Prac 2 Flashcards
Required Practical 2: Measuring an Enthalpy Change
Put the polystyrene cup with a thermometer and lid in a beaker for insulation and support. Add 25 cm³ of deionised water.
Stirrer: Weigh the sample bottle containing the required mass of solid on a 2 d.p. balance.
Measure the initial temperature of the solution every minute for four minutes.
On the fifth minute, transfer the contents of the weighing bottle to the polystyrene cup and stir for the remainder of the fifth minute.
Continue to record the temperature of the solution every minute until you reach 15 minutes.
Reweigh the sample bottle and record the difference in mass.
Draw a graph, extrapolate, and do your calculations.
Why Polystyrene cup, lid, and insulation?
All prevent heat loss which would affect the results.
Why Draw a graph and extrapolate your line of best fit?
You deliberately miss recording the temperature on the fifth minute as you are busy stirring the mixture. You
extrapolate your LOBF back to the fifth minute to estimate the temperature change at this point.
Why Reweigh sample bottle and record the difference in mass?
Some solid could remain in the weighing bottle. Reweighing ensures you only include solid transferred to the cup in your calculations.
Calculations?
moles = mass / Mr to calculate the moles of solid added to the water. Q = MCΔT to calculate energy (J) (/1000 to convert to kJ) finally kJ mol⁻¹
Why would the energy change not be as expected?
Incomplete combustion
Evaporation of fuel
Experiment not completed under standard conditions
Tripod and gauze/ glass used reducing heat transfer
