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N Chemistry A Level > Core Pracs > Flashcards

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

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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

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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

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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
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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
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6
Q

what is a standard solution?

A

solution of known concentration

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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

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8
Q

how to increase accuracy of weighing solids?

A

mass of difference technique

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9
Q

how to detect when a titration reaction has reached completion?

A

use acid-base indicators, colour change indicates end point

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10
Q

what are concordant results?

A

titres within 0.1cm3 of each other

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11
Q

how to calculate conc?

A

mol/dm3

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12
Q

what reaction is hydrolysis of halogenoalkanes?

A

nucleophilic substitution

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13
Q

Ag+ (aq) + X- (aq) ->

A

AgX (s)

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14
Q

what is added as a solvent for hydrolysis of halogenoalkanes?

A

ethanol
so reactants dissolve and mix

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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

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15
Q

what are the ppt of silver halides?

A

AgCl - white
AgBr - cream
AgI - yellow

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16
Q

what are the rates of primary, secondary and tertiary halogenoalkane hydrolysis?

A

tertiary is the fastest
primary slowest

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17
Q

why are water baths used for hydrolysis of halogenoalkanes?

A

to keep temp constant
as a control variable

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18
Q

how to decrease uncertainty in time taken?

A

use lower temp so RoR decreases, so % uncertainty will be lower

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19
Q

how to produce ethanal from ethanol?

A

distillation

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20
Q

how to produce ethanoic acid from ethanol?

A

reflux + potassium dichromate

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21
Q

what reactants is needed to oxidise alcohol?

A

K2Cr2O7

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22
Q

what is oxidation?

A

loss of electrons
oxidation num increases

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23
Q

what colour change in potassium dichromate to indicate aldehyde and carboxylic acid?

A

orange to green

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24
Q

why is heating under reflux used?

A
  • allow heating for a long period of time
  • prevents volatile reactants/products escaping
  • ensures even heating
  • increase RoR
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25
Q

why are anti-bumping granules added/start by heating gently before heating under reflux or distillation?

A

to prevent spitting
allow smooth boiling

26
Q

how do you separate organic layer from aqueous layer?

A

transfer using separating funnel
open the tap, keep organic layer in separating funnel

27
Q

how to remove unreacted HCl?

A

add NaHCO3, shake and open tap to release pressure

28
Q

how to test for acid or base?

A
  • indicator paper
    red = acid
    blue = alkali
  • pH probe
29
Q

how to conduct flame tests?

A
  • dip nichrome wire
  • into conc HCl solution
  • blue bunsen flame
30
Q

how to test for carbonate ions? CO3 2- or HCO3-

A

Ca(OH)2
clear to cloudy from CaCO3 ppt

31
Q

how to test for ammonium? NH4+

A

add aqueous NaOH
warm mixture
ammonia gas turns red litmus paper blue

32
Q

how to test for sulfate ions? SO4^2-

A

Add nitric acid (to remove carbonate ions)
then add barium chloride/nitrate
BaSO4 white ppt

33
Q

how to test for bromides?

A

add chlorine water
turns orange if present

Or
Add nitric acid and silver nitrate
Cream ppt if positive

34
Q

how to test for -OH groups?

A

PCl5
misty white fumes

35
Q

how to test for alkenes?

A

bromine water
orange to colourless

36
Q

how to test for aldehydes and ketones?

A

Fehling’s solution
aldehyde: blue to red
ketone: stays blue

37
Q

how to test for alcohol (primary, secondary) and aldehydes?

A

H2SO4 + K2Cr2O7
orange to green
in warm water bath

38
Q

how to calculate enthalpy change of a reaction?

A

Q=mcΔT
%1000 into kJ
% moles of limiting reactant

39
Q

+/- xx kJ
what do the signs mean?

A

+ is endothermic
- is exothermic

40
Q

how to prevent heat loss to surroundings in a reaction?

A
  • insulate using polystyrene cup with lid
  • avoid large temp differences between surrounding and equipment (calorimeter)
  • use a bomb calorimeter
41
Q

how to increase accuracy of calorimetry experiment?

A
  • stir solution so heat is evenly distributed
  • use digital thermometer
  • use greater concs and masses, greater temp change reduces % errors
42
Q

what is accuracy?

A

closeness to actual value

43
Q

Suggest a method to separate pure ethanoic acid, boiling temperature 118C, from the water.

A

Fractional distillation
Remove water from ethanoic acid at 98-102C

44
Q

How to make sure all water of crystallisation has been removed?

A

Heat to constant mass
Keep heating and weighing

45
Q

Why is NaHCO3 added to organic layer?

A

To neutralise acid
Remove unreacted acid like HCl

46
Q

What are 2 drying agents?

A

Calcium chloride or sodium sulfate
Becomes solid and filter off

47
Q

How can you tell if a reaction is completed? Give 2 examples (2 marks)

A
  • effervescence stops
  • solid disappears
48
Q

How can you tell if a reaction is completed? Give 2 examples (2 marks)

A
  • effervescence stops
  • solid disappears
49
Q

What is the role of sodium thiosulfate in iodine clock reaction?

A

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
Q

Why is a sodium hydroxide solution not the best candidate for making a standard solution?

A

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
Q

Why are anti-bumping granules used?

A

prevent boiling too vigorously and splashing into condenser

52
Q

Explain why crude oil can be separated with fractional distillation. (2 marks)

A
  • they have different boiling points
  • since they have different chain lengths and IMF
53
Q

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)

A
  • greater transfer loss from more repeats
  • takes more time
  • higher % error (calculate with given numbers)
54
Q

How to lower percentage errors?

A
  • use bigger mass of solids / sample size
  • use balance with more decimal places
55
Q

Identify a safety issue associated with this producing hydrogen iodide.

A

It is corrosive

56
Q

+ - which one is endothermic and ex other is

A

+ endo
- exo

57
Q

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.

A
  • 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
Q

What apparatus is used for distillation?

A

Quickfit apparatus
- round bottomed flask or pear
- connected w T head
- liebeg condenser
- collecting vessel

59
Q

How to remove any unreacted water from organic liquid?

A

Add anhydrous Na2SO4

60
Q

Why are acid-base indicators used?

A

To detect when a reaction has reached its equivalence point

61
Q

How do you calibrate a pH probe?

A

Submerge in buffer solutions

62
Q

What are 2 methods to measure rate of reaction?

A
  • initial rate reaction (like iodine clock)
  • continuous method (measuring change in vol over a period of time)
63
Q

What are 2 methods to measure rate of reaction?

A
  • initial rate reaction (like iodine clock)
  • continuous method (measuring change in vol over a period of time)

N Chemistry A Level (21 decks)

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