Core Pracs Flashcards
1
Q
how to calculate percentage errors?
A
(uncertainty/quantity measured) x 100
if 2 readings taken (e.g. measuring change)
(2xuncertainty/quantity measured)x100
2
Q
Weak vs strong acids?
A
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
3
Q
what is avogadro’s law?
A
equal vol of gas at same temp and pressure will contain same number of molecules
6.02 x 10^23
4
Q
how to carry out an experiment to measure the molar volume of gas?
A
- react ethanoic acid and solid CaCO3
- gas syringe to measure vol of gas released
- repeat for increasing mass of CaCO3
5
Q
what are some issues with measuring volume of gas (CO2) with gas syringe?
A
- 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
6
Q
what is a standard solution?
A
solution of known concentration
7
Q
how to make a standard solution?
A
measure mass of solid with balance
transfer to volumetric flask, rinse with water
fill to line, bottom of meniscus touches line
invert to mix
8
Q
how to increase accuracy of weighing solids?
A
mass of difference technique
9
Q
how to detect when a titration reaction has reached completion?
A
use acid-base indicators, colour change indicates end point
10
Q
what are concordant results?
A
titres within 0.1cm3 of each other
11
Q
how to calculate conc?
A
mol/dm3
12
Q
what reaction is hydrolysis of halogenoalkanes?
A
nucleophilic substitution
13
Q
Ag+ (aq) + X- (aq) ->
A
AgX (s)
14
Q
what is added as a solvent for hydrolysis of halogenoalkanes?
A
ethanol
so reactants dissolve and mix
15
Q
what silver halide ppt forms the quickest and why?
A
AgI fastest, AgCl slowest
C-I is the weakest bond (large ionic radius of I)(in a halogenoalkane) so easier to react
15
Q
what are the ppt of silver halides?
A
AgCl - white
AgBr - cream
AgI - yellow
16
Q
what are the rates of primary, secondary and tertiary halogenoalkane hydrolysis?
A
tertiary is the fastest
primary slowest
17
Q
why are water baths used for hydrolysis of halogenoalkanes?
A
to keep temp constant
as a control variable
18
Q
how to decrease uncertainty in time taken?
A
use lower temp so RoR decreases, so % uncertainty will be lower
19
Q
how to produce ethanal from ethanol?
A
distillation
20
Q
how to produce ethanoic acid from ethanol?
A
reflux + potassium dichromate
21
Q
what reactants is needed to oxidise alcohol?
A
K2Cr2O7
22
Q
what is oxidation?
A
loss of electrons
oxidation num increases
23
Q
what colour change in potassium dichromate to indicate aldehyde and carboxylic acid?
A
orange to green
24
why is heating under reflux used?
- allow heating for a long period of time
- prevents volatile reactants/products escaping
- ensures even heating
- increase RoR
25
why are anti-bumping granules added/start by heating gently before heating under reflux or distillation?
How does it work?
to prevent spitting
- **distribute heat more evenly, form smaller bubbles to form**
26
how do you separate organic layer from aqueous layer?
transfer using separating funnel
open the tap, keep organic layer in separating funnel
27
how to remove unreacted HCl?
add NaHCO3, shake and open tap to release pressure
28
how to test for acid or base?
- indicator paper
red = acid
blue = alkali
- pH probe
29
how to conduct flame tests?
- dip nichrome wire
- into conc HCl solution
- blue bunsen flame
30
how to test for carbonate ions? CO3 2- or HCO3-
Ca(OH)2
clear to cloudy from CaCO3 ppt
31
how to test for ammonium? NH4+
add aqueous NaOH
warm mixture
ammonia gas turns red litmus paper blue
32
how to test for sulfate ions? SO4^2-
Add nitric acid (to remove carbonate ions)
then add barium chloride/nitrate
BaSO4 white ppt
33
how to test for bromides?
add chlorine water
turns orange if present
Or
Add nitric acid and silver nitrate
Cream ppt if positive
34
how to test for -OH groups?
PCl5
misty white fumes
35
how to test for alkenes?
bromine water
orange to colourless
36
how to test for aldehydes and ketones?
