RP2 - Measurement of an Enthalpy Change Flashcards
Energy change (J) =
mass of solution (g) x heat capacity x temperature change (K)
How is temperature change determined in calorimetry
Plot a graph of temperature against time
Extrapolate to the point of addition
Determine ΔT at the point of addition
Why is a polystyrene cup used instead of glass beaker
Polystyrene is a better insulator than glass so less heat loss
Why is copper used instead of glass
Copper is a better conductor
How to decrease uncertainty in calorimetry
Increase concentrations of solutions - MAIN ONE
Increase temperature change
Errors in flame calorimetry
Incomplete combustion of fuel
Evaporation of fuel after weighing
Measurements not carried out under standard conditions
Describe how you would carry out an experiment to determine the enthalpy of solution of anhydrous magnesium chloride. You should use about 0.8 g of anhydrous magnesium chloride. Explain how your results could be used to calculate the enthalpy of solution.
Measures water with named appropriate apparatus
Suitable volume/mass / volume/mass in range 10 – 200 cm3/g
Into insulated container / polystyrene cup (NOT just ‘lid’)
Add known mass of MgCl2(s)
Use of ‘before and after’ weighing method. NOT ‘added with washings’
Record initial temperature (min 2 measurements)
Record T at regular timed intervals for 5+ mins / until trend seen
Plot T vs time
Extrapolate lines to when solid added (to find initial and final T)
Tfinal – Tinitial = ΔT / idea of finding ΔT from graph at point of addition
q = mcΔT
amount = mass/Mr (0.80/95.3 = 8.39 × 10−3 mol)
ΔHsoln = –q/8.39 × 10−3 or in words
Explain how you can measure out 50g of water without using a balance
Measure 50cm^3 of water out using a measuring cylinder as water has a know density of 1g/cm^3
