RP2 - measurement of an enthalpy change Flashcards
(11 cards)
what’s the calorimetric method
- wash the equipment with the solutions to be used
- dry cup after washing
- put polystyrene cup in a 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 into solution
- measure initial temp of solution . Do this every min for 2-3 min
- at minute 3 transfer second reagent to cup . If solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance
- stir mixture ( ensures that all of the solution is same temp )
- record temp every minute after addition for several mins
what are some errors in the calorimetric method
- energy transfer from surroundings
- approximation in specific heat capacity of solution . Method assumes all solutions have the heat capacity of water
- neglecting the specific heat capacity of the calorimeter - we ignore any energy absorbed by apparatus
- reaction or dissolving may be incomplete or slow
- density of solution is taken to be the same as water
what is the flame calorimeter used for
measuring enthalpy of combustion
- the fuel is burnt and the flame is used to heat up water in a metal cup
Need to measure :
- mass of spirit burner before and after
- temperature change of water
- volume of water in cup
what are some errors in using flame calorimetry
- energy losses from calorimeter
- incomplete combustion of fuel
- incomplete transfer of energy
- evaporation of fuel after weighing
- heat capacity of calorimeter not included
- measurements not carried out under standard conditions as H2O is gas , not liquid in this experiment
EQ : explain why it’s important that the reaction mixture is stirred before recording each temperature (1)
Ensures that all of the solution is the same temperature
why should u not use a plastic sirring rod when mixing
It’s a poor conductor of heat and it may melt
Why should you not use a glass beaker and what should you use instead
Glass is a poor insulator leading to significant heat loss
use a polystyrene cup instead
EQ : Outline a simple practical experiment that the engineer could use to, including a he measurements to be taken in order to determine the enthalpy of combustion for one of the unknown alcohols (5)
- Weigh the spirit burner ( alcohol ) before and after combustion
- water in a calorimeter / beaker
- burn the alcohol to heat the water
- measure temperature rise in water
EQ : A student determines the enthalpy change for reaction between calcium carbonate and hcl
Method :
- measure out aq hcl using a measuring cylinder and pour acid into a glass beaker
- weigh out solid on a watch glass and tip solid into acid
- stir mixture with thermometer
- record maximum temp reached
Explain how the experimental method and use of apparatus can be improved to provide more accurate data . Describe how this data from improved method can be used to determine an accurate value for temp change (6)
- use a burette / pipette instead of a measuring cylinder
- Use a polystyrene cup instead of a beaker
- reweigh watch glass after adding the solid
- use powdered solid
- Measure and record the initial temp of solution for a few mins before addition
- measure and record temp after addition at regular intervals for 8+ mins
- plot a graph of temp against time
- extrapolate to point of addition
- determine change in temp at point of addition
EQ : what are two disadvantages of using a glass beaker on a tripod and gauze (2)
- glass is a poor conductor of heat
- tripod and gauze would reduce heat transfer
EQ : suggest two reasons why the value of enthalpy of combustion from this experiment is less exothermic than a data book value (2)
- heat lost to surroundings
- incomplete combustion
