RP2- Calorimetry Flashcards
Give 2 sources of error in this practical?
- heat/energy lost to surroundings
- incomplete reaction/combustion (for flame calorimetry)
- approximation of specific heat capacities
How could you improve accuracy of calorimetry?
use a bomb calorimeter
How do you measure temperature changes during calorimetry?
- using a thermometer
- taking readings at regular intervals at eye level
Why do you put the cup in a beaker?
insulation and support
What should you do before mixing reactants together?
take initial temperatures (at regular intervals)
Why do you stir the mixture?
ensures solution is at the same temperature
Why might the correct temperatures be difficult to obtain for a slow reaction?
cooling occurs at the same time as the reaction
How do we counteract any cooling that has happened simultaneously?
- regular readings
- extrapolate graph back to point when reactants mixed
What needs to be measured in an investigation measuring enthalpies of combustion?
- mass of spirit burner before and after
- temperature change of water
- volume of water
What does q stand for in q = mcΔT? Give units
energy (Joules, J)
What is the value of c for water?
4.18 J/g/K
What does m mean in q = mcΔT?
mass of water in g
Explain how to calculate enthalpy change from an energy change and moles reacted
ΔH = q / moles reacted
Describe the steps you would take to determine an accurate minimum temperature to calculate the enthalpy of solution of an ionic solid that is not influenced by heat from the surroundings
- measure the temperature of water every 30 seconds for 4 minutes
- measure the temperature of the solution every 30 seconds for 10 minutes after the addition of solid
- plot this data on a graph
- extrapolate the temperature change back to the start of the reaction and the average start temperature
- find the change in temperature
Why is measuring accurate temperature change difficult?
- as soon as a reaction starts the temperature of the surroundings starts to have an effect almost immediately
- this can be heat lost to the surroundings or heat absorbed from the surroundings
How could you change a method to reduce percentage uncertainty in temperature change?
- increase magnitude of temperature change
- by increasing concentration/amount of solid/alkali/acid
Describe the method for a calorimetry practical
- measure 25cm3 water with measuring cylinder/volumetric pipette
- put in to insulated container/polystyrene cup
- add (mass of) solid
- weigh by difference
Describe the measurements in a calorimetry practical
- record initial temperature at least twice
- record T at regular timed intervals (every 30 seconds) for 10 minutes
- plot a graph of T vs Time
Describe how you would use results in a calorimetry practical
- draw line of best fit
- extrapolate line to point when solid added
- T final - T initial = change in Temp
- q = mcΔT
- ΔH = q / moles reacted
How can heat loss be minimised in calorimetry?
use polystyrene cup.insulated beaker
Why might calorimetry be less accurate when hydrating a salt?
no way to stop some of the salt dissolving/the salt is soluble
Why might the enthalpy change of neutralisation be different from what was obtained in an experiment?
- weak acid/not fully dissociated
- more energy needed to complete dissociation
State why the heat change calculated from the bomb calorimeter experiment is not an
enthalpy change
not under constant pressure
What are disadvantages of using a glass beaker and tripod and gauze in this experiment?
- Glass is a poorer conductor than copper
- Tripod and gauze would reduce heat transfer (from combustion)
