Energy changes
Read our general notes on Risk Assessment
I tried to plan and design an experiment to find the molar enthalpy change for the reaction between magnesium powder and aqueous copper(II)
sulphate. I decided to measure the sulphate into a polystyrene cup and add an excess of magnesium powder, then record the temperature. What should I do next?
Corrie writes ....
From your temperature readings before and after mixing you can work out the temperature rise of the contents of the cup. Use this to work out the amount of heat released by the reaction, using the well-known formula:
Heat energy = mass (g) x specific heat (J g-1 deg-1) x temperature (deg) change. (Note: no + or - sign at this stage - just an amount of heat in J)
In this type of experiment we take the mass being heated/cooled as approximately equal to the mass of water in the cup (knowing the volume of solution used, and that 1 cm3 water has a mass of 1 g). Similarly we assume the specific heat of the mixture is the same as that of water, viz. 4.2 J g-1 deg-1.
Convert the amount of energy you've calculated into kJ.
Work out the amount (moles) of copper sulphate you used. This is the amount that gave the amount of energy above. Use it to work out how much energy would have been involved if you had used a mole of copper sulphate.
Give this a energy a sign, based on whether the reaction was exo- or endothermic. This is the enthalpy change for the reaction, in kJ mol-1.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 2 ratings so far
, rated at 
updated: 18 December 2007
