temperature. 
  • As we observe the states of elements going down Group VII (halogens), we observe gas to liquid to solid: F2(g), Cl2(g), Br2(l), I2(s).
  • Carbon reacts with hydrogen, oxygen, nitrogen, phosphorus, sulfur and some other elements to form organic molecules. I highly recommend you revise compounds of carbon if you are a little rusty.  The below tables should help you out:

  • Last, but definitely not least, memorise your common cations and anions - their elemental composition AND charge, which are shown in the below table.  Know how to write chemical formulas from cations and anions by balancing the charges of cations and anions into a neutral compound.  I cannot stress this point enough - it is usually a large sticking point for most students and one you can't get around.  Spend the time memorising the below table and you will save yourself a lot of stress.  There are a few tips and tricks to memorising this table though, take note:
    • The top four rows (plus sulfide) all pertain to main group elements which have a charge relevant to their position on the periodic table - Group II elements have a +2 charge; Group VII elements have a -1 charge, etc.
    • Transition elements often have more than one oxidation state, however only the most common ones have been listed on the table.  Note that copper, iron and tin each have two common ions so be sure you know which one you are dealing with.
    • Some of the polyatomic anions (sulfide, sulfite, sulfate, carbonate, phosphate) also have associated hydrogenated compounds.  If you can memorise the non-hydrogenated forms and their charges, then you can work out the chemical formula for every hydrogen added - add one hydrogen and add +1 to the charge of the compound.

Types of reactions

It is super important to first identify the type of reaction you are looking at - this will give you vital cues as to the products of the reaction in question.  I have summarised the types of reactions pertinent to VCE below, excluding a couple related to the reactions of alkenes which is covered later in Year 12.

Combustion reactions:
These reactions are essentially burning a compound - often a carbon/hydrogen/oxygen based compound or sometimes solid metal - in air.  What does a fire need to burn?  Fuel, a source of ignition, and of course OXYGEN GAS.  Therefore, a chemical reaction representing the combustion of a compound is really a reaction with oxygen gas:
The reaction of a carbon compound with oxygen gas will ALWAYS produce carbon dioxide and water as products.   The reaction of a metal with oxygen is actually a redox reaction involving the oxidation of the metal, and will always produce a metal oxide.

Precipitation reactions:
The formation of a precipitate in plain English is the formation of a solid compound in solution, and can be used as a quantitative determination which is termed gravimetric analysis.  Gravimetric analysis is useful because the solid formed as a result of a reaction can be separated from the liquid by filtration, and then can be dried and weighed.  Through stoichiometry you can determine the amount/concentration of the reactant(s).

To form a precipitate, two salts undergo a metathesis reaction - the swapping of cations/anions - which produces a salt which is insoluble in the solvent. It is important to note that the formation of a solid is related to its solubility in that particular solvent.

There are some great solubility tables published in countless textbooks and on the internet - not only are there plenty of these about, but I believe they're not needed. At least for the purposes of VCE and basic chemistry courses, the products of a precipitation reaction are not going to be ambiguous.  The vital information you need to know is:
All salts of Li+, Na+, K+, NH4+, CH3COO- and NO3- are ALWAYS going to be soluble in water. 
If you KNOW the reaction is definitely a precipitation reaction, then the other salt MUST be the solid.
As you practice, you will come to know the common insoluble salts. e.g. PbI2, BaSO4, AgCl, etc.

Acid-base reactions:
There are several different types of acid-base reactions, which can be summarised in the below graphic.

The most common type of acid-base reaction encountered in VCE chemistry is the one in light blue/aqua.  In acid-base titrations, usually it is not the amount of product measured, but the amount of one reactant to bring the reaction to the equivalence point - the point at which you have stoichiometrically equal amounts of the acid and the base.  This is determined by observation of the colour of an indicator; the colour change is termed the end point.  An indicator is chosen so that the equivalence point and end point are very close.
Sometimes reactions involving carbonates or hydrogen carbonates are used quantitatively by measuring the loss of CO2 gas from the reaction system - the mass of CO2 lost can be weighed on a set of scales.

