Over the last couple of months I have posted some ideas of lab work for you to help with the mandatory labs or mandatory skills that the IB has stipulated are carried out over the duration of the IB course.
So far we have looked at labs for deducing the empirical formula of compounds, examples of titration labs and labs to calculate the Mr of a substance using the ideal gas equation.
This months post will take up where the others have left off and look at the next skill. This deals with topic 5.1, a calorimetry experiment for an enthalpy of reaction. The guidance goes on to that that the results should also be evaluated.
As in previous posts, it goes without saying that you, the teacher are fully responsible for ensuring these labs are carries out safely and it is imperative that you carry out the appropriate risk assessments before engaging in any of the labs listed.
So what labs can you carry out that slot into this area of the syllabus. What makes enthalpy labs good ones to carry out is that the labs generate data that needs to be processed using a number of different skills.
The very nature of these labs means that temperature data is collected and if this is collected over time, the data can be graphically displayed, lines of best can be produced, extrapolated and further data can be obtained from these graphs (eg, a maximum temperature change) which can then be used to calculate a heat change.
All the labs will rely on the use of the equation:
q = m c ΔT
In my experience there are three types of labs that will give you good (and not so good results) that can also be evaluated.
1, A thermometric titration / enthalpy of neutralisation:
This lab involves using an acid and an alkali. One is put in a burette, a know quantity of the other is pipetted into a conical flask. Small portions (say, 1mL) are titrated into the conical flask (note, no indicator is used).
The conical flask is swirled and the temperature of the solution is recorded. A digital thermometer / data logger is best. This is carried out until the burette is emptied. Typically, 25mL is put in the conical flask and the burette holds 50mL.
A graph is plotted of temperature v volume and two lines of best fit are drawn, one for the increase in temperature and one the decrease. Both lines are drawn so they intersect and this means that a maximum change in temperature can be recorded.
2, An enthalpy of combustion:
This will involve using some sort of fuel (alcohol) burner to heat up a known mass of water in a calorimeter – the calorimeter is usually made of copper but I have seen versions of this lab where an aluminium Coke can is used instead. Either should obtain workable results.
Here are some images of the apparatus typically used to carry out this type of lab:
3, An enthalpy of solution:
This type of lab uses a Styrofoam or polystyrene cup to insulate the reaction and keep heat loss to a minimum. It could be a neutralisation, formation of a solution (literally, dissolving NaCl in water) or a reaction with something like Mg and HCl (although it will be hard to obtain theoretical data for the latter).
So what do you think? Have you used these labs or not? Do you have any other ideas or suggestions for good labs to carry out with regards to this section of the course?