On old A Level and IB exams there were commonly questions that straight-up asked for definitions. Literally, “Define … X”. Common examples of X were things such as “Enthalpy of Formation”, or “First Ionization Energy”. As far as I can tell, even long before the modern, relatively edubabble-filled AP exam, these types of questions were never popular (if they came up at ALL) on the American version of the high school gold standard for chemistry. This is a real shame because definitions are important – VERY important – since they open up understanding in so many areas.
This was brought to the fore three times in the last few days via emails to me. Each message was from a chemistry teacher asking me a chemistry question, with one of the questions being asked by two different people. The connection? Each question is solved via a knowledge of specific definitions. Here are those chemistry teacher questions.
- “Looking at heats of neutralization. As I was talking in class baking soda and vinegar came to mind. But this is not an acid base neutralization reaction? It is endothermic.”
- “You do not multiply Br2(l) and Cl2(g) by 0.5, because your answer is 218 kJ. I’m getting 101.8 kJ using my method. Could you possibly help me out?”, and same question with a slight different emphasis, “Because 14.7 is the enthalpy of formation value for BrCl, and that by definition is for one mole, and because the given atomization values for the diatomics are given in kJ/mol, we thought those values should be halved because the coefficients are halved in the BrCl formation equation.
- In a third message, the transition of Br2(l) to Br2(g)was described as “the heat of formation of gas.”
To give context to the two questions in the second bulletpoint above, they each emanate from a question on one of my AP Worksheets that reads, “The standard enthalpy of formation of BrCl(g) is 14.7 kJmol-1. The standard enthalpies for the atomization of Br2(l) and Cl2(g) are +112 and +121.0 kJ per mol of atoms respectively, calculate the average enthalpy of the Br-Cl bond in BrCl(g).”
My answers to these inquiries (below) explain that each problem is solved via a knowledge of definitions, hence definitions are important. It’s a shame that AP shuns such pure, recall knowledge in favor of edubabble ideas.
- Enthalpy of neutralization has a specific definition and is the enthalpy change when one mole of water is formed from a reaction between one more of acid (H+) and one mole of base (OH–). The reaction you are talking about, while certainly ‘neutralizing’ the acid, does not fit that definition, so does not have the same ∆H. Well, it’s because the net ionic equation is NOT, H+ + OH– → H2O.
- It’s because the enthalpy of atomization is defined in terms of forming ONE mole of Br atoms, so there is no need to divide it by two. Same for Cl. Comes down to definitions.
- There’s no such thing as, “Heat of formation of a gas”.