Core Pracs Flashcards
how to calculate percentage errors?
(uncertainty/quantity measured) x 100
if 2 readings taken (e.g. measuring change)
(2xuncertainty/quantity measured)x100
Weak vs strong acids?
the extent to which they dissociate into ions in solution.
Strong acids dissociate completely, producing a high conc of H+
weak acids dissociate partially, lower H+ conc produced
what is avogadro’s law?
equal vol of gas at same temp and pressure will contain same number of molecules
6.02 x 10^23
how to carry out an experiment to measure the molar volume of gas?
- react ethanoic acid and solid CaCO3
- gas syringe to measure vol of gas released
- repeat for increasing mass of CaCO3
what are some issues with measuring volume of gas (CO2) with gas syringe?
- gas may escape before bung is added
- solid reactant may be of diff SA
- CO2 is slightly soluble in water, so exact gas produced not measured
what is a standard solution?
solution of known concentration
how to make a standard solution?
measure mass of solid with balance
transfer to volumetric flask, rinse with water
fill to line, bottom of meniscus touches line
invert to mix
how to increase accuracy of weighing solids?
mass of difference technique
how to detect when a titration reaction has reached completion?
use acid-base indicators, colour change indicates end point
what are concordant results?
titres within 0.1cm3 of each other
how to calculate conc?
mol/dm3
what reaction is hydrolysis of halogenoalkanes?
nucleophilic substitution
Ag+ (aq) + X- (aq) ->
AgX (s)
what is added as a solvent for hydrolysis of halogenoalkanes?
ethanol
so reactants dissolve and mix
what silver halide ppt forms the quickest and why?
AgI fastest, AgCl slowest
C-I is the weakest bond (large ionic radius of I)(in a halogenoalkane) so easier to react
what are the ppt of silver halides?
AgCl - white
AgBr - cream
AgI - yellow
what are the rates of primary, secondary and tertiary halogenoalkane hydrolysis?
tertiary is the fastest
primary slowest
why are water baths used for hydrolysis of halogenoalkanes?
to keep temp constant
as a control variable
how to decrease uncertainty in time taken?
use lower temp so RoR decreases, so % uncertainty will be lower
how to produce ethanal from ethanol?
distillation
how to produce ethanoic acid from ethanol?
reflux + potassium dichromate
what reactants is needed to oxidise alcohol?
K2Cr2O7
what is oxidation?
loss of electrons
oxidation num increases
what colour change in potassium dichromate to indicate aldehyde and carboxylic acid?
orange to green
why is heating under reflux used?
- allow heating for a long period of time
- prevents volatile reactants/products escaping
- ensures even heating
- increase RoR
why are anti-bumping granules added/start by heating gently before heating under reflux or distillation?
to prevent spitting
allow smooth boiling
how do you separate organic layer from aqueous layer?
transfer using separating funnel
open the tap, keep organic layer in separating funnel
how to remove unreacted HCl?
add NaHCO3, shake and open tap to release pressure
how to test for acid or base?
- indicator paper
red = acid
blue = alkali - pH probe
how to conduct flame tests?
- dip nichrome wire
- into conc HCl solution
- blue bunsen flame
how to test for carbonate ions? CO3 2- or HCO3-
Ca(OH)2
clear to cloudy from CaCO3 ppt
how to test for ammonium? NH4+
add aqueous NaOH
warm mixture
ammonia gas turns red litmus paper blue
how to test for sulfate ions? SO4^2-
Add nitric acid (to remove carbonate ions)
then add barium chloride/nitrate
BaSO4 white ppt
how to test for bromides?
add chlorine water
turns orange if present
Or
Add nitric acid and silver nitrate
Cream ppt if positive
how to test for -OH groups?
PCl5
misty white fumes
how to test for alkenes?
bromine water
orange to colourless
how to test for aldehydes and ketones?
Fehling’s solution
aldehyde: blue to red
ketone: stays blue
how to test for alcohol (primary, secondary) and aldehydes?
H2SO4 + K2Cr2O7
orange to green
in warm water bath
how to calculate enthalpy change of a reaction?
Q=mcΔT
%1000 into kJ
% moles of limiting reactant
+/- xx kJ
what do the signs mean?
+ is endothermic
- is exothermic
how to prevent heat loss to surroundings in a reaction?
- insulate using polystyrene cup with lid
- avoid large temp differences between surrounding and equipment (calorimeter)
- use a bomb calorimeter
how to increase accuracy of calorimetry experiment?
- stir solution so heat is evenly distributed
- use digital thermometer
- use greater concs and masses, greater temp change reduces % errors
what is accuracy?
closeness to actual value
Suggest a method to separate pure ethanoic acid, boiling temperature 118C, from the water.
Fractional distillation
Remove water from ethanoic acid at 98-102C
How to make sure all water of crystallisation has been removed?
Heat to constant mass
Keep heating and weighing
Why is NaHCO3 added to organic layer?
To neutralise acid
Remove unreacted acid like HCl
What are 2 drying agents?
(anhydrous)
Calcium chloride or sodium sulfate
Becomes solid and filter off
How can you tell if a reaction is completed? Give 2 examples (2 marks)
- effervescence stops
- solid disappears
How can you tell if a reaction is completed? Give 2 examples (2 marks)
- effervescence stops
- solid disappears
What is the role of sodium thiosulfate in iodine clock reaction?