Fehling's solution
aldehyde: blue to red
ketone: stays blue
37
how to test for alcohol (primary, secondary) and aldehydes?
H2SO4 + K2Cr2O7
orange to green
in warm water bath
38
how to calculate enthalpy change of a reaction?
Q=mcΔT
%1000 into kJ
% moles of limiting reactant
39
+/- xx kJ
what do the signs mean?
+ is endothermic
- is exothermic
40
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
41
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
42
what is accuracy?
closeness to actual value
43
Suggest a method to separate pure ethanoic acid, boiling temperature 118C, from the water.
Fractional distillation
Remove water from ethanoic acid at 98-102C
44
How to make sure all water of crystallisation has been removed?
Heat to constant mass
Keep heating and weighing
45
Why is NaHCO3 added to organic layer?
To neutralise acid
Remove unreacted acid like HCl
46
What are 2 drying agents?
(anhydrous)
Calcium chloride or sodium sulfate
Becomes solid and filter off
47
How can you tell if a reaction is completed? Give 2 examples (2 marks)
- effervescence stops
- solid disappears
48
How can you tell if a reaction is completed? Give 2 examples (2 marks)
- effervescence stops
- solid disappears
49
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
50
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.
51
Why are anti-bumping granules used?
prevent boiling too vigorously and splashing into condenser
52
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
53
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)
54
How to lower percentage errors? (2 marks)
- use bigger mass of solids / sample size
- use balance with more decimal places
55
Identify a safety issue associated with this producing hydrogen iodide.
It is corrosive
56
+ - which one is endothermic and ex other is
+ endo
- exo
57
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
58
What apparatus is used for distillation?
Quickfit apparatus
- round bottomed flask or pear
- connected w T head
- liebeg condenser
- collecting vessel
59
How to remove any unreacted water from organic liquid?
Add anhydrous Na2SO4
60
Why are acid-base indicators used?
To detect when a reaction has reached its equivalence point
61
How do you calibrate a pH probe?
Submerge in buffer solutions
62
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)
63
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)
64
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
65
State 2 hazards associated with ammonia. (1 mark)
toxic and corrosive
66
What precaution must be used to work with ammonia gas?
Fume cupboard (and wear gloves)
67
what are halides?
chlorRIDE
broMIDE
iodIDE
68
What is the advantage of diluting a sample in titration?
can repeat titrations and find more valid results
with concordant titres
69
What is the disadvantage of diluting a sample in titration?
High % uncertainty
70
Give one ad and disad for applying ammonia directly into the soil as a fertiliser. (2 marks)
ad - high % of N
disad - corrosive / toxic
71
What is the type of reaction when
NH3 + HNO3 -> NH4NO3
neutralisation
/ acid base
as salt is made
72
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
73
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
74
State an experimental technique which could be used to monitor the change in phenolphalein concentration. (1 mark)
colorimetry
75
What is the best solid to absorb water vapour?
anhydrous calcium chloride
76
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
77
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
78
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
79
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
80
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
81
Why in recrystallisation, 1. minimum hot solvent is used and 2. ice bath is used?
1. to obtain a saturated solution of crystallised compound
2. maximise recrystallization, separate desired compound from impurities
83
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.
84
What is the apparatus called where water enters from the bottom and out from the top? Why?
Condenser!!!
To have EFFICIENT COOLING, prevent AIRLOCK
**must elaborate to max out marks**
85
What is potassium dichromate(VI) solution usually acidified with?
H2SO4
86
Explain the difference between experimental and actual value for enthalpy change of combustion of methanol.
Give 3 reasons (3 marks)
- heat lost to surroundings
- incomplete combustion of methanol
- evaporation of water
87
What is the maximum temperature of water that it can be heated to?
100C
CN be used to link to Qs
88
What are the safety issues associated with the experiment
H2 (g) + I2 (g) -> 2HI (g)
I2 vapour is toxic
HI is corrosive
H2SO4 flammable/explosive
89
IO3^- + 5I- + 6H+ -> 3I2 + 3H2O
State why the order of reaction with respect to iodide cannot be five, even though 5 mol of iodide ions are in the equation. (1 mark)
The chance of five or more ions colliding in the same RDS is negligible
91
