Home, Chemistry, Physics

ISSR CLASSES
Checkpoint Science
iGCSE Chemistry
iGCSE Physics
iGCSE Coordinated Science
A-Level Chemistry

PRACTICALS
Practicals Home
Practicals A-Z
Study Plan for Practical Work

Teacher and technician notes (2.8)

Measuring the time it takes for a small strip of magnesium to dissolve in dilute hydrochloric acid gives some useful results to illustrate the effects of temperature and concentration on rate of reaction. It gives an excellent introduction to errors when students consider the consistency of their results. Discussions regarding the effects of stirring and temperature change caused by the exothermic reaction reveal a number of weaknesses in students' understanding. There are some lovely dynamics in this experiment with gas bubbles adhering to the surface of the metal and the metal being lifted to the surface and the sides of the beaker. All of these lead to significant variations in the time for each "identical" run without some intervention from stirring. It really can get students thinking much more deeply about the practical they are carrying out.

It will also get the students thinking how to dilute the stock solution of acid to get the various concentrations they want. If this is not possible in the timescale you have, you may want to make up a number of different concentrations of acid for them.

Although it is not required at this stage, the accuracy of results can be checked by plotting an activation energy graph of ln (rate) against the reciprocal of absolute temperature for the temperatures data. We should get a straight line with negative gradient. Similarly, as the reaction is second order, a plot of rate against the square of the concentration should give a straight line with positive gradient for the concentration data.

There is no reason why the students cannot do the catalyst demonstration themselves, but timing might make this difficult.

Materials (Mg & acid):

Students' worksheet

Materials (Catalyst demo. ):

Risk assessment