Dissolution reactions:
Dissolution reactions are simply those where a solute is dissolving in a solvent to form a solution - or said another way, where compounds are breaking apart into their constituent anions and cations in the presence of a solvent, usually water:

They can also be used to depict an acid dissociating in water.  For a monoprotic acid, this is a simple task:

For a polyprotic acid, there are several steps involved, where each reaction involves only the loss of one  hydrogen for the acid.  Note that the first dissociation reaction for a triprotic acid is shown as a strong acid and therefore complete dissociation is observed, denoted by the "normal" arrow pointing to the right. Whereas in the second and third reactions, dihydrogen species and hydrogen species are weak acids and thus incomplete dissociation is observed, denoted by the equilibrium arrows.

Balancing equations
There are no hard and fast rules for balancing equations - everyone has their own way which works for them.  However, there are a few tips and tricks:

  • Keep a tally of all the elements on each side of the equation; change your tally when you add a stoichiometric coefficient (the number in front of each compound).
  • I also recommend that if a polyatomic anion/cation does not break apart during the reaction, count it as one entity, rather than its constituent element
    • e.g. treat phosphate as one entity with a 3- charge rather than one phosphorus and four oxygens
  • Start with elements other than carbon, hydrogen and oxygen (if applicable).
  • Then proceed with C, H and O in the most appropriate order
  • When balancing combustion reactions, I recommend you balance carbon first, then hydrogen, and finally oxygen.
  • If there is one element by itself, e.g. O2, leave this one to last.
  • Don't forget to add states at the end!

The most important factor in consolidating your understanding of predicting reaction products and balancing equations is PRACTICE.  I have written a worksheet to help you out, click here to access it.  The answers can be found on the Free Resources page.

With just over 2 weeks until 2012 VCE Chemistry Exam, here are some final tips and tricks for before, during and after the big day.

The preparation stage

Know your learning style.  I have covered this in more detail in a previous post.  Some people consolidate their knowledge through writing, some by listening, some by explaining, others by drawing diagrams or concept maps.  Figure out which ways help information stick into your head.  This is step 1 – don’t skip over it!

Organise your time.  Pre-plan your study times, allocate study sessions appropriately to all of your subjects, and try to stick to them as much as possible.  Give some thought as to what time of the day your brain takes in information the best, and work with it, not against it.  If you are a morning person, DON’T stay up past midnight studying!  Don’t forget to schedule in down-time - especially exercise - which will kill off stress hormones (such as cortisol), and produce happy hormones (endorphins).

Do questions! Do as many exam-style questions as you can get your hands on, over and over. Seek out the ones that really give you lots of trouble and try to understand what’s going on. The best way to learn is to make mistakes and understand where you went wrong.

Identify problem areas, and go back to revise. Doing questions and getting things wrong before the exam is a blessing because you still have time to fill in knowledge blanks.  Work out topics and question types that are difficult, and spend more time understanding them fully.  Take time to go and see your teacher, refer to classmates, or get a tutor for this step if it’s causing you serious grief.

Study by yourself and in groups. Studying by yourself is great because you can study YOUR problem areas.  A group environment encourages everyone to be able to put what they understand into words. You can help each other understand tricky concepts, and discuss answers to questions that you all found difficult. If you understand a concept well, the best way to consolidate your knowledge is to teach it to someone else!  Sometimes the process of trying to explain something to someone enables the proverbial light bulb to go off in your head, and all of a sudden everything is clear.

Do practice exams under exam conditions. Nothing gets you prepared for exam time management than actually doing practice exams.  I highly recommend to my students doing VCAA exams last, which will give you a good idea of how a proper VCE exam feels when you are close to doing your own.

