Theory behind this practical:

We know that reactions are either endothermic or exothermic. We also know that they are not equal in how much heat energy they give out or absorb. Here, we will not only find out if the reaction is exothermic or endothermic, we will take measurements so that we can quote the energy change per gram or calcium chloride or, for higher tier, the energy change per mole.

It is also very important to use this experiment as an opportunity to compare results with the rest of the class and the official figures. You then need to look at how we could improve the result – equipment, procedure, repeats & mean, remove anomalies…

Method:

  • Into a polystyrene cup, add 50cm3 of tap water
  • For improved stability, stand the cup inside a glass beaker
  • Place a thermometer into the water and record the initial temperature
  • Carefully measure out 5g of calcium chloride (replace the lid)
  • Add the calcium chloride to the water and place plastic lid on and put the thermometer through the hole into the solution
  • Record the highest/lowest temperature achieved – final temperature.
  • Calculate the temperature change = initial temperaturefinal temperature

Once completed, your teacher may ask you to repeat the same experiment to assess accuracy or to repeat with a different mass of calcium chloride to see if you get the same energy grange per gram (or mole), which you should.

Energy change = mass of water (50) x temperature change x 4.2 (SHC of water)

To find the energy change per gram, divide the above answer by 5 (as 5g added).

Technician notes:

  • Small pots of calcium chloride with screw on lids
  • Spatulas
  • Balances
  • Weighing boats
  • Polystyrene cups
  • Glass beakers into which the cups can stand
  • Plastic lids with centre holes
  • short thermometers (best stability)

Safety:

Usual lab rules must be adhered to:

  • Lose hair tied up
  • Goggles on throughout the lesson (Calcium chloride – dangerous in eyes)
  • Bags away and stools tucked under
  • Notify the teacher immediately if any spills or breakages occur

For further/more detailed information, check all reagents and practices at CLEAPSS.