Extending time taken for reaction to take place to obtain more accurate results
It reacts with one of the products immediately, colour change to blue-black when used up
CP13a
By reducing iodine back to iodide before the iodine can complex with the starch to form the characteristic blue-black color
Why is a sodium hydroxide solution not the best candidate for making a standard solution?
Hydroxide absorbs moisture and gases from the atmosphere, meaning that the initial known concentration would no longer be accurate once the solution has been left for some time.
Why are anti-bumping granules used?
prevent boiling too vigorously and splashing into condenser
Explain why crude oil can be separated with fractional distillation. (2 marks)
- they have different boiling points
- since they have different chain lengths and IMF
Compare and contrast the use of a 250cm3 measuring cylinder to measure out the 150cm3 distilled water than using 25cm3 measuring cylinder 6 times. (3 marks)
- greater transfer loss from more repeats
- takes more time
- higher % error (calculate with given numbers)
How to lower percentage errors?
- use bigger mass of solids / sample size
- use balance with more decimal places
Identify a safety issue associated with this producing hydrogen iodide.
It is corrosive
+ - which one is endothermic and ex other is
+ endo
- exo
Devise a procedure to obtain a pure dry sample of cyclohexene from the distillate + include reason.
Cyclohexene (BP 84) is prepared by reaction cyclohexanol + H3PO4.
- separate w separating funnel
- remove lower layer as it has higher density
- drying agent CaCl2
- filter out using filter paper
- collect residue by Bunsen burning
What apparatus is used for distillation?
Quickfit apparatus
- round bottomed flask or pear
- connected w T head
- liebeg condenser
- collecting vessel
How to remove any unreacted water from organic liquid?
Add anhydrous Na2SO4
Why are acid-base indicators used?
To detect when a reaction has reached its equivalence point
How do you calibrate a pH probe?
Submerge in buffer solutions
What are 2 methods to measure rate of reaction?
- initial rate reaction (like iodine clock)
- continuous method (measuring change in vol over a period of time)
What are 2 methods to measure rate of reaction?
- initial rate reaction (like iodine clock)
- continuous method (measuring change in vol over a period of time)
How to test for ammonia and how to know all the ammonia has been evolved? (2 marks)
- damp red litmus paper turns blue
- repeat with fresh damp red litmus paper until no change to blue
State 2 hazards associated with ammonia. (1 mark)
toxic and corrosive
What precaution must be used to work with ammonia gas?
Fume cupboard (and wear gloves)
what are halides?
chlorRIDE
broMIDE
iodIDE
What is the advantage of diluting a sample in titration?
can repeat titrations and find more valid results
with concordant titres
What is the disadvantage of diluting a sample in titration?
High % uncertainty
Give one ad and disad for applying ammonia directly into the soil as a fertiliser. (2 marks)
ad - high % of N
disad - corrosive / toxic
What is the type of reaction when
NH3 + HNO3 -> NH4NO3
neutralisation
/ acid base
as salt is made
Given ammonium nitrate is soluble in water.
How will ammonium nitrate interact with water molecules?
- form bonds of hydration
- NH4+ ions will form bonds of hydration with lone pairs on O of water
- NO3- will form bonds of hydration with δ+ H on water
Give a reason why a burette is used to add sodium hydroxide instead of a volumetric pipette in a titration. (1 mark)
Because volume can be added drop by drop to a smaller resolution using a burette
pipette has a fixed and larger volume
State an experimental technique which could be used to monitor the change in phenolphalein concentration. (1 mark)
colorimetry
What is the best solid to absorb water vapour?
anhydrous calcium chloride
Give a reason why the absorption of water vapour must occur before the absorption of carbon dioxide in a soda lime test? (1 mark)
because water will also react with soda lime
Explain why the industrial Haber process making ammonium uses a temperature of 700K rather than higher given reaction is exothermic. (3 marks)
- favours endothermic reaction which is backwards, we want ammonium
- though RoR will be increased, as rate of successful collisions will increase
- however higher temp costs more, reducing profit
- so 700K compromises good yield at good rate at fair cost
Samples of a reaction mixture are removed at timed intervals and titrated with alkali to determine the concentration of H+ (aq).
State and explain what must be done to each sample before it is titrated with alkali. (2 marks)
QUENCH sample
(Since titration with alkali, cannot add base to quench like usual)
Place in ice bucket to cool rapidly and slow RoR to negligible rate
Devise how to use a series of experiments to determine the order of a reaction in respect to A.
X is added that reacts rapidly with E.
A + B + C -> D + E
(This is a clock reaction, X is the mop)
- measure 5cm3 of 0.1 moldm-3 of A,B,C and X
- using a volumetric pipette
- mix all together in a conical flask
- use stopwatch to time how long for blue colour to appear
- repeat, with everything the same but changing the vol of A used. (4cm3 A + 1cm3 H2O, 3+2, 2+3, 1+4)
- record time taken
- 1/time to find rate and compare
- control temp using water bath
Describe the apparatus used to compare the decomposition of metal carbonates. Include how rate of decomposition would be compared. (2 marks)
- Two test tubes sealed connected with delivery tube, one submerge to bubble gas in in limewater, one heated
- Time taken for limewater to go cloudy
Why in recrystallisation, 1. minimum hot solvent is used and 2. ice bath is used?
- to obtain a saturated solution of crystallised compound
- maximise recrystallization, separate desired compound from impurities
Suggest a reason why recrystallisation will slightly reduce the yield of x. (1 mark)
Some retired products remain dissolved in cold solvent, and lost in separation process.
