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VCE Chemistry – Area of Study 2 – Common Errors

• Not taking into account the mole of hydrocarbon burnt to produce the ?H reported for a thermochemical reaction.

• Not doing step 3 in calorimetry. In most calorimetry questions, an amount such as 0.235g of reactant is used. Therefore the energy change calculated in step 2 would be for that 0.235g, whereas the answer will usually be required in energy per g or per mole.

• Trying to reduce ions that are not present, i.e. in a galvanic cell when the electrode is Ag but the electrolyte is NaCl for example. If Ag is the cathode, Ag+ ions are not present and could not be reduced. In this case water will be reduced.

• Not taking notice of whether a given equation is discharging (spontaneous reaction) or recharging (non-spontaneous reaction) in a question.

• Forgetting to include H2O as a possible oxidant or reductant when predicting the half-equations for electrolysis. Remember that water may well be both the reactant and the oxidant in a particular electrolytic cell.

• Misreading masses given in kg and not converting to g

• Confusing terms anode and cathode and defining them in terms of charge, rather than the process that occurs at each.

• Not realising a difference between alkaline and acid fuel cells.

• Forgetting to check and use the mole ratio of electrons to product in a half equation. Also, recognising the simple connection between electrons in half equation and charge on the ion.

• Not realising that even when H2O is oxidised or reduced preferentially in electrolysis of a solution, the ions present will still move to one or other of the electrodes.

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