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
The partial oxidation of propan- 1- ol
- Sulphuric acid, potassium dichromate and a few anti bumping granules. Shake the contents of the flask until the solution is complete.
- Add propan-1-ol in drops from a dropping pipette, shaking the flask so as to mix the contents.
- Assemble distillation apparatus
- Gently heat
Distillation potential errors
- Bulb of the thermometer should be in the T junction connecting to the condenser to measure the correct boiling point
- Water goes in from the bottom of the condenser to go against gravity
Why does Water goes in from the bottom of the condenser?
This allows more efficient cooling and prevents the backflow of water
Reflux: full oxidation of primary alcohols
- Measure water into a boiling tube. Add sodium dichromate, shake and set aside to dissolve.
Put propan-1-ol into a round bottomed flask and add water and anti bumping granules. Put condenser on the flask for reflux. - Add concentrated sulphuric acid down the condenser in drops from a dropping pipette.
- Add a drop at a time so that the mixture continues to boil without external heating.
- When all the sodium dichromate solution has been added, use a low bunsen burner flame to keep the mixture boiling for 10 minutes, not allowing any vapour to escape.
- Distill
Why should you never seal the end of the condenser
The build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up
Anti-bumping granules are added to the flask in both distillation and reflux to
prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles
A data book value for the H–H bond enthalpy is 436 kJ mol−1
Suggest one reason why this value is different from your answer to part (a).
Mean bond enthalpies are not the same as the actual bond enthalpies in CO2
(and / or methanol and / or water)
Using a volumetric pipette is more accurate than a measuring cylinder because
it has a smaller uncertainty
Titrate solution A with solution B
A- conical flask
B- burette
Phenolphthalein
If acid is added from the burette the colour change would be pink (alkali) to colourless (acid): end point pink colour just disappears
Use with titrations using strong ALKALIS
Methyl Orange
Methyl orange is a suitable indicator for neutralisation
reactions where strong acids are used.
It is red in acid and yellow in alkali. It is orange at the end
point
% uncertainty or error
uncertainty / measurement made on apparatus x 100
% uncertainty < % difference
discrepancy in the result due to other errors
% uncertainty > % difference
no discrepancy and all errors in the results can be explained by the sensitivity of the equipment