Practice relaxation techniques.  If you are prone to stress and anxiety in exams then I highly recommend practicing meditation or relaxation techniques long before exam time.  The goal of starting early is so that you can be practiced enough to be able to “switch on” your calm when you really need it.  In an exam you want to be able to concentrate on the task at hand in a calm and positive manner, and meditation is an excellent vehicle to get you there - my choice is zen meditation.  If you are new to meditation, check out this article.  But any modality (prayer, practicing gratitude, breathing) that relaxes you is absolutely fine as long as it doesn’t interrupt the exam environment.

Think about how you might structure your time in each exam.  In early practice, take the time to answer questions and understand them fully, but in your final preparations and in the exam, if a multiple choice question is taking you 5 minutes, its well and truly time to move on.  Don’t get to the end of your time and realize there were questions you could have answered easily but didn’t have time to do them. 

Take breaks when studying long hours. Get up, stretch, get food and drink, and most importantly, keep your body moving!  Get a dose of oxygen and you will return to study clear-headed and energized.

On the day of the exam

Come in well rested. Spend the day prior to the exam doing light revision and allow yourself to wind down properly before trying to sleep. RELAX. Ensure your body clock is not on nightshift if you have a 9am exam. Get some light exercise, eat some brain foods (e.g. fish, eggs, vegetables, nuts and seeds). Stressing out is not going to help your cause, and last minute cramming – forget it!

Be early.  If you are running late, or unsure where you need to go, your stress levels will be through the roof before you even arrive at the exam venue.  If you are not attending an exam venue at your school, perhaps even visit the venue the day before if you’re concerned about finding your way.  Aim to be 15-30 mins early, then at least you have a buffer if trains/buses are late or traffic is a nightmare (and don’t forget money for parking if you are driving!).  Worst case scenario, is you’ll have some time to kill before the exam… To meditate ;)

Be confident!  You have spent so much time preparing for this – right?  Trust that you have prepared well and remember that everyone is in the same boat.  There will always be times you go blank and questions that stump you.  The important point is to keep a clear head, take a deep breath and battle through. 

On a related note,
Stay away from groups of people who are stressing out – it’s contagious! Even if they’re your friends, don’t get sucked in by the negativity. 

Minimize distractions. Wear earplugs if you can’t stand someone clicking a pen or blowing their nose every 2 seconds next to you.  Be prepared for all types of temperatures in the exam room - take a jumper, even in hot weather, in case the air-con is too cold; and wear a singlet so you can strip off if the room is stuffy.  If you are taking public transport to get to the exam and the weather looks dicey – take a raincoat/umbrella so you’re not forced to take the exam sopping wet.

Avoid too much caffeine. ESPECIALLY if you are the type of person who normally avoids caffeine!  Use classical music, aromatherapy and/or herbs to enhance your brain power if that’s your preference.  Experiment with these in your study sessions prior to the exam day.

Don’t forget to bring:
School/Photo ID
Several working pencils, pens, eraser, sharpener
Calculator (check battery status the day before!)
A clear water bottle (with the label removed)
A watch to keep on track
A jumper
Painkillers (in case of headache)

In the exam

Follow instructions.  Read the instructions carefully and ensure you fill out your name/student number and multiple choice sheets properly!  Listen to supervisors instructions carefully. Enough said really.

Use reading time effectively. Take this time to read the exam thoroughly and decide how you are going to tackle it. Multiple choice or short answer first?  Give some thought as to which questions are worth the most amount of marks, and how much time you should allocate them.  Remember you do not have to complete the exam in the order in which it is written.  

Work rapidly but thoroughly. Push yourself to work a little faster in the exam (and practice exams) to ensure you have enough time to attempt all questions.  However, don’t work so fast that you make silly mistakes.  Find a happy medium.

Eliminate multiple choice answers that are incorrect as soon as possible.  Usually, there are patterns in the answers.  If you do eliminate incorrect answers, do not fail to reconsider them or change your mind later either if you think its fitting.  Sometimes the purpose of similar and/or exact opposite answers is there to overwhelm and confuse you.

Approach exam questions with a clear mind.  Don’t get caught up in excessive jargon and/or complicated diagrams.  Be able to read the information provided, and then step back from it and view the questions with a sense of clarity.  The purpose of the complicated information is to overwhelm you – don’t get sucked in!

