practical Flashcards
Why is volume of water varied? (all)
To keep total volume constant so that any change in concentration is proportional to the change in volume of the reagent.
Why does the reaction mixture contain both ethanoic acid and sodium ethanoate?
Both ethanoic acid and sodium ethanoate form an acidic buffer, ensuring that [H+] remains relatively constant throughout the reaction
Graphically analyse the results (for Kinetics)
Plot the graph with recorded data and then sketch graphs for zero, first and second order. Compare and deduce.
Formula to deduce volume of reagent (of known concentration) if there is a need to change the concentration.
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Steps to change the concentration (assuming volume is 50cm^3 and below)
c1v1 = c2v2
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Use a 50.00cm^3 burette to measure accurately (volume) of (concentration) (reagent) into a 100cm^3 volumetric flask.
Top up to the mark with distilled water
Insert the stopper and shake it well to form a homogeneous solution of (new concentration)
What reagent is used to slow down the iodine clock reaction and prevent the immediate colouration of the solution
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How does it work
Sodium thiosulfate (or any group I thiosulfate compound)
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The thiosulfate ion reduces the Iodine produced from the other reaction back into iodide before the iodine forms a complex with the starch molecules to form the characteristic blue-black colour.
General Procedure for Gravimetric Experiments (weigh mass)
- Weigh a clean and dry boiling tube/crucible.
- Add a given amount of solid into the tube/crucible and reweigh.
- Heat the mixture strongly to decompose the solid. (Heat the whole length of the tube to prevent condensation and cracking of the tube: only for boiling tube)
- Cool the tube before re-weighing to find the mass of the tube and residue
- Repeat the heating, cooling and reweighing processes until constant mass is achieved where the consecutive mass difference is less than 0.050g
How to increase reliability of results for energetics experiments (styrofoam cup)
Heat loss to surroundings:
Can be compensated by providing lagging on the body of the calorimeter /use a lid to cover the calorimeter
Inaccuracy in temperature measurement due to the use of a 1’C thermometer:
Use a thermometer with 0.2’C graduation that measures 0.1’C to improve the accuracy
Major source of error when determining enthalpy change for reaction between magnesium ribbon and HCl
There was a loss of heat to the surrounding and hence temperature rise was lower than actual.
The Magnesium ribbon is coated with a layer of oxide due to oxidation in air, hence less magnesium was used resulting in a lower temperature rise.
Comment on the consistency of the titre values. (state the difference in titre values for both readings and point out criterion of consistent results)
As the titre values 1 & 2 (are the same)/(have a difference of ___), and hence within/outside of the 0.10cm^3 range. As such, the readings are consistent/inconsistent.
Usually, you are talking about your own readings, so definitely go for the consistent one:
As the titre values 1 & 2 are the same/ have a difference of 0.05cm^3, and hence within the 0.10cm^3 range, the readings are consistent.
Test for Chlorine gas
pungent yellow gas evolved turns moist blue litmus paper red
Test for Hydrogen gas
odourless and colourless gas evolved extinguishes lighted splint with a ‘pop’ sound
Test for Ammonia gas
pungent gas evolved turns moist red litmus paper blue
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probably out of syllabus
dip a glass rod in concentrated HCl, when the glass rod is brought close to the mouth of the test tube, the gas evolved forms dense white fumes around the glass rod
Test for Carbonate anion
Use dropper to suck out the gas evolved (CO2) and bubble into limewater. White ppt is formed.
