Calorimetry can be a confusing topic. Avoid common errors by following these essential tips:
- Always label the units of E (kJ or J) above the E. This is the most common source of error in calorimetry calculations. Try this quick way to remember the required units of E: If there’s ΔH in the equation, the units are kJ; otherwise, the units are J.
- In E=mcΔT, all the variables refer to the mass of water being heated. A common error among students is to use the mass of limiting reactant instead of the mass of water. Generally, m in this equation is 100 g or a similar round number.
- Never convert ΔT to kelvin. Temperature changes are the same in kelvin and celcius… never add 273 when finding ΔT.
- No calibration step? Use m×c instead. Because E=mcΔT and E=CfΔT, it therefore follows that Cf ≡ m×c. For example, if we’re heating a 100.0 g of water without a Cf, we should use Cf = 100×4.18 = 418 J K-1 instead.
- In ΔH = E/n, n denotes the number of moles of limiting reactant. Never add up the number of moles of reactants: use the number of moles of limiting reagent only.
- Calculate twice. Students most often make mistakes when converting hours or days into seconds. Many answers are therefore wrong by a factor of 60. Do your calculations twice: once while doing the question and again when you check over your answers at the end of the SAC or examination.
- Know a ballpark figure. Neutralisation and solubility reactions tend to have 2-digit ΔH values; combustion reactions tend to have a 3-digit ΔH and explosive reactions tend to have a 4-digit ΔH. If you get a 5-digit ΔH value, you’ve probably forgotten to convert your answer into kilojoules!
- Remember the ‘+’ or ‘-‘ sign! The calculator doesn’t know whether the answer should be positive or negative. Think about it yourself instead: endothermic reactions need a ‘+’ sign and exothermic need a ‘-‘ sign. VCAA awards a whole mark for getting the ‘+’ or ‘-‘ sign correct! It’s possibly the easiest mark in the whole paper.