Do the questions you KNOW you can do well, first.  This is a great confidence builder.  Also, many students explain to me that they spend a lot of time on harder questions, only to get to the end of the exam and rush through questions they can do well, and make silly mistakes.  Don’t enter into the psychological game that examiners play to put you off-guard – stay emotionally detached from difficult questions; skip them and come back to them when you have time at the end.  Make time for questions that are quick and easy for you so you can optimize your time usage.

Keep your calculations organized.  ALWAYS write down the equation you are using – they are usually worth a mark.  Try to write down your calculations in a logical fashion and write clearly and simply.

Use jargon words knowledgeably.  For a lot of short answer (worded) questions, examiners are looking for key phrases and words in your answer.  Definitely use them if it’s appropriate, but only if you know what they mean.

Make an educated guess for unknown answers.  If you absolutely don’t know the answer to a question, give it your best shot.  Don’t leave the question blank; write SOMETHING!  You may just score yourself one extra mark without truly knowing the correct answer.

Use spare time to check your answers. Keep a clear mind when rechecking.  Be wary, because rechecking answers introduces the element of doubt – often our first instincts are correct.  That being said, spare time also often allows students to rectify silly mistakes.  Check calculations with your calculator again if it was a complicated process to get the answer.  Double check significant figures. Also make sure your handwriting is legible for the examiner.

Double check the last page! Unfortunately, I myself have fallen victim to not realizing there was the last question on the last page – after the exam was over.  Learn from my mistake!  Double check that you haven’t missed entire questions “hidden” on the last page.  All VCAA exams have “END OF QUESTION AND ANSWER BOOK” on the last page of the exam.

Write down anything you can before pens down.  If you are out of time, and you know how to answer a question, but don’t have time to finish, at least get the equation you would have used down on paper, and if you can, rearrange the equation and put the values into the right spots.  Often for a 3-4 mark question, you will get marks for using the right equation, and in the right way.  The final answer is usually only worth 1 mark.

After the exam

Celebrate! Pat yourself on the back for getting through a challenging exam.

Avoid post-exam debrief sessions with your friends.  These can be both good and bad for your confidence.  Good when you are talking about answers you all agree on, but bad when someone says “Did you get ____ for Question X?” and you didn’t get that answer.  My general advice is stay away from these types of discussions, especially immediately after the exam.

Don’t stress over the result.  You did the best you could on the day, and there is nothing more you can do about it.  If your mark fails to get you to where you need to be, remember that there are many more ways to get to where you want to be, it just might take a bit longer.  If you want it badly enough, you will get there, I know it.

When doing calculations involving the ideal gas equation, it is extremely important to be able to convert between units.  This is because when using PV=nRT, R is a constant value which is known, but varies depending on which units are used for Pressure (P), Volume (V), and Temperature (T).  The number of moles (n) will always be simply the number of moles (mol)

For VCE students, the ideal gas constant used is 8.31 J/mol.K, for which:

  • Pressure must be expressed in the units kiloPascals (or kPa) 
  • Volume must be expressed in Litres (L), and 
  • Temperature must be expressed in Kelvin (K)

Usually, in a given question, some or all of these variables may not be in the desired units, and thus, conversion is required.  Volume may be in milliLitres (mL), and Temperature may be in degrees Celsius (oC), however conversion for these are straightforward:

However, pressure may be give in any one of five units:
  • Atmospheres (atm)
  • Pascals (Pa)
  • kiloPascals (kPa)
  • Millimetres Mercury (mm Hg)
  • Bar (bar)
NB: There are many more units of pressure, however none that will be used for VCE Chemistry.

I use the following schematic to help students convert between the five units of pressure.  It is important to note that converting to atmospheres is central to all other conversions. i.e., if you were to convert from mmHg to kPa, you must first convert from mmHg to atm, and then atm to kPa.

There are, of course other methods of converting between these units, however I have found this methodology to be the most user-friendly, I hope you do too!

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