Practical Skills Flashcards
What is a standard solution?
A solution of known concentration
Why are acid-base indicators used?
To detect when a reaction reaches completion/ becomes neutral, usually by the presence of a color change
What are concordant results?
Titres that are within 0.1 cm3 of each other
risk in glassware hazards
may break and cut you
risk in chemical hazards
may be flammable,
may be toxic,
and may be an irritant or corrosive, causing irritation to skin, eyes, lungs.
For a Hess’s cycle the sum of the clockwise enthalpy changes … the sum of the anticlockwise enthalpy changes
equals
how can you reduce the uncertainty in the mass measurement?
use a balance with a greater resolution,
use a larger mass
percentage uncertainty
100 x absolute uncertainty/ calculated value
why may an experimental value for enthalpy change be different from the theoretical value?
heat loss to surroundings,
incomplete combustion,
non-standard conditions,
evaporation of alcohol/ water.
how do you prevent heat loss to surroundings/ apparatus?
insulate the beaker by placing it in a polystyrene cup with a lid,
avoid large temperature differences between surroundings and calorimeter
use a bomb calorimeter
other than preventing heat loss, how can the accuracy of this experiment be improved?
read the thermometer at eye level to avoid parallax errors,
stir the solution so the temperature is evenly distributed,
use a digital thermometer for more accurate and faster readings,
use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty.
what is accuracy?
the more accurate the data, the closer it is to the actual value.
how can the rate of reaction be measured?
initial rates method – i.e. the iodine clock reaction,
a continuous monitoring method – i.e. measuring the volume of gas released in a reaction over time
BaCl2
initial
after 10 drops of NaOH
excess NaOH
colorless solution
colorless solution
colorless solution
CaBr2
initial
after 10 drops of NaOH
excess NaOH
colorless solution
slight white ppt
slight white ppt
MgCl2
initial
after 10 drops of NaOH
excess NaOH
colorless solution
slight white ppt
white ppt
SrCl2
initial
after 10 drops of NaOH
excess NaOH
colorless solution
slight white ppt
slight white ppt
BaCl2
initial
after 10 drops of H2SO4
excess H2SO4
colorless solution
white ppt
white ppt
CaBr2
initial
after 10 drops of H2SO4
excess H2SO4
colorless solution
slight white ppt
slight white ppt
MgCl2
initial
after 10 drops of H2SO4
excess H2SO4
colorless solution
slight white ppt
colorless solution
SrCl2
initial
after 10 drops of H2SO4
excess H2SO4
colorless solution
white ppt
white ppt
carbonate ions + HCl, in delivery tube with open end, added into Ca(OH)2. If Ca(OH)2 goes cloudy, meaning carbonate ions were present in the other test tube.
bubububutterfly
Add HCl and BaCl2 to the suspended sulfate solution. If sulfate ions are present, a white ppt of BaSO4 will form.
flowers and grass and berries and nectar
what is the order of testing ions? (carbonate, halide and sulfate). explain.
carbonate -> sulfate -> halide
this prevents false positive results occurring i.e. unexpected insoluble precipitates such as Ag2SO4, Ag2CO3 and BaCO3 could form.
methyl orange
red=acid
yellow= base
phenolphthalein
colorless=acid
pink=base
why are anti-bumping granules used in distillation?
to allow smooth boiling – preventing bubbles caused by vapor to cause upwards splashing in the flask
testing for an alcohol
add (1cm3?) of sample to a test tube, and add a small piece of Na. presence of alcohol is indicated by effervescence and a white crystalline solid is left. if a phenolphthalein indicator was added, in a positive result the colorless solution will turn pink, due to basic salt formed.
testing for carboxylic acid
add na2co3 to the sample. if a carboxylic acid is present, co2 will be given off.
https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/practical-skills/organic-tests-flashcards/ slide 13, mls
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rate of reaction
the change in concentration of reactants or products over time
what is an initial rates method
if want2 ngertiin example: https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/practical-skills/measuring-rate-of-reaction-flashcards/ slide 9
the method involves measuring the initial rate of reaction for multiple different concentrations to observe how rate of reaction varies.
https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/practical-skills/measuring-rate-of-reaction-flashcards/ initial rates method example
what is the issue with this experiment?
Some low I- concentrations may take too long to react,
delayed stopwatch reactions,
concentrations may not be exact due to measuring apparatus
what is a continuous monitoring method
https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/practical-skills/measuring-rate-of-reaction-flashcards/ slide 15
this involves measuring the change in concentration of a reactant or product over time (or measuring the volume of gas released) as reaction progresses
https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/practical-skills/measuring-rate-of-reaction-flashcards/ slide 15
issues?
some gas may escape before the bung is added,
the Mg strips may be of different mass, surface area, etc. which will affect the rate of reaction.
https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/practical-skills/measuring-rate-of-reaction-flashcards/ slide 15
how do u prevent gas escaping?
place the solid reactant upright inside a sample tube in the conical flask, tipping the tube over by moving the conical flask around to start the reaction
why does a pH probe need to be calibrated
so that the pH values for each pH reading are accurate
what are the main steps in producing a pure organic solid?
- synthesis of the compound (usually using reflux, distillation, etc.)
- filtration (usually vacuum filtration)
- purification (recrystallization)
why is heating under reflux used?
allows heating for a long period of time,
prevents the flask from boiling dry,
prevents volatile reactants/ products escaping,
ensures even heating.
how do you use laboratory equipment to filter under reduced pressure?
using a buchner funnel and a buchner flask, connected by rubber tubing to the vacuum source.
the funnel contains a layer of filter paper,
pour the substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask,
the solid will remain on the paper
how do you purify a solid product?
by recrystallisation.
- add minimum amount of warm solvent to the impure sample until it has dissolved
- allow to cool, crystals should form
- when no more form you can filter under reduced pressure to obtain a dry crystalline solid.
organic layer
liquids less dense than water
how do you purify a liquid organic product?
- you can use a separating funnel to isolate the organic layer from the aqueous layer
- add anhydrous MgSO4 to the organic product to get rid of all water
- you can then purify the liquid by distillation, which separates the substances by boiling point
what can be reacted with transition metals for a precipitation reaction to occur?
aqueous NaOH,
NH3.
what are the uses of thin layer or paper chromatography?
to separate a liquids mixture into its constituent components for analysis. This allows identification by the calculation and comparison of Rf values.
Rf value
distance travelled by component: distance travelled by solvent
why must any lines drawn on the tlc plate be in pencil?
because if drawn in ink, this will dissolve in the solvent and run up the plate with the other components, contaminating the plate.
why should the solvent be below the pencil line?
if above the pencil line, the solvent will dissolve all samples and they will not run up the plate
in tlc, paper chromatography, what happens if your sample is too concentrated?
the spots overlap
in tlc, paper chromatography, what happens if you use less solvent and have a high baseline?
you will get large spots
how do you view the spots on a tlc plate?
some spots may be visible, others may only be visible under UV light.
sometimes the plate is sprayed with a chemical/ locating agent to show invisible spots.
universal indicator
shades of yellow orange red = acid
shades of blue green = base
percentage error
margin of error/ actual or mean measurement x 100%
