RP2 - Measurement of an Enthalpy Change Flashcards
Energy change (J) =
mass of solution (g) x heat capacity x temperature change (K)
Where mass of solution is either mass of water or mass of acid + base
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
Energy loss to surroundings
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
How do you find delta H after finding out Q
Enthalpy change = Q / moles of reactant NOT in excess only
Place a minus sign if temperature increases
Describe the general method for calorimetry for a solid dissolving/ reacting in a solution/ two solutions reacting together
washes the equipment (cup and pipettes etc) with the solutions to be used
dry the cup after washing
put polystyrene cup in a glass beaker for insulation and support
Measure out desired volumes of solutions with volumetric pipettes and transfer to
insulated cup
clamp thermometer into place making sure the thermometer bulb is immersed in
solution
measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
At minute 3 transfer second reagent to cup.
If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.
If using a solid reagent then use ‘before and after’ weighing method
stirs mixture (ensures that all of the solution is at the same temperature)
Record temperature every minute after addition for several minutes
Plot a graph of temperature against time
Extrapolate to the point of addition
Determine ΔT at the point of addition
Errors in calorimetry
Energy loss to surroundings
Energy absorbed by apparatus
Reaction may be incomplete/slow
Density of solution assumed to be same as water
Why do we take readings at regular time intervals and extrapolate the temperature line back to the time of mixing
If reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction
