Required Practicals Flashcards
Making a titration solution
Step 1 making the solution
- Add required mass of solid to beaker
- Add distilled water and stir using a glass rod
- Pour solution into a graduated volumetric flask via a funnel
- Rinse the beaker and funnel and add washings from these to the volumetric flask
- Make up to the mark with distilled water using a dropping pipette or the last few drops
- Invert flask several times to ensure uniform solution
In the preparation of a titration solution why must the flask not be heated
the heat would cause the flask to expand and the volume would then be incorrect
Making a titration solution
Step 2
General Method
- Rinse equipment
- Pipette known volume of alkali into conical flask
- Make sure the jet space is filled with acid
- Add a few drops of indicator
Use white tile underneath the flask to help observe colour change - Add acid to alkali whilst swirling the mixture and add acid dropwise at end point
- Note burette reading before and after the addition of the acid
- Repeat titration until at least two concordant readings are obtained
Rinse a burette with
acid
Rinse a pipette with
alkali
Rinse the conical flask with
distilled water
Why does the equipment need to be rinsed in a titration experiment
If not rinsed the alkali or acid may be diluted by residual water or react with leftover substances. Result in a lower or larger titre reading.
Why must the jet space be filled with acid
Not filled properly before the titration (bubbles present) and fills during then the titre reading will be larger than expected
Why can too much indicator affect results
because they are weak acids
Concordant readings
two readings which are within 0.1cm3 of each other
Method to measure enthalpy change
Calorimetric method
- 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 in solution
- Measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes
- Temperature of the reactants a few minutes before they are added gets a better average temperature.
- 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
- Stir mixture (ensures that all of the solution is at the same temperature)
- Record temperature every minute after addition for several minutes
Errors in calorimetry experiment
- Density of solution is taken to be the same as water.
- energy transfer from surroundings (usually loss)
- approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water.
- neglecting the specific heat capacity of the calorimeter- we ignore any energy absorbed by the apparatus.
- reaction or dissolving may be incomplete or slow.
This Hess’s law is used to work out the enthalpy change to form a hydrated salt from an anhydrous salt. This cannot be done experimentally because:
- it is impossible to add the exact amount of water without the solid dissolving
- it is not easy to measure the temperature change of a solid.
Measuring Enthalpies of Combustion using Flame Calorimetry
Generally 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
Errors of Measuring Enthalpies of Combustion 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
