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.
Consider getting a home tutor who can answer your questions and explain difficult concepts to you. Students learn much faster with a tutor than on their own.
Pleasantly optimistic but overly simplistic.
384 pages, ★★★
If only all the world’s energy needs and its climate worries could be solved by one author with a laptop. That’s what this book attempts to do. Even an optimist would struggle to believe that. It takes the collective action of many people’s mindsets and lifestyles, along with concerted action by business and governments to manage climatic change and build a reliable supply of renewable energy. Sustainable Energy Without the Hot Air simply aims too high.
This book analyses, then over-analyses and extrapolates wildly. (I forgive the author, as it is very difficult to do anything in the field of climate policy without sounding alarmist—although Bjørn Lomborg was one of few authors to do that successfully.) The fundamental data is abundant and extremely useful for policy-makers. The analysis is a little simplistic, and the numerical extrapolations are not to be believed at all. Of course it would be marvellous for the UK to become energy-independent on solar, wind and biomass by 2050, but investors won’t be encouraged by these hasty, lofty calculations. Gut instinct is enough.
Apart from the opportunities posed by building a giant dam across the Severn Estuary (a project probably beyond Britain’s capability right now), the reality is that Britain isn’t particularly well-suited to energy production. The UK has no more coal, no more gas, and no extreme weather that would make solar power or wind turbines highly profitable (try the Sahara or northern Europe for that). Perhaps Britain should focus on other industries (like a high-voltage, intercontinental direct current electric grid) and import energy instead?
The dodgy references worry me. They’re all internet-based and are written bewilderingly as a series of short URLs (like bit.ly/4dgf82). I know from experience that there’s enough material on Google to support basically any thesis—or even a pair of contradicting theses.
Sustainable Energy Without the Hot Air is a collection of good ideas—but take the quantified results with a pinch of salt. Three stars for optimism and for effort. This book isn’t the magic bullet it appears to be. ★★★