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.

Leave a Reply

Powered by Blogger.